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 * {@code java.io}. Suppose the internals of the {@code add} method * can throw an {@link java.io.IOException IOException}. The implementation * can communicate the details of the {@code IOException} to its caller * while conforming to the {@code Collection} interface by wrapping the * {@code 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 * {@code Throwable} constructors that takes a cause. For example: *
* try {
* lowLevelOp();
* } catch (LowLevelException le) {
* throw new HighLevelException(le); // Chaining-aware constructor
* }
*
* Because the {@code 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
* {@code 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 {@code 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 * {@code initCause} method, but it is more convenient and expressive to * delegate to a constructor that takes a cause. * *
By convention, class {@code Throwable} and its subclasses have two * constructors, one that takes no arguments and one that takes a * {@code 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 * {@code Throwable} (the cause), and one that takes a * {@code String} (the detail message) and a {@code 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 {@code getStackTrace}
* and {@code 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.)
* @jls3 11.2 Compile-Time Checking of Exceptions
* @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
* {@code 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.
*/
/**
* The list of suppressed exceptions, as returned by
* {@link #getSuppressedExceptions()}.
*
* @serial
* @since 1.7
*/
private List 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
* {@code 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 {@code 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 {@code (cause==null ? null : cause.toString())} (which
* typically contains the class and detail message of {@code 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 {@code 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 {@code Throwable} instance
* (which may be {@code 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
* {@code getMessage()}.
*
* @return The localized description of this throwable.
* @since JDK1.1
*/
public String getLocalizedMessage() {
return getMessage();
}
/**
* Returns the cause of this throwable or {@code 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 {@code 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 {@code Throwable}. Note that it is not
* necessary to override any of the {@code PrintStackTrace} methods,
* all of which invoke the {@code getCause} method to determine the
* cause of a throwable.
*
* @return the cause of this throwable or {@code null} if the
* cause is nonexistent or unknown.
* @since 1.4
*/
public synchronized 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 {@code null} value is
* permitted, and indicates that the cause is nonexistent or
* unknown.)
* @return a reference to this {@code Throwable} instance.
* @throws IllegalArgumentException if {@code 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:
* An exception can have both a cause and one or more suppressed
* exceptions:
* 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
* {@code 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 {@code 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 {@code Throwable}'s stack
* trace.
*
* @throws NullPointerException if {@code stackTrace} is
* {@code null}, or if any of the elements of
* {@code stackTrace} are {@code 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 + "]");
synchronized (this) {
this.stackTrace = defensiveCopy;
}
}
/**
* Returns the number of elements in the stack trace (or 0 if the stack
* trace is unavailable).
*
* package-protection for use by SharedSecrets.
*/
native int getStackTraceDepth();
/**
* Returns the specified element of the stack trace.
*
* package-protection for use by SharedSecrets.
*
* @param index index of the element to return.
* @throws IndexOutOfBoundsException if {@code index < 0 ||
* index >= getStackTraceDepth() }
*/
native StackTraceElement getStackTraceElement(int index);
private void readObject(ObjectInputStream s)
throws IOException, ClassNotFoundException {
s.defaultReadObject(); // read in all fields
List Note that when one exception {@linkplain
* #initCause(Throwable) causes} another exception, the first
* exception is usually caught and then the second exception is
* thrown in response. In contrast, when one exception suppresses
* another, two exceptions are thrown in sibling code blocks, such
* as in a {@code try} block and in its {@code finally} block, and
* control flow can only continue with one exception so the second
* is recorded as a suppressed exception of the first.
*
* @param exception the exception to be added to the list of
* suppressed exceptions
* @throws NullPointerException if {@code exception} is null
* @throws IllegalArgumentException if {@code exception} is this
* throwable; a throwable cannot suppress itself.
* @since 1.7
*/
public synchronized void addSuppressedException(Throwable exception) {
if (exception == null)
throw new NullPointerException(NULL_CAUSE_MESSAGE);
if (exception == this)
throw new IllegalArgumentException("Self-suppression not permitted");
if (suppressedExceptions == null)
suppressedExceptions = new ArrayList
*
* If {@code getLocalizedMessage} returns {@code null}, then just
* the class name is returned.
*
* @return a string representation of this throwable.
*/
public String toString() {
String s = getClass().getName();
String message = getLocalizedMessage();
return (message != null) ? (s + ": " + message) : s;
}
/**
* Prints this throwable and its backtrace to the
* standard error stream. This method prints a stack trace for this
* {@code Throwable} object on the error output stream that is
* the value of the field {@code 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:
*
* This example was produced by running the program:
*
* 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 {@code "..."}.
* 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 {
* }
*
* As of release 7, the platform supports the notion of
* suppressed exceptions (in conjunction with the {@code
* try}-with-resources statement). Any exceptions that were
* suppressed in order to deliver an exception are printed out
* beneath the stack trace. The format of this information
* depends on the implementation, but the following example may be
* regarded as typical:
*
*
* Exception in thread "main" java.lang.Exception: Something happened
* at Foo.bar(Foo.java:10)
* at Foo.main(Foo.java:5)
* Suppressed: Resource$CloseFailException: Resource ID = 0
* at Resource.close(Resource.java:26)
* at Foo.bar(Foo.java:9)
* ... 1 more
*
* Note that the "... n more" notation is used on suppressed exceptions
* just at it is used on causes. Unlike causes, suppressed exceptions are
* indented beyond their "containing exceptions."
*
*
* Exception in thread "main" java.lang.Exception: Main block
* at Foo3.main(Foo3.java:7)
* Suppressed: Resource$CloseFailException: Resource ID = 2
* at Resource.close(Resource.java:26)
* at Foo3.main(Foo3.java:5)
* Suppressed: Resource$CloseFailException: Resource ID = 1
* at Resource.close(Resource.java:26)
* at Foo3.main(Foo3.java:5)
* Caused by: java.lang.Exception: I did it
* at Foo3.main(Foo3.java:8)
*
* Likewise, a suppressed exception can have a cause:
*
* Exception in thread "main" java.lang.Exception: Main block
* at Foo4.main(Foo4.java:6)
* Suppressed: Resource2$CloseFailException: Resource ID = 1
* at Resource2.close(Resource2.java:20)
* at Foo4.main(Foo4.java:5)
* Caused by: java.lang.Exception: Rats, you caught me
* at Resource2$CloseFailException.
*/
public void printStackTrace() {
printStackTrace(System.err);
}
/**
* Prints this throwable and its backtrace to the specified print stream.
*
* @param s {@code PrintStream} to use for output
*/
public void printStackTrace(PrintStream s) {
printStackTrace(new WrappedPrintStream(s));
}
private void printStackTrace(PrintStreamOrWriter s) {
// Guard against malicious overrides of Throwable.equals by
// using a Set with identity equality semantics.
Set