throwable.java

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   *       {
   *         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
   * {
   * }
   * </pre>
   * <p>
   * <pre>
   *  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
   * </pre>
   */
  public void printStackTrace()
  {
    printStackTrace(System.err);
  }

  /**
   * Print a stack trace to the specified PrintStream. See
   * {@link #printStackTrace()} for the sample format.
   *
   * @param s the PrintStream to write the trace to
   */
  public void printStackTrace(PrintStream s)
  {
    s.print(stackTraceString());
  }

  /**
   * Prints the exception, the detailed message and the stack trace
   * associated with this Throwable to the given <code>PrintWriter</code>.
   * The actual output written is implemention specific. Use the result of
   * <code>getStackTrace()</code> when more precise information is needed.
   *
   * <p>This implementation first prints a line with the result of this
   * object's <code>toString()</code> method.
   * <br>
   * Then for all elements given by <code>getStackTrace</code> it prints
   * a line containing three spaces, the string "at " and the result of calling
   * the <code>toString()</code> method on the <code>StackTraceElement</code>
   * object. If <code>getStackTrace()</code> returns an empty array it prints
   * a line containing three spaces and the string
   * "&lt;&lt;No stacktrace available&gt;&gt;".
   * <br>
   * Then if <code>getCause()</code> doesn't return null it adds a line
   * starting with "Caused by: " and the result of calling
   * <code>toString()</code> on the cause.
   * <br>
   * Then for every cause (of a cause, etc) the stacktrace is printed the
   * same as for the top level <code>Throwable</code> 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.
   *
   * @param w the PrintWriter to write the trace to
   * @since 1.1
   */
  public void printStackTrace (PrintWriter pw)
  {
    pw.print(stackTraceString());
  }

  private static final String nl = System.getProperty("line.separator");
  // Create whole stack trace in a stringbuffer so we don't have to print
  // it line by line. This prevents printing multiple stack traces from
  // different threads to get mixed up when written to the same PrintWriter.
  private String stackTraceString()
  {
    StringBuffer sb = new StringBuffer();

    // Main stacktrace
    StackTraceElement[] stack = getStackTrace();
    stackTraceStringBuffer(sb, this.toString(), stack, 0);

    // The cause(s)
    Throwable cause = getCause();
    while (cause != null)
      {
	// Cause start first line
        sb.append("Caused by: ");

        // Cause stacktrace
        StackTraceElement[] parentStack = stack;
        stack = cause.getStackTrace();
	if (parentStack == null || parentStack.length == 0)
	  stackTraceStringBuffer(sb, cause.toString(), stack, 0);
	else
	  {
	    int equal = 0; // Count how many of the last stack frames are equal
	    int frame = stack.length-1;
	    int parentFrame = parentStack.length-1;
	    while (frame > 0 && parentFrame > 0)
	      {
		if (stack[frame].equals(parentStack[parentFrame]))
		  {
		    equal++;
		    frame--;
		    parentFrame--;
		  }
		else
		  break;
	      }
	    stackTraceStringBuffer(sb, cause.toString(), stack, equal);
	  }
        cause = cause.getCause();
      }

    return sb.toString();
  }

  // Adds to the given StringBuffer a line containing the name and
  // all stacktrace elements minus the last equal ones.
  private static void stackTraceStringBuffer(StringBuffer sb, String name,
					StackTraceElement[] stack, int equal)
  {
    // (finish) first line
    sb.append(name);
    sb.append(nl);

    // The stacktrace
    if (stack == null || stack.length == 0)
      {
	sb.append("   <<No stacktrace available>>");
	sb.append(nl);
      }
    else
      {
	for (int i = 0; i < stack.length-equal; i++)
	  {
	    sb.append("   at ");
	    sb.append(stack[i] == null ? "<<Unknown>>" : stack[i].toString());
	    sb.append(nl);
	  }
	if (equal > 0)
	  {
	    sb.append("   ...");
	    sb.append(equal);
	    sb.append(" more");
	    sb.append(nl);
	  }
      }
  }

  /**
   * Fill in the stack trace with the current execution stack.
   *
   * @return this same throwable
   * @see #printStackTrace()
   */
  public Throwable fillInStackTrace()
  {
    vmState = VMThrowable.fillInStackTrace(this);
    stackTrace = null; // Should be regenerated when used.

    return this;
  }

  /**
   * Provides access to the information printed in {@link #printStackTrace()}.
   * The array is non-null, with no null entries, although the virtual
   * machine is allowed to skip stack frames.  If the array is not 0-length,
   * then slot 0 holds the information on the stack frame where the Throwable
   * was created (or at least where <code>fillInStackTrace()</code> was
   * called).
   *
   * @return an array of stack trace information, as available from the VM
   * @since 1.4
   */
  public StackTraceElement[] getStackTrace()
  {
    if (stackTrace == null)
      if (vmState == null)
	stackTrace = new StackTraceElement[0];
      else 
	{
	  stackTrace = vmState.getStackTrace(this);
	  vmState = null; // No longer needed
	}

    return stackTrace;
  }

  /**
   * Change the stack trace manually. This method is designed for remote
   * procedure calls, which intend to alter the stack trace before or after
   * serialization according to the context of the remote call.
   * <p>
   * The contents of the given stacktrace is copied so changes to the
   * original array do not change the stack trace elements of this
   * throwable.
   *
   * @param stackTrace the new trace to use
   * @throws NullPointerException if stackTrace is null or has null elements
   * @since 1.4
   */
  public void setStackTrace(StackTraceElement[] stackTrace)
  {
    int i = stackTrace.length;
    StackTraceElement[] st = new StackTraceElement[i];

    while (--i >= 0)
      {
	st[i] = stackTrace[i];
	if (st[i] == null)
	  throw new NullPointerException("Element " + i + " null");
      }

    this.stackTrace = st;
  }

  /**
   * VM state when fillInStackTrace was called.
   * Used by getStackTrace() to get an array of StackTraceElements.
   * Cleared when no longer needed.
   */
  private transient VMThrowable vmState;
}