rev 5703 : [mq]: throwableevent

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  13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  14  * version 2 for more details (a copy is included in the LICENSE file that
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  25 
  26 package java.lang;
  27 import  java.io.*;
  28 import  java.util.*;
  29 
  30 import sun.misc.ThrowableTrace;
  31 
  32 /**
  33  * The {@code Throwable} class is the superclass of all errors and
  34  * exceptions in the Java language. Only objects that are instances of this
  35  * class (or one of its subclasses) are thrown by the Java Virtual Machine or
  36  * can be thrown by the Java {@code throw} statement. Similarly, only
  37  * this class or one of its subclasses can be the argument type in a
  38  * {@code catch} clause.
  39  *
  40  * For the purposes of compile-time checking of exceptions, {@code
  41  * Throwable} and any subclass of {@code Throwable} that is not also a
  42  * subclass of either {@link RuntimeException} or {@link Error} are
  43  * regarded as checked exceptions.
  44  *
  45  * <p>Instances of two subclasses, {@link java.lang.Error} and
  46  * {@link java.lang.Exception}, are conventionally used to indicate
  47  * that exceptional situations have occurred. Typically, these instances
  48  * are freshly created in the context of the exceptional situation so
  49  * as to include relevant information (such as stack trace data).
  50  *
  51  * <p>A throwable contains a snapshot of the execution stack of its
  52  * thread at the time it was created. It can also contain a message
  53  * string that gives more information about the error. Over time, a
  54  * throwable can {@linkplain Throwable#addSuppressed suppress} other
  55  * throwables from being propagated.  Finally, the throwable can also
  56  * contain a <i>cause</i>: another throwable that caused this
  57  * throwable to be constructed.  The recording of this causal information
  58  * is referred to as the <i>chained exception</i> facility, as the
  59  * cause can, itself, have a cause, and so on, leading to a "chain" of
  60  * exceptions, each caused by another.
  61  *
  62  * <p>One reason that a throwable may have a cause is that the class that
  63  * throws it is built atop a lower layered abstraction, and an operation on
  64  * the upper layer fails due to a failure in the lower layer.  It would be bad
  65  * design to let the throwable thrown by the lower layer propagate outward, as
  66  * it is generally unrelated to the abstraction provided by the upper layer.
  67  * Further, doing so would tie the API of the upper layer to the details of
  68  * its implementation, assuming the lower layer's exception was a checked
  69  * exception.  Throwing a "wrapped exception" (i.e., an exception containing a
  70  * cause) allows the upper layer to communicate the details of the failure to
  71  * its caller without incurring either of these shortcomings.  It preserves
  72  * the flexibility to change the implementation of the upper layer without
  73  * changing its API (in particular, the set of exceptions thrown by its
  74  * methods).
  75  *
  76  * <p>A second reason that a throwable may have a cause is that the method
  77  * that throws it must conform to a general-purpose interface that does not
  78  * permit the method to throw the cause directly.  For example, suppose
  79  * a persistent collection conforms to the {@link java.util.Collection
  80  * Collection} interface, and that its persistence is implemented atop
  81  * {@code java.io}.  Suppose the internals of the {@code add} method
  82  * can throw an {@link java.io.IOException IOException}.  The implementation
  83  * can communicate the details of the {@code IOException} to its caller
  84  * while conforming to the {@code Collection} interface by wrapping the
  85  * {@code IOException} in an appropriate unchecked exception.  (The
  86  * specification for the persistent collection should indicate that it is
  87  * capable of throwing such exceptions.)
  88  *
  89  * <p>A cause can be associated with a throwable in two ways: via a
  90  * constructor that takes the cause as an argument, or via the
  91  * {@link #initCause(Throwable)} method.  New throwable classes that
  92  * wish to allow causes to be associated with them should provide constructors
  93  * that take a cause and delegate (perhaps indirectly) to one of the
  94  * {@code Throwable} constructors that takes a cause.
  95  *
  96  * Because the {@code initCause} method is public, it allows a cause to be
  97  * associated with any throwable, even a "legacy throwable" whose
  98  * implementation predates the addition of the exception chaining mechanism to
  99  * {@code Throwable}.
 100  *
 101  * <p>By convention, class {@code Throwable} and its subclasses have two
 102  * constructors, one that takes no arguments and one that takes a
 103  * {@code String} argument that can be used to produce a detail message.
 104  * Further, those subclasses that might likely have a cause associated with
 105  * them should have two more constructors, one that takes a
 106  * {@code Throwable} (the cause), and one that takes a
 107  * {@code String} (the detail message) and a {@code Throwable} (the
 108  * cause).
 109  *
 110  * @author  unascribed
 111  * @author  Josh Bloch (Added exception chaining and programmatic access to
 112  *          stack trace in 1.4.)
 113  * @jls 11.2 Compile-Time Checking of Exceptions
 114  * @since JDK1.0
 115  */
 116 public class Throwable implements Serializable {
 117     /** use serialVersionUID from JDK 1.0.2 for interoperability */
 118     private static final long serialVersionUID = -3042686055658047285L;
 119 
 120     /**
 121      * Native code saves some indication of the stack backtrace in this slot.
 122      */
 123     private transient Object backtrace;
 124 
 125     /**
 126      * Specific details about the Throwable.  For example, for
 127      * {@code FileNotFoundException}, this contains the name of
 128      * the file that could not be found.
 129      *
 130      * @serial
 131      */
 132     private String detailMessage;
 133 
 134 
 135     /**
 136      * Holder class to defer initializing sentinel objects only used
 137      * for serialization.
 138      */
 139     private static class SentinelHolder {
 140         /**
 141          * {@linkplain #setStackTrace(StackTraceElement[]) Setting the
 142          * stack trace} to a one-element array containing this sentinel
 143          * value indicates future attempts to set the stack trace will be
 144          * ignored.  The sentinal is equal to the result of calling:<br>
 145          * {@code new StackTraceElement("", "", null, Integer.MIN_VALUE)}
 146          */
 147         public static final StackTraceElement STACK_TRACE_ELEMENT_SENTINEL =
 148             new StackTraceElement("", "", null, Integer.MIN_VALUE);
 149 
 150         /**
 151          * Sentinel value used in the serial form to indicate an immutable
 152          * stack trace.
 153          */
 154         public static final StackTraceElement[] STACK_TRACE_SENTINEL =
 155             new StackTraceElement[] {STACK_TRACE_ELEMENT_SENTINEL};
 156     }
 157 
 158     /**
 159      * A shared value for an empty stack.
 160      */
 161     private static final StackTraceElement[] UNASSIGNED_STACK = new StackTraceElement[0];
 162 
 163     /*
 164      * To allow Throwable objects to be made immutable and safely
 165      * reused by the JVM, such as OutOfMemoryErrors, fields of
 166      * Throwable that are writable in response to user actions, cause,
 167      * stackTrace, and suppressedExceptions obey the following
 168      * protocol:
 169      *
 170      * 1) The fields are initialized to a non-null sentinel value
 171      * which indicates the value has logically not been set.
 172      *
 173      * 2) Writing a null to the field indicates further writes
 174      * are forbidden
 175      *
 176      * 3) The sentinel value may be replaced with another non-null
 177      * value.
 178      *
 179      * For example, implementations of the HotSpot JVM have
 180      * preallocated OutOfMemoryError objects to provide for better
 181      * diagnosability of that situation.  These objects are created
 182      * without calling the constructor for that class and the fields
 183      * in question are initialized to null.  To support this
 184      * capability, any new fields added to Throwable that require
 185      * being initialized to a non-null value require a coordinated JVM
 186      * change.
 187      */
 188 
 189     /**
 190      * The throwable that caused this throwable to get thrown, or null if this
 191      * throwable was not caused by another throwable, or if the causative
 192      * throwable is unknown.  If this field is equal to this throwable itself,
 193      * it indicates that the cause of this throwable has not yet been
 194      * initialized.
 195      *
 196      * @serial
 197      * @since 1.4
 198      */
 199     private Throwable cause = this;
 200 
 201     /**
 202      * The stack trace, as returned by {@link #getStackTrace()}.
 203      *
 204      * The field is initialized to a zero-length array.  A {@code
 205      * null} value of this field indicates subsequent calls to {@link
 206      * #setStackTrace(StackTraceElement[])} and {@link
 207      * #fillInStackTrace()} will be be no-ops.
 208      *
 209      * @serial
 210      * @since 1.4
 211      */
 212     private StackTraceElement[] stackTrace = UNASSIGNED_STACK;
 213 
 214     // Setting this static field introduces an acceptable
 215     // initialization dependency on a few java.util classes.
 216     private static final List<Throwable> SUPPRESSED_SENTINEL =
 217         Collections.unmodifiableList(new ArrayList<Throwable>(0));
 218 
 219     /**
 220      * The list of suppressed exceptions, as returned by {@link
 221      * #getSuppressed()}.  The list is initialized to a zero-element
 222      * unmodifiable sentinel list.  When a serialized Throwable is
 223      * read in, if the {@code suppressedExceptions} field points to a
 224      * zero-element list, the field is reset to the sentinel value.
 225      *
 226      * @serial
 227      * @since 1.7
 228      */
 229     private List<Throwable> suppressedExceptions = SUPPRESSED_SENTINEL;
 230 
 231     /** Message for trying to suppress a null exception. */
 232     private static final String NULL_CAUSE_MESSAGE = "Cannot suppress a null exception.";
 233 
 234     /** Message for trying to suppress oneself. */
 235     private static final String SELF_SUPPRESSION_MESSAGE = "Self-suppression not permitted";
 236 
 237     /** Caption  for labeling causative exception stack traces */
 238     private static final String CAUSE_CAPTION = "Caused by: ";
 239 
 240     /** Caption for labeling suppressed exception stack traces */
 241     private static final String SUPPRESSED_CAPTION = "Suppressed: ";
 242 
 243     /**
 244      * Constructs a new throwable with {@code null} as its detail message.
 245      * The cause is not initialized, and may subsequently be initialized by a
 246      * call to {@link #initCause}.
 247      *
 248      * <p>The {@link #fillInStackTrace()} method is called to initialize
 249      * the stack trace data in the newly created throwable.
 250      */
 251     public Throwable() {
 252         fillInStackTrace();
 253         ThrowableTrace.traceThrowable(this);
 254     }
 255 
 256     /**
 257      * Constructs a new throwable with the specified detail message.  The
 258      * cause is not initialized, and may subsequently be initialized by
 259      * a call to {@link #initCause}.
 260      *
 261      * <p>The {@link #fillInStackTrace()} method is called to initialize
 262      * the stack trace data in the newly created throwable.
 263      *
 264      * @param   message   the detail message. The detail message is saved for
 265      *          later retrieval by the {@link #getMessage()} method.
 266      */
 267     public Throwable(String message) {
 268         fillInStackTrace();
 269         detailMessage = message;
 270         ThrowableTrace.traceThrowable(this);
 271     }
 272 
 273     /**
 274      * Constructs a new throwable with the specified detail message and
 275      * cause.  <p>Note that the detail message associated with
 276      * {@code cause} is <i>not</i> automatically incorporated in
 277      * this throwable's detail message.
 278      *
 279      * <p>The {@link #fillInStackTrace()} method is called to initialize
 280      * the stack trace data in the newly created throwable.
 281      *
 282      * @param  message the detail message (which is saved for later retrieval
 283      *         by the {@link #getMessage()} method).
 284      * @param  cause the cause (which is saved for later retrieval by the
 285      *         {@link #getCause()} method).  (A {@code null} value is
 286      *         permitted, and indicates that the cause is nonexistent or
 287      *         unknown.)
 288      * @since  1.4
 289      */
 290     public Throwable(String message, Throwable cause) {
 291         fillInStackTrace();
 292         detailMessage = message;
 293         this.cause = cause;
 294         ThrowableTrace.traceThrowable(this);
 295     }
 296 
 297     /**
 298      * Constructs a new throwable with the specified cause and a detail
 299      * message of {@code (cause==null ? null : cause.toString())} (which
 300      * typically contains the class and detail message of {@code cause}).
 301      * This constructor is useful for throwables that are little more than
 302      * wrappers for other throwables (for example, {@link
 303      * java.security.PrivilegedActionException}).
 304      *
 305      * <p>The {@link #fillInStackTrace()} method is called to initialize
 306      * the stack trace data in the newly created throwable.
 307      *
 308      * @param  cause the cause (which is saved for later retrieval by the
 309      *         {@link #getCause()} method).  (A {@code null} value is
 310      *         permitted, and indicates that the cause is nonexistent or
 311      *         unknown.)
 312      * @since  1.4
 313      */
 314     public Throwable(Throwable cause) {
 315         fillInStackTrace();
 316         detailMessage = (cause==null ? null : cause.toString());
 317         this.cause = cause;
 318         ThrowableTrace.traceThrowable(this);
 319     }
 320 
 321     /**
 322      * Constructs a new throwable with the specified detail message,
 323      * cause, {@linkplain #addSuppressed suppression} enabled or
 324      * disabled, and writable stack trace enabled or disabled.  If
 325      * suppression is disabled, {@link #getSuppressed} for this object
 326      * will return a zero-length array and calls to {@link
 327      * #addSuppressed} that would otherwise append an exception to the
 328      * suppressed list will have no effect.  If the writable stack
 329      * trace is false, this constructor will not call {@link
 330      * #fillInStackTrace()}, a {@code null} will be written to the
 331      * {@code stackTrace} field, and subsequent calls to {@code
 332      * fillInStackTrace} and {@link
 333      * #setStackTrace(StackTraceElement[])} will not set the stack
 334      * trace.  If the writable stack trace is false, {@link
 335      * #getStackTrace} will return a zero length array.
 336      *
 337      * <p>Note that the other constructors of {@code Throwable} treat
 338      * suppression as being enabled and the stack trace as being
 339      * writable.  Subclasses of {@code Throwable} should document any
 340      * conditions under which suppression is disabled and document
 341      * conditions under which the stack trace is not writable.
 342      * Disabling of suppression should only occur in exceptional
 343      * circumstances where special requirements exist, such as a
 344      * virtual machine reusing exception objects under low-memory
 345      * situations.  Circumstances where a given exception object is
 346      * repeatedly caught and rethrown, such as to implement control
 347      * flow between two sub-systems, is another situation where
 348      * immutable throwable objects would be appropriate.
 349      *
 350      * @param  message the detail message.
 351      * @param cause the cause.  (A {@code null} value is permitted,
 352      * and indicates that the cause is nonexistent or unknown.)
 353      * @param enableSuppression whether or not suppression is enabled or disabled
 354      * @param writableStackTrace whether or not the stack trace should be
 355      *                           writable
 356      *
 357      * @see OutOfMemoryError
 358      * @see NullPointerException
 359      * @see ArithmeticException
 360      * @since 1.7
 361      */
 362     protected Throwable(String message, Throwable cause,
 363                         boolean enableSuppression,
 364                         boolean writableStackTrace) {
 365         if (writableStackTrace) {
 366             fillInStackTrace();
 367         } else {
 368             stackTrace = null;
 369         }
 370         detailMessage = message;
 371         this.cause = cause;
 372         if (!enableSuppression)
 373             suppressedExceptions = null;
 374         ThrowableTrace.traceThrowable(this);
 375     }
 376 
 377     /**
 378      * Returns the detail message string of this throwable.
 379      *
 380      * @return  the detail message string of this {@code Throwable} instance
 381      *          (which may be {@code null}).
 382      */
 383     public String getMessage() {
 384         return detailMessage;
 385     }
 386 
 387     /**
 388      * Creates a localized description of this throwable.
 389      * Subclasses may override this method in order to produce a
 390      * locale-specific message.  For subclasses that do not override this
 391      * method, the default implementation returns the same result as
 392      * {@code getMessage()}.
 393      *
 394      * @return  The localized description of this throwable.
 395      * @since   JDK1.1
 396      */
 397     public String getLocalizedMessage() {
 398         return getMessage();
 399     }
 400 
 401     /**
 402      * Returns the cause of this throwable or {@code null} if the
 403      * cause is nonexistent or unknown.  (The cause is the throwable that
 404      * caused this throwable to get thrown.)
 405      *
 406      * <p>This implementation returns the cause that was supplied via one of
 407      * the constructors requiring a {@code Throwable}, or that was set after
 408      * creation with the {@link #initCause(Throwable)} method.  While it is
 409      * typically unnecessary to override this method, a subclass can override
 410      * it to return a cause set by some other means.  This is appropriate for
 411      * a "legacy chained throwable" that predates the addition of chained
 412      * exceptions to {@code Throwable}.  Note that it is <i>not</i>
 413      * necessary to override any of the {@code PrintStackTrace} methods,
 414      * all of which invoke the {@code getCause} method to determine the
 415      * cause of a throwable.
 416      *
 417      * @return  the cause of this throwable or {@code null} if the
 418      *          cause is nonexistent or unknown.
 419      * @since 1.4
 420      */
 421     public synchronized Throwable getCause() {
 422         return (cause==this ? null : cause);
 423     }
 424 
 425     /**
 426      * Initializes the <i>cause</i> of this throwable to the specified value.
 427      * (The cause is the throwable that caused this throwable to get thrown.)
 428      *
 429      * <p>This method can be called at most once.  It is generally called from
 430      * within the constructor, or immediately after creating the
 431      * throwable.  If this throwable was created
 432      * with {@link #Throwable(Throwable)} or
 433      * {@link #Throwable(String,Throwable)}, this method cannot be called
 434      * even once.
 435      *
 436      * <p>An example of using this method on a legacy throwable type
 437      * without other support for setting the cause is:
 438      *
 439      * <pre>
 440      * try {
 441      *     lowLevelOp();
 442      * } catch (LowLevelException le) {
 443      *     throw (HighLevelException)
 444      *           new HighLevelException().initCause(le); // Legacy constructor
 445      * }
 446      * </pre>
 447      *
 448      * @param  cause the cause (which is saved for later retrieval by the
 449      *         {@link #getCause()} method).  (A {@code null} value is
 450      *         permitted, and indicates that the cause is nonexistent or
 451      *         unknown.)
 452      * @return  a reference to this {@code Throwable} instance.
 453      * @throws IllegalArgumentException if {@code cause} is this
 454      *         throwable.  (A throwable cannot be its own cause.)
 455      * @throws IllegalStateException if this throwable was
 456      *         created with {@link #Throwable(Throwable)} or
 457      *         {@link #Throwable(String,Throwable)}, or this method has already
 458      *         been called on this throwable.
 459      * @since  1.4
 460      */
 461     public synchronized Throwable initCause(Throwable cause) {
 462         if (this.cause != this)
 463             throw new IllegalStateException("Can't overwrite cause");
 464         if (cause == this)
 465             throw new IllegalArgumentException("Self-causation not permitted");
 466         this.cause = cause;
 467         return this;
 468     }
 469 
 470     /**
 471      * Returns a short description of this throwable.
 472      * The result is the concatenation of:
 473      * <ul>
 474      * <li> the {@linkplain Class#getName() name} of the class of this object
 475      * <li> ": " (a colon and a space)
 476      * <li> the result of invoking this object's {@link #getLocalizedMessage}
 477      *      method
 478      * </ul>
 479      * If {@code getLocalizedMessage} returns {@code null}, then just
 480      * the class name is returned.
 481      *
 482      * @return a string representation of this throwable.
 483      */
 484     public String toString() {
 485         String s = getClass().getName();
 486         String message = getLocalizedMessage();
 487         return (message != null) ? (s + ": " + message) : s;
 488     }
 489 
 490     /**
 491      * Prints this throwable and its backtrace to the
 492      * standard error stream. This method prints a stack trace for this
 493      * {@code Throwable} object on the error output stream that is
 494      * the value of the field {@code System.err}. The first line of
 495      * output contains the result of the {@link #toString()} method for
 496      * this object.  Remaining lines represent data previously recorded by
 497      * the method {@link #fillInStackTrace()}. The format of this
 498      * information depends on the implementation, but the following
 499      * example may be regarded as typical:
 500      * <blockquote><pre>
 501      * java.lang.NullPointerException
 502      *         at MyClass.mash(MyClass.java:9)
 503      *         at MyClass.crunch(MyClass.java:6)
 504      *         at MyClass.main(MyClass.java:3)
 505      * </pre></blockquote>
 506      * This example was produced by running the program:
 507      * <pre>
 508      * class MyClass {
 509      *     public static void main(String[] args) {
 510      *         crunch(null);
 511      *     }
 512      *     static void crunch(int[] a) {
 513      *         mash(a);
 514      *     }
 515      *     static void mash(int[] b) {
 516      *         System.out.println(b[0]);
 517      *     }
 518      * }
 519      * </pre>
 520      * The backtrace for a throwable with an initialized, non-null cause
 521      * should generally include the backtrace for the cause.  The format
 522      * of this information depends on the implementation, but the following
 523      * example may be regarded as typical:
 524      * <pre>
 525      * HighLevelException: MidLevelException: LowLevelException
 526      *         at Junk.a(Junk.java:13)
 527      *         at Junk.main(Junk.java:4)
 528      * Caused by: MidLevelException: LowLevelException
 529      *         at Junk.c(Junk.java:23)
 530      *         at Junk.b(Junk.java:17)
 531      *         at Junk.a(Junk.java:11)
 532      *         ... 1 more
 533      * Caused by: LowLevelException
 534      *         at Junk.e(Junk.java:30)
 535      *         at Junk.d(Junk.java:27)
 536      *         at Junk.c(Junk.java:21)
 537      *         ... 3 more
 538      * </pre>
 539      * Note the presence of lines containing the characters {@code "..."}.
 540      * These lines indicate that the remainder of the stack trace for this
 541      * exception matches the indicated number of frames from the bottom of the
 542      * stack trace of the exception that was caused by this exception (the
 543      * "enclosing" exception).  This shorthand can greatly reduce the length
 544      * of the output in the common case where a wrapped exception is thrown
 545      * from same method as the "causative exception" is caught.  The above
 546      * example was produced by running the program:
 547      * <pre>
 548      * public class Junk {
 549      *     public static void main(String args[]) {
 550      *         try {
 551      *             a();
 552      *         } catch(HighLevelException e) {
 553      *             e.printStackTrace();
 554      *         }
 555      *     }
 556      *     static void a() throws HighLevelException {
 557      *         try {
 558      *             b();
 559      *         } catch(MidLevelException e) {
 560      *             throw new HighLevelException(e);
 561      *         }
 562      *     }
 563      *     static void b() throws MidLevelException {
 564      *         c();
 565      *     }
 566      *     static void c() throws MidLevelException {
 567      *         try {
 568      *             d();
 569      *         } catch(LowLevelException e) {
 570      *             throw new MidLevelException(e);
 571      *         }
 572      *     }
 573      *     static void d() throws LowLevelException {
 574      *        e();
 575      *     }
 576      *     static void e() throws LowLevelException {
 577      *         throw new LowLevelException();
 578      *     }
 579      * }
 580      *
 581      * class HighLevelException extends Exception {
 582      *     HighLevelException(Throwable cause) { super(cause); }
 583      * }
 584      *
 585      * class MidLevelException extends Exception {
 586      *     MidLevelException(Throwable cause)  { super(cause); }
 587      * }
 588      *
 589      * class LowLevelException extends Exception {
 590      * }
 591      * </pre>
 592      * As of release 7, the platform supports the notion of
 593      * <i>suppressed exceptions</i> (in conjunction with the {@code
 594      * try}-with-resources statement). Any exceptions that were
 595      * suppressed in order to deliver an exception are printed out
 596      * beneath the stack trace.  The format of this information
 597      * depends on the implementation, but the following example may be
 598      * regarded as typical:
 599      *
 600      * <pre>
 601      * Exception in thread "main" java.lang.Exception: Something happened
 602      *  at Foo.bar(Foo.java:10)
 603      *  at Foo.main(Foo.java:5)
 604      *  Suppressed: Resource$CloseFailException: Resource ID = 0
 605      *          at Resource.close(Resource.java:26)
 606      *          at Foo.bar(Foo.java:9)
 607      *          ... 1 more
 608      * </pre>
 609      * Note that the "... n more" notation is used on suppressed exceptions
 610      * just at it is used on causes. Unlike causes, suppressed exceptions are
 611      * indented beyond their "containing exceptions."
 612      *
 613      * <p>An exception can have both a cause and one or more suppressed
 614      * exceptions:
 615      * <pre>
 616      * Exception in thread "main" java.lang.Exception: Main block
 617      *  at Foo3.main(Foo3.java:7)
 618      *  Suppressed: Resource$CloseFailException: Resource ID = 2
 619      *          at Resource.close(Resource.java:26)
 620      *          at Foo3.main(Foo3.java:5)
 621      *  Suppressed: Resource$CloseFailException: Resource ID = 1
 622      *          at Resource.close(Resource.java:26)
 623      *          at Foo3.main(Foo3.java:5)
 624      * Caused by: java.lang.Exception: I did it
 625      *  at Foo3.main(Foo3.java:8)
 626      * </pre>
 627      * Likewise, a suppressed exception can have a cause:
 628      * <pre>
 629      * Exception in thread "main" java.lang.Exception: Main block
 630      *  at Foo4.main(Foo4.java:6)
 631      *  Suppressed: Resource2$CloseFailException: Resource ID = 1
 632      *          at Resource2.close(Resource2.java:20)
 633      *          at Foo4.main(Foo4.java:5)
 634      *  Caused by: java.lang.Exception: Rats, you caught me
 635      *          at Resource2$CloseFailException.&lt;init&gt;(Resource2.java:45)
 636      *          ... 2 more
 637      * </pre>
 638      */
 639     public void printStackTrace() {
 640         printStackTrace(System.err);
 641     }
 642 
 643     /**
 644      * Prints this throwable and its backtrace to the specified print stream.
 645      *
 646      * @param s {@code PrintStream} to use for output
 647      */
 648     public void printStackTrace(PrintStream s) {
 649         printStackTrace(new WrappedPrintStream(s));
 650     }
 651 
 652     private void printStackTrace(PrintStreamOrWriter s) {
 653         // Guard against malicious overrides of Throwable.equals by
 654         // using a Set with identity equality semantics.
 655         Set<Throwable> dejaVu =
 656             Collections.newSetFromMap(new IdentityHashMap<Throwable, Boolean>());
 657         dejaVu.add(this);
 658 
 659         synchronized (s.lock()) {
 660             // Print our stack trace
 661             s.println(this);
 662             StackTraceElement[] trace = getOurStackTrace();
 663             for (StackTraceElement traceElement : trace)
 664                 s.println("\tat " + traceElement);
 665 
 666             // Print suppressed exceptions, if any
 667             for (Throwable se : getSuppressed())
 668                 se.printEnclosedStackTrace(s, trace, SUPPRESSED_CAPTION, "\t", dejaVu);
 669 
 670             // Print cause, if any
 671             Throwable ourCause = getCause();
 672             if (ourCause != null)
 673                 ourCause.printEnclosedStackTrace(s, trace, CAUSE_CAPTION, "", dejaVu);
 674         }
 675     }
 676 
 677     /**
 678      * Print our stack trace as an enclosed exception for the specified
 679      * stack trace.
 680      */
 681     private void printEnclosedStackTrace(PrintStreamOrWriter s,
 682                                          StackTraceElement[] enclosingTrace,
 683                                          String caption,
 684                                          String prefix,
 685                                          Set<Throwable> dejaVu) {
 686         assert Thread.holdsLock(s.lock());
 687         if (dejaVu.contains(this)) {
 688             s.println("\t[CIRCULAR REFERENCE:" + this + "]");
 689         } else {
 690             dejaVu.add(this);
 691             // Compute number of frames in common between this and enclosing trace
 692             StackTraceElement[] trace = getOurStackTrace();
 693             int m = trace.length - 1;
 694             int n = enclosingTrace.length - 1;
 695             while (m >= 0 && n >=0 && trace[m].equals(enclosingTrace[n])) {
 696                 m--; n--;
 697             }
 698             int framesInCommon = trace.length - 1 - m;
 699 
 700             // Print our stack trace
 701             s.println(prefix + caption + this);
 702             for (int i = 0; i <= m; i++)
 703                 s.println(prefix + "\tat " + trace[i]);
 704             if (framesInCommon != 0)
 705                 s.println(prefix + "\t... " + framesInCommon + " more");
 706 
 707             // Print suppressed exceptions, if any
 708             for (Throwable se : getSuppressed())
 709                 se.printEnclosedStackTrace(s, trace, SUPPRESSED_CAPTION,
 710                                            prefix +"\t", dejaVu);
 711 
 712             // Print cause, if any
 713             Throwable ourCause = getCause();
 714             if (ourCause != null)
 715                 ourCause.printEnclosedStackTrace(s, trace, CAUSE_CAPTION, prefix, dejaVu);
 716         }
 717     }
 718 
 719     /**
 720      * Prints this throwable and its backtrace to the specified
 721      * print writer.
 722      *
 723      * @param s {@code PrintWriter} to use for output
 724      * @since   JDK1.1
 725      */
 726     public void printStackTrace(PrintWriter s) {
 727         printStackTrace(new WrappedPrintWriter(s));
 728     }
 729 
 730     /**
 731      * Wrapper class for PrintStream and PrintWriter to enable a single
 732      * implementation of printStackTrace.
 733      */
 734     private abstract static class PrintStreamOrWriter {
 735         /** Returns the object to be locked when using this StreamOrWriter */
 736         abstract Object lock();
 737 
 738         /** Prints the specified string as a line on this StreamOrWriter */
 739         abstract void println(Object o);
 740     }
 741 
 742     private static class WrappedPrintStream extends PrintStreamOrWriter {
 743         private final PrintStream printStream;
 744 
 745         WrappedPrintStream(PrintStream printStream) {
 746             this.printStream = printStream;
 747         }
 748 
 749         Object lock() {
 750             return printStream;
 751         }
 752 
 753         void println(Object o) {
 754             printStream.println(o);
 755         }
 756     }
 757 
 758     private static class WrappedPrintWriter extends PrintStreamOrWriter {
 759         private final PrintWriter printWriter;
 760 
 761         WrappedPrintWriter(PrintWriter printWriter) {
 762             this.printWriter = printWriter;
 763         }
 764 
 765         Object lock() {
 766             return printWriter;
 767         }
 768 
 769         void println(Object o) {
 770             printWriter.println(o);
 771         }
 772     }
 773 
 774     /**
 775      * Fills in the execution stack trace. This method records within this
 776      * {@code Throwable} object information about the current state of
 777      * the stack frames for the current thread.
 778      *
 779      * <p>If the stack trace of this {@code Throwable} {@linkplain
 780      * Throwable#Throwable(String, Throwable, boolean, boolean) is not
 781      * writable}, calling this method has no effect.
 782      *
 783      * @return  a reference to this {@code Throwable} instance.
 784      * @see     java.lang.Throwable#printStackTrace()
 785      */
 786     public synchronized Throwable fillInStackTrace() {
 787         if (stackTrace != null ||
 788             backtrace != null /* Out of protocol state */ ) {
 789             fillInStackTrace(0);
 790             stackTrace = UNASSIGNED_STACK;
 791         }
 792         return this;
 793     }
 794 
 795     private native Throwable fillInStackTrace(int dummy);
 796 
 797     /**
 798      * Provides programmatic access to the stack trace information printed by
 799      * {@link #printStackTrace()}.  Returns an array of stack trace elements,
 800      * each representing one stack frame.  The zeroth element of the array
 801      * (assuming the array's length is non-zero) represents the top of the
 802      * stack, which is the last method invocation in the sequence.  Typically,
 803      * this is the point at which this throwable was created and thrown.
 804      * The last element of the array (assuming the array's length is non-zero)
 805      * represents the bottom of the stack, which is the first method invocation
 806      * in the sequence.
 807      *
 808      * <p>Some virtual machines may, under some circumstances, omit one
 809      * or more stack frames from the stack trace.  In the extreme case,
 810      * a virtual machine that has no stack trace information concerning
 811      * this throwable is permitted to return a zero-length array from this
 812      * method.  Generally speaking, the array returned by this method will
 813      * contain one element for every frame that would be printed by
 814      * {@code printStackTrace}.  Writes to the returned array do not
 815      * affect future calls to this method.
 816      *
 817      * @return an array of stack trace elements representing the stack trace
 818      *         pertaining to this throwable.
 819      * @since  1.4
 820      */
 821     public StackTraceElement[] getStackTrace() {
 822         return getOurStackTrace().clone();
 823     }
 824 
 825     private synchronized StackTraceElement[] getOurStackTrace() {
 826         // Initialize stack trace field with information from
 827         // backtrace if this is the first call to this method
 828         if (stackTrace == UNASSIGNED_STACK ||
 829             (stackTrace == null && backtrace != null) /* Out of protocol state */) {
 830             int depth = getStackTraceDepth();
 831             stackTrace = new StackTraceElement[depth];
 832             for (int i=0; i < depth; i++)
 833                 stackTrace[i] = getStackTraceElement(i);
 834         } else if (stackTrace == null) {
 835             return UNASSIGNED_STACK;
 836         }
 837         return stackTrace;
 838     }
 839 
 840     /**
 841      * Sets the stack trace elements that will be returned by
 842      * {@link #getStackTrace()} and printed by {@link #printStackTrace()}
 843      * and related methods.
 844      *
 845      * This method, which is designed for use by RPC frameworks and other
 846      * advanced systems, allows the client to override the default
 847      * stack trace that is either generated by {@link #fillInStackTrace()}
 848      * when a throwable is constructed or deserialized when a throwable is
 849      * read from a serialization stream.
 850      *
 851      * <p>If the stack trace of this {@code Throwable} {@linkplain
 852      * Throwable#Throwable(String, Throwable, boolean, boolean) is not
 853      * writable}, calling this method has no effect other than
 854      * validating its argument.
 855      *
 856      * @param   stackTrace the stack trace elements to be associated with
 857      * this {@code Throwable}.  The specified array is copied by this
 858      * call; changes in the specified array after the method invocation
 859      * returns will have no affect on this {@code Throwable}'s stack
 860      * trace.
 861      *
 862      * @throws NullPointerException if {@code stackTrace} is
 863      *         {@code null} or if any of the elements of
 864      *         {@code stackTrace} are {@code null}
 865      *
 866      * @since  1.4
 867      */
 868     public void setStackTrace(StackTraceElement[] stackTrace) {
 869         // Validate argument
 870         StackTraceElement[] defensiveCopy = stackTrace.clone();
 871         for (int i = 0; i < defensiveCopy.length; i++) {
 872             if (defensiveCopy[i] == null)
 873                 throw new NullPointerException("stackTrace[" + i + "]");
 874         }
 875 
 876         synchronized (this) {
 877             if (this.stackTrace == null && // Immutable stack
 878                 backtrace == null) // Test for out of protocol state
 879                 return;
 880             this.stackTrace = defensiveCopy;
 881         }
 882     }
 883 
 884     /**
 885      * Returns the number of elements in the stack trace (or 0 if the stack
 886      * trace is unavailable).
 887      *
 888      * package-protection for use by SharedSecrets.
 889      */
 890     native int getStackTraceDepth();
 891 
 892     /**
 893      * Returns the specified element of the stack trace.
 894      *
 895      * package-protection for use by SharedSecrets.
 896      *
 897      * @param index index of the element to return.
 898      * @throws IndexOutOfBoundsException if {@code index < 0 ||
 899      *         index >= getStackTraceDepth() }
 900      */
 901     native StackTraceElement getStackTraceElement(int index);
 902 
 903     /**
 904      * Reads a {@code Throwable} from a stream, enforcing
 905      * well-formedness constraints on fields.  Null entries and
 906      * self-pointers are not allowed in the list of {@code
 907      * suppressedExceptions}.  Null entries are not allowed for stack
 908      * trace elements.  A null stack trace in the serial form results
 909      * in a zero-length stack element array. A single-element stack
 910      * trace whose entry is equal to {@code new StackTraceElement("",
 911      * "", null, Integer.MIN_VALUE)} results in a {@code null} {@code
 912      * stackTrace} field.
 913      *
 914      * Note that there are no constraints on the value the {@code
 915      * cause} field can hold; both {@code null} and {@code this} are
 916      * valid values for the field.
 917      */
 918     private void readObject(ObjectInputStream s)
 919         throws IOException, ClassNotFoundException {
 920         s.defaultReadObject();     // read in all fields
 921         if (suppressedExceptions != null) {
 922             List<Throwable> suppressed = null;
 923             if (suppressedExceptions.isEmpty()) {
 924                 // Use the sentinel for a zero-length list
 925                 suppressed = SUPPRESSED_SENTINEL;
 926             } else { // Copy Throwables to new list
 927                 suppressed = new ArrayList<>(1);
 928                 for (Throwable t : suppressedExceptions) {
 929                     // Enforce constraints on suppressed exceptions in
 930                     // case of corrupt or malicious stream.
 931                     if (t == null)
 932                         throw new NullPointerException(NULL_CAUSE_MESSAGE);
 933                     if (t == this)
 934                         throw new IllegalArgumentException(SELF_SUPPRESSION_MESSAGE);
 935                     suppressed.add(t);
 936                 }
 937             }
 938             suppressedExceptions = suppressed;
 939         } // else a null suppressedExceptions field remains null
 940 
 941         /*
 942          * For zero-length stack traces, use a clone of
 943          * UNASSIGNED_STACK rather than UNASSIGNED_STACK itself to
 944          * allow identity comparison against UNASSIGNED_STACK in
 945          * getOurStackTrace.  The identity of UNASSIGNED_STACK in
 946          * stackTrace indicates to the getOurStackTrace method that
 947          * the stackTrace needs to be constructed from the information
 948          * in backtrace.
 949          */
 950         if (stackTrace != null) {
 951             if (stackTrace.length == 0) {
 952                 stackTrace = UNASSIGNED_STACK.clone();
 953             }  else if (stackTrace.length == 1 &&
 954                         // Check for the marker of an immutable stack trace
 955                         SentinelHolder.STACK_TRACE_ELEMENT_SENTINEL.equals(stackTrace[0])) {
 956                 stackTrace = null;
 957             } else { // Verify stack trace elements are non-null.
 958                 for(StackTraceElement ste : stackTrace) {
 959                     if (ste == null)
 960                         throw new NullPointerException("null StackTraceElement in serial stream. ");
 961                 }
 962             }
 963         } else {
 964             // A null stackTrace field in the serial form can result
 965             // from an exception serialized without that field in
 966             // older JDK releases; treat such exceptions as having
 967             // empty stack traces.
 968             stackTrace = UNASSIGNED_STACK.clone();
 969         }
 970     }
 971 
 972     /**
 973      * Write a {@code Throwable} object to a stream.
 974      *
 975      * A {@code null} stack trace field is represented in the serial
 976      * form as a one-element array whose element is equal to {@code
 977      * new StackTraceElement("", "", null, Integer.MIN_VALUE)}.
 978      */
 979     private synchronized void writeObject(ObjectOutputStream s)
 980         throws IOException {
 981         // Ensure that the stackTrace field is initialized to a
 982         // non-null value, if appropriate.  As of JDK 7, a null stack
 983         // trace field is a valid value indicating the stack trace
 984         // should not be set.
 985         getOurStackTrace();
 986 
 987         StackTraceElement[] oldStackTrace = stackTrace;
 988         try {
 989             if (stackTrace == null)
 990                 stackTrace = SentinelHolder.STACK_TRACE_SENTINEL;
 991             s.defaultWriteObject();
 992         } finally {
 993             stackTrace = oldStackTrace;
 994         }
 995     }
 996 
 997     /**
 998      * Appends the specified exception to the exceptions that were
 999      * suppressed in order to deliver this exception. This method is
1000      * thread-safe and typically called (automatically and implicitly)
1001      * by the {@code try}-with-resources statement.
1002      *
1003      * <p>The suppression behavior is enabled <em>unless</em> disabled
1004      * {@linkplain #Throwable(String, Throwable, boolean, boolean) via
1005      * a constructor}.  When suppression is disabled, this method does
1006      * nothing other than to validate its argument.
1007      *
1008      * <p>Note that when one exception {@linkplain
1009      * #initCause(Throwable) causes} another exception, the first
1010      * exception is usually caught and then the second exception is
1011      * thrown in response.  In other words, there is a causal
1012      * connection between the two exceptions.
1013      *
1014      * In contrast, there are situations where two independent
1015      * exceptions can be thrown in sibling code blocks, in particular
1016      * in the {@code try} block of a {@code try}-with-resources
1017      * statement and the compiler-generated {@code finally} block
1018      * which closes the resource.
1019      *
1020      * In these situations, only one of the thrown exceptions can be
1021      * propagated.  In the {@code try}-with-resources statement, when
1022      * there are two such exceptions, the exception originating from
1023      * the {@code try} block is propagated and the exception from the
1024      * {@code finally} block is added to the list of exceptions
1025      * suppressed by the exception from the {@code try} block.  As an
1026      * exception unwinds the stack, it can accumulate multiple
1027      * suppressed exceptions.
1028      *
1029      * <p>An exception may have suppressed exceptions while also being
1030      * caused by another exception.  Whether or not an exception has a
1031      * cause is semantically known at the time of its creation, unlike
1032      * whether or not an exception will suppress other exceptions
1033      * which is typically only determined after an exception is
1034      * thrown.
1035      *
1036      * <p>Note that programmer written code is also able to take
1037      * advantage of calling this method in situations where there are
1038      * multiple sibling exceptions and only one can be propagated.
1039      *
1040      * @param exception the exception to be added to the list of
1041      *        suppressed exceptions
1042      * @throws IllegalArgumentException if {@code exception} is this
1043      *         throwable; a throwable cannot suppress itself.
1044      * @throws NullPointerException if {@code exception} is {@code null}
1045      * @since 1.7
1046      */
1047     public final synchronized void addSuppressed(Throwable exception) {
1048         if (exception == this)
1049             throw new IllegalArgumentException(SELF_SUPPRESSION_MESSAGE);
1050 
1051         if (exception == null)
1052             throw new NullPointerException(NULL_CAUSE_MESSAGE);
1053 
1054         if (suppressedExceptions == null) // Suppressed exceptions not recorded
1055             return;
1056 
1057         if (suppressedExceptions == SUPPRESSED_SENTINEL)
1058             suppressedExceptions = new ArrayList<>(1);
1059 
1060         suppressedExceptions.add(exception);
1061     }
1062 
1063     private static final Throwable[] EMPTY_THROWABLE_ARRAY = new Throwable[0];
1064 
1065     /**
1066      * Returns an array containing all of the exceptions that were
1067      * suppressed, typically by the {@code try}-with-resources
1068      * statement, in order to deliver this exception.
1069      *
1070      * If no exceptions were suppressed or {@linkplain
1071      * #Throwable(String, Throwable, boolean, boolean) suppression is
1072      * disabled}, an empty array is returned.  This method is
1073      * thread-safe.  Writes to the returned array do not affect future
1074      * calls to this method.
1075      *
1076      * @return an array containing all of the exceptions that were
1077      *         suppressed to deliver this exception.
1078      * @since 1.7
1079      */
1080     public final synchronized Throwable[] getSuppressed() {
1081         if (suppressedExceptions == SUPPRESSED_SENTINEL ||
1082             suppressedExceptions == null)
1083             return EMPTY_THROWABLE_ARRAY;
1084         else
1085             return suppressedExceptions.toArray(EMPTY_THROWABLE_ARRAY);
1086     }
1087 }
--- EOF ---