001/* java.lang.Throwable -- Root class for all Exceptions and Errors
002   Copyright (C) 1998, 1999, 2002, 2004, 2005  Free Software Foundation, Inc.
003
004This file is part of GNU Classpath.
005
006GNU Classpath is free software; you can redistribute it and/or modify
007it under the terms of the GNU General Public License as published by
008the Free Software Foundation; either version 2, or (at your option)
009any later version.
010
011GNU Classpath is distributed in the hope that it will be useful, but
012WITHOUT ANY WARRANTY; without even the implied warranty of
013MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
014General Public License for more details.
015
016You should have received a copy of the GNU General Public License
017along with GNU Classpath; see the file COPYING.  If not, write to the
018Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
01902110-1301 USA.
020
021Linking this library statically or dynamically with other modules is
022making a combined work based on this library.  Thus, the terms and
023conditions of the GNU General Public License cover the whole
024combination.
025
026As a special exception, the copyright holders of this library give you
027permission to link this library with independent modules to produce an
028executable, regardless of the license terms of these independent
029modules, and to copy and distribute the resulting executable under
030terms of your choice, provided that you also meet, for each linked
031independent module, the terms and conditions of the license of that
032module.  An independent module is a module which is not derived from
033or based on this library.  If you modify this library, you may extend
034this exception to your version of the library, but you are not
035obligated to do so.  If you do not wish to do so, delete this
036exception statement from your version. */
037
038package java.lang;
039
040import gnu.classpath.SystemProperties;
041
042import gnu.java.lang.CPStringBuilder;
043
044import java.io.PrintStream;
045import java.io.PrintWriter;
046import java.io.Serializable;
047
048/**
049 * Throwable is the superclass of all exceptions that can be raised.
050 *
051 * <p>There are two special cases: {@link Error} and {@link RuntimeException}:
052 * these two classes (and their subclasses) are considered unchecked
053 * exceptions, and are either frequent enough or catastrophic enough that you
054 * do not need to declare them in <code>throws</code> clauses.  Everything
055 * else is a checked exception, and is ususally a subclass of
056 * {@link Exception}; these exceptions have to be handled or declared.
057 *
058 * <p>Instances of this class are usually created with knowledge of the
059 * execution context, so that you can get a stack trace of the problem spot
060 * in the code.  Also, since JDK 1.4, Throwables participate in "exception
061 * chaining."  This means that one exception can be caused by another, and
062 * preserve the information of the original.
063 *
064 * <p>One reason this is useful is to wrap exceptions to conform to an
065 * interface.  For example, it would be bad design to require all levels
066 * of a program interface to be aware of the low-level exceptions thrown
067 * at one level of abstraction. Another example is wrapping a checked
068 * exception in an unchecked one, to communicate that failure occured
069 * while still obeying the method throws clause of a superclass.
070 *
071 * <p>A cause is assigned in one of two ways; but can only be assigned once
072 * in the lifetime of the Throwable.  There are new constructors added to
073 * several classes in the exception hierarchy that directly initialize the
074 * cause, or you can use the <code>initCause</code> method. This second
075 * method is especially useful if the superclass has not been retrofitted
076 * with new constructors:<br>
077 * <pre>
078 * try
079 *   {
080 *     lowLevelOp();
081 *   }
082 * catch (LowLevelException lle)
083 *   {
084 *     throw (HighLevelException) new HighLevelException().initCause(lle);
085 *   }
086 * </pre>
087 * Notice the cast in the above example; without it, your method would need
088 * a throws clase that declared Throwable, defeating the purpose of chainig
089 * your exceptions.
090 *
091 * <p>By convention, exception classes have two constructors: one with no
092 * arguments, and one that takes a String for a detail message.  Further,
093 * classes which are likely to be used in an exception chain also provide
094 * a constructor that takes a Throwable, with or without a detail message
095 * string.
096 *
097 * <p>Another 1.4 feature is the StackTrace, a means of reflection that
098 * allows the program to inspect the context of the exception, and which is
099 * serialized, so that remote procedure calls can correctly pass exceptions.
100 *
101 * @author Brian Jones
102 * @author John Keiser
103 * @author Mark Wielaard
104 * @author Tom Tromey
105 * @author Eric Blake (ebb9@email.byu.edu)
106 * @since 1.0
107 * @status updated to 1.4
108 */
109public class Throwable implements Serializable
110{
111  /**
112   * Compatible with JDK 1.0+.
113   */
114  private static final long serialVersionUID = -3042686055658047285L;
115
116  /**
117   * The detail message.
118   *
119   * @serial specific details about the exception, may be null
120   */
121  private final String detailMessage;
122
123  /**
124   * The cause of the throwable, including null for an unknown or non-chained
125   * cause. This may only be set once; so the field is set to
126   * <code>this</code> until initialized.
127   *
128   * @serial the cause, or null if unknown, or this if not yet set
129   * @since 1.4
130   */
131  private Throwable cause = this;
132
133  /**
134   * The stack trace, in a serialized form.
135   *
136   * @serial the elements of the stack trace; this is non-null, and has
137   *         no null entries
138   * @since 1.4
139   */
140  private StackTraceElement[] stackTrace;
141
142  /**
143   * Instantiate this Throwable with an empty message. The cause remains
144   * uninitialized.  {@link #fillInStackTrace()} will be called to set
145   * up the stack trace.
146   */
147  public Throwable()
148  {
149    this((String) null);
150  }
151
152  /**
153   * Instantiate this Throwable with the given message. The cause remains
154   * uninitialized.  {@link #fillInStackTrace()} will be called to set
155   * up the stack trace.
156   *
157   * @param message the message to associate with the Throwable
158   */
159  public Throwable(String message)
160  {
161    fillInStackTrace();
162    detailMessage = message;
163  }
164
165  /**
166   * Instantiate this Throwable with the given message and cause. Note that
167   * the message is unrelated to the message of the cause.
168   * {@link #fillInStackTrace()} will be called to set up the stack trace.
169   *
170   * @param message the message to associate with the Throwable
171   * @param cause the cause, may be null
172   * @since 1.4
173   */
174  public Throwable(String message, Throwable cause)
175  {
176    this(message);
177    this.cause = cause;
178  }
179
180  /**
181   * Instantiate this Throwable with the given cause. The message is then
182   * built as <code>cause == null ? null : cause.toString()</code>.
183   * {@link #fillInStackTrace()} will be called to set up the stack trace.
184   *
185   * @param cause the cause, may be null
186   * @since 1.4
187   */
188  public Throwable(Throwable cause)
189  {
190    this(cause == null ? null : cause.toString(), cause);
191  }
192
193  /**
194   * Get the message associated with this Throwable.
195   *
196   * @return the error message associated with this Throwable, may be null
197   */
198  public String getMessage()
199  {
200    return detailMessage;
201  }
202
203  /**
204   * Get a localized version of this Throwable's error message.
205   * This method must be overridden in a subclass of Throwable
206   * to actually produce locale-specific methods.  The Throwable
207   * implementation just returns getMessage().
208   *
209   * @return a localized version of this error message
210   * @see #getMessage()
211   * @since 1.1
212   */
213  public String getLocalizedMessage()
214  {
215    return getMessage();
216  }
217
218  /**
219   * Returns the cause of this exception, or null if the cause is not known
220   * or non-existant. This cause is initialized by the new constructors,
221   * or by calling initCause.
222   *
223   * @return the cause of this Throwable
224   * @since 1.4
225   */
226  public Throwable getCause()
227  {
228    return cause == this ? null : cause;
229  }
230
231  /**
232   * Initialize the cause of this Throwable.  This may only be called once
233   * during the object lifetime, including implicitly by chaining
234   * constructors.
235   *
236   * @param cause the cause of this Throwable, may be null
237   * @return this
238   * @throws IllegalArgumentException if cause is this (a Throwable can't be
239   *         its own cause!)
240   * @throws IllegalStateException if the cause has already been set
241   * @since 1.4
242   */
243  public Throwable initCause(Throwable cause)
244  {
245    if (cause == this)
246      throw new IllegalArgumentException();
247    if (this.cause != this)
248      throw new IllegalStateException();
249    this.cause = cause;
250    return this;
251  }
252
253  /**
254   * Get a human-readable representation of this Throwable. The detail message
255   * is retrieved by getLocalizedMessage().  Then, with a null detail
256   * message, this string is simply the object's class name; otherwise
257   * the string is <code>getClass().getName() + ": " + message</code>.
258   *
259   * @return a human-readable String represting this Throwable
260   */
261  public String toString()
262  {
263    String msg = getLocalizedMessage();
264    return getClass().getName() + (msg == null ? "" : ": " + msg);
265  }
266
267  /**
268   * Print a stack trace to the standard error stream. This stream is the
269   * current contents of <code>System.err</code>. The first line of output
270   * is the result of {@link #toString()}, and the remaining lines represent
271   * the data created by {@link #fillInStackTrace()}. While the format is
272   * unspecified, this implementation uses the suggested format, demonstrated
273   * by this example:<br>
274   * <pre>
275   * public class Junk
276   * {
277   *   public static void main(String args[])
278   *   {
279   *     try
280   *       {
281   *         a();
282   *       }
283   *     catch(HighLevelException e)
284   *       {
285   *         e.printStackTrace();
286   *       }
287   *   }
288   *   static void a() throws HighLevelException
289   *   {
290   *     try
291   *       {
292   *         b();
293   *       }
294   *     catch(MidLevelException e)
295   *       {
296   *         throw new HighLevelException(e);
297   *       }
298   *   }
299   *   static void b() throws MidLevelException
300   *   {
301   *     c();
302   *   }
303   *   static void c() throws MidLevelException
304   *   {
305   *     try
306   *       {
307   *         d();
308   *       }
309   *     catch(LowLevelException e)
310   *       {
311   *         throw new MidLevelException(e);
312   *       }
313   *   }
314   *   static void d() throws LowLevelException
315   *   {
316   *     e();
317   *   }
318   *   static void e() throws LowLevelException
319   *   {
320   *     throw new LowLevelException();
321   *   }
322   * }
323   * class HighLevelException extends Exception
324   * {
325   *   HighLevelException(Throwable cause) { super(cause); }
326   * }
327   * class MidLevelException extends Exception
328   * {
329   *   MidLevelException(Throwable cause)  { super(cause); }
330   * }
331   * class LowLevelException extends Exception
332   * {
333   * }
334   * </pre>
335   * <p>
336   * <pre>
337   *  HighLevelException: MidLevelException: LowLevelException
338   *          at Junk.a(Junk.java:13)
339   *          at Junk.main(Junk.java:4)
340   *  Caused by: MidLevelException: LowLevelException
341   *          at Junk.c(Junk.java:23)
342   *          at Junk.b(Junk.java:17)
343   *          at Junk.a(Junk.java:11)
344   *          ... 1 more
345   *  Caused by: LowLevelException
346   *          at Junk.e(Junk.java:30)
347   *          at Junk.d(Junk.java:27)
348   *          at Junk.c(Junk.java:21)
349   *          ... 3 more
350   * </pre>
351   */
352  public void printStackTrace()
353  {
354    printStackTrace(System.err);
355  }
356
357  /**
358   * Print a stack trace to the specified PrintStream. See
359   * {@link #printStackTrace()} for the sample format.
360   *
361   * @param s the PrintStream to write the trace to
362   */
363  public void printStackTrace(PrintStream s)
364  {
365    s.print(stackTraceString());
366  }
367
368  /**
369   * Prints the exception, the detailed message and the stack trace
370   * associated with this Throwable to the given <code>PrintWriter</code>.
371   * The actual output written is implemention specific. Use the result of
372   * <code>getStackTrace()</code> when more precise information is needed.
373   *
374   * <p>This implementation first prints a line with the result of this
375   * object's <code>toString()</code> method.
376   * <br>
377   * Then for all elements given by <code>getStackTrace</code> it prints
378   * a line containing three spaces, the string "at " and the result of calling
379   * the <code>toString()</code> method on the <code>StackTraceElement</code>
380   * object. If <code>getStackTrace()</code> returns an empty array it prints
381   * a line containing three spaces and the string
382   * "&lt;&lt;No stacktrace available&gt;&gt;".
383   * <br>
384   * Then if <code>getCause()</code> doesn't return null it adds a line
385   * starting with "Caused by: " and the result of calling
386   * <code>toString()</code> on the cause.
387   * <br>
388   * Then for every cause (of a cause, etc) the stacktrace is printed the
389   * same as for the top level <code>Throwable</code> except that as soon
390   * as all the remaining stack frames of the cause are the same as the
391   * the last stack frames of the throwable that the cause is wrapped in
392   * then a line starting with three spaces and the string "... X more" is
393   * printed, where X is the number of remaining stackframes.
394   *
395   * @param pw the PrintWriter to write the trace to
396   * @since 1.1
397   */
398  public void printStackTrace (PrintWriter pw)
399  {
400    pw.print(stackTraceString());
401  }
402
403  /*
404   * We use inner class to avoid a static initializer in this basic class.
405   */
406  private static class StaticData
407  {
408    static final String nl = SystemProperties.getProperty("line.separator");
409  }
410
411  // Create whole stack trace in a stringbuffer so we don't have to print
412  // it line by line. This prevents printing multiple stack traces from
413  // different threads to get mixed up when written to the same PrintWriter.
414  private String stackTraceString()
415  {
416    CPStringBuilder sb = new CPStringBuilder();
417
418    // Main stacktrace
419    StackTraceElement[] stack = getStackTrace();
420    stackTraceStringBuffer(sb, this.toString(), stack, 0);
421
422    // The cause(s)
423    Throwable cause = getCause();
424    while (cause != null)
425      {
426        // Cause start first line
427        sb.append("Caused by: ");
428
429        // Cause stacktrace
430        StackTraceElement[] parentStack = stack;
431        stack = cause.getStackTrace();
432        if (parentStack == null || parentStack.length == 0)
433          stackTraceStringBuffer(sb, cause.toString(), stack, 0);
434        else
435          {
436            int equal = 0; // Count how many of the last stack frames are equal
437            int frame = stack.length-1;
438            int parentFrame = parentStack.length-1;
439            while (frame > 0 && parentFrame > 0)
440              {
441                if (stack[frame].equals(parentStack[parentFrame]))
442                  {
443                    equal++;
444                    frame--;
445                    parentFrame--;
446                  }
447                else
448                  break;
449              }
450            stackTraceStringBuffer(sb, cause.toString(), stack, equal);
451          }
452        cause = cause.getCause();
453      }
454
455    return sb.toString();
456  }
457
458  // Adds to the given StringBuffer a line containing the name and
459  // all stacktrace elements minus the last equal ones.
460  private static void stackTraceStringBuffer(CPStringBuilder sb, String name,
461                                        StackTraceElement[] stack, int equal)
462  {
463    String nl = StaticData.nl;
464    // (finish) first line
465    sb.append(name);
466    sb.append(nl);
467
468    // The stacktrace
469    if (stack == null || stack.length == 0)
470      {
471        sb.append("   <<No stacktrace available>>");
472        sb.append(nl);
473      }
474    else
475      {
476        for (int i = 0; i < stack.length-equal; i++)
477          {
478            sb.append("   at ");
479            sb.append(stack[i] == null ? "<<Unknown>>" : stack[i].toString());
480            sb.append(nl);
481          }
482        if (equal > 0)
483          {
484            sb.append("   ...");
485            sb.append(equal);
486            sb.append(" more");
487            sb.append(nl);
488          }
489      }
490  }
491
492  /**
493   * Fill in the stack trace with the current execution stack.
494   *
495   * @return this same throwable
496   * @see #printStackTrace()
497   */
498  public Throwable fillInStackTrace()
499  {
500    vmState = VMThrowable.fillInStackTrace(this);
501    stackTrace = null; // Should be regenerated when used.
502
503    return this;
504  }
505
506  /**
507   * Provides access to the information printed in {@link #printStackTrace()}.
508   * The array is non-null, with no null entries, although the virtual
509   * machine is allowed to skip stack frames.  If the array is not 0-length,
510   * then slot 0 holds the information on the stack frame where the Throwable
511   * was created (or at least where <code>fillInStackTrace()</code> was
512   * called).
513   *
514   * @return an array of stack trace information, as available from the VM
515   * @since 1.4
516   */
517  public StackTraceElement[] getStackTrace()
518  {
519    if (stackTrace == null)
520      if (vmState == null)
521        stackTrace = new StackTraceElement[0];
522      else
523        {
524          stackTrace = vmState.getStackTrace(this);
525          vmState = null; // No longer needed
526        }
527
528    return stackTrace;
529  }
530
531  /**
532   * Change the stack trace manually. This method is designed for remote
533   * procedure calls, which intend to alter the stack trace before or after
534   * serialization according to the context of the remote call.
535   * <p>
536   * The contents of the given stacktrace is copied so changes to the
537   * original array do not change the stack trace elements of this
538   * throwable.
539   *
540   * @param stackTrace the new trace to use
541   * @throws NullPointerException if stackTrace is null or has null elements
542   * @since 1.4
543   */
544  public void setStackTrace(StackTraceElement[] stackTrace)
545  {
546    int i = stackTrace.length;
547    StackTraceElement[] st = new StackTraceElement[i];
548
549    while (--i >= 0)
550      {
551        st[i] = stackTrace[i];
552        if (st[i] == null)
553          throw new NullPointerException("Element " + i + " null");
554      }
555
556    this.stackTrace = st;
557  }
558
559  /**
560   * VM state when fillInStackTrace was called.
561   * Used by getStackTrace() to get an array of StackTraceElements.
562   * Cleared when no longer needed.
563   */
564  private transient VMThrowable vmState;
565}