Class Throwable

java.lang.Object
java.lang.Throwable
All Implemented Interfaces:
Serializable
Direct Known Subclasses:
Error,Exception
public classThrowableextendsObjectimplementsSerializable
TheThrowable 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 Javathrow statement. Similarly, only this class or one of its subclasses can be the argument type in acatch clause. For the purposes of compile-time checking of exceptions, Throwable and any subclass ofThrowable that is not also a subclass of eitherRuntimeException orError are regarded as checked exceptions.

Instances of two subclasses,Error andException, 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. Over time, a throwable cansuppress other throwables from being propagated. Finally, the throwable can also contain acause: another throwable that caused this throwable to be constructed. The recording of this causal information is referred to as thechained 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 theCollection interface, and that its persistence is implemented atopjava.io. Suppose the internals of theadd method can throw anIOException. The implementation can communicate the details of theIOException to its caller while conforming to theCollection interface by wrapping theIOException 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 theinitCause(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 theThrowable constructors that takes a cause. Because theinitCause 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 toThrowable.

By convention, classThrowable and its subclasses have two constructors, one that takes no arguments and one that takes aString 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 aThrowable (the cause), and one that takes aString (the detail message) and aThrowable (the cause).

SeeJava Language Specification:
11.2 Compile-Time Checking of Exceptions
Since:
1.0
See Also:

  • Constructor Details

    • Throwable

      public Throwable()
      Constructs a new throwable withnull as its detail message. The cause is not initialized, and may subsequently be initialized by a call toinitCause(java.lang.Throwable).

      ThefillInStackTrace() method is called to initialize the stack trace data in the newly created throwable.

    • Throwable

      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 toinitCause(java.lang.Throwable).

      ThefillInStackTrace() method is called to initialize the stack trace data in the newly created throwable.

      Parameters:
      message - the detail message. The detail message is saved for later retrieval by thegetMessage() method.

    • Throwable

      public Throwable(String message,Throwable cause)
      Constructs a new throwable with the specified detail message and cause.

      Note that the detail message associated withcause isnot automatically incorporated in this throwable's detail message.

      ThefillInStackTrace() method is called to initialize the stack trace data in the newly created throwable.

      Parameters:
      message - the detail message (which is saved for later retrieval by thegetMessage() method).
      cause - the cause (which is saved for later retrieval by thegetCause() method). (Anull value is permitted, and indicates that the cause is nonexistent or unknown.)
      Since:
      1.4

    • Throwable

      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 ofcause). This constructor is useful for throwables that are little more than wrappers for other throwables (for example,PrivilegedActionException).

      ThefillInStackTrace() method is called to initialize the stack trace data in the newly created throwable.

      Parameters:
      cause - the cause (which is saved for later retrieval by thegetCause() method). (Anull value is permitted, and indicates that the cause is nonexistent or unknown.)
      Since:
      1.4

    • Throwable

      protected Throwable(String message,Throwable cause, boolean enableSuppression, boolean writableStackTrace)
      Constructs a new throwable with the specified detail message, cause,suppression enabled or disabled, and writable stack trace enabled or disabled. If suppression is disabled,getSuppressed() for this object will return a zero-length array and calls toaddSuppressed(java.lang.Throwable) that would otherwise append an exception to the suppressed list will have no effect. If the writable stack trace is false, this constructor will not callfillInStackTrace(), anull will be written to thestackTrace field, and subsequent calls to fillInStackTrace andsetStackTrace(StackTraceElement[]) will not set the stack trace. If the writable stack trace is false,getStackTrace() will return a zero length array.

      Note that the other constructors ofThrowable treat suppression as being enabled and the stack trace as being writable. Subclasses ofThrowable should document any conditions under which suppression is disabled and document conditions under which the stack trace is not writable. Disabling of suppression should only occur in exceptional circumstances where special requirements exist, such as a virtual machine reusing exception objects under low-memory situations. Circumstances where a given exception object is repeatedly caught and rethrown, such as to implement control flow between two sub-systems, is another situation where immutable throwable objects would be appropriate.

      Parameters:
      message - the detail message.
      cause - the cause. (Anull value is permitted, and indicates that the cause is nonexistent or unknown.)
      enableSuppression - whether or not suppression is enabled
      writableStackTrace - whether or not the stack trace is writable
      Since:
      1.7
      See Also:

  • Method Details

    • getMessage

      public String getMessage()
      Returns the detail message string of this throwable.
      Returns:
      the detail message string of thisThrowable instance (which may benull).

    • getLocalizedMessage

      public String getLocalizedMessage()
      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 asgetMessage().
      Returns:
      The localized description of this throwable.
      Since:
      1.1

    • getCause

      public Throwable getCause()
      Returns the cause of this throwable ornull 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 aThrowable, or that was set after creation with theinitCause(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 toThrowable. Note that it isnot necessary to override any of thePrintStackTrace methods, all of which invoke thegetCause method to determine the cause of a throwable.

      Returns:
      the cause of this throwable ornull if the cause is nonexistent or unknown.
      Since:
      1.4

    • initCause

      public Throwable initCause(Throwable cause)
      Initializes thecause 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 withThrowable(Throwable) orThrowable(String,Throwable), this method cannot be called even once.

      An example of using this method on a legacy throwable type without other support for setting the cause is:

       try {     lowLevelOp(); } catch (LowLevelException le) {     throw (HighLevelException)           new HighLevelException().initCause(le); // Legacy constructor }

      Parameters:
      cause - the cause (which is saved for later retrieval by thegetCause() method). (Anull value is permitted, and indicates that the cause is nonexistent or unknown.)
      Returns:
      a reference to thisThrowable instance.
      Throws:
      IllegalArgumentException - ifcause is this throwable. (A throwable cannot be its own cause.)
      IllegalStateException - if this throwable was created withThrowable(Throwable) orThrowable(String,Throwable), or this method has already been called on this throwable.
      Since:
      1.4

    • toString

      public String toString()
      Returns a short description of this throwable. The result is the concatenation of:
      • thename of the class of this object
      • ": " (a colon and a space)
      • the result of invoking this object'sgetLocalizedMessage() method
      IfgetLocalizedMessage returnsnull, then just the class name is returned.
      Overrides:
      toString in class Object
      Returns:
      a string representation of this throwable.

    • printStackTrace

      public void printStackTrace()
      Prints this throwable and its backtrace to the standard error stream. This method prints a stack trace for thisThrowable object on the error output stream that is the value of the fieldSystem.err. The first line of output contains the result of thetoString() method for this object. Remaining lines represent data previously recorded by the methodfillInStackTrace(). 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 the 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 ofsuppressed exceptions (in conjunction with the 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 as it is used on causes. Unlike causes, suppressed exceptions are indented beyond their "containing exceptions."

      An exception can have both a cause and one or more suppressed 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.<init>(Resource2.java:45)                 ... 2 more

    • printStackTrace

      public void printStackTrace(PrintStream s)
      Prints this throwable and its backtrace to the specified print stream.
      Parameters:
      s -PrintStream to use for output

    • printStackTrace

      public void printStackTrace(PrintWriter s)
      Prints this throwable and its backtrace to the specified print writer.
      Parameters:
      s -PrintWriter to use for output
      Since:
      1.1

    • fillInStackTrace

      public Throwable fillInStackTrace()
      Fills in the execution stack trace. This method records within thisThrowable object information about the current state of the stack frames for the current thread.

      If the stack trace of thisThrowableis not writable, calling this method has no effect.

      Returns:
      a reference to thisThrowable instance.
      See Also:

    • getStackTrace

      public StackTraceElement[] getStackTrace()
      Provides programmatic access to the stack trace information printed byprintStackTrace(). 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 byprintStackTrace. Writes to the returned array do not affect future calls to this method.

      Returns:
      an array of stack trace elements representing the stack trace pertaining to this throwable.
      Since:
      1.4

    • setStackTrace

      public void setStackTrace(StackTraceElement[] stackTrace)
      Sets the stack trace elements that will be returned bygetStackTrace() and printed byprintStackTrace() 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 byfillInStackTrace() when a throwable is constructed or deserialized when a throwable is read from a serialization stream.

      If the stack trace of thisThrowableis not writable, calling this method has no effect other than validating its argument.

      Parameters:
      stackTrace - the stack trace elements to be associated with thisThrowable. The specified array is copied by this call; changes in the specified array after the method invocation returns will have no effect on thisThrowable's stack trace.
      Throws:
      NullPointerException - ifstackTrace isnull or if any of the elements ofstackTrace arenull
      Since:
      1.4

    • addSuppressed

      public final void addSuppressed(Throwable exception)
      Appends the specified exception to the exceptions that were suppressed in order to deliver this exception. This method is thread-safe and typically called (automatically and implicitly) by thetry-with-resources statement.

      The suppression behavior is enabledunless disabledvia a constructor. When suppression is disabled, this method does nothing other than to validate its argument.

      Note that when one exceptioncauses another exception, the first exception is usually caught and then the second exception is thrown in response. In other words, there is a causal connection between the two exceptions. In contrast, there are situations where two independent exceptions can be thrown in sibling code blocks, in particular in thetry block of atry-with-resources statement and the compiler-generatedfinally block which closes the resource. In these situations, only one of the thrown exceptions can be propagated. In thetry-with-resources statement, when there are two such exceptions, the exception originating from thetry block is propagated and the exception from thefinally block is added to the list of exceptions suppressed by the exception from thetry block. As an exception unwinds the stack, it can accumulate multiple suppressed exceptions.

      An exception may have suppressed exceptions while also being caused by another exception. Whether or not an exception has a cause is semantically known at the time of its creation, unlike whether or not an exception will suppress other exceptions which is typically only determined after an exception is thrown.

      Note that programmer written code is also able to take advantage of calling this method in situations where there are multiple sibling exceptions and only one can be propagated.

      Parameters:
      exception - the exception to be added to the list of suppressed exceptions
      Throws:
      IllegalArgumentException - ifexception is this throwable; a throwable cannot suppress itself.
      NullPointerException - ifexception isnull
      Since:
      1.7

    • getSuppressed

      public final Throwable[] getSuppressed()
      Returns an array containing all of the exceptions that were suppressed, typically by thetry-with-resources statement, in order to deliver this exception. If no exceptions were suppressed orsuppression is disabled, an empty array is returned. This method is thread-safe. Writes to the returned array do not affect future calls to this method.
      Returns:
      an array containing all of the exceptions that were suppressed to deliver this exception.
      Since:
      1.7