1 /*
   2  * Copyright (c) 1994, 2011, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.  Oracle designates this
   8  * particular file as subject to the "Classpath" exception as provided
   9  * by Oracle in the LICENSE file that accompanied this code.
  10  *
  11  * This code is distributed in the hope that it will be useful, but WITHOUT
  12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  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
  15  * accompanied this code).
  16  *
  17  * You should have received a copy of the GNU General Public License version
  18  * 2 along with this work; if not, write to the Free Software Foundation,
  19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  20  *
  21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  22  * or visit www.oracle.com if you need additional information or have any
  23  * questions.
  24  */
  25 
  26 package java.lang;
  27 import  java.io.*;
  28 import  java.util.*;
  29 
  30 /**
  31  * The {@code Throwable} class is the superclass of all errors and
  32  * exceptions in the Java language. Only objects that are instances of this
  33  * class (or one of its subclasses) are thrown by the Java Virtual Machine or
  34  * can be thrown by the Java {@code throw} statement. Similarly, only
  35  * this class or one of its subclasses can be the argument type in a
  36  * {@code catch} clause.
  37  *
  38  * For the purposes of compile-time checking of exceptions, {@code
  39  * Throwable} and any subclass of {@code Throwable} that is not also a
  40  * subclass of either {@link RuntimeException} or {@link Error} are
  41  * regarded as checked exceptions.
  42  *
  43  * <p>Instances of two subclasses, {@link java.lang.Error} and
  44  * {@link java.lang.Exception}, are conventionally used to indicate
  45  * that exceptional situations have occurred. Typically, these instances
  46  * are freshly created in the context of the exceptional situation so
  47  * as to include relevant information (such as stack trace data).
  48  *
  49  * <p>A throwable contains a snapshot of the execution stack of its
  50  * thread at the time it was created. It can also contain a message
  51  * string that gives more information about the error. Over time, a
  52  * throwable can {@linkplain Throwable#addSuppressed suppress} other
  53  * throwables from being propagated.  Finally, the throwable can also
  54  * contain a <i>cause</i>: another throwable that caused this
  55  * throwable to be constructed.  The recording of this causal information
  56  * is referred to as the <i>chained exception</i> facility, as the
  57  * cause can, itself, have a cause, and so on, leading to a "chain" of
  58  * exceptions, each caused by another.
  59  *
  60  * <p>One reason that a throwable may have a cause is that the class that
  61  * throws it is built atop a lower layered abstraction, and an operation on
  62  * the upper layer fails due to a failure in the lower layer.  It would be bad
  63  * design to let the throwable thrown by the lower layer propagate outward, as
  64  * it is generally unrelated to the abstraction provided by the upper layer.
  65  * Further, doing so would tie the API of the upper layer to the details of
  66  * its implementation, assuming the lower layer's exception was a checked
  67  * exception.  Throwing a "wrapped exception" (i.e., an exception containing a
  68  * cause) allows the upper layer to communicate the details of the failure to
  69  * its caller without incurring either of these shortcomings.  It preserves
  70  * the flexibility to change the implementation of the upper layer without
  71  * changing its API (in particular, the set of exceptions thrown by its
  72  * methods).
  73  *
  74  * <p>A second reason that a throwable may have a cause is that the method
  75  * that throws it must conform to a general-purpose interface that does not
  76  * permit the method to throw the cause directly.  For example, suppose
  77  * a persistent collection conforms to the {@link java.util.Collection
  78  * Collection} interface, and that its persistence is implemented atop
  79  * {@code java.io}.  Suppose the internals of the {@code add} method
  80  * can throw an {@link java.io.IOException IOException}.  The implementation
  81  * can communicate the details of the {@code IOException} to its caller
  82  * while conforming to the {@code Collection} interface by wrapping the
  83  * {@code IOException} in an appropriate unchecked exception.  (The
  84  * specification for the persistent collection should indicate that it is
  85  * capable of throwing such exceptions.)
  86  *
  87  * <p>A cause can be associated with a throwable in two ways: via a
  88  * constructor that takes the cause as an argument, or via the
  89  * {@link #initCause(Throwable)} method.  New throwable classes that
  90  * wish to allow causes to be associated with them should provide constructors
  91  * that take a cause and delegate (perhaps indirectly) to one of the
  92  * {@code Throwable} constructors that takes a cause.
  93  *
  94  * Because the {@code initCause} method is public, it allows a cause to be
  95  * associated with any throwable, even a "legacy throwable" whose
  96  * implementation predates the addition of the exception chaining mechanism to
  97  * {@code Throwable}.
  98  *
  99  * <p>By convention, class {@code Throwable} and its subclasses have two
 100  * constructors, one that takes no arguments and one that takes a
 101  * {@code String} argument that can be used to produce a detail message.
 102  * Further, those subclasses that might likely have a cause associated with
 103  * them should have two more constructors, one that takes a
 104  * {@code Throwable} (the cause), and one that takes a
 105  * {@code String} (the detail message) and a {@code Throwable} (the
 106  * cause).
 107  *
 108  * @author  unascribed
 109  * @author  Josh Bloch (Added exception chaining and programmatic access to
 110  *          stack trace in 1.4.)
 111  * @jls3 11.2 Compile-Time Checking of Exceptions
 112  * @since JDK1.0
 113  */
 114 public class Throwable implements Serializable {
 115     /** use serialVersionUID from JDK 1.0.2 for interoperability */
 116     private static final long serialVersionUID = -3042686055658047285L;
 117 
 118     /**
 119      * Native code saves some indication of the stack backtrace in this slot.
 120      */
 121     private transient Object backtrace;
 122 
 123     /**
 124      * Specific details about the Throwable.  For example, for
 125      * {@code FileNotFoundException}, this contains the name of
 126      * the file that could not be found.
 127      *
 128      * @serial
 129      */
 130     private String detailMessage;
 131 
 132     
 133     /**
 134      * Holder class to defer initializing sentinel objects only used
 135      * for serialization.
 136      */
 137     private static class SentinelHolder {
 138         /**
 139          * {@linkplain #setStackTrace(StackTraceElement[]) Setting the
 140          * stack trace} to a one-element array containing this sentinel
 141          * value indicates future attempts to set the stack trace will be
 142          * ignored.  The sentinal is equal to the result of calling:<br>
 143          * {@code new StackTraceElement("", "", null, Integer.MIN_VALUE)}
 144          */
 145         public static final StackTraceElement STACK_TRACE_ELEMENT_SENTINEL =
 146             new StackTraceElement("", "", null, Integer.MIN_VALUE);
 147 
 148         /**
 149          * Sentinel value used in the serial form to indicate an immutable
 150          * stack trace.
 151          */
 152         public static final StackTraceElement[] STACK_TRACE_SENTINEL = 
 153             new StackTraceElement[] {STACK_TRACE_ELEMENT_SENTINEL};
 154     }
 155 
 156     /**
 157      * A value indicating that the logical stack trace has been
 158      * populated into the backtrace field.
 159      */
 160     private static final StackTraceElement[] FILLED_IN_STACK =
 161         new StackTraceElement[] {new StackTraceElement("FILLED_IN", "STACK", null, -1)};
 162 
 163     /**
 164      * A shared value for an empty stack.
 165      */
 166     private static final StackTraceElement[] EMPTY_STACK = new StackTraceElement[0];
 167 
 168     /*
 169      * To allow Throwable objects to be made immutable and safely
 170      * reused by the JVM, such as OutOfMemoryErrors, fields of
 171      * Throwable that are writable in response to user actions, cause,
 172      * stackTrace, and suppressedExceptions obey the following
 173      * protocol:
 174      *
 175      * 1) The fields are initialized to a non-null sentinel value
 176      * which indicates the value has logically not been set.
 177      *
 178      * 2) Writing a null to the field indicates further writes
 179      * are forbidden
 180      *
 181      * 3) The sentinel value may be replaced with another non-null
 182      * value.
 183      *
 184      * For example, implementations of the HotSpot JVM have
 185      * preallocated OutOfMemoryError objects to provide for better
 186      * diagnosability of that situation.  These objects are created
 187      * without calling the constructor for that class and the fields
 188      * in question are initialized to null.  To support this
 189      * capability, any new fields added to Throwable that require
 190      * being initialized to a non-null value require a coordinated JVM
 191      * change.
 192      */
 193 
 194     /**
 195      * The throwable that caused this throwable to get thrown, or null if this
 196      * throwable was not caused by another throwable, or if the causative
 197      * throwable is unknown.  If this field is equal to this throwable itself,
 198      * it indicates that the cause of this throwable has not yet been
 199      * initialized.
 200      *
 201      * @serial
 202      * @since 1.4
 203      */
 204     private Throwable cause = this;
 205 
 206     /**
 207      * The stack trace, as returned by {@link #getStackTrace()}.
 208      *
 209      * The field is initialized to a zero-length array.  A {@code
 210      * null} value of this field indicates subsequent calls to {@link
 211      * #setStackTrace()} and {@link #fillInStackTrace} will be be no-ops.
 212      *
 213      * @serial
 214      * @since 1.4
 215      */
 216     private StackTraceElement[] stackTrace = EMPTY_STACK;
 217 
 218     // Setting this static field introduces an acceptable
 219     // initialization dependency on a few java.util classes.
 220     private static final List<Throwable> SUPPRESSED_SENTINEL =
 221         Collections.unmodifiableList(new ArrayList<Throwable>(0));
 222 
 223     /**
 224      * The list of suppressed exceptions, as returned by {@link
 225      * #getSuppressed()}.  The list is initialized to a zero-element
 226      * unmodifiable sentinel list.  When a serialized Throwable is
 227      * read in, if the {@code suppressedExceptions} field points to a
 228      * zero-element list, the field is reset to the sentinel value.
 229      *
 230      * @serial
 231      * @since 1.7
 232      */
 233     private List<Throwable> suppressedExceptions = SUPPRESSED_SENTINEL;
 234 
 235     /** Message for trying to suppress a null exception. */
 236     private static final String NULL_CAUSE_MESSAGE = "Cannot suppress a null exception.";
 237 
 238     /** Message for trying to suppress oneself. */
 239     private static final String SELF_SUPPRESSION_MESSAGE = "Self-suppression not permitted";
 240 
 241     /** Caption  for labeling causative exception stack traces */
 242     private static final String CAUSE_CAPTION = "Caused by: ";
 243 
 244     /** Caption for labeling suppressed exception stack traces */
 245     private static final String SUPPRESSED_CAPTION = "Suppressed: ";
 246 
 247     /**
 248      * Constructs a new throwable with {@code null} as its detail message.
 249      * The cause is not initialized, and may subsequently be initialized by a
 250      * call to {@link #initCause}.
 251      *
 252      * <p>The {@link #fillInStackTrace()} method is called to initialize
 253      * the stack trace data in the newly created throwable.
 254      */
 255     public Throwable() {
 256         fillInStackTrace();
 257     }
 258 
 259     /**
 260      * Constructs a new throwable with the specified detail message.  The
 261      * cause is not initialized, and may subsequently be initialized by
 262      * a call to {@link #initCause}.
 263      *
 264      * <p>The {@link #fillInStackTrace()} method is called to initialize
 265      * the stack trace data in the newly created throwable.
 266      *
 267      * @param   message   the detail message. The detail message is saved for
 268      *          later retrieval by the {@link #getMessage()} method.
 269      */
 270     public Throwable(String message) {
 271         fillInStackTrace();
 272         detailMessage = message;
 273     }
 274 
 275     /**
 276      * Constructs a new throwable with the specified detail message and
 277      * cause.  <p>Note that the detail message associated with
 278      * {@code cause} is <i>not</i> automatically incorporated in
 279      * this throwable's detail message.
 280      *
 281      * <p>The {@link #fillInStackTrace()} method is called to initialize
 282      * the stack trace data in the newly created throwable.
 283      *
 284      * @param  message the detail message (which is saved for later retrieval
 285      *         by the {@link #getMessage()} method).
 286      * @param  cause the cause (which is saved for later retrieval by the
 287      *         {@link #getCause()} method).  (A {@code null} value is
 288      *         permitted, and indicates that the cause is nonexistent or
 289      *         unknown.)
 290      * @since  1.4
 291      */
 292     public Throwable(String message, Throwable cause) {
 293         fillInStackTrace();
 294         detailMessage = message;
 295         this.cause = cause;
 296     }
 297 
 298     /**
 299      * Constructs a new throwable with the specified cause and a detail
 300      * message of {@code (cause==null ? null : cause.toString())} (which
 301      * typically contains the class and detail message of {@code cause}).
 302      * This constructor is useful for throwables that are little more than
 303      * wrappers for other throwables (for example, {@link
 304      * java.security.PrivilegedActionException}).
 305      *
 306      * <p>The {@link #fillInStackTrace()} method is called to initialize
 307      * the stack trace data in the newly created throwable.
 308      *
 309      * @param  cause the cause (which is saved for later retrieval by the
 310      *         {@link #getCause()} method).  (A {@code null} value is
 311      *         permitted, and indicates that the cause is nonexistent or
 312      *         unknown.)
 313      * @since  1.4
 314      */
 315     public Throwable(Throwable cause) {
 316         fillInStackTrace();
 317         detailMessage = (cause==null ? null : cause.toString());
 318         this.cause = cause;
 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()} and subsequent calls to {@code
 331      * fillInStackTrace} and {@link
 332      * #setStackTrace(StackTraceElement[])} will not set the stack
 333      * trace.  If the writable stack trace is false, {@link
 334      * #getStackTrace} will return a zero length array.
 335      *
 336      * <p>Note that the other constructors of {@code Throwable} treat
 337      * suppression as being enabled and the stack trace as being
 338      * writable.  Subclasses of {@code Throwable} should document any
 339      * conditions under which suppression is disabled.  Disabling of
 340      * suppression should only occur in exceptional circumstances
 341      * where special requirements exist, such as a virtual machine
 342      * reusing exception objects under low-memory situations.
 343      *
 344      * @param  message the detail message.
 345      * @param cause the cause.  (A {@code null} value is permitted,
 346      * and indicates that the cause is nonexistent or unknown.)
 347      * @param enableSuppression whether or not suppression is enabled or disabled
 348      * @param writableStackTrace whether or not the stack trace should be 
 349      *                           writable
 350      *
 351      * @see OutOfMemoryError
 352      * @see NullPointerException
 353      * @see ArithmeticException
 354      * @since 1.7
 355      */
 356     protected Throwable(String message, Throwable cause,
 357                         boolean enableSuppression,
 358                         boolean writableStackTrace) {
 359         if (writableStackTrace) {
 360             fillInStackTrace();
 361         } else {
 362             stackTrace = null;
 363         }
 364         detailMessage = message;
 365         this.cause = cause;
 366         if (!enableSuppression)
 367             suppressedExceptions = null;
 368     }
 369 
 370     /**
 371      * Returns the detail message string of this throwable.
 372      *
 373      * @return  the detail message string of this {@code Throwable} instance
 374      *          (which may be {@code null}).
 375      */
 376     public String getMessage() {
 377         return detailMessage;
 378     }
 379 
 380     /**
 381      * Creates a localized description of this throwable.
 382      * Subclasses may override this method in order to produce a
 383      * locale-specific message.  For subclasses that do not override this
 384      * method, the default implementation returns the same result as
 385      * {@code getMessage()}.
 386      *
 387      * @return  The localized description of this throwable.
 388      * @since   JDK1.1
 389      */
 390     public String getLocalizedMessage() {
 391         return getMessage();
 392     }
 393 
 394     /**
 395      * Returns the cause of this throwable or {@code null} if the
 396      * cause is nonexistent or unknown.  (The cause is the throwable that
 397      * caused this throwable to get thrown.)
 398      *
 399      * <p>This implementation returns the cause that was supplied via one of
 400      * the constructors requiring a {@code Throwable}, or that was set after
 401      * creation with the {@link #initCause(Throwable)} method.  While it is
 402      * typically unnecessary to override this method, a subclass can override
 403      * it to return a cause set by some other means.  This is appropriate for
 404      * a "legacy chained throwable" that predates the addition of chained
 405      * exceptions to {@code Throwable}.  Note that it is <i>not</i>
 406      * necessary to override any of the {@code PrintStackTrace} methods,
 407      * all of which invoke the {@code getCause} method to determine the
 408      * cause of a throwable.
 409      *
 410      * @return  the cause of this throwable or {@code null} if the
 411      *          cause is nonexistent or unknown.
 412      * @since 1.4
 413      */
 414     public synchronized Throwable getCause() {
 415         return (cause==this ? null : cause);
 416     }
 417 
 418     /**
 419      * Initializes the <i>cause</i> of this throwable to the specified value.
 420      * (The cause is the throwable that caused this throwable to get thrown.)
 421      *
 422      * <p>This method can be called at most once.  It is generally called from
 423      * within the constructor, or immediately after creating the
 424      * throwable.  If this throwable was created
 425      * with {@link #Throwable(Throwable)} or
 426      * {@link #Throwable(String,Throwable)}, this method cannot be called
 427      * even once.
 428      *
 429      * @param  cause the cause (which is saved for later retrieval by the
 430      *         {@link #getCause()} method).  (A {@code null} value is
 431      *         permitted, and indicates that the cause is nonexistent or
 432      *         unknown.)
 433      * @return  a reference to this {@code Throwable} instance.
 434      * @throws IllegalArgumentException if {@code cause} is this
 435      *         throwable.  (A throwable cannot be its own cause.)
 436      * @throws IllegalStateException if this throwable was
 437      *         created with {@link #Throwable(Throwable)} or
 438      *         {@link #Throwable(String,Throwable)}, or this method has already
 439      *         been called on this throwable.
 440      * @since  1.4
 441      */
 442     public synchronized Throwable initCause(Throwable cause) {
 443         if (this.cause != this)
 444             throw new IllegalStateException("Can't overwrite cause");
 445         if (cause == this)
 446             throw new IllegalArgumentException("Self-causation not permitted");
 447         this.cause = cause;
 448         return this;
 449     }
 450 
 451     /**
 452      * Returns a short description of this throwable.
 453      * The result is the concatenation of:
 454      * <ul>
 455      * <li> the {@linkplain Class#getName() name} of the class of this object
 456      * <li> ": " (a colon and a space)
 457      * <li> the result of invoking this object's {@link #getLocalizedMessage}
 458      *      method
 459      * </ul>
 460      * If {@code getLocalizedMessage} returns {@code null}, then just
 461      * the class name is returned.
 462      *
 463      * @return a string representation of this throwable.
 464      */
 465     public String toString() {
 466         String s = getClass().getName();
 467         String message = getLocalizedMessage();
 468         return (message != null) ? (s + ": " + message) : s;
 469     }
 470 
 471     /**
 472      * Prints this throwable and its backtrace to the
 473      * standard error stream. This method prints a stack trace for this
 474      * {@code Throwable} object on the error output stream that is
 475      * the value of the field {@code System.err}. The first line of
 476      * output contains the result of the {@link #toString()} method for
 477      * this object.  Remaining lines represent data previously recorded by
 478      * the method {@link #fillInStackTrace()}. The format of this
 479      * information depends on the implementation, but the following
 480      * example may be regarded as typical:
 481      * <blockquote><pre>
 482      * java.lang.NullPointerException
 483      *         at MyClass.mash(MyClass.java:9)
 484      *         at MyClass.crunch(MyClass.java:6)
 485      *         at MyClass.main(MyClass.java:3)
 486      * </pre></blockquote>
 487      * This example was produced by running the program:
 488      * <pre>
 489      * class MyClass {
 490      *     public static void main(String[] args) {
 491      *         crunch(null);
 492      *     }
 493      *     static void crunch(int[] a) {
 494      *         mash(a);
 495      *     }
 496      *     static void mash(int[] b) {
 497      *         System.out.println(b[0]);
 498      *     }
 499      * }
 500      * </pre>
 501      * The backtrace for a throwable with an initialized, non-null cause
 502      * should generally include the backtrace for the cause.  The format
 503      * of this information depends on the implementation, but the following
 504      * example may be regarded as typical:
 505      * <pre>
 506      * HighLevelException: MidLevelException: LowLevelException
 507      *         at Junk.a(Junk.java:13)
 508      *         at Junk.main(Junk.java:4)
 509      * Caused by: MidLevelException: LowLevelException
 510      *         at Junk.c(Junk.java:23)
 511      *         at Junk.b(Junk.java:17)
 512      *         at Junk.a(Junk.java:11)
 513      *         ... 1 more
 514      * Caused by: LowLevelException
 515      *         at Junk.e(Junk.java:30)
 516      *         at Junk.d(Junk.java:27)
 517      *         at Junk.c(Junk.java:21)
 518      *         ... 3 more
 519      * </pre>
 520      * Note the presence of lines containing the characters {@code "..."}.
 521      * These lines indicate that the remainder of the stack trace for this
 522      * exception matches the indicated number of frames from the bottom of the
 523      * stack trace of the exception that was caused by this exception (the
 524      * "enclosing" exception).  This shorthand can greatly reduce the length
 525      * of the output in the common case where a wrapped exception is thrown
 526      * from same method as the "causative exception" is caught.  The above
 527      * example was produced by running the program:
 528      * <pre>
 529      * public class Junk {
 530      *     public static void main(String args[]) {
 531      *         try {
 532      *             a();
 533      *         } catch(HighLevelException e) {
 534      *             e.printStackTrace();
 535      *         }
 536      *     }
 537      *     static void a() throws HighLevelException {
 538      *         try {
 539      *             b();
 540      *         } catch(MidLevelException e) {
 541      *             throw new HighLevelException(e);
 542      *         }
 543      *     }
 544      *     static void b() throws MidLevelException {
 545      *         c();
 546      *     }
 547      *     static void c() throws MidLevelException {
 548      *         try {
 549      *             d();
 550      *         } catch(LowLevelException e) {
 551      *             throw new MidLevelException(e);
 552      *         }
 553      *     }
 554      *     static void d() throws LowLevelException {
 555      *        e();
 556      *     }
 557      *     static void e() throws LowLevelException {
 558      *         throw new LowLevelException();
 559      *     }
 560      * }
 561      *
 562      * class HighLevelException extends Exception {
 563      *     HighLevelException(Throwable cause) { super(cause); }
 564      * }
 565      *
 566      * class MidLevelException extends Exception {
 567      *     MidLevelException(Throwable cause)  { super(cause); }
 568      * }
 569      *
 570      * class LowLevelException extends Exception {
 571      * }
 572      * </pre>
 573      * As of release 7, the platform supports the notion of
 574      * <i>suppressed exceptions</i> (in conjunction with the {@code
 575      * try}-with-resources statement). Any exceptions that were
 576      * suppressed in order to deliver an exception are printed out
 577      * beneath the stack trace.  The format of this information
 578      * depends on the implementation, but the following example may be
 579      * regarded as typical:
 580      *
 581      * <pre>
 582      * Exception in thread "main" java.lang.Exception: Something happened
 583      *  at Foo.bar(Foo.java:10)
 584      *  at Foo.main(Foo.java:5)
 585      *  Suppressed: Resource$CloseFailException: Resource ID = 0
 586      *          at Resource.close(Resource.java:26)
 587      *          at Foo.bar(Foo.java:9)
 588      *          ... 1 more
 589      * </pre>
 590      * Note that the "... n more" notation is used on suppressed exceptions
 591      * just at it is used on causes. Unlike causes, suppressed exceptions are
 592      * indented beyond their "containing exceptions."
 593      *
 594      * <p>An exception can have both a cause and one or more suppressed
 595      * exceptions:
 596      * <pre>
 597      * Exception in thread "main" java.lang.Exception: Main block
 598      *  at Foo3.main(Foo3.java:7)
 599      *  Suppressed: Resource$CloseFailException: Resource ID = 2
 600      *          at Resource.close(Resource.java:26)
 601      *          at Foo3.main(Foo3.java:5)
 602      *  Suppressed: Resource$CloseFailException: Resource ID = 1
 603      *          at Resource.close(Resource.java:26)
 604      *          at Foo3.main(Foo3.java:5)
 605      * Caused by: java.lang.Exception: I did it
 606      *  at Foo3.main(Foo3.java:8)
 607      * </pre>
 608      * Likewise, a suppressed exception can have a cause:
 609      * <pre>
 610      * Exception in thread "main" java.lang.Exception: Main block
 611      *  at Foo4.main(Foo4.java:6)
 612      *  Suppressed: Resource2$CloseFailException: Resource ID = 1
 613      *          at Resource2.close(Resource2.java:20)
 614      *          at Foo4.main(Foo4.java:5)
 615      *  Caused by: java.lang.Exception: Rats, you caught me
 616      *          at Resource2$CloseFailException.<init>(Resource2.java:45)
 617      *          ... 2 more
 618      * </pre>
 619      */
 620     public void printStackTrace() {
 621         printStackTrace(System.err);
 622     }
 623 
 624     /**
 625      * Prints this throwable and its backtrace to the specified print stream.
 626      *
 627      * @param s {@code PrintStream} to use for output
 628      */
 629     public void printStackTrace(PrintStream s) {
 630         printStackTrace(new WrappedPrintStream(s));
 631     }
 632 
 633     private void printStackTrace(PrintStreamOrWriter s) {
 634         // Guard against malicious overrides of Throwable.equals by
 635         // using a Set with identity equality semantics.
 636         Set<Throwable> dejaVu =
 637             Collections.newSetFromMap(new IdentityHashMap<Throwable, Boolean>());
 638         dejaVu.add(this);
 639 
 640         synchronized (s.lock()) {
 641             // Print our stack trace
 642             s.println(this);
 643             StackTraceElement[] trace = getOurStackTrace();
 644             for (StackTraceElement traceElement : trace)
 645                 s.println("\tat " + traceElement);
 646 
 647             // Print suppressed exceptions, if any
 648             for (Throwable se : getSuppressed())
 649                 se.printEnclosedStackTrace(s, trace, SUPPRESSED_CAPTION, "\t", dejaVu);
 650 
 651             // Print cause, if any
 652             Throwable ourCause = getCause();
 653             if (ourCause != null)
 654                 ourCause.printEnclosedStackTrace(s, trace, CAUSE_CAPTION, "", dejaVu);
 655         }
 656     }
 657 
 658     /**
 659      * Print our stack trace as an enclosed exception for the specified
 660      * stack trace.
 661      */
 662     private void printEnclosedStackTrace(PrintStreamOrWriter s,
 663                                          StackTraceElement[] enclosingTrace,
 664                                          String caption,
 665                                          String prefix,
 666                                          Set<Throwable> dejaVu) {
 667         assert Thread.holdsLock(s.lock());
 668         if (dejaVu.contains(this)) {
 669             s.println("\t[CIRCULAR REFERENCE:" + this + "]");
 670         } else {
 671             dejaVu.add(this);
 672             // Compute number of frames in common between this and enclosing trace
 673             StackTraceElement[] trace = getOurStackTrace();
 674             int m = trace.length - 1;
 675             int n = enclosingTrace.length - 1;
 676             while (m >= 0 && n >=0 && trace[m].equals(enclosingTrace[n])) {
 677                 m--; n--;
 678             }
 679             int framesInCommon = trace.length - 1 - m;
 680 
 681             // Print our stack trace
 682             s.println(prefix + caption + this);
 683             for (int i = 0; i <= m; i++)
 684                 s.println(prefix + "\tat " + trace[i]);
 685             if (framesInCommon != 0)
 686                 s.println(prefix + "\t... " + framesInCommon + " more");
 687 
 688             // Print suppressed exceptions, if any
 689             for (Throwable se : getSuppressed())
 690                 se.printEnclosedStackTrace(s, trace, SUPPRESSED_CAPTION,
 691                                            prefix +"\t", dejaVu);
 692 
 693             // Print cause, if any
 694             Throwable ourCause = getCause();
 695             if (ourCause != null)
 696                 ourCause.printEnclosedStackTrace(s, trace, CAUSE_CAPTION, prefix, dejaVu);
 697         }
 698     }
 699 
 700     /**
 701      * Prints this throwable and its backtrace to the specified
 702      * print writer.
 703      *
 704      * @param s {@code PrintWriter} to use for output
 705      * @since   JDK1.1
 706      */
 707     public void printStackTrace(PrintWriter s) {
 708         printStackTrace(new WrappedPrintWriter(s));
 709     }
 710 
 711     /**
 712      * Wrapper class for PrintStream and PrintWriter to enable a single
 713      * implementation of printStackTrace.
 714      */
 715     private abstract static class PrintStreamOrWriter {
 716         /** Returns the object to be locked when using this StreamOrWriter */
 717         abstract Object lock();
 718 
 719         /** Prints the specified string as a line on this StreamOrWriter */
 720         abstract void println(Object o);
 721     }
 722 
 723     private static class WrappedPrintStream extends PrintStreamOrWriter {
 724         private final PrintStream printStream;
 725 
 726         WrappedPrintStream(PrintStream printStream) {
 727             this.printStream = printStream;
 728         }
 729 
 730         Object lock() {
 731             return printStream;
 732         }
 733 
 734         void println(Object o) {
 735             printStream.println(o);
 736         }
 737     }
 738 
 739     private static class WrappedPrintWriter extends PrintStreamOrWriter {
 740         private final PrintWriter printWriter;
 741 
 742         WrappedPrintWriter(PrintWriter printWriter) {
 743             this.printWriter = printWriter;
 744         }
 745 
 746         Object lock() {
 747             return printWriter;
 748         }
 749 
 750         void println(Object o) {
 751             printWriter.println(o);
 752         }
 753     }
 754 
 755     /**
 756      * Fills in the execution stack trace. This method records within this
 757      * {@code Throwable} object information about the current state of
 758      * the stack frames for the current thread.
 759      *
 760      * <p>If the stack trace of this {@code Throwable} {@linkplain
 761      * Throwable#Throwable(String, Throwable, boolean, boolean) is not
 762      * writable}, calling this method has no effect.
 763      *
 764      * @return  a reference to this {@code Throwable} instance.
 765      * @see     java.lang.Throwable#printStackTrace()
 766      */
 767     public synchronized Throwable fillInStackTrace() {
 768         if (stackTrace != null) {
 769             fillInStackTrace(0);
 770             stackTrace = FILLED_IN_STACK;
 771         }
 772         return this;
 773     }
 774 
 775     private native Throwable fillInStackTrace(int dummy);
 776 
 777     /**
 778      * Provides programmatic access to the stack trace information printed by
 779      * {@link #printStackTrace()}.  Returns an array of stack trace elements,
 780      * each representing one stack frame.  The zeroth element of the array
 781      * (assuming the array's length is non-zero) represents the top of the
 782      * stack, which is the last method invocation in the sequence.  Typically,
 783      * this is the point at which this throwable was created and thrown.
 784      * The last element of the array (assuming the array's length is non-zero)
 785      * represents the bottom of the stack, which is the first method invocation
 786      * in the sequence.
 787      *
 788      * <p>Some virtual machines may, under some circumstances, omit one
 789      * or more stack frames from the stack trace.  In the extreme case,
 790      * a virtual machine that has no stack trace information concerning
 791      * this throwable is permitted to return a zero-length array from this
 792      * method.  Generally speaking, the array returned by this method will
 793      * contain one element for every frame that would be printed by
 794      * {@code printStackTrace}.
 795      *
 796      * @return an array of stack trace elements representing the stack trace
 797      *         pertaining to this throwable.
 798      * @since  1.4
 799      */
 800     public StackTraceElement[] getStackTrace() {
 801         return getOurStackTrace().clone();
 802     }
 803 
 804     private synchronized StackTraceElement[] getOurStackTrace() {
 805         // Initialize stack trace field with information from
 806         // backtrace if this is the first call to this method
 807         if (stackTrace == FILLED_IN_STACK) {
 808             int depth = getStackTraceDepth();
 809             stackTrace = new StackTraceElement[depth];
 810             for (int i=0; i < depth; i++)
 811                 stackTrace[i] = getStackTraceElement(i);
 812         } else if (stackTrace == null) {
 813             return EMPTY_STACK; 
 814         }
 815         return stackTrace;
 816     }
 817 
 818     /**
 819      * Sets the stack trace elements that will be returned by
 820      * {@link #getStackTrace()} and printed by {@link #printStackTrace()}
 821      * and related methods.
 822      *
 823      * This method, which is designed for use by RPC frameworks and other
 824      * advanced systems, allows the client to override the default
 825      * stack trace that is either generated by {@link #fillInStackTrace()}
 826      * when a throwable is constructed or deserialized when a throwable is
 827      * read from a serialization stream.
 828      *
 829      * <p>If the stack trace of this {@code Throwable} {@linkplain
 830      * Throwable#Throwable(String, Throwable, boolean, boolean) is not
 831      * writable}, calling this method has no effect other than
 832      * validating its argument.
 833      *
 834      * @param   stackTrace the stack trace elements to be associated with
 835      * this {@code Throwable}.  The specified array is copied by this
 836      * call; changes in the specified array after the method invocation
 837      * returns will have no affect on this {@code Throwable}'s stack
 838      * trace.
 839      *
 840      * @throws NullPointerException if {@code stackTrace} is
 841      *         {@code null} or if any of the elements of
 842      *         {@code stackTrace} are {@code null}
 843      *
 844      * @since  1.4
 845      */
 846     public void setStackTrace(StackTraceElement[] stackTrace) {
 847         Objects.requireNonNull(stackTrace);
 848 
 849         if (this.stackTrace == null) // Immutable stack
 850             return;
 851 
 852         StackTraceElement[] defensiveCopy = stackTrace.clone();
 853         for (int i = 0; i < defensiveCopy.length; i++) {
 854             if (defensiveCopy[i] == null)
 855                 throw new NullPointerException("stackTrace[" + i + "]");
 856         }
 857 
 858         synchronized (this) {
 859             this.stackTrace = defensiveCopy;
 860         }
 861     }
 862 
 863     /**
 864      * Returns the number of elements in the stack trace (or 0 if the stack
 865      * trace is unavailable).
 866      *
 867      * package-protection for use by SharedSecrets.
 868      */
 869     native int getStackTraceDepth();
 870 
 871     /**
 872      * Returns the specified element of the stack trace.
 873      *
 874      * package-protection for use by SharedSecrets.
 875      *
 876      * @param index index of the element to return.
 877      * @throws IndexOutOfBoundsException if {@code index < 0 ||
 878      *         index >= getStackTraceDepth() }
 879      */
 880     native StackTraceElement getStackTraceElement(int index);
 881 
 882     /**
 883      * Reads a {@code Throwable} from a stream, enforcing
 884      * well-formedness constraints on fields.  Null entries and
 885      * self-pointers are not allowed in the list of {@code
 886      * suppressedExceptions}.  Null entries are not allowed for stack
 887      * trace elements.  A single-element stack trace whose entry is
 888      * equal to {@code new StackTraceElement("", "", null,
 889      * Integer.MIN_VALUE)} results in a {@code null} {@code
 890      * stackTrace} field.
 891      *
 892      * Note that there are no constraints on the value the {@code
 893      * cause} field can hold; both {@code null} and {@code this} are
 894      * valid values for the field.
 895      */
 896     private void readObject(ObjectInputStream s)
 897         throws IOException, ClassNotFoundException {
 898         s.defaultReadObject();     // read in all fields
 899         if (suppressedExceptions != null) {
 900             List<Throwable> suppressed = null;
 901             if (suppressedExceptions.isEmpty()) {
 902                 // Use the sentinel for a zero-length list
 903                 suppressed = SUPPRESSED_SENTINEL;
 904             } else { // Copy Throwables to new list
 905                 suppressed = new ArrayList<>(1);
 906                 for (Throwable t : suppressedExceptions) {
 907                     // Enforce constraints on suppressed exceptions in
 908                     // case of corrupt or malicious stream.
 909                     if (t == null)
 910                         throw new NullPointerException(NULL_CAUSE_MESSAGE);
 911                     if (t == this)
 912                         throw new IllegalArgumentException(SELF_SUPPRESSION_MESSAGE);
 913                     suppressed.add(t);
 914                 }
 915             }
 916             suppressedExceptions = suppressed;
 917         } // else a null suppressedExceptions field remains null
 918 
 919         // Check for the marker of an immutable stack trace
 920         if (stackTrace != null) {
 921             // Share zero-length stack traces
 922             if (stackTrace.length == 0) {
 923                 stackTrace = EMPTY_STACK;
 924             }  else if (stackTrace.length == 1 &&
 925                 SentinelHolder.STACK_TRACE_ELEMENT_SENTINEL.equals(stackTrace[0])) {
 926                 stackTrace = null;
 927             } else { // Verify stack trace elements are non-null.
 928                 for(StackTraceElement ste : stackTrace) {
 929                     if (ste == null)
 930                         throw new NullPointerException("null StackTraceElement in serial stream. ");
 931                 }
 932             }
 933         }
 934         // A null stackTrace field in the serial form can result from
 935         // an exception serialized without that field.  Such exceptions
 936         // are now treated as having immutable stack traces.
 937     }
 938 
 939     /**
 940      * Write a {@code Throwable} object to a stream.
 941      *
 942      * A {@code null} stack trace field is represented in the serial
 943      * form as a one-element array whose element is equal to {@code
 944      * new StackTraceElement("", "", null, Integer.MIN_VALUE)}.
 945      */
 946     private synchronized void writeObject(ObjectOutputStream s)
 947         throws IOException {
 948         // Ensure that the stackTrace field is initialized to a
 949         // non-null value, if appropriate.  As of JDK 7, a null stack
 950         // trace field is a valid value indicating the stack trace
 951         // should not be set.
 952         getOurStackTrace();
 953         ObjectOutputStream.PutField fields = s.putFields();
 954 
 955         fields.put("detailMessage", detailMessage);
 956         fields.put("cause", cause);
 957         // Serialize a null stacktrace using the stack trace sentinel.
 958         if (stackTrace == null)
 959             fields.put("stackTrace", SentinelHolder.STACK_TRACE_SENTINEL);
 960         else
 961             fields.put("stackTrace", stackTrace);
 962         fields.put("suppressedExceptions", suppressedExceptions);
 963 
 964         s.writeFields();
 965     }
 966 
 967     /**
 968      * Appends the specified exception to the exceptions that were
 969      * suppressed in order to deliver this exception. This method is
 970      * typically called (automatically and implicitly) by the {@code
 971      * try}-with-resources statement.
 972      *
 973      * <p>The suppression behavior is enabled <em>unless</em> disabled
 974      * {@linkplain #Throwable(String, Throwable, boolean) via a
 975      * constructor}.  When suppression is disabled, this method does
 976      * nothing other than to validate its argument.
 977      *
 978      * <p>Note that when one exception {@linkplain
 979      * #initCause(Throwable) causes} another exception, the first
 980      * exception is usually caught and then the second exception is
 981      * thrown in response.  In other words, there is a causal
 982      * connection between the two exceptions.
 983      *
 984      * In contrast, there are situations where two independent
 985      * exceptions can be thrown in sibling code blocks, in particular
 986      * in the {@code try} block of a {@code try}-with-resources
 987      * statement and the compiler-generated {@code finally} block
 988      * which closes the resource.
 989      *
 990      * In these situations, only one of the thrown exceptions can be
 991      * propagated.  In the {@code try}-with-resources statement, when
 992      * there are two such exceptions, the exception originating from
 993      * the {@code try} block is propagated and the exception from the
 994      * {@code finally} block is added to the list of exceptions
 995      * suppressed by the exception from the {@code try} block.  As an
 996      * exception unwinds the stack, it can accumulate multiple
 997      * suppressed exceptions.
 998      *
 999      * <p>An exception may have suppressed exceptions while also being
1000      * caused by another exception.  Whether or not an exception has a
1001      * cause is semantically known at the time of its creation, unlike
1002      * whether or not an exception will suppress other exceptions
1003      * which is typically only determined after an exception is
1004      * thrown.
1005      *
1006      * <p>Note that programmer written code is also able to take
1007      * advantage of calling this method in situations where there are
1008      * multiple sibling exceptions and only one can be propagated.
1009      *
1010      * @param exception the exception to be added to the list of
1011      *        suppressed exceptions
1012      * @throws IllegalArgumentException if {@code exception} is this
1013      *         throwable; a throwable cannot suppress itself.
1014      * @throws NullPointerException if {@code exception} is {@code null}
1015      * @since 1.7
1016      */
1017     public final synchronized void addSuppressed(Throwable exception) {
1018         if (exception == this)
1019             throw new IllegalArgumentException(SELF_SUPPRESSION_MESSAGE);
1020 
1021         if (exception == null)
1022             throw new NullPointerException(NULL_CAUSE_MESSAGE);
1023 
1024         if (suppressedExceptions == null) // Suppressed exceptions not recorded
1025             return;
1026 
1027         if (suppressedExceptions == SUPPRESSED_SENTINEL)
1028             suppressedExceptions = new ArrayList<>(1);
1029 
1030         suppressedExceptions.add(exception);
1031     }
1032 
1033     private static final Throwable[] EMPTY_THROWABLE_ARRAY = new Throwable[0];
1034 
1035     /**
1036      * Returns an array containing all of the exceptions that were
1037      * suppressed, typically by the {@code try}-with-resources
1038      * statement, in order to deliver this exception.
1039      *
1040      * If no exceptions were suppressed or {@linkplain
1041      * #Throwable(String, Throwable, boolean) suppression is disabled},
1042      * an empty array is returned.
1043      *
1044      * @return an array containing all of the exceptions that were
1045      *         suppressed to deliver this exception.
1046      * @since 1.7
1047      */
1048     public final synchronized Throwable[] getSuppressed() {
1049         if (suppressedExceptions == SUPPRESSED_SENTINEL ||
1050             suppressedExceptions == null)
1051             return EMPTY_THROWABLE_ARRAY;
1052         else
1053             return suppressedExceptions.toArray(EMPTY_THROWABLE_ARRAY);
1054     }
1055 }