ClassLoader.java

/*
 * Copyright 1994-2009 Sun Microsystems, Inc.  All Rights Reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Sun designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Sun in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
 * CA 95054 USA or visit www.sun.com if you need additional information or
 * have any questions.
 */
package java.lang;

import java.io.InputStream;
import java.io.IOException;
import java.io.File;
import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;
import java.net.MalformedURLException;
import java.net.URL;
import java.security.AccessController;
import java.security.AccessControlContext;
import java.security.CodeSource;
import java.security.Policy;
import java.security.PrivilegedAction;
import java.security.PrivilegedActionException;
import java.security.PrivilegedExceptionAction;
import java.security.ProtectionDomain;
import java.security.cert.Certificate;
import java.util.Collections;
import java.util.Enumeration;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Set;
import java.util.Stack;
import java.util.Map;
import java.util.Vector;
import java.util.Hashtable;
import java.util.WeakHashMap;
import java.util.concurrent.ConcurrentHashMap;
import sun.misc.ClassFileTransformer;
import sun.misc.CompoundEnumeration;
import sun.misc.Resource;
import sun.misc.URLClassPath;
import sun.misc.VM;
import sun.reflect.Reflection;
import sun.security.util.SecurityConstants;

/**
 * A class loader is an object that is responsible for loading classes. The
 * class <tt>ClassLoader</tt> is an abstract class.  Given the <a
 * href="#name">binary name</a> of a class, a class loader should attempt to
 * locate or generate data that constitutes a definition for the class.  A
 * typical strategy is to transform the name into a file name and then read a
 * "class file" of that name from a file system.
 *
 * <p> Every {@link Class <tt>Class</tt>} object contains a {@link
 * Class#getClassLoader() reference} to the <tt>ClassLoader</tt> that defined
 * it.
 *
 * <p> <tt>Class</tt> objects for array classes are not created by class
 * loaders, but are created automatically as required by the Java runtime.
 * The class loader for an array class, as returned by {@link
 * Class#getClassLoader()} is the same as the class loader for its element
 * type; if the element type is a primitive type, then the array class has no
 * class loader.
 *
 * <p> Applications implement subclasses of <tt>ClassLoader</tt> in order to
 * extend the manner in which the Java virtual machine dynamically loads
 * classes.
 *
 * <p> Class loaders may typically be used by security managers to indicate
 * security domains.
 *
 * <p> The <tt>ClassLoader</tt> class uses a delegation model to search for
 * classes and resources.  Each instance of <tt>ClassLoader</tt> has an
 * associated parent class loader.  When requested to find a class or
 * resource, a <tt>ClassLoader</tt> instance will delegate the search for the
 * class or resource to its parent class loader before attempting to find the
 * class or resource itself.  The virtual machine's built-in class loader,
 * called the "bootstrap class loader", does not itself have a parent but may
 * serve as the parent of a <tt>ClassLoader</tt> instance.
 *
 * <p> Class loaders that support concurrent loading of classes are known as
 * <em>parallel capable</em> class loaders and are required to register
 * themselves at their class initialization time by invoking the
 * {@link
 * #registerAsParallelCapable <tt>ClassLoader.registerAsParallelCapable</tt>}
 * method. In environments in which the delegation model is not strictly
 * hierarchical, class loaders need to be parallel capable, otherise class
 * loading can lead to deadlocks because the loader lock is held for the
 * duration of the class loading process (see {@link #loadClass
 * <tt>loadClass</tt>} methods).
 *
 * <p> Normally, the Java virtual machine loads classes from the local file
 * system in a platform-dependent manner.  For example, on UNIX systems, the
 * virtual machine loads classes from the directory defined by the
 * <tt>CLASSPATH</tt> environment variable.
 *
 * <p> However, some classes may not originate from a file; they may originate
 * from other sources, such as the network, or they could be constructed by an
 * application.  The method {@link #defineClass(String, byte[], int, int)
 * <tt>defineClass</tt>} converts an array of bytes into an instance of class
 * <tt>Class</tt>. Instances of this newly defined class can be created using
 * {@link Class#newInstance <tt>Class.newInstance</tt>}.
 *
 * <p> The methods and constructors of objects created by a class loader may
 * reference other classes.  To determine the class(es) referred to, the Java
 * virtual machine invokes the {@link #loadClass <tt>loadClass</tt>} method of
 * the class loader that originally created the class.
 *
 * <p> For example, an application could create a network class loader to
 * download class files from a server.  Sample code might look like:
 *
 * <blockquote><pre>
 *   ClassLoader loader&nbsp;= new NetworkClassLoader(host,&nbsp;port);
 *   Object main&nbsp;= loader.loadClass("Main", true).newInstance();
 *       &nbsp;.&nbsp;.&nbsp;.
 * </pre></blockquote>
 *
 * <p> The network class loader subclass must define the methods {@link
 * #findClass <tt>findClass</tt>} and <tt>loadClassData</tt> to load a class
 * from the network.  Once it has downloaded the bytes that make up the class,
 * it should use the method {@link #defineClass <tt>defineClass</tt>} to
 * create a class instance.  A sample implementation is:
 *
 * <blockquote><pre>
 *     class NetworkClassLoader extends ClassLoader {
 *         String host;
 *         int port;
 *
 *         public Class findClass(String name) {
 *             byte[] b = loadClassData(name);
 *             return defineClass(name, b, 0, b.length);
 *         }
 *
 *         private byte[] loadClassData(String name) {
 *             // load the class data from the connection
 *             &nbsp;.&nbsp;.&nbsp;.
 *         }
 *     }
 * </pre></blockquote>
 *
 * <h4> <a name="name">Binary names</a> </h4>
 *
 * <p> Any class name provided as a {@link String} parameter to methods in
 * <tt>ClassLoader</tt> must be a binary name as defined by the <a
 * href="http://java.sun.com/docs/books/jls/">Java Language Specification</a>.
 *
 * <p> Examples of valid class names include:
 * <blockquote><pre>
 *   "java.lang.String"
 *   "javax.swing.JSpinner$DefaultEditor"
 *   "java.security.KeyStore$Builder$FileBuilder$1"
 *   "java.net.URLClassLoader$3$1"
 * </pre></blockquote>
 *
 * @see      #resolveClass(Class)
 * @since 1.0
 */
public abstract class ClassLoader {

    private static native void registerNatives();

    // Set of classes which are registered as parallel capable class loaders
    private static final Set<Class<? extends ClassLoader>> parallelLoaders
        = Collections.newSetFromMap(Collections.synchronizedMap
              (new WeakHashMap<Class<? extends ClassLoader>, Boolean>()));

    static {
        registerNatives();
        parallelLoaders.add(ClassLoader.class);
    }

    // If initialization succeed this is set to true and security checks will
    // succeed.  Otherwise the object is not initialized and the object is
    // useless.
    private final boolean initialized;

    // The parent class loader for delegation
    // Note: VM hardcoded the offset of this field, thus all new fields
    // must be added *after* it.
    private final ClassLoader parent;

    // Maps class name to the corresponding lock object when the current
    // class loader is parallel capable.
    // Note: VM also uses this field to decide if the current class loader
    // is parallel capable and the appropriate lock object for class loading.
    private final ConcurrentHashMap<String, Object> parallelLockMap;

    // Hashtable that maps packages to certs
    private final Map <String, Certificate[]> package2certs;

    // Shared among all packages with unsigned classes
    private static final Certificate[] nocerts = new Certificate[0];

    // The classes loaded by this class loader. The only purpose of this table
    // is to keep the classes from being GC'ed until the loader is GC'ed.
    private final Vector<Class<?>> classes = new Vector<Class<?>>();

    // The "default" domain. Set as the default ProtectionDomain on newly
    // created classes.
    private final ProtectionDomain defaultDomain =
        new ProtectionDomain(new CodeSource(null, (Certificate[]) null),
                             null, this, null);

    // The initiating protection domains for all classes loaded by this loader
    private final Set<ProtectionDomain> domains;

    // Invoked by the VM to record every loaded class with this loader.
    void addClass(Class c) {
        classes.addElement(c);
    }

    // The packages defined in this class loader.  Each package name is mapped
    // to its corresponding Package object.
    // @GuardedBy("itself")
    private final HashMap<String, Package> packages =
        new HashMap<String, Package>();

    /**
     * Creates a new class loader using the specified parent class loader for
     * delegation.
     *
     * <p> If there is a security manager, its {@link
     * SecurityManager#checkCreateClassLoader()
     * <tt>checkCreateClassLoader</tt>} method is invoked.  This may result in
     * a security exception.  </p>
     *
     * @param  parent
     *         The parent class loader
     *
     * @throws  SecurityException
     *          If a security manager exists and its
     *          <tt>checkCreateClassLoader</tt> method doesn't allow creation
     *          of a new class loader.
     *
     * @since  1.2
     */
    protected ClassLoader(ClassLoader parent) {
        SecurityManager security = System.getSecurityManager();
        if (security != null) {
            security.checkCreateClassLoader();
        }
        this.parent = parent;
        if (parallelLoaders.contains(this.getClass())) {
            parallelLockMap = new ConcurrentHashMap<String, Object>();
            package2certs = new ConcurrentHashMap<String, Certificate[]>();
            domains =
                Collections.synchronizedSet(new HashSet<ProtectionDomain>());
            assertionLock = new Object();
        } else {
            // no finer-grained lock; lock on the classloader instance
            parallelLockMap = null;
            package2certs = new Hashtable<String, Certificate[]>();
            domains = new HashSet<ProtectionDomain>();
            assertionLock = this;
        }
        initialized = true;
    }

    /**
     * Creates a new class loader using the <tt>ClassLoader</tt> returned by
     * the method {@link #getSystemClassLoader()
     * <tt>getSystemClassLoader()</tt>} as the parent class loader.
     *
     * <p> If there is a security manager, its {@link
     * SecurityManager#checkCreateClassLoader()
     * <tt>checkCreateClassLoader</tt>} method is invoked.  This may result in
     * a security exception.  </p>
     *
     * @throws  SecurityException
     *          If a security manager exists and its
     *          <tt>checkCreateClassLoader</tt> method doesn't allow creation
     *          of a new class loader.
     */
    protected ClassLoader() {
        SecurityManager security = System.getSecurityManager();
        if (security != null) {
            security.checkCreateClassLoader();
        }
        this.parent = getSystemClassLoader();
        if (parallelLoaders.contains(this.getClass())) {
            parallelLockMap = new ConcurrentHashMap<String, Object>();
            package2certs = new ConcurrentHashMap<String, Certificate[]>();
            domains =
                Collections.synchronizedSet(new HashSet<ProtectionDomain>());
            assertionLock = new Object();
        } else {
            // no finer-grained lock; lock on the classloader instance
            parallelLockMap = null;
            package2certs = new Hashtable<String, Certificate[]>();
            domains = new HashSet<ProtectionDomain>();
            assertionLock = this;
        }
        initialized = true;
    }

    // -- Class --

    /**
     * Loads the class with the specified <a href="#name">binary name</a>.
     * This method searches for classes in the same manner as the {@link
     * #loadClass(String, boolean)} method.  It is invoked by the Java virtual
     * machine to resolve class references.  Invoking this method is equivalent
     * to invoking {@link #loadClass(String, boolean) <tt>loadClass(name,
     * false)</tt>}.  </p>
     *
     * @param  name
     *         The <a href="#name">binary name</a> of the class
     *
     * @return  The resulting <tt>Class</tt> object
     *
     * @throws  ClassNotFoundException
     *          If the class was not found
     */
    public Class<?> loadClass(String name) throws ClassNotFoundException {
        return loadClass(name, false);
    }

    /**
     * Loads the class with the specified <a href="#name">binary name</a>.  The
     * default implementation of this method searches for classes in the
     * following order:
     *
     * <p><ol>
     *
     *   <li><p> Invoke {@link #findLoadedClass(String)} to check if the class
     *   has already been loaded.  </p></li>
     *
     *   <li><p> Invoke the {@link #loadClass(String) <tt>loadClass</tt>} method
     *   on the parent class loader.  If the parent is <tt>null</tt> the class
     *   loader built-in to the virtual machine is used, instead.  </p></li>
     *
     *   <li><p> Invoke the {@link #findClass(String)} method to find the
     *   class.  </p></li>
     *
     * </ol>
     *
     * <p> If the class was found using the above steps, and the
     * <tt>resolve</tt> flag is true, this method will then invoke the {@link
     * #resolveClass(Class)} method on the resulting <tt>Class</tt> object.
     *
     * <p> Subclasses of <tt>ClassLoader</tt> are encouraged to override {@link
     * #findClass(String)}, rather than this method.  </p>
     *
     * <p> Unless overridden, this method synchronizes on the result of
     * {@link #getClassLoadingLock <tt>getClassLoadingLock</tt>} method
     * during the entire class loading process.
     *
     * @param  name
     *         The <a href="#name">binary name</a> of the class
     *
     * @param  resolve
     *         If <tt>true</tt> then resolve the class
     *
     * @return  The resulting <tt>Class</tt> object
     *
     * @throws  ClassNotFoundException
     *          If the class could not be found
     */
    protected Class<?> loadClass(String name, boolean resolve)
        throws ClassNotFoundException
    {
        synchronized (getClassLoadingLock(name)) {
            // First, check if the class has already been loaded
            Class c = findLoadedClass(name);
            if (c == null) {
                try {
                    if (parent != null) {
                        c = parent.loadClass(name, false);
                    } else {
                        c = findBootstrapClass0(name);
                    }
                } catch (ClassNotFoundException e) {
                    // If still not found, then invoke findClass in order
                    // to find the class.
                    c = findClass(name);
                }
            }
            if (resolve) {
                resolveClass(c);
            }
            return c;
        }
    }

    /**
     * Returns the lock object for class loading operations.
     * For backward compatibility, the default implementation of this method
     * behaves as follows. If this ClassLoader object is registered as
     * parallel capable, the method returns a dedicated object associated
     * with the specified class name. Otherwise, the method returns this
     * ClassLoader object. </p>
     *
     * @param  className
     *         The name of the to-be-loaded class
     *
     * @return the lock for class loading operations
     *
     * @throws NullPointerException
     *         If registered as parallel capable and <tt>className</tt> is null
     *
     * @see #loadClass(String, boolean)
     *
     * @since  1.7
     */
    protected Object getClassLoadingLock(String className) {
        Object lock = this;
        if (parallelLockMap != null) {
            Object newLock = new Object();
            lock = parallelLockMap.putIfAbsent(className, newLock);
            if (lock == null) {
                lock = newLock;
            }
        }
        return lock;
    }

    // This method is invoked by the virtual machine to load a class.
    private Class loadClassInternal(String name)
        throws ClassNotFoundException
    {
        // For backward compatibility, explicitly lock on 'this' when
        // the current class loader is not parallel capable.
        if (parallelLockMap == null) {
            synchronized (this) {
                 return loadClass(name);
            }
        } else {
            return loadClass(name);
        }
    }

    // Invoked by the VM after loading class with this loader.
    private void checkPackageAccess(Class cls, ProtectionDomain pd) {
        final SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            final String name = cls.getName();
            final int i = name.lastIndexOf('.');
            if (i != -1) {
                AccessController.doPrivileged(new PrivilegedAction<Void>() {
                    public Void run() {
                        sm.checkPackageAccess(name.substring(0, i));
                        return null;
                    }
                }, new AccessControlContext(new ProtectionDomain[] {pd}));
            }
        }
        domains.add(pd);
    }

    /**
     * Finds the class with the specified <a href="#name">binary name</a>.
     * This method should be overridden by class loader implementations that
     * follow the delegation model for loading classes, and will be invoked by
     * the {@link #loadClass <tt>loadClass</tt>} method after checking the
     * parent class loader for the requested class.  The default implementation
     * throws a <tt>ClassNotFoundException</tt>.  </p>
     *
     * @param  name
     *         The <a href="#name">binary name</a> of the class
     *
     * @return  The resulting <tt>Class</tt> object
     *
     * @throws  ClassNotFoundException
     *          If the class could not be found
     *
     * @since  1.2
     */
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        throw new ClassNotFoundException(name);
    }

    /**
     * Converts an array of bytes into an instance of class <tt>Class</tt>.
     * Before the <tt>Class</tt> can be used it must be resolved.  This method
     * is deprecated in favor of the version that takes a <a
     * href="#name">binary name</a> as its first argument, and is more secure.
     *
     * @param  b
     *         The bytes that make up the class data.  The bytes in positions
     *         <tt>off</tt> through <tt>off+len-1</tt> should have the format
     *         of a valid class file as defined by the <a
     *         href="http://java.sun.com/docs/books/vmspec/">Java Virtual
     *         Machine Specification</a>.
     *
     * @param  off
     *         The start offset in <tt>b</tt> of the class data
     *
     * @param  len
     *         The length of the class data
     *
     * @return  The <tt>Class</tt> object that was created from the specified
     *          class data
     *
     * @throws  ClassFormatError
     *          If the data did not contain a valid class
     *
     * @throws  IndexOutOfBoundsException
     *          If either <tt>off</tt> or <tt>len</tt> is negative, or if
     *          <tt>off+len</tt> is greater than <tt>b.length</tt>.
     *
     * @see  #loadClass(String, boolean)
     * @see  #resolveClass(Class)
     *
     * @deprecated  Replaced by {@link #defineClass(String, byte[], int, int)
     * defineClass(String, byte[], int, int)}
     */
    @Deprecated
    protected final Class<?> defineClass(byte[] b, int off, int len)
        throws ClassFormatError
    {
        return defineClass(null, b, off, len, null);
    }

    /**
     * Converts an array of bytes into an instance of class <tt>Class</tt>.
     * Before the <tt>Class</tt> can be used it must be resolved.
     *
     * <p> This method assigns a default {@link java.security.ProtectionDomain
     * <tt>ProtectionDomain</tt>} to the newly defined class.  The
     * <tt>ProtectionDomain</tt> is effectively granted the same set of
     * permissions returned when {@link
     * java.security.Policy#getPermissions(java.security.CodeSource)
     * <tt>Policy.getPolicy().getPermissions(new CodeSource(null, null))</tt>}
     * is invoked.  The default domain is created on the first invocation of
     * {@link #defineClass(String, byte[], int, int) <tt>defineClass</tt>},
     * and re-used on subsequent invocations.
     *
     * <p> To assign a specific <tt>ProtectionDomain</tt> to the class, use
     * the {@link #defineClass(String, byte[], int, int,
     * java.security.ProtectionDomain) <tt>defineClass</tt>} method that takes a
     * <tt>ProtectionDomain</tt> as one of its arguments.  </p>
     *
     * @param  name
     *         The expected <a href="#name">binary name</a> of the class, or
     *         <tt>null</tt> if not known
     *
     * @param  b
     *         The bytes that make up the class data.  The bytes in positions
     *         <tt>off</tt> through <tt>off+len-1</tt> should have the format
     *         of a valid class file as defined by the <a
     *         href="http://java.sun.com/docs/books/vmspec/">Java Virtual
     *         Machine Specification</a>.
     *
     * @param  off
     *         The start offset in <tt>b</tt> of the class data
     *
     * @param  len
     *         The length of the class data
     *
     * @return  The <tt>Class</tt> object that was created from the specified
     *          class data.
     *
     * @throws  ClassFormatError
     *          If the data did not contain a valid class
     *
     * @throws  IndexOutOfBoundsException
     *          If either <tt>off</tt> or <tt>len</tt> is negative, or if
     *          <tt>off+len</tt> is greater than <tt>b.length</tt>.
     *
     * @throws  SecurityException
     *          If an attempt is made to add this class to a package that
     *          contains classes that were signed by a different set of
     *          certificates than this class (which is unsigned), or if
     *          <tt>name</tt> begins with "<tt>java.</tt>".
     *
     * @see  #loadClass(String, boolean)
     * @see  #resolveClass(Class)
     * @see  java.security.CodeSource
     * @see  java.security.SecureClassLoader
     *
     * @since  1.1
     */
    protected final Class<?> defineClass(String name, byte[] b, int off, int len)
        throws ClassFormatError
    {
        return defineClass(name, b, off, len, null);
    }

    /* Determine protection domain, and check that:
        - not define java.* class,
        - signer of this class matches signers for the rest of the classes in
          package.
    */
    private ProtectionDomain preDefineClass(String name,
                                            ProtectionDomain pd)
    {
        if (!checkName(name))
            throw new NoClassDefFoundError("IllegalName: " + name);

        if ((name != null) && name.startsWith("java.")) {
            throw new SecurityException
                ("Prohibited package name: " +
                 name.substring(0, name.lastIndexOf('.')));
        }
        if (pd == null) {
            pd = defaultDomain;
        }

        if (name != null) checkCerts(name, pd.getCodeSource());

        return pd;
    }

    private String defineClassSourceLocation(ProtectionDomain pd)
    {
        CodeSource cs = pd.getCodeSource();
        String source = null;
        if (cs != null && cs.getLocation() != null) {
            source = cs.getLocation().toString();
        }
        return source;
    }

    private Class defineTransformedClass(String name, byte[] b, int off, int len,
                                         ProtectionDomain pd,
                                         ClassFormatError cfe, String source)
      throws ClassFormatError
    {
        // Class format error - try to transform the bytecode and
        // define the class again
        //
        ClassFileTransformer[] transformers =
            ClassFileTransformer.getTransformers();
        Class c = null;

        if (transformers != null) {
            for (ClassFileTransformer transformer : transformers) {
                try {
                    // Transform byte code using transformer
                    byte[] tb = transformer.transform(b, off, len);
                    c = defineClass1(name, tb, 0, tb.length,
                                     pd, source);
                    break;
                } catch (ClassFormatError cfe2)     {
                    // If ClassFormatError occurs, try next transformer
                }
            }
        }

        // Rethrow original ClassFormatError if unable to transform
        // bytecode to well-formed
        //
        if (c == null)
            throw cfe;

        return c;
    }

    private void postDefineClass(Class c, ProtectionDomain pd)
    {
        if (pd.getCodeSource() != null) {
            Certificate certs[] = pd.getCodeSource().getCertificates();
            if (certs != null)
                setSigners(c, certs);
        }
    }

    /**
     * Converts an array of bytes into an instance of class <tt>Class</tt>,
     * with an optional <tt>ProtectionDomain</tt>.  If the domain is
     * <tt>null</tt>, then a default domain will be assigned to the class as
     * specified in the documentation for {@link #defineClass(String, byte[],
     * int, int)}.  Before the class can be used it must be resolved.
     *
     * <p> The first class defined in a package determines the exact set of
     * certificates that all subsequent classes defined in that package must
     * contain.  The set of certificates for a class is obtained from the
     * {@link java.security.CodeSource <tt>CodeSource</tt>} within the
     * <tt>ProtectionDomain</tt> of the class.  Any classes added to that
     * package must contain the same set of certificates or a
     * <tt>SecurityException</tt> will be thrown.  Note that if
     * <tt>name</tt> is <tt>null</tt>, this check is not performed.
     * You should always pass in the <a href="#name">binary name</a> of the
     * class you are defining as well as the bytes.  This ensures that the
     * class you are defining is indeed the class you think it is.
     *
     * <p> The specified <tt>name</tt> cannot begin with "<tt>java.</tt>", since
     * all classes in the "<tt>java.*</tt> packages can only be defined by the
     * bootstrap class loader.  If <tt>name</tt> is not <tt>null</tt>, it
     * must be equal to the <a href="#name">binary name</a> of the class
     * specified by the byte array "<tt>b</tt>", otherwise a {@link
     * <tt>NoClassDefFoundError</tt>} will be thrown.  </p>
     *
     * @param  name
     *         The expected <a href="#name">binary name</a> of the class, or
     *         <tt>null</tt> if not known
     *
     * @param  b
     *         The bytes that make up the class data. The bytes in positions
     *         <tt>off</tt> through <tt>off+len-1</tt> should have the format
     *         of a valid class file as defined by the <a
     *         href="http://java.sun.com/docs/books/vmspec/">Java Virtual
     *         Machine Specification</a>.
     *
     * @param  off
     *         The start offset in <tt>b</tt> of the class data
     *
     * @param  len
     *         The length of the class data
     *
     * @param  protectionDomain
     *         The ProtectionDomain of the class
     *
     * @return  The <tt>Class</tt> object created from the data,
     *          and optional <tt>ProtectionDomain</tt>.
     *
     * @throws  ClassFormatError
     *          If the data did not contain a valid class
     *
     * @throws  NoClassDefFoundError
     *          If <tt>name</tt> is not equal to the <a href="#name">binary
     *          name</a> of the class specified by <tt>b</tt>
     *
     * @throws  IndexOutOfBoundsException
     *          If either <tt>off</tt> or <tt>len</tt> is negative, or if
     *          <tt>off+len</tt> is greater than <tt>b.length</tt>.
     *
     * @throws  SecurityException
     *          If an attempt is made to add this class to a package that
     *          contains classes that were signed by a different set of
     *          certificates than this class, or if <tt>name</tt> begins with
     *          "<tt>java.</tt>".
     */
    protected final Class<?> defineClass(String name, byte[] b, int off, int len,
                                         ProtectionDomain protectionDomain)
        throws ClassFormatError
    {
        check();
        protectionDomain = preDefineClass(name, protectionDomain);

        Class c = null;
        String source = defineClassSourceLocation(protectionDomain);

        try {
            c = defineClass1(name, b, off, len, protectionDomain, source);
        } catch (ClassFormatError cfe) {
            c = defineTransformedClass(name, b, off, len, protectionDomain, cfe,
                                       source);
        }

        postDefineClass(c, protectionDomain);
        return c;
    }

    /**
     * Converts a {@link java.nio.ByteBuffer <tt>ByteBuffer</tt>}
     * into an instance of class <tt>Class</tt>,
     * with an optional <tt>ProtectionDomain</tt>.  If the domain is
     * <tt>null</tt>, then a default domain will be assigned to the class as
     * specified in the documentation for {@link #defineClass(String, byte[],
     * int, int)}.  Before the class can be used it must be resolved.
     *
     * <p>The rules about the first class defined in a package determining the
     * set of certificates for the package, and the restrictions on class names
     * are identical to those specified in the documentation for {@link
     * #defineClass(String, byte[], int, int, ProtectionDomain)}.
     *
     * <p> An invocation of this method of the form
     * <i>cl</i><tt>.defineClass(</tt><i>name</i><tt>,</tt>
     * <i>bBuffer</i><tt>,</tt> <i>pd</i><tt>)</tt> yields exactly the same
     * result as the statements
     *
     * <blockquote><tt>
     * ...<br>
     * byte[] temp = new byte[</tt><i>bBuffer</i><tt>.{@link
     * java.nio.ByteBuffer#remaining remaining}()];<br>
     *     </tt><i>bBuffer</i><tt>.{@link java.nio.ByteBuffer#get(byte[])
     * get}(temp);<br>
     *     return {@link #defineClass(String, byte[], int, int, ProtectionDomain)
     * </tt><i>cl</i><tt>.defineClass}(</tt><i>name</i><tt>, temp, 0,
     * temp.length, </tt><i>pd</i><tt>);<br>
     * </tt></blockquote>
     *
     * @param  name
     *         The expected <a href="#name">binary name</a. of the class, or
     *         <tt>null</tt> if not known
     *
     * @param  b
     *         The bytes that make up the class data. The bytes from positions
     *         <tt>b.position()</tt> through <tt>b.position() + b.limit() -1
     *         </tt> should have the format of a valid class file as defined by
     *         the <a href="http://java.sun.com/docs/books/vmspec/">Java Virtual
     *         Machine Specification</a>.
     *
     * @param  protectionDomain
     *         The ProtectionDomain of the class, or <tt>null</tt>.
     *
     * @return  The <tt>Class</tt> object created from the data,
     *          and optional <tt>ProtectionDomain</tt>.
     *
     * @throws  ClassFormatError
     *          If the data did not contain a valid class.
     *
     * @throws  NoClassDefFoundError
     *          If <tt>name</tt> is not equal to the <a href="#name">binary
     *          name</a> of the class specified by <tt>b</tt>
     *
     * @throws  SecurityException
     *          If an attempt is made to add this class to a package that
     *          contains classes that were signed by a different set of
     *          certificates than this class, or if <tt>name</tt> begins with
     *          "<tt>java.</tt>".
     *
     * @see      #defineClass(String, byte[], int, int, ProtectionDomain)
     *
     * @since  1.5
     */
    protected final Class<?> defineClass(String name, java.nio.ByteBuffer b,
                                         ProtectionDomain protectionDomain)
        throws ClassFormatError
    {
        check();

        int len = b.remaining();

        // Use byte[] if not a direct ByteBufer:
        if (!b.isDirect()) {
            if (b.hasArray()) {
                return defineClass(name, b.array(),
                                   b.position() + b.arrayOffset(), len,
                                   protectionDomain);
            } else {
                // no array, or read-only array
                byte[] tb = new byte[len];
                b.get(tb);  // get bytes out of byte buffer.
                return defineClass(name, tb, 0, len, protectionDomain);
            }
        }

        protectionDomain = preDefineClass(name, protectionDomain);

        Class c = null;
        String source = defineClassSourceLocation(protectionDomain);

        try {
            c = defineClass2(name, b, b.position(), len, protectionDomain,
                             source);
        } catch (ClassFormatError cfe) {
            byte[] tb = new byte[len];
            b.get(tb);  // get bytes out of byte buffer.
            c = defineTransformedClass(name, tb, 0, len, protectionDomain, cfe,
                                       source);
        }

        postDefineClass(c, protectionDomain);
        return c;
    }

    private native Class defineClass0(String name, byte[] b, int off, int len,
                                      ProtectionDomain pd);

    private native Class defineClass1(String name, byte[] b, int off, int len,
                                      ProtectionDomain pd, String source);

    private native Class defineClass2(String name, java.nio.ByteBuffer b,
                                      int off, int len, ProtectionDomain pd,
                                      String source);

    // true if the name is null or has the potential to be a valid binary name
    private boolean checkName(String name) {
        if ((name == null) || (name.length() == 0))
            return true;
        if ((name.indexOf('/') != -1)
            || (!VM.allowArraySyntax() && (name.charAt(0) == '[')))
            return false;
        return true;
    }

    private void checkCerts(String name, CodeSource cs) {
        int i = name.lastIndexOf('.');
        String pname = (i == -1) ? "" : name.substring(0, i);

        Certificate[] certs = null;
        if (cs != null) {
            certs = cs.getCertificates();
        }
        Certificate[] pcerts = null;
        if (parallelLockMap == null) {
            synchronized (this) {
                pcerts = package2certs.get(pname);
                if (pcerts == null) {
                    package2certs.put(pname, (certs == null? nocerts:certs));
                }
            }
        } else {
            pcerts = ((ConcurrentHashMap<String, Certificate[]>)package2certs).
                putIfAbsent(pname, (certs == null? nocerts:certs));
        }
        if (pcerts != null && !compareCerts(pcerts, certs)) {
            throw new SecurityException("class \""+ name +
                 "\"'s signer information does not match signer information of other classes in the same package");
        }
    }

    /**
     * check to make sure the certs for the new class (certs) are the same as
     * the certs for the first class inserted in the package (pcerts)
     */
    private boolean compareCerts(Certificate[] pcerts,
                                 Certificate[] certs)
    {
        // certs can be null, indicating no certs.
        if ((certs == null) || (certs.length == 0)) {
            return pcerts.length == 0;
        }

        // the length must be the same at this point
        if (certs.length != pcerts.length)
            return false;

        // go through and make sure all the certs in one array
        // are in the other and vice-versa.
        boolean match;
        for (int i = 0; i < certs.length; i++) {
            match = false;
            for (int j = 0; j < pcerts.length; j++) {
                if (certs[i].equals(pcerts[j])) {
                    match = true;
                    break;
                }
            }
            if (!match) return false;
        }

        // now do the same for pcerts
        for (int i = 0; i < pcerts.length; i++) {
            match = false;
            for (int j = 0; j < certs.length; j++) {
                if (pcerts[i].equals(certs[j])) {
                    match = true;
                    break;
                }
            }
            if (!match) return false;
        }

        return true;
    }

    /**
     * Links the specified class.  This (misleadingly named) method may be
     * used by a class loader to link a class.  If the class <tt>c</tt> has
     * already been linked, then this method simply returns. Otherwise, the
     * class is linked as described in the "Execution" chapter of the <a
     * href="http://java.sun.com/docs/books/jls/">Java Language
     * Specification</a>.
     * </p>
     *
     * @param  c
     *         The class to link
     *
     * @throws  NullPointerException
     *          If <tt>c</tt> is <tt>null</tt>.
     *
     * @see  #defineClass(String, byte[], int, int)
     */
    protected final void resolveClass(Class<?> c) {
        check();
        resolveClass0(c);
    }

    private native void resolveClass0(Class c);

    /**
     * Finds a class with the specified <a href="#name">binary name</a>,
     * loading it if necessary.
     *
     * <p> This method loads the class through the system class loader (see
     * {@link #getSystemClassLoader()}).  The <tt>Class</tt> object returned
     * might have more than one <tt>ClassLoader</tt> associated with it.
     * Subclasses of <tt>ClassLoader</tt> need not usually invoke this method,
     * because most class loaders need to override just {@link
     * #findClass(String)}.  </p>
     *
     * @param  name
     *         The <a href="#name">binary name</a> of the class
     *
     * @return  The <tt>Class</tt> object for the specified <tt>name</tt>
     *
     * @throws  ClassNotFoundException
     *          If the class could not be found
     *
     * @see  #ClassLoader(ClassLoader)
     * @see  #getParent()
     */
    protected final Class<?> findSystemClass(String name)
        throws ClassNotFoundException
    {
        check();
        ClassLoader system = getSystemClassLoader();
        if (system == null) {
            if (!checkName(name))
                throw new ClassNotFoundException(name);
            return findBootstrapClass(name);
        }
        return system.loadClass(name);
    }

    private Class findBootstrapClass0(String name)
        throws ClassNotFoundException
    {
        check();
        if (!checkName(name))
            throw new ClassNotFoundException(name);
        return findBootstrapClass(name);
    }

    private native Class findBootstrapClass(String name)
        throws ClassNotFoundException;

    // Check to make sure the class loader has been initialized.
    private void check() {
        if (!initialized) {
            throw new SecurityException("ClassLoader object not initialized");
        }
    }

    /**
     * Returns the class with the given <a href="#name">binary name</a> if this
     * loader has been recorded by the Java virtual machine as an initiating
     * loader of a class with that <a href="#name">binary name</a>.  Otherwise
     * <tt>null</tt> is returned.  </p>
     *
     * @param  name
     *         The <a href="#name">binary name</a> of the class
     *
     * @return  The <tt>Class</tt> object, or <tt>null</tt> if the class has
     *          not been loaded
     *
     * @since  1.1
     */
    protected final Class<?> findLoadedClass(String name) {
        check();
        if (!checkName(name))
            return null;
        return findLoadedClass0(name);
    }

    private native final Class findLoadedClass0(String name);

    /**
     * Sets the signers of a class.  This should be invoked after defining a
     * class.  </p>
     *
     * @param  c
     *         The <tt>Class</tt> object
     *
     * @param  signers
     *         The signers for the class
     *
     * @since  1.1
     */
    protected final void setSigners(Class<?> c, Object[] signers) {
        check();
        c.setSigners(signers);
    }


    // -- Resource --

    /**
     * Finds the resource with the given name.  A resource is some data
     * (images, audio, text, etc) that can be accessed by class code in a way
     * that is independent of the location of the code.
     *
     * <p> The name of a resource is a '<tt>/</tt>'-separated path name that
     * identifies the resource.
     *
     * <p> This method will first search the parent class loader for the
     * resource; if the parent is <tt>null</tt> the path of the class loader
     * built-in to the virtual machine is searched.  That failing, this method
     * will invoke {@link #findResource(String)} to find the resource.  </p>
     *
     * @param  name
     *         The resource name
     *
     * @return  A <tt>URL</tt> object for reading the resource, or
     *          <tt>null</tt> if the resource could not be found or the invoker
     *          doesn't have adequate  privileges to get the resource.
     *
     * @since  1.1
     */
    public URL getResource(String name) {
        URL url;
        if (parent != null) {
            url = parent.getResource(name);
        } else {
            url = getBootstrapResource(name);
        }
        if (url == null) {
            url = findResource(name);
        }
        return url;
    }

    /**
     * Finds all the resources with the given name. A resource is some data
     * (images, audio, text, etc) that can be accessed by class code in a way
     * that is independent of the location of the code.
     *
     * <p>The name of a resource is a <tt>/</tt>-separated path name that
     * identifies the resource.
     *
     * <p> The search order is described in the documentation for {@link
     * #getResource(String)}.  </p>
     *
     * @param  name
     *         The resource name
     *
     * @return  An enumeration of {@link java.net.URL <tt>URL</tt>} objects for
     *          the resource.  If no resources could  be found, the enumeration
     *          will be empty.  Resources that the class loader doesn't have
     *          access to will not be in the enumeration.
     *
     * @throws  IOException
     *          If I/O errors occur
     *
     * @see  #findResources(String)
     *
     * @since  1.2
     */
    public Enumeration<URL> getResources(String name) throws IOException {
        Enumeration[] tmp = new Enumeration[2];
        if (parent != null) {
            tmp[0] = parent.getResources(name);
        } else {
            tmp[0] = getBootstrapResources(name);
        }
        tmp[1] = findResources(name);

        return new CompoundEnumeration<URL>(tmp);
    }

    /**
     * Finds the resource with the given name. Class loader implementations
     * should override this method to specify where to find resources.  </p>
     *
     * @param  name
     *         The resource name
     *
     * @return  A <tt>URL</tt> object for reading the resource, or
     *          <tt>null</tt> if the resource could not be found
     *
     * @since  1.2
     */
    protected URL findResource(String name) {
        return null;
    }

    /**
     * Returns an enumeration of {@link java.net.URL <tt>URL</tt>} objects
     * representing all the resources with the given name. Class loader
     * implementations should override this method to specify where to load
     * resources from.  </p>
     *
     * @param  name
     *         The resource name
     *
     * @return  An enumeration of {@link java.net.URL <tt>URL</tt>} objects for
     *          the resources
     *
     * @throws  IOException
     *          If I/O errors occur
     *
     * @since  1.2
     */
    protected Enumeration<URL> findResources(String name) throws IOException {
        return java.util.Collections.emptyEnumeration();
    }

    // index 0: java.lang.ClassLoader.class
    // index 1: the immediate caller of index 0.
    // index 2: the immediate caller of index 1.
    private static native Class<? extends ClassLoader> getCaller(int index);

    /**
     * Registers the caller class loader as parallel capable.
     * In order for the registration to succeed, all super classes
     * of the caller class loader must also be registered as
     * parallel capable when this method is called. </p>
     * Note that once a class loader is registered as
     * parallel capable, there is no way to change it back.
     * In addition, registration should be done statically before
     * any instance of the caller classloader being constructed. </p>
     *
     * @return  true if the caller is successfully registered as
     *          parallel capable and false if otherwise.
     *
     * @since   1.7
     */
    protected static boolean registerAsParallelCapable() {
        Class<? extends ClassLoader> caller = getCaller(1);
        Class superCls = caller.getSuperclass();
        boolean result = false;
        // Explicit synchronization needed for composite action
        synchronized (parallelLoaders) {
            if (!parallelLoaders.contains(caller)) {
                if (parallelLoaders.contains(superCls)) {
                    // register the immediate caller as parallel capable
                    // if and only if all of its super classes are.
                    // Note: given current classloading sequence, if
                    // the immediate super class is parallel capable,
                    // all the super classes higher up must be too.
                    result = true;
                    parallelLoaders.add(caller);
                }
            } else result = true;
        }
        return result;
    }

    /**
     * Find a resource of the specified name from the search path used to load
     * classes.  This method locates the resource through the system class
     * loader (see {@link #getSystemClassLoader()}).  </p>
     *
     * @param  name
     *         The resource name
     *
     * @return  A {@link java.net.URL <tt>URL</tt>} object for reading the
     *          resource, or <tt>null</tt> if the resource could not be found
     *
     * @since  1.1
     */
    public static URL getSystemResource(String name) {
        ClassLoader system = getSystemClassLoader();
        if (system == null) {
            return getBootstrapResource(name);
        }
        return system.getResource(name);
    }

    /**
     * Finds all resources of the specified name from the search path used to
     * load classes.  The resources thus found are returned as an
     * {@link java.util.Enumeration <tt>Enumeration</tt>} of {@link
     * java.net.URL <tt>URL</tt>} objects.
     *
     * <p> The search order is described in the documentation for {@link
     * #getSystemResource(String)}.  </p>
     *
     * @param  name
     *         The resource name
     *
     * @return  An enumeration of resource {@link java.net.URL <tt>URL</tt>}
     *          objects
     *
     * @throws  IOException
     *          If I/O errors occur

     * @since  1.2
     */
    public static Enumeration<URL> getSystemResources(String name)
        throws IOException
    {
        ClassLoader system = getSystemClassLoader();
        if (system == null) {
            return getBootstrapResources(name);
        }
        return system.getResources(name);
    }

    /**
     * Find resources from the VM's built-in classloader.
     */
    private static URL getBootstrapResource(String name) {
        URLClassPath ucp = getBootstrapClassPath();
        Resource res = ucp.getResource(name);
        return res != null ? res.getURL() : null;
    }

    /**
     * Find resources from the VM's built-in classloader.
     */
    private static Enumeration<URL> getBootstrapResources(String name)
        throws IOException
    {
        final Enumeration<Resource> e =
            getBootstrapClassPath().getResources(name);
        return new Enumeration<URL> () {
            public URL nextElement() {
                return e.nextElement().getURL();
            }
            public boolean hasMoreElements() {
                return e.hasMoreElements();
            }
        };
    }

    // Returns the URLClassPath that is used for finding system resources.
    static URLClassPath getBootstrapClassPath() {
        if (bootstrapClassPath == null) {
            bootstrapClassPath = sun.misc.Launcher.getBootstrapClassPath();
        }
        return bootstrapClassPath;
    }

    private static URLClassPath bootstrapClassPath;

    /**
     * Returns an input stream for reading the specified resource.
     *
     * <p> The search order is described in the documentation for {@link
     * #getResource(String)}.  </p>
     *
     * @param  name
     *         The resource name
     *
     * @return  An input stream for reading the resource, or <tt>null</tt>
     *          if the resource could not be found
     *
     * @since  1.1
     */
    public InputStream getResourceAsStream(String name) {
        URL url = getResource(name);
        try {
            return url != null ? url.openStream() : null;
        } catch (IOException e) {
            return null;
        }
    }

    /**
     * Open for reading, a resource of the specified name from the search path
     * used to load classes.  This method locates the resource through the
     * system class loader (see {@link #getSystemClassLoader()}).  </p>
     *
     * @param  name
     *         The resource name
     *
     * @return  An input stream for reading the resource, or <tt>null</tt>
     *          if the resource could not be found
     *
     * @since  1.1
     */
    public static InputStream getSystemResourceAsStream(String name) {
        URL url = getSystemResource(name);
        try {
            return url != null ? url.openStream() : null;
        } catch (IOException e) {
            return null;
        }
    }


    // -- Hierarchy --

    /**
     * Returns the parent class loader for delegation. Some implementations may
     * use <tt>null</tt> to represent the bootstrap class loader. This method
     * will return <tt>null</tt> in such implementations if this class loader's
     * parent is the bootstrap class loader.
     *
     * <p> If a security manager is present, and the invoker's class loader is
     * not <tt>null</tt> and is not an ancestor of this class loader, then this
     * method invokes the security manager's {@link
     * SecurityManager#checkPermission(java.security.Permission)
     * <tt>checkPermission</tt>} method with a {@link
     * RuntimePermission#RuntimePermission(String)
     * <tt>RuntimePermission("getClassLoader")</tt>} permission to verify
     * access to the parent class loader is permitted.  If not, a
     * <tt>SecurityException</tt> will be thrown.  </p>
     *
     * @return  The parent <tt>ClassLoader</tt>
     *
     * @throws  SecurityException
     *          If a security manager exists and its <tt>checkPermission</tt>
     *          method doesn't allow access to this class loader's parent class
     *          loader.
     *
     * @since  1.2
     */
    public final ClassLoader getParent() {
        if (parent == null)
            return null;
        SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            ClassLoader ccl = getCallerClassLoader();
            if (ccl != null && !isAncestor(ccl)) {
                sm.checkPermission(SecurityConstants.GET_CLASSLOADER_PERMISSION);
            }
        }
        return parent;
    }

    /**
     * Returns the system class loader for delegation.  This is the default
     * delegation parent for new <tt>ClassLoader</tt> instances, and is
     * typically the class loader used to start the application.
     *
     * <p> This method is first invoked early in the runtime's startup
     * sequence, at which point it creates the system class loader and sets it
     * as the context class loader of the invoking <tt>Thread</tt>.
     *
     * <p> The default system class loader is an implementation-dependent
     * instance of this class.
     *
     * <p> If the system property "<tt>java.system.class.loader</tt>" is defined
     * when this method is first invoked then the value of that property is
     * taken to be the name of a class that will be returned as the system
     * class loader.  The class is loaded using the default system class loader
     * and must define a public constructor that takes a single parameter of
     * type <tt>ClassLoader</tt> which is used as the delegation parent.  An
     * instance is then created using this constructor with the default system
     * class loader as the parameter.  The resulting class loader is defined
     * to be the system class loader.
     *
     * <p> If a security manager is present, and the invoker's class loader is
     * not <tt>null</tt> and the invoker's class loader is not the same as or
     * an ancestor of the system class loader, then this method invokes the
     * security manager's {@link
     * SecurityManager#checkPermission(java.security.Permission)
     * <tt>checkPermission</tt>} method with a {@link
     * RuntimePermission#RuntimePermission(String)
     * <tt>RuntimePermission("getClassLoader")</tt>} permission to verify
     * access to the system class loader.  If not, a
     * <tt>SecurityException</tt> will be thrown.  </p>
     *
     * @return  The system <tt>ClassLoader</tt> for delegation, or
     *          <tt>null</tt> if none
     *
     * @throws  SecurityException
     *          If a security manager exists and its <tt>checkPermission</tt>
     *          method doesn't allow access to the system class loader.
     *
     * @throws  IllegalStateException
     *          If invoked recursively during the construction of the class
     *          loader specified by the "<tt>java.system.class.loader</tt>"
     *          property.
     *
     * @throws  Error
     *          If the system property "<tt>java.system.class.loader</tt>"
     *          is defined but the named class could not be loaded, the
     *          provider class does not define the required constructor, or an
     *          exception is thrown by that constructor when it is invoked. The
     *          underlying cause of the error can be retrieved via the
     *          {@link Throwable#getCause()} method.
     *
     * @revised  1.4
     */
    public static ClassLoader getSystemClassLoader() {
        initSystemClassLoader();
        if (scl == null) {
            return null;
        }
        SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            ClassLoader ccl = getCallerClassLoader();
            if (ccl != null && ccl != scl && !scl.isAncestor(ccl)) {
                sm.checkPermission(SecurityConstants.GET_CLASSLOADER_PERMISSION);
            }
        }
        return scl;
    }

    private static synchronized void initSystemClassLoader() {
        if (!sclSet) {
            if (scl != null)
                throw new IllegalStateException("recursive invocation");
            sun.misc.Launcher l = sun.misc.Launcher.getLauncher();
            if (l != null) {
                Throwable oops = null;
                scl = l.getClassLoader();
                try {
                    scl = AccessController.doPrivileged(
                        new SystemClassLoaderAction(scl));
                } catch (PrivilegedActionException pae) {
                    oops = pae.getCause();
                    if (oops instanceof InvocationTargetException) {
                        oops = oops.getCause();
                    }
                }
                if (oops != null) {
                    if (oops instanceof Error) {
                        throw (Error) oops;
                    } else {
                        // wrap the exception
                        throw new Error(oops);
                    }
                }
            }
            sclSet = true;
        }
    }

    // Returns true if the specified class loader can be found in this class
    // loader's delegation chain.
    boolean isAncestor(ClassLoader cl) {
        ClassLoader acl = this;
        do {
            acl = acl.parent;
            if (cl == acl) {
                return true;
            }
        } while (acl != null);
        return false;
    }

    // Returns the invoker's class loader, or null if none.
    // NOTE: This must always be invoked when there is exactly one intervening
    // frame from the core libraries on the stack between this method's
    // invocation and the desired invoker.
    static ClassLoader getCallerClassLoader() {
        // NOTE use of more generic Reflection.getCallerClass()
        Class caller = Reflection.getCallerClass(3);
        // This can be null if the VM is requesting it
        if (caller == null) {
            return null;
        }
        // Circumvent security check since this is package-private
        return caller.getClassLoader0();
    }

    // The class loader for the system
    // @GuardedBy("ClassLoader.class")
    private static ClassLoader scl;

    // Set to true once the system class loader has been set
    // @GuardedBy("ClassLoader.class")
    private static boolean sclSet;


    // -- Package --

    /**
     * Defines a package by name in this <tt>ClassLoader</tt>.  This allows
     * class loaders to define the packages for their classes. Packages must
     * be created before the class is defined, and package names must be
     * unique within a class loader and cannot be redefined or changed once
     * created.  </p>
     *
     * @param  name
     *         The package name
     *
     * @param  specTitle
     *         The specification title
     *
     * @param  specVersion
     *         The specification version
     *
     * @param  specVendor
     *         The specification vendor
     *
     * @param  implTitle
     *         The implementation title
     *
     * @param  implVersion
     *         The implementation version
     *
     * @param  implVendor
     *         The implementation vendor
     *
     * @param  sealBase
     *         If not <tt>null</tt>, then this package is sealed with
     *         respect to the given code source {@link java.net.URL
     *         <tt>URL</tt>}  object.  Otherwise, the package is not sealed.
     *
     * @return  The newly defined <tt>Package</tt> object
     *
     * @throws  IllegalArgumentException
     *          If package name duplicates an existing package either in this
     *          class loader or one of its ancestors
     *
     * @since  1.2
     */
    protected Package definePackage(String name, String specTitle,
                                    String specVersion, String specVendor,
                                    String implTitle, String implVersion,
                                    String implVendor, URL sealBase)
        throws IllegalArgumentException
    {
        synchronized (packages) {
            Package pkg = getPackage(name);
            if (pkg != null) {
                throw new IllegalArgumentException(name);
            }
            pkg = new Package(name, specTitle, specVersion, specVendor,
                              implTitle, implVersion, implVendor,
                              sealBase, this);
            packages.put(name, pkg);
            return pkg;
        }
    }

    /**
     * Returns a <tt>Package</tt> that has been defined by this class loader
     * or any of its ancestors.  </p>
     *
     * @param  name
     *         The package name
     *
     * @return  The <tt>Package</tt> corresponding to the given name, or
     *          <tt>null</tt> if not found
     *
     * @since  1.2
     */
    protected Package getPackage(String name) {
        synchronized (packages) {
            Package pkg = packages.get(name);
            if (pkg == null) {
                if (parent != null) {
                    pkg = parent.getPackage(name);
                } else {
                    pkg = Package.getSystemPackage(name);
                }
                if (pkg != null) {
                    packages.put(name, pkg);
                }
            }
            return pkg;
        }
    }

    /**
     * Returns all of the <tt>Packages</tt> defined by this class loader and
     * its ancestors.  </p>
     *
     * @return  The array of <tt>Package</tt> objects defined by this
     *          <tt>ClassLoader</tt>
     *
     * @since  1.2
     */
    protected Package[] getPackages() {
        Map<String, Package> map;
        synchronized (packages) {
            map = new HashMap<String, Package>(packages);
        }
        Package[] pkgs;
        if (parent != null) {
            pkgs = parent.getPackages();
        } else {
            pkgs = Package.getSystemPackages();
        }
        if (pkgs != null) {
            for (int i = 0; i < pkgs.length; i++) {
                String pkgName = pkgs[i].getName();
                if (map.get(pkgName) == null) {
                    map.put(pkgName, pkgs[i]);
                }
            }
        }
        return map.values().toArray(new Package[map.size()]);
    }


    // -- Native library access --

    /**
     * Returns the absolute path name of a native library.  The VM invokes this
     * method to locate the native libraries that belong to classes loaded with
     * this class loader. If this method returns <tt>null</tt>, the VM
     * searches the library along the path specified as the
     * "<tt>java.library.path</tt>" property.  </p>
     *
     * @param  libname
     *         The library name
     *
     * @return  The absolute path of the native library
     *
     * @see  System#loadLibrary(String)
     * @see  System#mapLibraryName(String)
     *
     * @since  1.2
     */
    protected String findLibrary(String libname) {
        return null;
    }

    /**
     * The inner class NativeLibrary denotes a loaded native library instance.
     * Every classloader contains a vector of loaded native libraries in the
     * private field <tt>nativeLibraries</tt>.  The native libraries loaded
     * into the system are entered into the <tt>systemNativeLibraries</tt>
     * vector.
     *
     * <p> Every native library requires a particular version of JNI. This is
     * denoted by the private <tt>jniVersion</tt> field.  This field is set by
     * the VM when it loads the library, and used by the VM to pass the correct
     * version of JNI to the native methods.  </p>
     *
     * @see      ClassLoader
     * @since    1.2
     */
    static class NativeLibrary {
        // opaque handle to native library, used in native code.
        long handle;
        // the version of JNI environment the native library requires.
        private int jniVersion;
        // the class from which the library is loaded, also indicates
        // the loader this native library belongs.
        private Class fromClass;
        // the canonicalized name of the native library.
        String name;

        native void load(String name);
        native long find(String name);
        native void unload();

        public NativeLibrary(Class fromClass, String name) {
            this.name = name;
            this.fromClass = fromClass;
        }

        protected void finalize() {
            synchronized (loadedLibraryNames) {
                if (fromClass.getClassLoader() != null && handle != 0) {
                    /* remove the native library name */
                    int size = loadedLibraryNames.size();
                    for (int i = 0; i < size; i++) {
                        if (name.equals(loadedLibraryNames.elementAt(i))) {
                            loadedLibraryNames.removeElementAt(i);
                            break;
                        }
                    }
                    /* unload the library. */
                    ClassLoader.nativeLibraryContext.push(this);
                    try {
                        unload();
                    } finally {
                        ClassLoader.nativeLibraryContext.pop();
                    }
                }
            }
        }
        // Invoked in the VM to determine the context class in
        // JNI_Load/JNI_Unload
        static Class getFromClass() {
            return ClassLoader.nativeLibraryContext.peek().fromClass;
        }
    }

    // All native library names we've loaded.
    private static Vector<String> loadedLibraryNames
        = new Vector<String>();

    // Native libraries belonging to system classes.
    private static Vector<NativeLibrary> systemNativeLibraries
        = new Vector<NativeLibrary>();

    // Native libraries associated with the class loader.
    private Vector<NativeLibrary> nativeLibraries
        = new Vector<NativeLibrary>();

    // native libraries being loaded/unloaded.
    private static Stack<NativeLibrary> nativeLibraryContext
        = new Stack<NativeLibrary>();

    // The paths searched for libraries
    private static String usr_paths[];
    private static String sys_paths[];

    private static String[] initializePath(String propname) {
        String ldpath = System.getProperty(propname, "");
        String ps = File.pathSeparator;
        int ldlen = ldpath.length();
        int i, j, n;
        // Count the separators in the path
        i = ldpath.indexOf(ps);
        n = 0;
        while (i >= 0) {
            n++;
            i = ldpath.indexOf(ps, i + 1);
        }

        // allocate the array of paths - n :'s = n + 1 path elements
        String[] paths = new String[n + 1];

        // Fill the array with paths from the ldpath
        n = i = 0;
        j = ldpath.indexOf(ps);
        while (j >= 0) {
            if (j - i > 0) {
                paths[n++] = ldpath.substring(i, j);
            } else if (j - i == 0) {
                paths[n++] = ".";
            }
            i = j + 1;
            j = ldpath.indexOf(ps, i);
        }
        paths[n] = ldpath.substring(i, ldlen);
        return paths;
    }

    // Invoked in the java.lang.Runtime class to implement load and loadLibrary.
    static void loadLibrary(Class fromClass, String name,
                            boolean isAbsolute) {
        ClassLoader loader =
            (fromClass == null) ? null : fromClass.getClassLoader();
        if (sys_paths == null) {
            usr_paths = initializePath("java.library.path");
            sys_paths = initializePath("sun.boot.library.path");
        }
        if (isAbsolute) {
            if (loadLibrary0(fromClass, new File(name))) {
                return;
            }
            throw new UnsatisfiedLinkError("Can't load library: " + name);
        }
        if (loader != null) {
            String libfilename = loader.findLibrary(name);
            if (libfilename != null) {
                File libfile = new File(libfilename);
                if (!libfile.isAbsolute()) {
                    throw new UnsatisfiedLinkError(
    "ClassLoader.findLibrary failed to return an absolute path: " + libfilename);
                }
                if (loadLibrary0(fromClass, libfile)) {
                    return;
                }
                throw new UnsatisfiedLinkError("Can't load " + libfilename);
            }
        }
        for (int i = 0 ; i < sys_paths.length ; i++) {
            File libfile = new File(sys_paths[i], System.mapLibraryName(name));
            if (loadLibrary0(fromClass, libfile)) {
                return;
            }
        }
        if (loader != null) {
            for (int i = 0 ; i < usr_paths.length ; i++) {
                File libfile = new File(usr_paths[i],
                                        System.mapLibraryName(name));
                if (loadLibrary0(fromClass, libfile)) {
                    return;
                }
            }
        }
        // Oops, it failed
        throw new UnsatisfiedLinkError("no " + name + " in java.library.path");
    }

    private static boolean loadLibrary0(Class fromClass, final File file) {
        boolean exists = AccessController.doPrivileged(
            new PrivilegedAction<Object>() {
                public Object run() {
                    return file.exists() ? Boolean.TRUE : null;
                }})
            != null;
        if (!exists) {
            return false;
        }
        String name;
        try {
            name = file.getCanonicalPath();
        } catch (IOException e) {
            return false;
        }
        ClassLoader loader =
            (fromClass == null) ? null : fromClass.getClassLoader();
        Vector<NativeLibrary> libs =
            loader != null ? loader.nativeLibraries : systemNativeLibraries;
        synchronized (libs) {
            int size = libs.size();
            for (int i = 0; i < size; i++) {
                NativeLibrary lib = libs.elementAt(i);
                if (name.equals(lib.name)) {
                    return true;
                }
            }

            synchronized (loadedLibraryNames) {
                if (loadedLibraryNames.contains(name)) {
                    throw new UnsatisfiedLinkError
                        ("Native Library " +
                         name +
                         " already loaded in another classloader");
                }
                /* If the library is being loaded (must be by the same thread,
                 * because Runtime.load and Runtime.loadLibrary are
                 * synchronous). The reason is can occur is that the JNI_OnLoad
                 * function can cause another loadLibrary invocation.
                 *
                 * Thus we can use a static stack to hold the list of libraries
                 * we are loading.
                 *
                 * If there is a pending load operation for the library, we
                 * immediately return success; otherwise, we raise
                 * UnsatisfiedLinkError.
                 */
                int n = nativeLibraryContext.size();
                for (int i = 0; i < n; i++) {
                    NativeLibrary lib = nativeLibraryContext.elementAt(i);
                    if (name.equals(lib.name)) {
                        if (loader == lib.fromClass.getClassLoader()) {
                            return true;
                        } else {
                            throw new UnsatisfiedLinkError
                                ("Native Library " +
                                 name +
                                 " is being loaded in another classloader");
                        }
                    }
                }
                NativeLibrary lib = new NativeLibrary(fromClass, name);
                nativeLibraryContext.push(lib);
                try {
                    lib.load(name);
                } finally {
                    nativeLibraryContext.pop();
                }
                if (lib.handle != 0) {
                    loadedLibraryNames.addElement(name);
                    libs.addElement(lib);
                    return true;
                }
                return false;
            }
        }
    }

    // Invoked in the VM class linking code.
    static long findNative(ClassLoader loader, String name) {
        Vector<NativeLibrary> libs =
            loader != null ? loader.nativeLibraries : systemNativeLibraries;
        synchronized (libs) {
            int size = libs.size();
            for (int i = 0; i < size; i++) {
                NativeLibrary lib = libs.elementAt(i);
                long entry = lib.find(name);
                if (entry != 0)
                    return entry;
            }
        }
        return 0;
    }


    // -- Assertion management --

    final Object assertionLock;

    // The default toggle for assertion checking.
    // @GuardedBy("assertionLock")
    private boolean defaultAssertionStatus = false;

    // Maps String packageName to Boolean package default assertion status Note
    // that the default package is placed under a null map key.  If this field
    // is null then we are delegating assertion status queries to the VM, i.e.,
    // none of this ClassLoader's assertion status modification methods have
    // been invoked.
    // @GuardedBy("assertionLock")
    private Map<String, Boolean> packageAssertionStatus = null;

    // Maps String fullyQualifiedClassName to Boolean assertionStatus If this
    // field is null then we are delegating assertion status queries to the VM,
    // i.e., none of this ClassLoader's assertion status modification methods
    // have been invoked.
    // @GuardedBy("assertionLock")
    Map<String, Boolean> classAssertionStatus = null;

    /**
     * Sets the default assertion status for this class loader.  This setting
     * determines whether classes loaded by this class loader and initialized
     * in the future will have assertions enabled or disabled by default.
     * This setting may be overridden on a per-package or per-class basis by
     * invoking {@link #setPackageAssertionStatus(String, boolean)} or {@link
     * #setClassAssertionStatus(String, boolean)}.  </p>
     *
     * @param  enabled
     *         <tt>true</tt> if classes loaded by this class loader will
     *         henceforth have assertions enabled by default, <tt>false</tt>
     *         if they will have assertions disabled by default.
     *
     * @since  1.4
     */
    public void setDefaultAssertionStatus(boolean enabled) {
        synchronized (assertionLock) {
            if (classAssertionStatus == null)
                initializeJavaAssertionMaps();

            defaultAssertionStatus = enabled;
        }
    }

    /**
     * Sets the package default assertion status for the named package.  The
     * package default assertion status determines the assertion status for
     * classes initialized in the future that belong to the named package or
     * any of its "subpackages".
     *
     * <p> A subpackage of a package named p is any package whose name begins
     * with "<tt>p.</tt>".  For example, <tt>javax.swing.text</tt> is a
     * subpackage of <tt>javax.swing</tt>, and both <tt>java.util</tt> and
     * <tt>java.lang.reflect</tt> are subpackages of <tt>java</tt>.
     *
     * <p> In the event that multiple package defaults apply to a given class,
     * the package default pertaining to the most specific package takes
     * precedence over the others.  For example, if <tt>javax.lang</tt> and
     * <tt>javax.lang.reflect</tt> both have package defaults associated with
     * them, the latter package default applies to classes in
     * <tt>javax.lang.reflect</tt>.
     *
     * <p> Package defaults take precedence over the class loader's default
     * assertion status, and may be overridden on a per-class basis by invoking
     * {@link #setClassAssertionStatus(String, boolean)}.  </p>
     *
     * @param  packageName
     *         The name of the package whose package default assertion status
     *         is to be set. A <tt>null</tt> value indicates the unnamed
     *         package that is "current"
     *         (<a href="http://java.sun.com/docs/books/jls/">Java Language
     *         Specification</a>, section 7.4.2).
     *
     * @param  enabled
     *         <tt>true</tt> if classes loaded by this classloader and
     *         belonging to the named package or any of its subpackages will
     *         have assertions enabled by default, <tt>false</tt> if they will
     *         have assertions disabled by default.
     *
     * @since  1.4
     */
    public void setPackageAssertionStatus(String packageName,
                                          boolean enabled) {
        synchronized (assertionLock) {
            if (packageAssertionStatus == null)
                initializeJavaAssertionMaps();

            packageAssertionStatus.put(packageName, enabled);
        }
    }

    /**
     * Sets the desired assertion status for the named top-level class in this
     * class loader and any nested classes contained therein.  This setting
     * takes precedence over the class loader's default assertion status, and
     * over any applicable per-package default.  This method has no effect if
     * the named class has already been initialized.  (Once a class is
     * initialized, its assertion status cannot change.)
     *
     * <p> If the named class is not a top-level class, this invocation will
     * have no effect on the actual assertion status of any class. </p>
     *
     * @param  className
     *         The fully qualified class name of the top-level class whose
     *         assertion status is to be set.
     *
     * @param  enabled
     *         <tt>true</tt> if the named class is to have assertions
     *         enabled when (and if) it is initialized, <tt>false</tt> if the
     *         class is to have assertions disabled.
     *
     * @since  1.4
     */
    public void setClassAssertionStatus(String className, boolean enabled) {
        synchronized (assertionLock) {
            if (classAssertionStatus == null)
                initializeJavaAssertionMaps();

            classAssertionStatus.put(className, enabled);
        }
    }

    /**
     * Sets the default assertion status for this class loader to
     * <tt>false</tt> and discards any package defaults or class assertion
     * status settings associated with the class loader.  This method is
     * provided so that class loaders can be made to ignore any command line or
     * persistent assertion status settings and "start with a clean slate."
     * </p>
     *
     * @since  1.4
     */
    public void clearAssertionStatus() {
        /*
         * Whether or not "Java assertion maps" are initialized, set
         * them to empty maps, effectively ignoring any present settings.
         */
        synchronized (assertionLock) {
            classAssertionStatus = new HashMap<String, Boolean>();
            packageAssertionStatus = new HashMap<String, Boolean>();
            defaultAssertionStatus = false;
        }
    }

    /**
     * Returns the assertion status that would be assigned to the specified
     * class if it were to be initialized at the time this method is invoked.
     * If the named class has had its assertion status set, the most recent
     * setting will be returned; otherwise, if any package default assertion
     * status pertains to this class, the most recent setting for the most
     * specific pertinent package default assertion status is returned;
     * otherwise, this class loader's default assertion status is returned.
     * </p>
     *
     * @param  className
     *         The fully qualified class name of the class whose desired
     *         assertion status is being queried.
     *
     * @return  The desired assertion status of the specified class.
     *
     * @see  #setClassAssertionStatus(String, boolean)
     * @see  #setPackageAssertionStatus(String, boolean)
     * @see  #setDefaultAssertionStatus(boolean)
     *
     * @since  1.4
     */
    boolean desiredAssertionStatus(String className) {
        synchronized (assertionLock) {
            // assert classAssertionStatus   != null;
            // assert packageAssertionStatus != null;

            // Check for a class entry
            Boolean result = classAssertionStatus.get(className);
            if (result != null)
                return result.booleanValue();

            // Check for most specific package entry
            int dotIndex = className.lastIndexOf(".");
            if (dotIndex < 0) { // default package
                result = packageAssertionStatus.get(null);
                if (result != null)
                    return result.booleanValue();
            }
            while(dotIndex > 0) {
                className = className.substring(0, dotIndex);
                result = packageAssertionStatus.get(className);
                if (result != null)
                    return result.booleanValue();
                dotIndex = className.lastIndexOf(".", dotIndex-1);
            }

            // Return the classloader default
            return defaultAssertionStatus;
        }
    }

    // Set up the assertions with information provided by the VM.
    // Note: Should only be called inside a synchronized block
    private void initializeJavaAssertionMaps() {
        // assert Thread.holdsLock(assertionLock);

        classAssertionStatus = new HashMap<String, Boolean>();
        packageAssertionStatus = new HashMap<String, Boolean>();
        AssertionStatusDirectives directives = retrieveDirectives();

        for(int i = 0; i < directives.classes.length; i++)
            classAssertionStatus.put(directives.classes[i],
                                     directives.classEnabled[i]);

        for(int i = 0; i < directives.packages.length; i++)
            packageAssertionStatus.put(directives.packages[i],
                                       directives.packageEnabled[i]);

        defaultAssertionStatus = directives.deflt;
    }

    // Retrieves the assertion directives from the VM.
    private static native AssertionStatusDirectives retrieveDirectives();
}


class SystemClassLoaderAction
    implements PrivilegedExceptionAction<ClassLoader> {
    private ClassLoader parent;

    SystemClassLoaderAction(ClassLoader parent) {
        this.parent = parent;
    }

    public ClassLoader run() throws Exception {
        String cls = System.getProperty("java.system.class.loader");
        if (cls == null) {
            return parent;
        }

        Constructor ctor = Class.forName(cls, true, parent)
            .getDeclaredConstructor(new Class[] { ClassLoader.class });
        ClassLoader sys = (ClassLoader) ctor.newInstance(
            new Object[] { parent });
        Thread.currentThread().setContextClassLoader(sys);
        return sys;
    }
}