Source code: java/lang/Throwable.java
1 /* java.lang.Throwable -- Root class for all Exceptions and Errors
2 Copyright (C) 1998, 1999, 2002, 2004, 2005 Free Software Foundation, Inc.
3
4 This file is part of GNU Classpath.
5
6 GNU Classpath is free software; you can redistribute it and/or modify
7 it under the terms of the GNU General Public License as published by
8 the Free Software Foundation; either version 2, or (at your option)
9 any later version.
10
11 GNU Classpath is distributed in the hope that it will be useful, but
12 WITHOUT ANY WARRANTY; without even the implied warranty of
13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
14 General Public License for more details.
15
16 You should have received a copy of the GNU General Public License
17 along with GNU Classpath; see the file COPYING. If not, write to the
18 Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
19 02110-1301 USA.
20
21 Linking this library statically or dynamically with other modules is
22 making a combined work based on this library. Thus, the terms and
23 conditions of the GNU General Public License cover the whole
24 combination.
25
26 As a special exception, the copyright holders of this library give you
27 permission to link this library with independent modules to produce an
28 executable, regardless of the license terms of these independent
29 modules, and to copy and distribute the resulting executable under
30 terms of your choice, provided that you also meet, for each linked
31 independent module, the terms and conditions of the license of that
32 module. An independent module is a module which is not derived from
33 or based on this library. If you modify this library, you may extend
34 this exception to your version of the library, but you are not
35 obligated to do so. If you do not wish to do so, delete this
36 exception statement from your version. */
37
38 package java.lang;
39
40 import gnu.classpath.SystemProperties;
41
42 import java.io.PrintStream;
43 import java.io.PrintWriter;
44 import java.io.Serializable;
45
46 /**
47 * Throwable is the superclass of all exceptions that can be raised.
48 *
49 * <p>There are two special cases: {@link Error} and {@link RuntimeException}:
50 * these two classes (and their subclasses) are considered unchecked
51 * exceptions, and are either frequent enough or catastrophic enough that you
52 * do not need to declare them in <code>throws</code> clauses. Everything
53 * else is a checked exception, and is ususally a subclass of
54 * {@link Exception}; these exceptions have to be handled or declared.
55 *
56 * <p>Instances of this class are usually created with knowledge of the
57 * execution context, so that you can get a stack trace of the problem spot
58 * in the code. Also, since JDK 1.4, Throwables participate in "exception
59 * chaining." This means that one exception can be caused by another, and
60 * preserve the information of the original.
61 *
62 * <p>One reason this is useful is to wrap exceptions to conform to an
63 * interface. For example, it would be bad design to require all levels
64 * of a program interface to be aware of the low-level exceptions thrown
65 * at one level of abstraction. Another example is wrapping a checked
66 * exception in an unchecked one, to communicate that failure occured
67 * while still obeying the method throws clause of a superclass.
68 *
69 * <p>A cause is assigned in one of two ways; but can only be assigned once
70 * in the lifetime of the Throwable. There are new constructors added to
71 * several classes in the exception hierarchy that directly initialize the
72 * cause, or you can use the <code>initCause</code> method. This second
73 * method is especially useful if the superclass has not been retrofitted
74 * with new constructors:<br>
75 * <pre>
76 * try
77 * {
78 * lowLevelOp();
79 * }
80 * catch (LowLevelException lle)
81 * {
82 * throw (HighLevelException) new HighLevelException().initCause(lle);
83 * }
84 * </pre>
85 * Notice the cast in the above example; without it, your method would need
86 * a throws clase that declared Throwable, defeating the purpose of chainig
87 * your exceptions.
88 *
89 * <p>By convention, exception classes have two constructors: one with no
90 * arguments, and one that takes a String for a detail message. Further,
91 * classes which are likely to be used in an exception chain also provide
92 * a constructor that takes a Throwable, with or without a detail message
93 * string.
94 *
95 * <p>Another 1.4 feature is the StackTrace, a means of reflection that
96 * allows the program to inspect the context of the exception, and which is
97 * serialized, so that remote procedure calls can correctly pass exceptions.
98 *
99 * @author Brian Jones
100 * @author John Keiser
101 * @author Mark Wielaard
102 * @author Tom Tromey
103 * @author Eric Blake (ebb9@email.byu.edu)
104 * @since 1.0
105 * @status updated to 1.4
106 */
107 public class Throwable implements Serializable
108 {
109 /**
110 * Compatible with JDK 1.0+.
111 */
112 private static final long serialVersionUID = -3042686055658047285L;
113
114 /**
115 * The detail message.
116 *
117 * @serial specific details about the exception, may be null
118 */
119 private final String detailMessage;
120
121 /**
122 * The cause of the throwable, including null for an unknown or non-chained
123 * cause. This may only be set once; so the field is set to
124 * <code>this</code> until initialized.
125 *
126 * @serial the cause, or null if unknown, or this if not yet set
127 * @since 1.4
128 */
129 private Throwable cause = this;
130
131 /**
132 * The stack trace, in a serialized form.
133 *
134 * @serial the elements of the stack trace; this is non-null, and has
135 * no null entries
136 * @since 1.4
137 */
138 private StackTraceElement[] stackTrace;
139
140 /**
141 * Instantiate this Throwable with an empty message. The cause remains
142 * uninitialized. {@link #fillInStackTrace()} will be called to set
143 * up the stack trace.
144 */
145 public Throwable()
146 {
147 this((String) null);
148 }
149
150 /**
151 * Instantiate this Throwable with the given message. The cause remains
152 * uninitialized. {@link #fillInStackTrace()} will be called to set
153 * up the stack trace.
154 *
155 * @param message the message to associate with the Throwable
156 */
157 public Throwable(String message)
158 {
159 fillInStackTrace();
160 detailMessage = message;
161 }
162
163 /**
164 * Instantiate this Throwable with the given message and cause. Note that
165 * the message is unrelated to the message of the cause.
166 * {@link #fillInStackTrace()} will be called to set up the stack trace.
167 *
168 * @param message the message to associate with the Throwable
169 * @param cause the cause, may be null
170 * @since 1.4
171 */
172 public Throwable(String message, Throwable cause)
173 {
174 this(message);
175 this.cause = cause;
176 }
177
178 /**
179 * Instantiate this Throwable with the given cause. The message is then
180 * built as <code>cause == null ? null : cause.toString()</code>.
181 * {@link #fillInStackTrace()} will be called to set up the stack trace.
182 *
183 * @param cause the cause, may be null
184 * @since 1.4
185 */
186 public Throwable(Throwable cause)
187 {
188 this(cause == null ? null : cause.toString(), cause);
189 }
190
191 /**
192 * Get the message associated with this Throwable.
193 *
194 * @return the error message associated with this Throwable, may be null
195 */
196 public String getMessage()
197 {
198 return detailMessage;
199 }
200
201 /**
202 * Get a localized version of this Throwable's error message.
203 * This method must be overridden in a subclass of Throwable
204 * to actually produce locale-specific methods. The Throwable
205 * implementation just returns getMessage().
206 *
207 * @return a localized version of this error message
208 * @see #getMessage()
209 * @since 1.1
210 */
211 public String getLocalizedMessage()
212 {
213 return getMessage();
214 }
215
216 /**
217 * Returns the cause of this exception, or null if the cause is not known
218 * or non-existant. This cause is initialized by the new constructors,
219 * or by calling initCause.
220 *
221 * @return the cause of this Throwable
222 * @since 1.4
223 */
224 public Throwable getCause()
225 {
226 return cause == this ? null : cause;
227 }
228
229 /**
230 * Initialize the cause of this Throwable. This may only be called once
231 * during the object lifetime, including implicitly by chaining
232 * constructors.
233 *
234 * @param cause the cause of this Throwable, may be null
235 * @return this
236 * @throws IllegalArgumentException if cause is this (a Throwable can't be
237 * its own cause!)
238 * @throws IllegalStateException if the cause has already been set
239 * @since 1.4
240 */
241 public Throwable initCause(Throwable cause)
242 {
243 if (cause == this)
244 throw new IllegalArgumentException();
245 if (this.cause != this)
246 throw new IllegalStateException();
247 this.cause = cause;
248 return this;
249 }
250
251 /**
252 * Get a human-readable representation of this Throwable. The detail message
253 * is retrieved by getLocalizedMessage(). Then, with a null detail
254 * message, this string is simply the object's class name; otherwise
255 * the string is <code>getClass().getName() + ": " + message</code>.
256 *
257 * @return a human-readable String represting this Throwable
258 */
259 public String toString()
260 {
261 String msg = getLocalizedMessage();
262 return getClass().getName() + (msg == null ? "" : ": " + msg);
263 }
264
265 /**
266 * Print a stack trace to the standard error stream. This stream is the
267 * current contents of <code>System.err</code>. The first line of output
268 * is the result of {@link #toString()}, and the remaining lines represent
269 * the data created by {@link #fillInStackTrace()}. While the format is
270 * unspecified, this implementation uses the suggested format, demonstrated
271 * by this example:<br>
272 * <pre>
273 * public class Junk
274 * {
275 * public static void main(String args[])
276 * {
277 * try
278 * {
279 * a();
280 * }
281 * catch(HighLevelException e)
282 * {
283 * e.printStackTrace();
284 * }
285 * }
286 * static void a() throws HighLevelException
287 * {
288 * try
289 * {
290 * b();
291 * }
292 * catch(MidLevelException e)
293 * {
294 * throw new HighLevelException(e);
295 * }
296 * }
297 * static void b() throws MidLevelException
298 * {
299 * c();
300 * }
301 * static void c() throws MidLevelException
302 * {
303 * try
304 * {
305 * d();
306 * }
307 * catch(LowLevelException e)
308 * {
309 * throw new MidLevelException(e);
310 * }
311 * }
312 * static void d() throws LowLevelException
313 * {
314 * e();
315 * }
316 * static void e() throws LowLevelException
317 * {
318 * throw new LowLevelException();
319 * }
320 * }
321 * class HighLevelException extends Exception
322 * {
323 * HighLevelException(Throwable cause) { super(cause); }
324 * }
325 * class MidLevelException extends Exception
326 * {
327 * MidLevelException(Throwable cause) { super(cause); }
328 * }
329 * class LowLevelException extends Exception
330 * {
331 * }
332 * </pre>
333 * <p>
334 * <pre>
335 * HighLevelException: MidLevelException: LowLevelException
336 * at Junk.a(Junk.java:13)
337 * at Junk.main(Junk.java:4)
338 * Caused by: MidLevelException: LowLevelException
339 * at Junk.c(Junk.java:23)
340 * at Junk.b(Junk.java:17)
341 * at Junk.a(Junk.java:11)
342 * ... 1 more
343 * Caused by: LowLevelException
344 * at Junk.e(Junk.java:30)
345 * at Junk.d(Junk.java:27)
346 * at Junk.c(Junk.java:21)
347 * ... 3 more
348 * </pre>
349 */
350 public void printStackTrace()
351 {
352 printStackTrace(System.err);
353 }
354
355 /**
356 * Print a stack trace to the specified PrintStream. See
357 * {@link #printStackTrace()} for the sample format.
358 *
359 * @param s the PrintStream to write the trace to
360 */
361 public void printStackTrace(PrintStream s)
362 {
363 s.print(stackTraceString());
364 }
365
366 /**
367 * Prints the exception, the detailed message and the stack trace
368 * associated with this Throwable to the given <code>PrintWriter</code>.
369 * The actual output written is implemention specific. Use the result of
370 * <code>getStackTrace()</code> when more precise information is needed.
371 *
372 * <p>This implementation first prints a line with the result of this
373 * object's <code>toString()</code> method.
374 * <br>
375 * Then for all elements given by <code>getStackTrace</code> it prints
376 * a line containing three spaces, the string "at " and the result of calling
377 * the <code>toString()</code> method on the <code>StackTraceElement</code>
378 * object. If <code>getStackTrace()</code> returns an empty array it prints
379 * a line containing three spaces and the string
380 * "<<No stacktrace available>>".
381 * <br>
382 * Then if <code>getCause()</code> doesn't return null it adds a line
383 * starting with "Caused by: " and the result of calling
384 * <code>toString()</code> on the cause.
385 * <br>
386 * Then for every cause (of a cause, etc) the stacktrace is printed the
387 * same as for the top level <code>Throwable</code> except that as soon
388 * as all the remaining stack frames of the cause are the same as the
389 * the last stack frames of the throwable that the cause is wrapped in
390 * then a line starting with three spaces and the string "... X more" is
391 * printed, where X is the number of remaining stackframes.
392 *
393 * @param pw the PrintWriter to write the trace to
394 * @since 1.1
395 */
396 public void printStackTrace (PrintWriter pw)
397 {
398 pw.print(stackTraceString());
399 }
400
401 /*
402 * We use inner class to avoid a static initializer in this basic class.
403 */
404 private static class StaticData
405 {
406 static final String nl = SystemProperties.getProperty("line.separator");
407 }
408
409 // Create whole stack trace in a stringbuffer so we don't have to print
410 // it line by line. This prevents printing multiple stack traces from
411 // different threads to get mixed up when written to the same PrintWriter.
412 private String stackTraceString()
413 {
414 StringBuffer sb = new StringBuffer();
415
416 // Main stacktrace
417 StackTraceElement[] stack = getStackTrace();
418 stackTraceStringBuffer(sb, this.toString(), stack, 0);
419
420 // The cause(s)
421 Throwable cause = getCause();
422 while (cause != null)
423 {
424 // Cause start first line
425 sb.append("Caused by: ");
426
427 // Cause stacktrace
428 StackTraceElement[] parentStack = stack;
429 stack = cause.getStackTrace();
430 if (parentStack == null || parentStack.length == 0)
431 stackTraceStringBuffer(sb, cause.toString(), stack, 0);
432 else
433 {
434 int equal = 0; // Count how many of the last stack frames are equal
435 int frame = stack.length-1;
436 int parentFrame = parentStack.length-1;
437 while (frame > 0 && parentFrame > 0)
438 {
439 if (stack[frame].equals(parentStack[parentFrame]))
440 {
441 equal++;
442 frame--;
443 parentFrame--;
444 }
445 else
446 break;
447 }
448 stackTraceStringBuffer(sb, cause.toString(), stack, equal);
449 }
450 cause = cause.getCause();
451 }
452
453 return sb.toString();
454 }
455
456 // Adds to the given StringBuffer a line containing the name and
457 // all stacktrace elements minus the last equal ones.
458 private static void stackTraceStringBuffer(StringBuffer sb, String name,
459 StackTraceElement[] stack, int equal)
460 {
461 String nl = StaticData.nl;
462 // (finish) first line
463 sb.append(name);
464 sb.append(nl);
465
466 // The stacktrace
467 if (stack == null || stack.length == 0)
468 {
469 sb.append(" <<No stacktrace available>>");
470 sb.append(nl);
471 }
472 else
473 {
474 for (int i = 0; i < stack.length-equal; i++)
475 {
476 sb.append(" at ");
477 sb.append(stack[i] == null ? "<<Unknown>>" : stack[i].toString());
478 sb.append(nl);
479 }
480 if (equal > 0)
481 {
482 sb.append(" ...");
483 sb.append(equal);
484 sb.append(" more");
485 sb.append(nl);
486 }
487 }
488 }
489
490 /**
491 * Fill in the stack trace with the current execution stack.
492 *
493 * @return this same throwable
494 * @see #printStackTrace()
495 */
496 public Throwable fillInStackTrace()
497 {
498 vmState = VMThrowable.fillInStackTrace(this);
499 stackTrace = null; // Should be regenerated when used.
500
501 return this;
502 }
503
504 /**
505 * Provides access to the information printed in {@link #printStackTrace()}.
506 * The array is non-null, with no null entries, although the virtual
507 * machine is allowed to skip stack frames. If the array is not 0-length,
508 * then slot 0 holds the information on the stack frame where the Throwable
509 * was created (or at least where <code>fillInStackTrace()</code> was
510 * called).
511 *
512 * @return an array of stack trace information, as available from the VM
513 * @since 1.4
514 */
515 public StackTraceElement[] getStackTrace()
516 {
517 if (stackTrace == null)
518 if (vmState == null)
519 stackTrace = new StackTraceElement[0];
520 else
521 {
522 stackTrace = vmState.getStackTrace(this);
523 vmState = null; // No longer needed
524 }
525
526 return stackTrace;
527 }
528
529 /**
530 * Change the stack trace manually. This method is designed for remote
531 * procedure calls, which intend to alter the stack trace before or after
532 * serialization according to the context of the remote call.
533 * <p>
534 * The contents of the given stacktrace is copied so changes to the
535 * original array do not change the stack trace elements of this
536 * throwable.
537 *
538 * @param stackTrace the new trace to use
539 * @throws NullPointerException if stackTrace is null or has null elements
540 * @since 1.4
541 */
542 public void setStackTrace(StackTraceElement[] stackTrace)
543 {
544 int i = stackTrace.length;
545 StackTraceElement[] st = new StackTraceElement[i];
546
547 while (--i >= 0)
548 {
549 st[i] = stackTrace[i];
550 if (st[i] == null)
551 throw new NullPointerException("Element " + i + " null");
552 }
553
554 this.stackTrace = st;
555 }
556
557 /**
558 * VM state when fillInStackTrace was called.
559 * Used by getStackTrace() to get an array of StackTraceElements.
560 * Cleared when no longer needed.
561 */
562 private transient VMThrowable vmState;
563 }