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 andexceptions in the Java language. Only objects that are instances of thisclass (or one of its subclasses) are thrown by the Java Virtual Machine orcan be thrown by the Javathrow statement. Similarly, onlythis 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 asubclass of eitherRuntimeException orError areregarded as checked exceptions.

Instances of two subclasses,Error andException, are conventionally used to indicatethat exceptional situations have occurred. Typically, these instancesare freshly created in the context of the exceptional situation soas to include relevant information (such as stack trace data).

A throwable contains a snapshot of the execution stack of itsthread at the time it was created. It can also contain a messagestring that gives more information about the error. Over time, athrowable cansuppress otherthrowables from being propagated. Finally, the throwable can alsocontain acause: another throwable that caused thisthrowable to be constructed. The recording of this causal informationis referred to as thechained exception facility, as thecause can, itself, have a cause, and so on, leading to a "chain" ofexceptions, each caused by another.

One reason that a throwable may have a cause is that the class thatthrows it is built atop a lower layered abstraction, and an operation onthe upper layer fails due to a failure in the lower layer. It would be baddesign to let the throwable thrown by the lower layer propagate outward, asit 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 ofits implementation, assuming the lower layer's exception was a checkedexception. Throwing a "wrapped exception" (i.e., an exception containing acause) allows the upper layer to communicate the details of the failure toits caller without incurring either of these shortcomings. It preservesthe flexibility to change the implementation of the upper layer withoutchanging its API (in particular, the set of exceptions thrown by itsmethods).

A second reason that a throwable may have a cause is that the methodthat throws it must conform to a general-purpose interface that does notpermit the method to throw the cause directly. For example, supposea persistent collection conforms to theCollection interface, and that its persistence is implemented atopjava.io. Suppose the internals of theadd methodcan throw anIOException. The implementationcan communicate the details of theIOException to its callerwhile conforming to theCollection interface by wrapping theIOException in an appropriate unchecked exception. (Thespecification for the persistent collection should indicate that it iscapable of throwing such exceptions.)

A cause can be associated with a throwable in two ways: via aconstructor that takes the cause as an argument, or via theinitCause(Throwable) method. New throwable classes thatwish to allow causes to be associated with them should provide constructorsthat 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 beassociated with any throwable, even a "legacy throwable" whoseimplementation predates the addition of the exception chaining mechanism toThrowable.

By convention, classThrowable and its subclasses have twoconstructors, 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 withthem should have two more constructors, one that takes aThrowable (the cause), and one that takes aString (the detail message) and aThrowable (thecause).

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 acall toinitCause(Throwable).

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

    • Throwable

      public Throwable(String message)
      Constructs a new throwable with the specified detail message. Thecause is not initialized, and may subsequently be initialized bya call toinitCause(Throwable).

      ThefillInStackTrace() method is called to initializethe 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 andcause.

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

      ThefillInStackTrace() method is called to initializethe 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 detailmessage of(cause==null ? null : cause.toString()) (whichtypically contains the class and detail message ofcause).This constructor is useful for throwables that are little more thanwrappers for other throwables (for example,PrivilegedActionException).

      ThefillInStackTrace() method is called to initializethe 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 ordisabled, and writable stack trace enabled or disabled. Ifsuppression is disabled,getSuppressed() for this objectwill return a zero-length array and calls toaddSuppressed(Throwable) that would otherwise append an exception to thesuppressed list will have no effect. If the writable stacktrace is false, this constructor will not callfillInStackTrace(), anull will be written to thestackTrace field, and subsequent calls tofillInStackTrace andsetStackTrace(StackTraceElement[]) will not set the stacktrace. If the writable stack trace is false,getStackTrace() will return a zero length array.

      Note that the other constructors ofThrowable treatsuppression as being enabled and the stack trace as beingwritable. Subclasses ofThrowable should document anyconditions under which suppression is disabled and documentconditions under which the stack trace is not writable.Disabling of suppression should only occur in exceptionalcircumstances where special requirements exist, such as avirtual machine reusing exception objects under low-memorysituations. Circumstances where a given exception object isrepeatedly caught and rethrown, such as to implement controlflow between two sub-systems, is another situation whereimmutable 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 alocale-specific message. For subclasses that do not override thismethod, 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 thecause is nonexistent or unknown. (The cause is the throwable thatcaused this throwable to get thrown.)

      This implementation returns the cause that was supplied via one ofthe constructors requiring aThrowable, or that was set aftercreation with theinitCause(Throwable) method. While it istypically unnecessary to override this method, a subclass can overrideit to return a cause set by some other means. This is appropriate fora "legacy chained throwable" that predates the addition of chainedexceptions toThrowable. Note that it isnotnecessary to override any of thePrintStackTrace methods,all of which invoke thegetCause method to determine thecause 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 fromwithin the constructor, or immediately after creating thethrowable. If this throwable was createdwithThrowable(Throwable) orThrowable(String,Throwable), this method cannot be calledeven once.

      An example of using this method on a legacy throwable typewithout 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 justthe 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 thestandard error stream. This method prints a stack trace for thisThrowable object on the error output stream that isthe value of the fieldSystem.err. The first line ofoutput contains the result of thetoString() method forthis object. Remaining lines represent data previously recorded bythe methodfillInStackTrace(). The format of thisinformation depends on the implementation, but the followingexample 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 causeshould generally include the backtrace for the cause. The formatof this information depends on the implementation, but the followingexample 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 moreCaused 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 thisexception matches the indicated number of frames from the bottom of thestack trace of the exception that was caused by this exception (the"enclosing" exception). This shorthand can greatly reduce the lengthof the output in the common case where a wrapped exception is thrownfrom the same method as the "causative exception" is caught. The aboveexample 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 thetry-with-resources statement). Any exceptions that weresuppressed in order to deliver an exception are printed outbeneath the stack trace. The format of this informationdepends on the implementation, but the following example may beregarded 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 exceptionsjust as it is used on causes. Unlike causes, suppressed exceptions areindented beyond their "containing exceptions."

      An exception can have both a cause and one or more suppressedexceptions:

      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 specifiedprint 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 ofthe stack frames for the current thread.

      If the stack trace of thisThrowableis notwritable, 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 thestack, 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 invocationin the sequence.

      Some virtual machines may, under some circumstances, omit oneor more stack frames from the stack trace. In the extreme case,a virtual machine that has no stack trace information concerningthis throwable is permitted to return a zero-length array from thismethod. Generally speaking, the array returned by this method willcontain one element for every frame that would be printed byprintStackTrace. Writes to the returned array do notaffect 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 otheradvanced systems, allows the client to override the defaultstack trace that is either generated byfillInStackTrace()when a throwable is constructed or deserialized when a throwable isread from a serialization stream.

      If the stack trace of thisThrowableis notwritable, calling this method has no effect other thanvalidating its argument.

      Parameters:
      stackTrace - the stack trace elements to be associated withthisThrowable. The specified array is copied by thiscall; changes in the specified array after the method invocationreturns will have no effect on thisThrowable's stacktrace.
      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 weresuppressed in order to deliver this exception. This method isthread-safe and typically called (automatically and implicitly)by thetry-with-resources statement.

      The suppression behavior is enabledunless disabledviaa constructor. When suppression is disabled, this method doesnothing other than to validate its argument.

      Note that when one exceptioncauses another exception, the firstexception is usually caught and then the second exception isthrown in response. In other words, there is a causalconnection between the two exceptions.In contrast, there are situations where two independentexceptions can be thrown in sibling code blocks, in particularin thetry block of atry-with-resourcesstatement and the compiler-generatedfinally blockwhich closes the resource.In these situations, only one of the thrown exceptions can bepropagated. In thetry-with-resources statement, whenthere are two such exceptions, the exception originating fromthetry block is propagated and the exception from thefinally block is added to the list of exceptionssuppressed by the exception from thetry block. As anexception unwinds the stack, it can accumulate multiplesuppressed exceptions.

      An exception may have suppressed exceptions while also beingcaused by another exception. Whether or not an exception has acause is semantically known at the time of its creation, unlikewhether or not an exception will suppress other exceptionswhich is typically only determined after an exception isthrown.

      Note that programmer written code is also able to takeadvantage of calling this method in situations where there aremultiple 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 weresuppressed, typically by thetry-with-resourcesstatement, in order to deliver this exception.If no exceptions were suppressed orsuppression isdisabled, an empty array is returned. This method isthread-safe. Writes to the returned array do not affect futurecalls to this method.
      Returns:
      an array containing all of the exceptions that were suppressed to deliver this exception.
      Since:
      1.7