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Throwable.javaAPI DocJava SE 6 API26665Tue Jun 10 00:25:38 BST 2008java.lang

Throwable

public class Throwable extends Object implements Serializable
The 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.)
version
1.56, 03/07/06
since
JDK1.0

Fields Summary
private static final long
serialVersionUID
use serialVersionUID from JDK 1.0.2 for interoperability
private transient Object
backtrace
Native code saves some indication of the stack backtrace in this slot.
private String
detailMessage
Specific details about the Throwable. For example, for FileNotFoundException, this contains the name of the file that could not be found.
private Throwable
cause
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.
private StackTraceElement[]
stackTrace
The stack trace, as returned by {@link #getStackTrace()}.
Constructors Summary
public Throwable()
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.

    /*
     * This field is lazily initialized on first use or serialization and
     * nulled out when fillInStackTrace is called.
     */

                                                    
      
        fillInStackTrace();
public Throwable(String message)
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.

        fillInStackTrace();
        detailMessage = message;
public Throwable(String message, Throwable cause)
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

        fillInStackTrace();
        detailMessage = message;
        this.cause = cause;
public Throwable(Throwable 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

        fillInStackTrace();
        detailMessage = (cause==null ? null : cause.toString());
        this.cause = cause;
Methods Summary
public native synchronized java.lang.ThrowablefillInStackTrace()
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 java.lang.ThrowablegetCause()
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
        return (cause==this ? null : cause);
    
public java.lang.StringgetLocalizedMessage()
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
        return getMessage();
    
public java.lang.StringgetMessage()
Returns the detail message string of this throwable.
return
the detail message string of this Throwable instance
(which may be null).
        return detailMessage;
    
private synchronized java.lang.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;
    
public java.lang.StackTraceElement[]getStackTrace()
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
        return (StackTraceElement[]) getOurStackTrace().clone();
    
private native intgetStackTraceDepth()
Returns the number of elements in the stack trace (or 0 if the stack
trace is unavailable).
private native java.lang.StackTraceElementgetStackTraceElement(int index)
Returns the specified element of the stack trace.
param
index index of the element to return.
throws
IndexOutOfBoundsException if index < 0 ||
index >= getStackTraceDepth() 
public synchronized java.lang.ThrowableinitCause(java.lang.Throwable 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
        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;
    
public voidprintStackTrace()
Prints this throwable and its backtrace to the
standard error stream. This method prints a stack trace for this
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)


This example was produced by running the program:

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 {
}

        printStackTrace(System.err);
    
public voidprintStackTrace(java.io.PrintStream s)
Prints this throwable and its backtrace to the specified print stream.
param
s PrintStream to use for output
        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);
        }
    
public voidprintStackTrace(java.io.PrintWriter s)
Prints this throwable and its backtrace to the specified
print writer.
param
s PrintWriter to use for output
since
JDK1.1
        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);
        }
    
private voidprintStackTraceAsCause(java.io.PrintStream s, java.lang.StackTraceElement[] causedTrace)
Print our stack trace as a cause for the specified stack trace.
        // 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);
    
private voidprintStackTraceAsCause(java.io.PrintWriter s, java.lang.StackTraceElement[] causedTrace)
Print our stack trace as a cause for the specified stack trace.
        // 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);
    
public voidsetStackTrace(java.lang.StackTraceElement[] 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
        StackTraceElement[] defensiveCopy =
            (StackTraceElement[]) stackTrace.clone();
        for (int i = 0; i < defensiveCopy.length; i++)
            if (defensiveCopy[i] == null)
                throw new NullPointerException("stackTrace[" + i + "]");

        this.stackTrace = defensiveCopy;
    
public java.lang.StringtoString()
Returns a short description of this throwable.
The result is the concatenation of:

    

  •  the {@linkplain Class#getName() name} of the class of this object

  •  ": " (a colon and a space)

  •  the result of invoking this object's {@link #getLocalizedMessage}
method


If getLocalizedMessage returns null, then just
the class name is returned.
return
a string representation of this throwable.
        String s = getClass().getName();
        String message = getLocalizedMessage();
        return (message != null) ? (s + ": " + message) : s;
    
private synchronized voidwriteObject(java.io.ObjectOutputStream s)
        getOurStackTrace();  // Ensure that stackTrace field is initialized.
        s.defaultWriteObject();