FileSystemProvider.java

/*
 * Copyright (c) 2007, 2011, Oracle and/or its affiliates. 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.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle 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 Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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package java.nio.file.spi;

import java.nio.file.*;
import java.nio.file.attribute.*;
import java.nio.channels.*;
import java.net.URI;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.IOException;
import java.util.*;
import java.util.concurrent.ExecutorService;
import java.security.AccessController;
import java.security.PrivilegedAction;

/**
 * Service-provider class for file systems. The methods defined by the {@link
 * java.nio.file.Files} class will typically delegate to an instance of this
 * class.
 *
 * <p> A file system provider is a concrete implementation of this class that
 * implements the abstract methods defined by this class. A provider is
 * identified by a {@code URI} {@link #getScheme() scheme}. The default provider
 * is identified by the URI scheme "file". It creates the {@link FileSystem} that
 * provides access to the file systems accessible to the Java virtual machine.
 * The {@link FileSystems} class defines how file system providers are located
 * and loaded. The default provider is typically a system-default provider but
 * may be overridden if the system property {@code
 * java.nio.file.spi.DefaultFileSystemProvider} is set. In that case, the
 * provider has a one argument constructor whose formal parameter type is {@code
 * FileSystemProvider}. All other providers have a zero argument constructor
 * that initializes the provider.
 *
 * <p> A provider is a factory for one or more {@link FileSystem} instances. Each
 * file system is identified by a {@code URI} where the URI's scheme matches
 * the provider's {@link #getScheme scheme}. The default file system, for example,
 * is identified by the URI {@code "file:///"}. A memory-based file system,
 * for example, may be identified by a URI such as {@code "memory:///?name=logfs"}.
 * The {@link #newFileSystem newFileSystem} method may be used to create a file
 * system, and the {@link #getFileSystem getFileSystem} method may be used to
 * obtain a reference to an existing file system created by the provider. Where
 * a provider is the factory for a single file system then it is provider dependent
 * if the file system is created when the provider is initialized, or later when
 * the {@code newFileSystem} method is invoked. In the case of the default
 * provider, the {@code FileSystem} is created when the provider is initialized.
 *
 * <p> All of the methods in this class are safe for use by multiple concurrent
 * threads.
 *
 * @since 1.7
 */

public abstract class FileSystemProvider {
    // lock using when loading providers
    private static final Object lock = new Object();

    // installed providers
    private static volatile List<FileSystemProvider> installedProviders;

    // used to avoid recursive loading of instaled providers
    private static boolean loadingProviders  = false;

    private static Void checkPermission() {
        SecurityManager sm = System.getSecurityManager();
        if (sm != null)
            sm.checkPermission(new RuntimePermission("fileSystemProvider"));
        return null;
    }
    private FileSystemProvider(Void ignore) { }

    /**
     * Initializes a new instance of this class.
     *
     * <p> During construction a provider may safely access files associated
     * with the default provider but care needs to be taken to avoid circular
     * loading of other installed providers. If circular loading of installed
     * providers is detected then an unspecified error is thrown.
     *
     * @throws  SecurityException
     *          If a security manager has been installed and it denies
     *          {@link RuntimePermission}<tt>("fileSystemProvider")</tt>
     */
    protected FileSystemProvider() {
        this(checkPermission());
    }

    // loads all installed providers
    private static List<FileSystemProvider> loadInstalledProviders() {
        List<FileSystemProvider> list = new ArrayList<FileSystemProvider>();

        ServiceLoader<FileSystemProvider> sl = ServiceLoader
            .load(FileSystemProvider.class, ClassLoader.getSystemClassLoader());

        // ServiceConfigurationError may be throw here
        for (FileSystemProvider provider: sl) {
            String scheme = provider.getScheme();

            // add to list if the provider is not "file" and isn't a duplicate
            if (!scheme.equalsIgnoreCase("file")) {
                boolean found = false;
                for (FileSystemProvider p: list) {
                    if (p.getScheme().equalsIgnoreCase(scheme)) {
                        found = true;
                        break;
                    }
                }
                if (!found) {
                    list.add(provider);
                }
            }
        }
        return list;
    }

    /**
     * Returns a list of the installed file system providers.
     *
     * <p> The first invocation of this method causes the default provider to be
     * initialized (if not already initialized) and loads any other installed
     * providers as described by the {@link FileSystems} class.
     *
     * @return  An unmodifiable list of the installed file system providers. The
     *          list contains at least one element, that is the default file
     *          system provider
     *
     * @throws  ServiceConfigurationError
     *          When an error occurs while loading a service provider
     */
    public static List<FileSystemProvider> installedProviders() {
        if (installedProviders == null) {
            // ensure default provider is initialized
            FileSystemProvider defaultProvider = FileSystems.getDefault().provider();

            synchronized (lock) {
                if (installedProviders == null) {
                    if (loadingProviders) {
                        throw new Error("Circular loading of installed providers detected");
                    }
                    loadingProviders = true;

                    List<FileSystemProvider> list = AccessController
                        .doPrivileged(new PrivilegedAction<List<FileSystemProvider>>() {
                            @Override
                            public List<FileSystemProvider> run() {
                                return loadInstalledProviders();
                        }});

                    // insert the default provider at the start of the list
                    list.add(0, defaultProvider);

                    installedProviders = Collections.unmodifiableList(list);
                }
            }
        }
        return installedProviders;
    }

    /**
     * Returns the URI scheme that identifies this provider.
     *
     * @return  The URI scheme
     */
    public abstract String getScheme();

    /**
     * Constructs a new {@code FileSystem} object identified by a URI. This
     * method is invoked by the {@link FileSystems#newFileSystem(URI,Map)}
     * method to open a new file system identified by a URI.
     *
     * <p> The {@code uri} parameter is an absolute, hierarchical URI, with a
     * scheme equal (without regard to case) to the scheme supported by this
     * provider. The exact form of the URI is highly provider dependent. The
     * {@code env} parameter is a map of provider specific properties to configure
     * the file system.
     *
     * <p> This method throws {@link FileSystemAlreadyExistsException} if the
     * file system already exists because it was previously created by an
     * invocation of this method. Once a file system is {@link
     * java.nio.file.FileSystem#close closed} it is provider-dependent if the
     * provider allows a new file system to be created with the same URI as a
     * file system it previously created.
     *
     * @param   uri
     *          URI reference
     * @param   env
     *          A map of provider specific properties to configure the file system;
     *          may be empty
     *
     * @return  A new file system
     *
     * @throws  IllegalArgumentException
     *          If the pre-conditions for the {@code uri} parameter aren't met,
     *          or the {@code env} parameter does not contain properties required
     *          by the provider, or a property value is invalid
     * @throws  IOException
     *          An I/O error occurs creating the file system
     * @throws  SecurityException
     *          If a security manager is installed and it denies an unspecified
     *          permission required by the file system provider implementation
     * @throws  FileSystemAlreadyExistsException
     *          If the file system has already been created
     */
    public abstract FileSystem newFileSystem(URI uri, Map<String,?> env)
        throws IOException;

    /**
     * Returns an existing {@code FileSystem} created by this provider.
     *
     * <p> This method returns a reference to a {@code FileSystem} that was
     * created by invoking the {@link #newFileSystem(URI,Map) newFileSystem(URI,Map)}
     * method. File systems created the {@link #newFileSystem(Path,Map)
     * newFileSystem(Path,Map)} method are not returned by this method.
     * The file system is identified by its {@code URI}. Its exact form
     * is highly provider dependent. In the case of the default provider the URI's
     * path component is {@code "/"} and the authority, query and fragment components
     * are undefined (Undefined components are represented by {@code null}).
     *
     * <p> Once a file system created by this provider is {@link
     * java.nio.file.FileSystem#close closed} it is provider-dependent if this
     * method returns a reference to the closed file system or throws {@link
     * FileSystemNotFoundException}. If the provider allows a new file system to
     * be created with the same URI as a file system it previously created then
     * this method throws the exception if invoked after the file system is
     * closed (and before a new instance is created by the {@link #newFileSystem
     * newFileSystem} method).
     *
     * <p> If a security manager is installed then a provider implementation
     * may require to check a permission before returning a reference to an
     * existing file system. In the case of the {@link FileSystems#getDefault
     * default} file system, no permission check is required.
     *
     * @param   uri
     *          URI reference
     *
     * @return  The file system
     *
     * @throws  IllegalArgumentException
     *          If the pre-conditions for the {@code uri} parameter aren't met
     * @throws  FileSystemNotFoundException
     *          If the file system does not exist
     * @throws  SecurityException
     *          If a security manager is installed and it denies an unspecified
     *          permission.
     */
    public abstract FileSystem getFileSystem(URI uri);

    /**
     * Return a {@code Path} object by converting the given {@link URI}. The
     * resulting {@code Path} is associated with a {@link FileSystem} that
     * already exists or is constructed automatically.
     *
     * <p> The exact form of the URI is file system provider dependent. In the
     * case of the default provider, the URI scheme is {@code "file"} and the
     * given URI has a non-empty path component, and undefined query, and
     * fragment components. The resulting {@code Path} is associated with the
     * default {@link FileSystems#getDefault default} {@code FileSystem}.
     *
     * <p> If a security manager is installed then a provider implementation
     * may require to check a permission. In the case of the {@link
     * FileSystems#getDefault default} file system, no permission check is
     * required.
     *
     * @param   uri
     *          The URI to convert
     *
     * @return  The resulting {@code Path}
     *
     * @throws  IllegalArgumentException
     *          If the URI scheme does not identify this provider or other
     *          preconditions on the uri parameter do not hold
     * @throws  FileSystemNotFoundException
     *          The file system, identified by the URI, does not exist and
     *          cannot be created automatically
     * @throws  SecurityException
     *          If a security manager is installed and it denies an unspecified
     *          permission.
     */
    public abstract Path getPath(URI uri);

    /**
     * Constructs a new {@code FileSystem} to access the contents of a file as a
     * file system.
     *
     * <p> This method is intended for specialized providers of pseudo file
     * systems where the contents of one or more files is treated as a file
     * system. The {@code env} parameter is a map of provider specific properties
     * to configure the file system.
     *
     * <p> If this provider does not support the creation of such file systems
     * or if the provider does not recognize the file type of the given file then
     * it throws {@code UnsupportedOperationException}. The default implementation
     * of this method throws {@code UnsupportedOperationException}.
     *
     * @param   path
     *          The path to the file
     * @param   env
     *          A map of provider specific properties to configure the file system;
     *          may be empty
     *
     * @return  A new file system
     *
     * @throws  UnsupportedOperationException
     *          If this provider does not support access to the contents as a
     *          file system or it does not recognize the file type of the
     *          given file
     * @throws  IllegalArgumentException
     *          If the {@code env} parameter does not contain properties required
     *          by the provider, or a property value is invalid
     * @throws  IOException
     *          If an I/O error occurs
     * @throws  SecurityException
     *          If a security manager is installed and it denies an unspecified
     *          permission.
     */
    public FileSystem newFileSystem(Path path, Map<String,?> env)
        throws IOException
    {
        throw new UnsupportedOperationException();
    }

    /**
     * Opens a file, returning an input stream to read from the file. This
     * method works in exactly the manner specified by the {@link
     * Files#newInputStream} method.
     *
     * <p> The default implementation of this method opens a channel to the file
     * as if by invoking the {@link #newByteChannel} method and constructs a
     * stream that reads bytes from the channel. This method should be overridden
     * where appropriate.
     *
     * @param   path
     *          the path to the file to open
     * @param   options
     *          options specifying how the file is opened
     *
     * @return  a new input stream
     *
     * @throws  IllegalArgumentException
     *          if an invalid combination of options is specified
     * @throws  UnsupportedOperationException
     *          if an unsupported option is specified
     * @throws  IOException
     *          if an I/O error occurs
     * @throws  SecurityException
     *          In the case of the default provider, and a security manager is
     *          installed, the {@link SecurityManager#checkRead(String) checkRead}
     *          method is invoked to check read access to the file.
     */
    public InputStream newInputStream(Path path, OpenOption... options)
        throws IOException
    {
        if (options.length > 0) {
            for (OpenOption opt: options) {
                // All OpenOption values except for APPEND and WRITE are allowed
                if (opt == StandardOpenOption.APPEND ||
                    opt == StandardOpenOption.WRITE)
                    throw new UnsupportedOperationException("'" + opt + "' not allowed");
            }
        }
        return Channels.newInputStream(Files.newByteChannel(path, options));
    }

    /**
     * Opens or creates a file, returning an output stream that may be used to
     * write bytes to the file. This method works in exactly the manner
     * specified by the {@link Files#newOutputStream} method.
     *
     * <p> The default implementation of this method opens a channel to the file
     * as if by invoking the {@link #newByteChannel} method and constructs a
     * stream that writes bytes to the channel. This method should be overridden
     * where appropriate.
     *
     * @param   path
     *          the path to the file to open or create
     * @param   options
     *          options specifying how the file is opened
     *
     * @return  a new output stream
     *
     * @throws  IllegalArgumentException
     *          if {@code options} contains an invalid combination of options
     * @throws  UnsupportedOperationException
     *          if an unsupported option is specified
     * @throws  IOException
     *          if an I/O error occurs
     * @throws  SecurityException
     *          In the case of the default provider, and a security manager is
     *          installed, the {@link SecurityManager#checkWrite(String) checkWrite}
     *          method is invoked to check write access to the file. The {@link
     *          SecurityManager#checkDelete(String) checkDelete} method is
     *          invoked to check delete access if the file is opened with the
     *          {@code DELETE_ON_CLOSE} option.
     */
    public OutputStream newOutputStream(Path path, OpenOption... options)
        throws IOException
    {
        int len = options.length;
        Set<OpenOption> opts = new HashSet<OpenOption>(len + 3);
        if (len == 0) {
            opts.add(StandardOpenOption.CREATE);
            opts.add(StandardOpenOption.TRUNCATE_EXISTING);
        } else {
            for (OpenOption opt: options) {
                if (opt == StandardOpenOption.READ)
                    throw new IllegalArgumentException("READ not allowed");
                opts.add(opt);
            }
        }
        opts.add(StandardOpenOption.WRITE);
        return Channels.newOutputStream(newByteChannel(path, opts));
    }

    /**
     * Opens or creates a file for reading and/or writing, returning a file
     * channel to access the file. This method works in exactly the manner
     * specified by the {@link FileChannel#open(Path,Set,FileAttribute[])
     * FileChannel.open} method. A provider that does not support all the
     * features required to construct a file channel throws {@code
     * UnsupportedOperationException}. The default provider is required to
     * support the creation of file channels. When not overridden, the default
     * implementation throws {@code UnsupportedOperationException}.
     *
     * @param   path
     *          the path of the file to open or create
     * @param   options
     *          options specifying how the file is opened
     * @param   attrs
     *          an optional list of file attributes to set atomically when
     *          creating the file
     *
     * @return  a new file channel
     *
     * @throws  IllegalArgumentException
     *          If the set contains an invalid combination of options
     * @throws  UnsupportedOperationException
     *          If this provider that does not support creating file channels,
     *          or an unsupported open option or file attribute is specified
     * @throws  IOException
     *          If an I/O error occurs
     * @throws  SecurityException
     *          In the case of the default file system, the {@link
     *          SecurityManager#checkRead(String)} method is invoked to check
     *          read access if the file is opened for reading. The {@link
     *          SecurityManager#checkWrite(String)} method is invoked to check
     *          write access if the file is opened for writing
     */
    public FileChannel newFileChannel(Path path,
                                      Set<? extends OpenOption> options,
                                      FileAttribute<?>... attrs)
        throws IOException
    {
        throw new UnsupportedOperationException();
    }

    /**
     * Opens or creates a file for reading and/or writing, returning an
     * asynchronous file channel to access the file. This method works in
     * exactly the manner specified by the {@link
     * AsynchronousFileChannel#open(Path,Set,ExecutorService,FileAttribute[])
     * AsynchronousFileChannel.open} method.
     * A provider that does not support all the features required to construct
     * an asynchronous file channel throws {@code UnsupportedOperationException}.
     * The default provider is required to support the creation of asynchronous
     * file channels. When not overridden, the default implementation of this
     * method throws {@code UnsupportedOperationException}.
     *
     * @param   path
     *          the path of the file to open or create
     * @param   options
     *          options specifying how the file is opened
     * @param   executor
     *          the thread pool or {@code null} to associate the channel with
     *          the default thread pool
     * @param   attrs
     *          an optional list of file attributes to set atomically when
     *          creating the file
     *
     * @return  a new asynchronous file channel
     *
     * @throws  IllegalArgumentException
     *          If the set contains an invalid combination of options
     * @throws  UnsupportedOperationException
     *          If this provider that does not support creating asynchronous file
     *          channels, or an unsupported open option or file attribute is
     *          specified
     * @throws  IOException
     *          If an I/O error occurs
     * @throws  SecurityException
     *          In the case of the default file system, the {@link
     *          SecurityManager#checkRead(String)} method is invoked to check
     *          read access if the file is opened for reading. The {@link
     *          SecurityManager#checkWrite(String)} method is invoked to check
     *          write access if the file is opened for writing
     */
    public AsynchronousFileChannel newAsynchronousFileChannel(Path path,
                                                              Set<? extends OpenOption> options,
                                                              ExecutorService executor,
                                                              FileAttribute<?>... attrs)
        throws IOException
    {
        throw new UnsupportedOperationException();
    }

    /**
     * Opens or creates a file, returning a seekable byte channel to access the
     * file. This method works in exactly the manner specified by the {@link
     * Files#newByteChannel(Path,Set,FileAttribute[])} method.
     *
     * @param   path
     *          the path to the file to open or create
     * @param   options
     *          options specifying how the file is opened
     * @param   attrs
     *          an optional list of file attributes to set atomically when
     *          creating the file
     *
     * @return  a new seekable byte channel
     *
     * @throws  IllegalArgumentException
     *          if the set contains an invalid combination of options
     * @throws  UnsupportedOperationException
     *          if an unsupported open option is specified or the array contains
     *          attributes that cannot be set atomically when creating the file
     * @throws  FileAlreadyExistsException
     *          if a file of that name already exists and the {@link
     *          StandardOpenOption#CREATE_NEW CREATE_NEW} option is specified
     *          <i>(optional specific exception)</i>
     * @throws  IOException
     *          if an I/O error occurs
     * @throws  SecurityException
     *          In the case of the default provider, and a security manager is
     *          installed, the {@link SecurityManager#checkRead(String) checkRead}
     *          method is invoked to check read access to the path if the file is
     *          opened for reading. The {@link SecurityManager#checkWrite(String)
     *          checkWrite} method is invoked to check write access to the path
     *          if the file is opened for writing. The {@link
     *          SecurityManager#checkDelete(String) checkDelete} method is
     *          invoked to check delete access if the file is opened with the
     *          {@code DELETE_ON_CLOSE} option.
     */
    public abstract SeekableByteChannel newByteChannel(Path path,
        Set<? extends OpenOption> options, FileAttribute<?>... attrs) throws IOException;

    /**
     * Opens a directory, returning a {@code DirectoryStream} to iterate over
     * the entries in the directory. This method works in exactly the manner
     * specified by the {@link
     * Files#newDirectoryStream(java.nio.file.Path, java.nio.file.DirectoryStream.Filter)}
     * method.
     *
     * @param   dir
     *          the path to the directory
     * @param   filter
     *          the directory stream filter
     *
     * @return  a new and open {@code DirectoryStream} object
     *
     * @throws  NotDirectoryException
     *          if the file could not otherwise be opened because it is not
     *          a directory <i>(optional specific exception)</i>
     * @throws  IOException
     *          if an I/O error occurs
     * @throws  SecurityException
     *          In the case of the default provider, and a security manager is
     *          installed, the {@link SecurityManager#checkRead(String) checkRead}
     *          method is invoked to check read access to the directory.
     */
    public abstract DirectoryStream<Path> newDirectoryStream(Path dir,
         DirectoryStream.Filter<? super Path> filter) throws IOException;

    /**
     * Creates a new directory. This method works in exactly the manner
     * specified by the {@link Files#createDirectory} method.
     *
     * @param   dir
     *          the directory to create
     * @param   attrs
     *          an optional list of file attributes to set atomically when
     *          creating the directory
     *
     * @throws  UnsupportedOperationException
     *          if the array contains an attribute that cannot be set atomically
     *          when creating the directory
     * @throws  FileAlreadyExistsException
     *          if a directory could not otherwise be created because a file of
     *          that name already exists <i>(optional specific exception)</i>
     * @throws  IOException
     *          if an I/O error occurs or the parent directory does not exist
     * @throws  SecurityException
     *          In the case of the default provider, and a security manager is
     *          installed, the {@link SecurityManager#checkWrite(String) checkWrite}
     *          method is invoked to check write access to the new directory.
     */
    public abstract void createDirectory(Path dir, FileAttribute<?>... attrs)
        throws IOException;

    /**
     * Creates a symbolic link to a target. This method works in exactly the
     * manner specified by the {@link Files#createSymbolicLink} method.
     *
     * <p> The default implementation of this method throws {@code
     * UnsupportedOperationException}.
     *
     * @param   link
     *          the path of the symbolic link to create
     * @param   target
     *          the target of the symbolic link
     * @param   attrs
     *          the array of attributes to set atomically when creating the
     *          symbolic link
     *
     * @throws  UnsupportedOperationException
     *          if the implementation does not support symbolic links or the
     *          array contains an attribute that cannot be set atomically when
     *          creating the symbolic link
     * @throws  FileAlreadyExistsException
     *          if a file with the name already exists <i>(optional specific
     *          exception)</i>
     * @throws  IOException
     *          if an I/O error occurs
     * @throws  SecurityException
     *          In the case of the default provider, and a security manager
     *          is installed, it denies {@link LinkPermission}<tt>("symbolic")</tt>
     *          or its {@link SecurityManager#checkWrite(String) checkWrite}
     *          method denies write access to the path of the symbolic link.
     */
    public void createSymbolicLink(Path link, Path target, FileAttribute<?>... attrs)
        throws IOException
    {
        throw new UnsupportedOperationException();
    }

    /**
     * Creates a new link (directory entry) for an existing file. This method
     * works in exactly the manner specified by the {@link Files#createLink}
     * method.
     *
     * <p> The default implementation of this method throws {@code
     * UnsupportedOperationException}.
     *
     * @param   link
     *          the link (directory entry) to create
     * @param   existing
     *          a path to an existing file
     *
     * @throws  UnsupportedOperationException
     *          if the implementation does not support adding an existing file
     *          to a directory
     * @throws  FileAlreadyExistsException
     *          if the entry could not otherwise be created because a file of
     *          that name already exists <i>(optional specific exception)</i>
     * @throws  IOException
     *          if an I/O error occurs
     * @throws  SecurityException
     *          In the case of the default provider, and a security manager
     *          is installed, it denies {@link LinkPermission}<tt>("hard")</tt>
     *          or its {@link SecurityManager#checkWrite(String) checkWrite}
     *          method denies write access to either the  link or the
     *          existing file.
     */
    public void createLink(Path link, Path existing) throws IOException {
        throw new UnsupportedOperationException();
    }

    /**
     * Deletes a file. This method works in exactly the  manner specified by the
     * {@link Files#delete} method.
     *
     * @param   path
     *          the path to the file to delete
     *
     * @throws  NoSuchFileException
     *          if the file does not exist <i>(optional specific exception)</i>
     * @throws  DirectoryNotEmptyException
     *          if the file is a directory and could not otherwise be deleted
     *          because the directory is not empty <i>(optional specific
     *          exception)</i>
     * @throws  IOException
     *          if an I/O error occurs
     * @throws  SecurityException
     *          In the case of the default provider, and a security manager is
     *          installed, the {@link SecurityManager#checkDelete(String)} method
     *          is invoked to check delete access to the file
     */
    public abstract void delete(Path path) throws IOException;

    /**
     * Deletes a file if it exists. This method works in exactly the manner
     * specified by the {@link Files#deleteIfExists} method.
     *
     * <p> The default implementation of this method simply invokes {@link
     * #delete} ignoring the {@code NoSuchFileException} when the file does not
     * exist. It may be overridden where appropriate.
     *
     * @param   path
     *          the path to the file to delete
     *
     * @return  {@code true} if the file was deleted by this method; {@code
     *          false} if the file could not be deleted because it did not
     *          exist
     *
     * @throws  DirectoryNotEmptyException
     *          if the file is a directory and could not otherwise be deleted
     *          because the directory is not empty <i>(optional specific
     *          exception)</i>
     * @throws  IOException
     *          if an I/O error occurs
     * @throws  SecurityException
     *          In the case of the default provider, and a security manager is
     *          installed, the {@link SecurityManager#checkDelete(String)} method
     *          is invoked to check delete access to the file
     */
    public boolean deleteIfExists(Path path) throws IOException {
        try {
            delete(path);
            return true;
        } catch (NoSuchFileException ignore) {
            return false;
        }
    }

    /**
     * Reads the target of a symbolic link. This method works in exactly the
     * manner specified by the {@link Files#readSymbolicLink} method.
     *
     * <p> The default implementation of this method throws {@code
     * UnsupportedOperationException}.
     *
     * @param   link
     *          the path to the symbolic link
     *
     * @return  The target of the symbolic link
     *
     * @throws  UnsupportedOperationException
     *          if the implementation does not support symbolic links
     * @throws  NotLinkException
     *          if the target could otherwise not be read because the file
     *          is not a symbolic link <i>(optional specific exception)</i>
     * @throws  IOException
     *          if an I/O error occurs
     * @throws  SecurityException
     *          In the case of the default provider, and a security manager
     *          is installed, it checks that {@code FilePermission} has been
     *          granted with the "{@code readlink}" action to read the link.
     */
    public Path readSymbolicLink(Path link) throws IOException {
        throw new UnsupportedOperationException();
    }

    /**
     * Copy a file to a target file. This method works in exactly the manner
     * specified by the {@link Files#copy(Path,Path,CopyOption[])} method
     * except that both the source and target paths must be associated with
     * this provider.
     *
     * @param   source
     *          the path to the file to copy
     * @param   target
     *          the path to the target file
     * @param   options
     *          options specifying how the copy should be done
     *
     * @throws  UnsupportedOperationException
     *          if the array contains a copy option that is not supported
     * @throws  FileAlreadyExistsException
     *          if the target file exists but cannot be replaced because the
     *          {@code REPLACE_EXISTING} option is not specified <i>(optional
     *          specific exception)</i>
     * @throws  DirectoryNotEmptyException
     *          the {@code REPLACE_EXISTING} option is specified but the file
     *          cannot be replaced because it is a non-empty directory
     *          <i>(optional specific exception)</i>
     * @throws  IOException
     *          if an I/O error occurs
     * @throws  SecurityException
     *          In the case of the default provider, and a security manager is
     *          installed, the {@link SecurityManager#checkRead(String) checkRead}
     *          method is invoked to check read access to the source file, the
     *          {@link SecurityManager#checkWrite(String) checkWrite} is invoked
     *          to check write access to the target file. If a symbolic link is
     *          copied the security manager is invoked to check {@link
     *          LinkPermission}{@code ("symbolic")}.
     */
    public abstract void copy(Path source, Path target, CopyOption... options)
        throws IOException;

    /**
     * Move or rename a file to a target file. This method works in exactly the
     * manner specified by the {@link Files#move} method except that both the
     * source and target paths must be associated with this provider.
     *
     * @param   source
     *          the path to the file to move
     * @param   target
     *          the path to the target file
     * @param   options
     *          options specifying how the move should be done
     *
     * @throws  UnsupportedOperationException
     *          if the array contains a copy option that is not supported
     * @throws  FileAlreadyExistsException
     *          if the target file exists but cannot be replaced because the
     *          {@code REPLACE_EXISTING} option is not specified <i>(optional
     *          specific exception)</i>
     * @throws  DirectoryNotEmptyException
     *          the {@code REPLACE_EXISTING} option is specified but the file
     *          cannot be replaced because it is a non-empty directory
     *          <i>(optional specific exception)</i>
     * @throws  AtomicMoveNotSupportedException
     *          if the options array contains the {@code ATOMIC_MOVE} option but
     *          the file cannot be moved as an atomic file system operation.
     * @throws  IOException
     *          if an I/O error occurs
     * @throws  SecurityException
     *          In the case of the default provider, and a security manager is
     *          installed, the {@link SecurityManager#checkWrite(String) checkWrite}
     *          method is invoked to check write access to both the source and
     *          target file.
     */
    public abstract void move(Path source, Path target, CopyOption... options)
        throws IOException;

    /**
     * Tests if two paths locate the same file. This method works in exactly the
     * manner specified by the {@link Files#isSameFile} method.
     *
     * @param   path
     *          one path to the file
     * @param   path2
     *          the other path
     *
     * @return  {@code true} if, and only if, the two paths locate the same file
     *
     * @throws  IOException
     *          if an I/O error occurs
     * @throws  SecurityException
     *          In the case of the default provider, and a security manager is
     *          installed, the {@link SecurityManager#checkRead(String) checkRead}
     *          method is invoked to check read access to both files.
     */
    public abstract boolean isSameFile(Path path, Path path2)
        throws IOException;

    /**
     * Tells whether or not a file is considered <em>hidden</em>. This method
     * works in exactly the manner specified by the {@link Files#isHidden}
     * method.
     *
     * <p> This method is invoked by the {@link Files#isHidden isHidden} method.
     *
     * @param   path
     *          the path to the file to test
     *
     * @return  {@code true} if the file is considered hidden
     *
     * @throws  IOException
     *          if an I/O error occurs
     * @throws  SecurityException
     *          In the case of the default provider, and a security manager is
     *          installed, the {@link SecurityManager#checkRead(String) checkRead}
     *          method is invoked to check read access to the file.
     */
    public abstract boolean isHidden(Path path) throws IOException;

    /**
     * Returns the {@link FileStore} representing the file store where a file
     * is located. This method works in exactly the manner specified by the
     * {@link Files#getFileStore} method.
     *
     * @param   path
     *          the path to the file
     *
     * @return  the file store where the file is stored
     *
     * @throws  IOException
     *          if an I/O error occurs
     * @throws  SecurityException
     *          In the case of the default provider, and a security manager is
     *          installed, the {@link SecurityManager#checkRead(String) checkRead}
     *          method is invoked to check read access to the file, and in
     *          addition it checks {@link RuntimePermission}<tt>
     *          ("getFileStoreAttributes")</tt>
     */
    public abstract FileStore getFileStore(Path path) throws IOException;

    /**
     * Checks the existence, and optionally the accessibility, of a file.
     *
     * <p> This method may be used by the {@link Files#isReadable isReadable},
     * {@link Files#isWritable isWritable} and {@link Files#isExecutable
     * isExecutable} methods to check the accessibility of a file.
     *
     * <p> This method checks the existence of a file and that this Java virtual
     * machine has appropriate privileges that would allow it access the file
     * according to all of access modes specified in the {@code modes} parameter
     * as follows:
     *
     * <table border=1 cellpadding=5 summary="">
     * <tr> <th>Value</th> <th>Description</th> </tr>
     * <tr>
     *   <td> {@link AccessMode#READ READ} </td>
     *   <td> Checks that the file exists and that the Java virtual machine has
     *     permission to read the file. </td>
     * </tr>
     * <tr>
     *   <td> {@link AccessMode#WRITE WRITE} </td>
     *   <td> Checks that the file exists and that the Java virtual machine has
     *     permission to write to the file, </td>
     * </tr>
     * <tr>
     *   <td> {@link AccessMode#EXECUTE EXECUTE} </td>
     *   <td> Checks that the file exists and that the Java virtual machine has
     *     permission to {@link Runtime#exec execute} the file. The semantics
     *     may differ when checking access to a directory. For example, on UNIX
     *     systems, checking for {@code EXECUTE} access checks that the Java
     *     virtual machine has permission to search the directory in order to
     *     access file or subdirectories. </td>
     * </tr>
     * </table>
     *
     * <p> If the {@code modes} parameter is of length zero, then the existence
     * of the file is checked.
     *
     * <p> This method follows symbolic links if the file referenced by this
     * object is a symbolic link. Depending on the implementation, this method
     * may require to read file permissions, access control lists, or other
     * file attributes in order to check the effective access to the file. To
     * determine the effective access to a file may require access to several
     * attributes and so in some implementations this method may not be atomic
     * with respect to other file system operations.
     *
     * @param   path
     *          the path to the file to check
     * @param   modes
     *          The access modes to check; may have zero elements
     *
     * @throws  UnsupportedOperationException
     *          an implementation is required to support checking for
     *          {@code READ}, {@code WRITE}, and {@code EXECUTE} access. This
     *          exception is specified to allow for the {@code Access} enum to
     *          be extended in future releases.
     * @throws  NoSuchFileException
     *          if a file does not exist <i>(optional specific exception)</i>
     * @throws  AccessDeniedException
     *          the requested access would be denied or the access cannot be
     *          determined because the Java virtual machine has insufficient
     *          privileges or other reasons. <i>(optional specific exception)</i>
     * @throws  IOException
     *          if an I/O error occurs
     * @throws  SecurityException
     *          In the case of the default provider, and a security manager is
     *          installed, the {@link SecurityManager#checkRead(String) checkRead}
     *          is invoked when checking read access to the file or only the
     *          existence of the file, the {@link SecurityManager#checkWrite(String)
     *          checkWrite} is invoked when checking write access to the file,
     *          and {@link SecurityManager#checkExec(String) checkExec} is invoked
     *          when checking execute access.
     */
    public abstract void checkAccess(Path path, AccessMode... modes)
        throws IOException;

    /**
     * Returns a file attribute view of a given type. This method works in
     * exactly the manner specified by the {@link Files#getFileAttributeView}
     * method.
     *
     * @param   <V>
     *          The {@code FileAttributeView} type
     * @param   path
     *          the path to the file
     * @param   type
     *          the {@code Class} object corresponding to the file attribute view
     * @param   options
     *          options indicating how symbolic links are handled
     *
     * @return  a file attribute view of the specified type, or {@code null} if
     *          the attribute view type is not available
     */
    public abstract <V extends FileAttributeView> V
        getFileAttributeView(Path path, Class<V> type, LinkOption... options);

    /**
     * Reads a file's attributes as a bulk operation. This method works in
     * exactly the manner specified by the {@link
     * Files#readAttributes(Path,Class,LinkOption[])} method.
     *
     * @param   <A>
     *          The {@code BasicFileAttributes} type
     * @param   path
     *          the path to the file
     * @param   type
     *          the {@code Class} of the file attributes required
     *          to read
     * @param   options
     *          options indicating how symbolic links are handled
     *
     * @return  the file attributes
     *
     * @throws  UnsupportedOperationException
     *          if an attributes of the given type are not supported
     * @throws  IOException
     *          if an I/O error occurs
     * @throws  SecurityException
     *          In the case of the default provider, a security manager is
     *          installed, its {@link SecurityManager#checkRead(String) checkRead}
     *          method is invoked to check read access to the file
     */
    public abstract <A extends BasicFileAttributes> A
        readAttributes(Path path, Class<A> type, LinkOption... options) throws IOException;

    /**
     * Reads a set of file attributes as a bulk operation. This method works in
     * exactly the manner specified by the {@link
     * Files#readAttributes(Path,String,LinkOption[])} method.
     *
     * @param   path
     *          the path to the file
     * @param   attributes
     *          the attributes to read
     * @param   options
     *          options indicating how symbolic links are handled
     *
     * @return  a map of the attributes returned; may be empty. The map's keys
     *          are the attribute names, its values are the attribute values
     *
     * @throws  UnsupportedOperationException
     *          if the attribute view is not available
     * @throws  IllegalArgumentException
     *          if no attributes are specified or an unrecognized attributes is
     *          specified
     * @throws  IOException
     *          If an I/O error occurs
     * @throws  SecurityException
     *          In the case of the default provider, and a security manager is
     *          installed, its {@link SecurityManager#checkRead(String) checkRead}
     *          method denies read access to the file. If this method is invoked
     *          to read security sensitive attributes then the security manager
     *          may be invoke to check for additional permissions.
     */
    public abstract Map<String,Object> readAttributes(Path path, String attributes,
                                                      LinkOption... options)
        throws IOException;

    /**
     * Sets the value of a file attribute. This method works in exactly the
     * manner specified by the {@link Files#setAttribute} method.
     *
     * @param   path
     *          the path to the file
     * @param   attribute
     *          the attribute to set
     * @param   value
     *          the attribute value
     * @param   options
     *          options indicating how symbolic links are handled
     *
     * @throws  UnsupportedOperationException
     *          if the attribute view is not available
     * @throws  IllegalArgumentException
     *          if the attribute name is not specified, or is not recognized, or
     *          the attribute value is of the correct type but has an
     *          inappropriate value
     * @throws  ClassCastException
     *          If the attribute value is not of the expected type or is a
     *          collection containing elements that are not of the expected
     *          type
     * @throws  IOException
     *          If an I/O error occurs
     * @throws  SecurityException
     *          In the case of the default provider, and a security manager is
     *          installed, its {@link SecurityManager#checkWrite(String) checkWrite}
     *          method denies write access to the file. If this method is invoked
     *          to set security sensitive attributes then the security manager
     *          may be invoked to check for additional permissions.
     */
    public abstract void setAttribute(Path path, String attribute,
                                      Object value, LinkOption... options)
        throws IOException;
}