java.lang
Class Throwable

java.lang.Object
  java.lang.Throwable

Direct Known Subclasses:

Error, Exception

public class Throwable
extends Object

Throwable is the superclass of all exceptions that can be raised.

There are two special cases: Error and RuntimeException: these two classes (and their subclasses) are considered unchecked exceptions, and are either frequent enough or catastrophic enough that you do not need to declare them in throws clauses. Everything else is a checked exception, and is ususally a subclass of Exception; these exceptions have to be handled or declared.

Instances of this class are usually created with knowledge of the execution context, so that you can get a stack trace of the problem spot in the code. Also, since JDK 1.4, Throwables participate in "exception chaining." This means that one exception can be caused by another, and preserve the information of the original.

One reason this is useful is to wrap exceptions to conform to an interface. For example, it would be bad design to require all levels of a program interface to be aware of the low-level exceptions thrown at one level of abstraction. Another example is wrapping a checked exception in an unchecked one, to communicate that failure occured while still obeying the method throws clause of a superclass.

A cause is assigned in one of two ways; but can only be assigned once in the lifetime of the Throwable. There are new constructors added to several classes in the exception hierarchy that directly initialize the cause, or you can use the initCause method. This second method is especially useful if the superclass has not been retrofitted with new constructors:

 try
   {
     lowLevelOp();
   }
 catch (LowLevelException lle)
   {
     throw (HighLevelException) new HighLevelException().initCause(lle);
   }
 

Notice the cast in the above example; without it, your method would need a throws clase that declared Throwable, defeating the purpose of chainig your exceptions.

By convention, exception classes have two constructors: one with no arguments, and one that takes a String for a detail message. Further, classes which are likely to be used in an exception chain also provide a constructor that takes a Throwable, with or without a detail message string.

Another 1.4 feature is the StackTrace, a means of reflection that allows the program to inspect the context of the exception, and which is serialized, so that remote procedure calls can correctly pass exceptions.

Since:

1.0

Constructor Summary

Throwable()
Instantiate this Throwable with an empty message.

Throwable(String message)
Instantiate this Throwable with the given message.

Throwable(String message, Throwable cause)
Instantiate this Throwable with the given message and cause.

Throwable(Throwable cause)
Instantiate this Throwable with the given cause.

Method Summary

Throwable	getCause() Returns the cause of this exception, or null if the cause is not known or non-existant.
String	getLocalizedMessage() Get a localized version of this Throwable's error message.
String	getMessage() Get the message associated with this Throwable.
Throwable	initCause(Throwable cause) Initialize the cause of this Throwable.
void	printStackTrace() Print a stack trace to the standard error stream.
void	printStackTrace(PrintStream s) Print a stack trace to the specified PrintStream.
void	printStackTrace(PrintWriter pw) Prints the exception, the detailed message and the stack trace associated with this Throwable to the given PrintWriter.
String	toString() Get a human-readable representation of this Throwable.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Constructor Detail

Throwable

public Throwable()

Instantiate this Throwable with an empty message. The cause remains uninitialized. #fillInStackTrace() will be called to set up the stack trace.

Throwable

public Throwable(String message)

Instantiate this Throwable with the given message. The cause remains uninitialized. #fillInStackTrace() will be called to set up the stack trace.

Parameters:

message - the message to associate with the Throwable

Throwable

public Throwable(String message,
                 Throwable cause)

Instantiate this Throwable with the given message and cause. Note that the message is unrelated to the message of the cause. #fillInStackTrace() will be called to set up the stack trace.

Parameters:

message - the message to associate with the Throwable

cause - the cause, may be null

Since:

1.4

Throwable

public Throwable(Throwable cause)

Instantiate this Throwable with the given cause. The message is then built as cause == null ? null : cause.toString(). #fillInStackTrace() will be called to set up the stack trace.

Parameters:

cause - the cause, may be null

Since:

1.4

Method Detail

getMessage

public String getMessage()

Get the message associated with this Throwable.

Returns:

the error message associated with this Throwable, may be null

getLocalizedMessage

public String getLocalizedMessage()

Get a localized version of this Throwable's error message. This method must be overridden in a subclass of Throwable to actually produce locale-specific methods. The Throwable implementation just returns getMessage().

Returns:

a localized version of this error message

Since:

1.1

See Also:

getMessage()

getCause

public Throwable getCause()

Returns the cause of this exception, or null if the cause is not known or non-existant. This cause is initialized by the new constructors, or by calling initCause.

Returns:

the cause of this Throwable

Since:

1.4

initCause

public Throwable initCause(Throwable cause)

Initialize the cause of this Throwable. This may only be called once during the object lifetime, including implicitly by chaining constructors.

Parameters:

cause - the cause of this Throwable, may be null

Returns:

this

Throws:

IllegalArgumentException - if cause is this (a Throwable can't be its own cause!)

IllegalStateException - if the cause has already been set

Since:

1.4

toString

public String toString()

Get a human-readable representation of this Throwable. The detail message is retrieved by getLocalizedMessage(). Then, with a null detail message, this string is simply the object's class name; otherwise the string is getClass().getName() + ": " + message.

Overrides:

toString in class Object

Returns:

a human-readable String represting this Throwable

See Also:

Object.getClass(), Object.hashCode(), Class.getName(), Integer.toHexString(int)

printStackTrace

public void printStackTrace()

Print a stack trace to the standard error stream. This stream is the current contents of System.err. The first line of output is the result of toString(), and the remaining lines represent the data created by #fillInStackTrace(). While the format is unspecified, this implementation uses the suggested format, demonstrated by this example:

 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
 {
 }
 

  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
 

printStackTrace

public void printStackTrace(PrintStream s)

Print a stack trace to the specified PrintStream. See printStackTrace() for the sample format.

Parameters:

s - the PrintStream to write the trace to

printStackTrace

public void printStackTrace(PrintWriter pw)

Prints the exception, the detailed message and the stack trace associated with this Throwable to the given PrintWriter. The actual output written is implemention specific. Use the result of getStackTrace() when more precise information is needed.

This implementation first prints a line with the result of this object's toString() method.
Then for all elements given by getStackTrace it prints a line containing three spaces, the string "at " and the result of calling the toString() method on the StackTraceElement object. If getStackTrace() returns an empty array it prints a line containing three spaces and the string "<<No stacktrace available>>".
Then if getCause() doesn't return null it adds a line starting with "Caused by: " and the result of calling toString() on the cause.
Then for every cause (of a cause, etc) the stacktrace is printed the same as for the top level Throwable except that as soon as all the remaining stack frames of the cause are the same as the the last stack frames of the throwable that the cause is wrapped in then a line starting with three spaces and the string "... X more" is printed, where X is the number of remaining stackframes.

Parameters:

pw - the PrintWriter to write the trace to

Since:

1.1