Throwable class is the superclass of all errors and
* exceptions in the Java language. Only objects that are instances of this
* class (or one of its subclasses) are thrown by the Java Virtual Machine or
* can be thrown by the Java throw statement. Similarly, only
* this class or one of its subclasses can be the argument type in a
* catch clause.
*
* Instances of two subclasses, {@link java.lang.Error} and * {@link java.lang.Exception}, are conventionally used to indicate * that exceptional situations have occurred. Typically, these instances * are freshly created in the context of the exceptional situation so * as to include relevant information (such as stack trace data). * *
A throwable contains a snapshot of the execution stack of its thread at * the time it was created. It can also contain a message string that gives * more information about the error. Finally, it can contain a cause: * another throwable that caused this throwable to get thrown. The cause * facility is new in release 1.4. It is also known as the chained * exception facility, as the cause can, itself, have a cause, and so on, * leading to a "chain" of exceptions, each caused by another. * *
One reason that a throwable may have a cause is that the class that * throws it is built atop a lower layered abstraction, and an operation on * the upper layer fails due to a failure in the lower layer. It would be bad * design to let the throwable thrown by the lower layer propagate outward, as * it is generally unrelated to the abstraction provided by the upper layer. * Further, doing so would tie the API of the upper layer to the details of * its implementation, assuming the lower layer's exception was a checked * exception. Throwing a "wrapped exception" (i.e., an exception containing a * cause) allows the upper layer to communicate the details of the failure to * its caller without incurring either of these shortcomings. It preserves * the flexibility to change the implementation of the upper layer without * changing its API (in particular, the set of exceptions thrown by its * methods). * *
A second reason that a throwable may have a cause is that the method * that throws it must conform to a general-purpose interface that does not * permit the method to throw the cause directly. For example, suppose * a persistent collection conforms to the {@link java.util.Collection * Collection} interface, and that its persistence is implemented atop * java.io. Suppose the internals of the add method * can throw an {@link java.io.IOException IOException}. The implementation * can communicate the details of the IOException to its caller * while conforming to the Collection interface by wrapping the * IOException in an appropriate unchecked exception. (The * specification for the persistent collection should indicate that it is * capable of throwing such exceptions.) * *
A cause can be associated with a throwable in two ways: via a * constructor that takes the cause as an argument, or via the * {@link #initCause(Throwable)} method. New throwable classes that * wish to allow causes to be associated with them should provide constructors * that take a cause and delegate (perhaps indirectly) to one of the * Throwable constructors that takes a cause. For example: *
* try {
* lowLevelOp();
* } catch (LowLevelException le) {
* throw new HighLevelException(le); // Chaining-aware constructor
* }
*
* Because the initCause method is public, it allows a cause to be
* associated with any throwable, even a "legacy throwable" whose
* implementation predates the addition of the exception chaining mechanism to
* Throwable. For example:
*
* try {
* lowLevelOp();
* } catch (LowLevelException le) {
* throw (HighLevelException)
new HighLevelException().initCause(le); // Legacy constructor
* }
*
*
* Prior to release 1.4, there were many throwables that had their own * non-standard exception chaining mechanisms ( * {@link ExceptionInInitializerError}, {@link ClassNotFoundException}, * {@link java.lang.reflect.UndeclaredThrowableException}, * {@link java.lang.reflect.InvocationTargetException}, * {@link java.io.WriteAbortedException}, * {@link java.security.PrivilegedActionException}, * {@link java.awt.print.PrinterIOException}, * {@link java.rmi.RemoteException} and * {@link javax.naming.NamingException}). * All of these throwables have been retrofitted to * use the standard exception chaining mechanism, while continuing to * implement their "legacy" chaining mechanisms for compatibility. * *
Further, as of release 1.4, many general purpose Throwable * classes (for example {@link Exception}, {@link RuntimeException}, * {@link Error}) have been retrofitted with constructors that take * a cause. This was not strictly necessary, due to the existence of the * initCause method, but it is more convenient and expressive to * delegate to a constructor that takes a cause. * *
By convention, class Throwable and its subclasses have two
* constructors, one that takes no arguments and one that takes a
* String argument that can be used to produce a detail message.
* Further, those subclasses that might likely have a cause associated with
* them should have two more constructors, one that takes a
* Throwable (the cause), and one that takes a
* String (the detail message) and a Throwable (the
* cause).
*
*
Also introduced in release 1.4 is the {@link #getStackTrace()} method,
* which allows programmatic access to the stack trace information that was
* previously available only in text form, via the various forms of the
* {@link #printStackTrace()} method. This information has been added to the
* serialized representation of this class so getStackTrace
* and printStackTrace will operate properly on a throwable that
* was obtained by deserialization.
*
* @author unascribed
* @author Josh Bloch (Added exception chaining and programmatic access to
* stack trace in 1.4.)
* @since JDK1.0
*/
public class Throwable implements Serializable {
/** use serialVersionUID from JDK 1.0.2 for interoperability */
private static final long serialVersionUID = -3042686055658047285L;
/**
* Native code saves some indication of the stack backtrace in this slot.
*/
private transient Object backtrace;
/**
* Specific details about the Throwable. For example, for
* FileNotFoundException, this contains the name of
* the file that could not be found.
*
* @serial
*/
private String detailMessage;
/**
* The throwable that caused this throwable to get thrown, or null if this
* throwable was not caused by another throwable, or if the causative
* throwable is unknown. If this field is equal to this throwable itself,
* it indicates that the cause of this throwable has not yet been
* initialized.
*
* @serial
* @since 1.4
*/
private Throwable cause = this;
/**
* The stack trace, as returned by {@link #getStackTrace()}.
*
* @serial
* @since 1.4
*/
private StackTraceElement[] stackTrace;
/*
* This field is lazily initialized on first use or serialization and
* nulled out when fillInStackTrace is called.
*/
/**
* Constructs a new throwable with null as its detail message.
* The cause is not initialized, and may subsequently be initialized by a
* call to {@link #initCause}.
*
*
The {@link #fillInStackTrace()} method is called to initialize * the stack trace data in the newly created throwable. */ public Throwable() { fillInStackTrace(); } /** * Constructs a new throwable with the specified detail message. The * cause is not initialized, and may subsequently be initialized by * a call to {@link #initCause}. * *
The {@link #fillInStackTrace()} method is called to initialize * the stack trace data in the newly created throwable. * * @param message the detail message. The detail message is saved for * later retrieval by the {@link #getMessage()} method. */ public Throwable(String message) { fillInStackTrace(); detailMessage = message; } /** * Constructs a new throwable with the specified detail message and * cause.
Note that the detail message associated with
* cause is not automatically incorporated in
* this throwable's detail message.
*
*
The {@link #fillInStackTrace()} method is called to initialize * the stack trace data in the newly created throwable. * * @param message the detail message (which is saved for later retrieval * by the {@link #getMessage()} method). * @param cause the cause (which is saved for later retrieval by the * {@link #getCause()} method). (A null value is * permitted, and indicates that the cause is nonexistent or * unknown.) * @since 1.4 */ public Throwable(String message, Throwable cause) { fillInStackTrace(); detailMessage = message; this.cause = cause; } /** * Constructs a new throwable with the specified cause and a detail * message of (cause==null ? null : cause.toString()) (which * typically contains the class and detail message of cause). * This constructor is useful for throwables that are little more than * wrappers for other throwables (for example, {@link * java.security.PrivilegedActionException}). * *
The {@link #fillInStackTrace()} method is called to initialize
* the stack trace data in the newly created throwable.
*
* @param cause the cause (which is saved for later retrieval by the
* {@link #getCause()} method). (A null value is
* permitted, and indicates that the cause is nonexistent or
* unknown.)
* @since 1.4
*/
public Throwable(Throwable cause) {
fillInStackTrace();
detailMessage = (cause==null ? null : cause.toString());
this.cause = cause;
}
/**
* Returns the detail message string of this throwable.
*
* @return the detail message string of this Throwable instance
* (which may be null).
*/
public String getMessage() {
return detailMessage;
}
/**
* Creates a localized description of this throwable.
* Subclasses may override this method in order to produce a
* locale-specific message. For subclasses that do not override this
* method, the default implementation returns the same result as
* getMessage().
*
* @return The localized description of this throwable.
* @since JDK1.1
*/
public String getLocalizedMessage() {
return getMessage();
}
/**
* Returns the cause of this throwable or null if the
* cause is nonexistent or unknown. (The cause is the throwable that
* caused this throwable to get thrown.)
*
*
This implementation returns the cause that was supplied via one of
* the constructors requiring a Throwable, or that was set after
* creation with the {@link #initCause(Throwable)} method. While it is
* typically unnecessary to override this method, a subclass can override
* it to return a cause set by some other means. This is appropriate for
* a "legacy chained throwable" that predates the addition of chained
* exceptions to Throwable. Note that it is not
* necessary to override any of the PrintStackTrace methods,
* all of which invoke the getCause method to determine the
* cause of a throwable.
*
* @return the cause of this throwable or null if the
* cause is nonexistent or unknown.
* @since 1.4
*/
public Throwable getCause() {
return (cause==this ? null : cause);
}
/**
* Initializes the cause of this throwable to the specified value.
* (The cause is the throwable that caused this throwable to get thrown.)
*
*
This method can be called at most once. It is generally called from
* within the constructor, or immediately after creating the
* throwable. If this throwable was created
* with {@link #Throwable(Throwable)} or
* {@link #Throwable(String,Throwable)}, this method cannot be called
* even once.
*
* @param cause the cause (which is saved for later retrieval by the
* {@link #getCause()} method). (A null value is
* permitted, and indicates that the cause is nonexistent or
* unknown.)
* @return a reference to this Throwable instance.
* @throws IllegalArgumentException if cause is this
* throwable. (A throwable cannot be its own cause.)
* @throws IllegalStateException if this throwable was
* created with {@link #Throwable(Throwable)} or
* {@link #Throwable(String,Throwable)}, or this method has already
* been called on this throwable.
* @since 1.4
*/
public synchronized Throwable initCause(Throwable cause) {
if (this.cause != this)
throw new IllegalStateException("Can't overwrite cause");
if (cause == this)
throw new IllegalArgumentException("Self-causation not permitted");
this.cause = cause;
return this;
}
/**
* Returns a short description of this throwable.
* The result is the concatenation of:
*
Throwable object on the error output stream that is
* the value of the field System.err. The first line of
* output contains the result of the {@link #toString()} method for
* this object. Remaining lines represent data previously recorded by
* the method {@link #fillInStackTrace()}. The format of this
* information depends on the implementation, but the following
* example may be regarded as typical:
*
* java.lang.NullPointerException
* at MyClass.mash(MyClass.java:9)
* at MyClass.crunch(MyClass.java:6)
* at MyClass.main(MyClass.java:3)
*
* class MyClass {
* public static void main(String[] args) {
* crunch(null);
* }
* static void crunch(int[] a) {
* mash(a);
* }
* static void mash(int[] b) {
* System.out.println(b[0]);
* }
* }
*
* The backtrace for a throwable with an initialized, non-null cause
* should generally include the backtrace for the cause. The format
* of this information depends on the implementation, but the following
* example may be regarded as typical:
*
* HighLevelException: MidLevelException: LowLevelException
* at Junk.a(Junk.java:13)
* at Junk.main(Junk.java:4)
* Caused by: MidLevelException: LowLevelException
* at Junk.c(Junk.java:23)
* at Junk.b(Junk.java:17)
* at Junk.a(Junk.java:11)
* ... 1 more
* Caused by: LowLevelException
* at Junk.e(Junk.java:30)
* at Junk.d(Junk.java:27)
* at Junk.c(Junk.java:21)
* ... 3 more
*
* Note the presence of lines containing the characters "...".
* These lines indicate that the remainder of the stack trace for this
* exception matches the indicated number of frames from the bottom of the
* stack trace of the exception that was caused by this exception (the
* "enclosing" exception). This shorthand can greatly reduce the length
* of the output in the common case where a wrapped exception is thrown
* from same method as the "causative exception" is caught. The above
* example was produced by running the program:
*
* public class Junk {
* public static void main(String args[]) {
* try {
* a();
* } catch(HighLevelException e) {
* e.printStackTrace();
* }
* }
* static void a() throws HighLevelException {
* try {
* b();
* } catch(MidLevelException e) {
* throw new HighLevelException(e);
* }
* }
* static void b() throws MidLevelException {
* c();
* }
* static void c() throws MidLevelException {
* try {
* d();
* } catch(LowLevelException e) {
* throw new MidLevelException(e);
* }
* }
* static void d() throws LowLevelException {
* e();
* }
* static void e() throws LowLevelException {
* throw new LowLevelException();
* }
* }
*
* class HighLevelException extends Exception {
* HighLevelException(Throwable cause) { super(cause); }
* }
*
* class MidLevelException extends Exception {
* MidLevelException(Throwable cause) { super(cause); }
* }
*
* class LowLevelException extends Exception {
* }
*
*/
public void printStackTrace() {
printStackTrace(System.err);
}
/**
* Prints this throwable and its backtrace to the specified print stream.
*
* @param s PrintStream to use for output
*/
public void printStackTrace(PrintStream s) {
synchronized (s) {
s.println(this);
StackTraceElement[] trace = getOurStackTrace();
for (int i=0; i < trace.length; i++)
s.println("\tat " + trace[i]);
Throwable ourCause = getCause();
if (ourCause != null)
ourCause.printStackTraceAsCause(s, trace);
}
}
/**
* Print our stack trace as a cause for the specified stack trace.
*/
private void printStackTraceAsCause(PrintStream s,
StackTraceElement[] causedTrace)
{
// assert Thread.holdsLock(s);
// Compute number of frames in common between this and caused
StackTraceElement[] trace = getOurStackTrace();
int m = trace.length-1, n = causedTrace.length-1;
while (m >= 0 && n >=0 && trace[m].equals(causedTrace[n])) {
m--; n--;
}
int framesInCommon = trace.length - 1 - m;
s.println("Caused by: " + this);
for (int i=0; i <= m; i++)
s.println("\tat " + trace[i]);
if (framesInCommon != 0)
s.println("\t... " + framesInCommon + " more");
// Recurse if we have a cause
Throwable ourCause = getCause();
if (ourCause != null)
ourCause.printStackTraceAsCause(s, trace);
}
/**
* Prints this throwable and its backtrace to the specified
* print writer.
*
* @param s PrintWriter to use for output
* @since JDK1.1
*/
public void printStackTrace(PrintWriter s) {
synchronized (s) {
s.println(this);
StackTraceElement[] trace = getOurStackTrace();
for (int i=0; i < trace.length; i++)
s.println("\tat " + trace[i]);
Throwable ourCause = getCause();
if (ourCause != null)
ourCause.printStackTraceAsCause(s, trace);
}
}
/**
* Print our stack trace as a cause for the specified stack trace.
*/
private void printStackTraceAsCause(PrintWriter s,
StackTraceElement[] causedTrace)
{
// assert Thread.holdsLock(s);
// Compute number of frames in common between this and caused
StackTraceElement[] trace = getOurStackTrace();
int m = trace.length-1, n = causedTrace.length-1;
while (m >= 0 && n >=0 && trace[m].equals(causedTrace[n])) {
m--; n--;
}
int framesInCommon = trace.length - 1 - m;
s.println("Caused by: " + this);
for (int i=0; i <= m; i++)
s.println("\tat " + trace[i]);
if (framesInCommon != 0)
s.println("\t... " + framesInCommon + " more");
// Recurse if we have a cause
Throwable ourCause = getCause();
if (ourCause != null)
ourCause.printStackTraceAsCause(s, trace);
}
/**
* Fills in the execution stack trace. This method records within this
* Throwable object information about the current state of
* the stack frames for the current thread.
*
* @return a reference to this Throwable instance.
* @see java.lang.Throwable#printStackTrace()
*/
public synchronized native Throwable fillInStackTrace();
/**
* Provides programmatic access to the stack trace information printed by
* {@link #printStackTrace()}. Returns an array of stack trace elements,
* each representing one stack frame. The zeroth element of the array
* (assuming the array's length is non-zero) represents the top of the
* stack, which is the last method invocation in the sequence. Typically,
* this is the point at which this throwable was created and thrown.
* The last element of the array (assuming the array's length is non-zero)
* represents the bottom of the stack, which is the first method invocation
* in the sequence.
*
* Some virtual machines may, under some circumstances, omit one
* or more stack frames from the stack trace. In the extreme case,
* a virtual machine that has no stack trace information concerning
* this throwable is permitted to return a zero-length array from this
* method. Generally speaking, the array returned by this method will
* contain one element for every frame that would be printed by
* printStackTrace.
*
* @return an array of stack trace elements representing the stack trace
* pertaining to this throwable.
* @since 1.4
*/
public StackTraceElement[] getStackTrace() {
return getOurStackTrace().clone();
}
private synchronized StackTraceElement[] getOurStackTrace() {
// Initialize stack trace if this is the first call to this method
if (stackTrace == null) {
int depth = getStackTraceDepth();
stackTrace = new StackTraceElement[depth];
for (int i=0; i < depth; i++)
stackTrace[i] = getStackTraceElement(i);
}
return stackTrace;
}
/**
* Sets the stack trace elements that will be returned by
* {@link #getStackTrace()} and printed by {@link #printStackTrace()}
* and related methods.
*
* This method, which is designed for use by RPC frameworks and other
* advanced systems, allows the client to override the default
* stack trace that is either generated by {@link #fillInStackTrace()}
* when a throwable is constructed or deserialized when a throwable is
* read from a serialization stream.
*
* @param stackTrace the stack trace elements to be associated with
* this Throwable. The specified array is copied by this
* call; changes in the specified array after the method invocation
* returns will have no affect on this Throwable's stack
* trace.
*
* @throws NullPointerException if stackTrace is
* null, or if any of the elements of
* stackTrace are null
*
* @since 1.4
*/
public void setStackTrace(StackTraceElement[] stackTrace) {
StackTraceElement[] defensiveCopy = stackTrace.clone();
for (int i = 0; i < defensiveCopy.length; i++)
if (defensiveCopy[i] == null)
throw new NullPointerException("stackTrace[" + i + "]");
this.stackTrace = defensiveCopy;
}
/**
* Returns the number of elements in the stack trace (or 0 if the stack
* trace is unavailable).
*/
private native int getStackTraceDepth();
/**
* Returns the specified element of the stack trace.
*
* @param index index of the element to return.
* @throws IndexOutOfBoundsException if index < 0 ||
* index >= getStackTraceDepth()
*/
private native StackTraceElement getStackTraceElement(int index);
private synchronized void writeObject(java.io.ObjectOutputStream s)
throws IOException
{
getOurStackTrace(); // Ensure that stackTrace field is initialized.
s.defaultWriteObject();
}
}