1 /*
   2  * Copyright (c) 1994, 2010, 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 thread at
  50  * the time it was created. It can also contain a message string that gives
  51  * more information about the error. Finally, it can contain a <i>cause</i>:
  52  * another throwable that caused this throwable to get thrown.  The cause
  53  * facility is new in release 1.4.  It is also known as the <i>chained
  54  * exception</i> facility, as the cause can, itself, have a cause, and so on,
  55  * leading to a "chain" of exceptions, each caused by another.
  56  *
  57  * <p>One reason that a throwable may have a cause is that the class that
  58  * throws it is built atop a lower layered abstraction, and an operation on
  59  * the upper layer fails due to a failure in the lower layer.  It would be bad
  60  * design to let the throwable thrown by the lower layer propagate outward, as
  61  * it is generally unrelated to the abstraction provided by the upper layer.
  62  * Further, doing so would tie the API of the upper layer to the details of
  63  * its implementation, assuming the lower layer's exception was a checked
  64  * exception.  Throwing a "wrapped exception" (i.e., an exception containing a
  65  * cause) allows the upper layer to communicate the details of the failure to
  66  * its caller without incurring either of these shortcomings.  It preserves
  67  * the flexibility to change the implementation of the upper layer without
  68  * changing its API (in particular, the set of exceptions thrown by its
  69  * methods).
  70  *
  71  * <p>A second reason that a throwable may have a cause is that the method
  72  * that throws it must conform to a general-purpose interface that does not
  73  * permit the method to throw the cause directly.  For example, suppose
  74  * a persistent collection conforms to the {@link java.util.Collection
  75  * Collection} interface, and that its persistence is implemented atop
  76  * {@code java.io}.  Suppose the internals of the {@code add} method
  77  * can throw an {@link java.io.IOException IOException}.  The implementation
  78  * can communicate the details of the {@code IOException} to its caller
  79  * while conforming to the {@code Collection} interface by wrapping the
  80  * {@code IOException} in an appropriate unchecked exception.  (The
  81  * specification for the persistent collection should indicate that it is
  82  * capable of throwing such exceptions.)
  83  *
  84  * <p>A cause can be associated with a throwable in two ways: via a
  85  * constructor that takes the cause as an argument, or via the
  86  * {@link #initCause(Throwable)} method.  New throwable classes that
  87  * wish to allow causes to be associated with them should provide constructors
  88  * that take a cause and delegate (perhaps indirectly) to one of the
  89  * {@code Throwable} constructors that takes a cause.  For example:
  90  * <pre>
  91  *     try {
  92  *         lowLevelOp();
  93  *     } catch (LowLevelException le) {
  94  *         throw new HighLevelException(le);  // Chaining-aware constructor
  95  *     }
  96  * </pre>
  97  * Because the {@code initCause} method is public, it allows a cause to be
  98  * associated with any throwable, even a "legacy throwable" whose
  99  * implementation predates the addition of the exception chaining mechanism to
 100  * {@code Throwable}. For example:
 101  * <pre>
 102  *     try {
 103  *         lowLevelOp();
 104  *     } catch (LowLevelException le) {
 105  *         throw (HighLevelException)
 106  *               new HighLevelException().initCause(le);  // Legacy constructor
 107  *     }
 108  * </pre>
 109  *
 110  * <p>Prior to release 1.4, there were many throwables that had their own
 111  * non-standard exception chaining mechanisms (
 112  * {@link ExceptionInInitializerError}, {@link ClassNotFoundException},
 113  * {@link java.lang.reflect.UndeclaredThrowableException},
 114  * {@link java.lang.reflect.InvocationTargetException},
 115  * {@link java.io.WriteAbortedException},
 116  * {@link java.security.PrivilegedActionException},
 117  * {@link java.awt.print.PrinterIOException},
 118  * {@link java.rmi.RemoteException} and
 119  * {@link javax.naming.NamingException}).
 120  * All of these throwables have been retrofitted to
 121  * use the standard exception chaining mechanism, while continuing to
 122  * implement their "legacy" chaining mechanisms for compatibility.
 123  *
 124  * <p>Further, as of release 1.4, many general purpose {@code Throwable}
 125  * classes (for example {@link Exception}, {@link RuntimeException},
 126  * {@link Error}) have been retrofitted with constructors that take
 127  * a cause.  This was not strictly necessary, due to the existence of the
 128  * {@code initCause} method, but it is more convenient and expressive to
 129  * delegate to a constructor that takes a cause.
 130  *
 131  * <p>By convention, class {@code Throwable} and its subclasses have two
 132  * constructors, one that takes no arguments and one that takes a
 133  * {@code String} argument that can be used to produce a detail message.
 134  * Further, those subclasses that might likely have a cause associated with
 135  * them should have two more constructors, one that takes a
 136  * {@code Throwable} (the cause), and one that takes a
 137  * {@code String} (the detail message) and a {@code Throwable} (the
 138  * cause).
 139  *
 140  * <p>Also introduced in release 1.4 is the {@link #getStackTrace()} method,
 141  * which allows programmatic access to the stack trace information that was
 142  * previously available only in text form, via the various forms of the
 143  * {@link #printStackTrace()} method.  This information has been added to the
 144  * <i>serialized representation</i> of this class so {@code getStackTrace}
 145  * and {@code printStackTrace} will operate properly on a throwable that
 146  * was obtained by deserialization.
 147  *
 148  * @author  unascribed
 149  * @author  Josh Bloch (Added exception chaining and programmatic access to
 150  *          stack trace in 1.4.)
 151  * @jls3 11.2 Compile-Time Checking of Exceptions
 152  * @since JDK1.0
 153  */
 154 public class Throwable implements Serializable {
 155     /** use serialVersionUID from JDK 1.0.2 for interoperability */
 156     private static final long serialVersionUID = -3042686055658047285L;
 157 
 158     /**
 159      * Native code saves some indication of the stack backtrace in this slot.
 160      */
 161     private transient Object backtrace;
 162 
 163     /**
 164      * Specific details about the Throwable.  For example, for
 165      * {@code FileNotFoundException}, this contains the name of
 166      * the file that could not be found.
 167      *
 168      * @serial
 169      */
 170     private String detailMessage;
 171 
 172     /**
 173      * A shared value for an empty stack.
 174      */
 175     private static final StackTraceElement[] EMPTY_STACK = new StackTraceElement[0];
 176 
 177     /*
 178      * To allow Throwable objects to be made immutable and safely
 179      * reused by the JVM, such as OutOfMemoryErrors, fields of
 180      * Throwable that are writable in response to user actions, cause
 181      * and suppressedExceptions obey the following protocol:
 182      *
 183      * 1) The fields are initialized to a non-null sentinel value
 184      * which indicates the value has logically not been set.
 185      *
 186      * 2) Writing a null to the field indicates further writes
 187      * are forbidden
 188      *
 189      * 3) The sentinel value may be replaced with another non-null
 190      * value.
 191      *
 192      * For example, implementations of the HotSpot JVM have
 193      * preallocated OutOfMemoryError objects to provide for better
 194      * diagnosability of that situation.  These objects are created
 195      * without calling the constructor for that class and the fields
 196      * in question are initialized to null.  To support this
 197      * capability, any new fields added to Throwable that require
 198      * being initialized to a non-null value require a coordinated JVM
 199      * change.
 200      */
 201 
 202     /**
 203      * The throwable that caused this throwable to get thrown, or null if this
 204      * throwable was not caused by another throwable, or if the causative
 205      * throwable is unknown.  If this field is equal to this throwable itself,
 206      * it indicates that the cause of this throwable has not yet been
 207      * initialized.
 208      *
 209      * @serial
 210      * @since 1.4
 211      */
 212     private Throwable cause = this;
 213 
 214     /**
 215      * The stack trace, as returned by {@link #getStackTrace()}.
 216      *
 217      * @serial
 218      * @since 1.4
 219      */
 220     private StackTraceElement[] stackTrace;
 221 
 222     // Setting this static field introduces an acceptable
 223     // initialization dependency on a few java.util classes.
 224     private static final List<Throwable> SUPPRESSED_SENTINEL =
 225         Collections.unmodifiableList(new ArrayList<Throwable>(0));
 226 
 227     /**
 228      * The list of suppressed exceptions, as returned by {@link
 229      * #getSuppressed()}.  The list is initialized to a zero-element
 230      * unmodifiable sentinel list.  When a serialized Throwable is
 231      * read in, if the {@code suppressedExceptions} field points to a
 232      * zero-element list, the field is reset to the sentinel value.
 233      *
 234      * @serial
 235      * @since 1.7
 236      */
 237     private List<Throwable> suppressedExceptions = SUPPRESSED_SENTINEL;
 238 
 239     /** Message for trying to suppress a null exception. */
 240     private static final String NULL_CAUSE_MESSAGE = "Cannot suppress a null exception.";
 241 
 242     /** Message for trying to suppress oneself. */
 243     private static final String SELF_SUPPRESSION_MESSAGE = "Self-suppression not permitted";
 244 
 245     /** Caption  for labeling causative exception stack traces */
 246     private static final String CAUSE_CAPTION = "Caused by: ";
 247 
 248     /** Caption for labeling suppressed exception stack traces */
 249     private static final String SUPPRESSED_CAPTION = "Suppressed: ";
 250 
 251     /**
 252      * Constructs a new throwable with {@code null} as its detail message.
 253      * The cause is not initialized, and may subsequently be initialized by a
 254      * call to {@link #initCause}.
 255      *
 256      * <p>The {@link #fillInStackTrace()} method is called to initialize
 257      * the stack trace data in the newly created throwable.
 258      */
 259     public Throwable() {
 260         fillInStackTrace();
 261     }
 262 
 263     /**
 264      * Constructs a new throwable with the specified detail message.  The
 265      * cause is not initialized, and may subsequently be initialized by
 266      * a call to {@link #initCause}.
 267      *
 268      * <p>The {@link #fillInStackTrace()} method is called to initialize
 269      * the stack trace data in the newly created throwable.
 270      *
 271      * @param   message   the detail message. The detail message is saved for
 272      *          later retrieval by the {@link #getMessage()} method.
 273      */
 274     public Throwable(String message) {
 275         fillInStackTrace();
 276         detailMessage = message;
 277     }
 278 
 279     /**
 280      * Constructs a new throwable with the specified detail message and
 281      * cause.  <p>Note that the detail message associated with
 282      * {@code cause} is <i>not</i> automatically incorporated in
 283      * this throwable's detail message.
 284      *
 285      * <p>The {@link #fillInStackTrace()} method is called to initialize
 286      * the stack trace data in the newly created throwable.
 287      *
 288      * @param  message the detail message (which is saved for later retrieval
 289      *         by the {@link #getMessage()} method).
 290      * @param  cause the cause (which is saved for later retrieval by the
 291      *         {@link #getCause()} method).  (A {@code null} value is
 292      *         permitted, and indicates that the cause is nonexistent or
 293      *         unknown.)
 294      * @since  1.4
 295      */
 296     public Throwable(String message, Throwable cause) {
 297         fillInStackTrace();
 298         detailMessage = message;
 299         this.cause = cause;
 300     }
 301 
 302     /**
 303      * Constructs a new throwable with the specified cause and a detail
 304      * message of {@code (cause==null ? null : cause.toString())} (which
 305      * typically contains the class and detail message of {@code cause}).
 306      * This constructor is useful for throwables that are little more than
 307      * wrappers for other throwables (for example, {@link
 308      * java.security.PrivilegedActionException}).
 309      *
 310      * <p>The {@link #fillInStackTrace()} method is called to initialize
 311      * the stack trace data in the newly created throwable.
 312      *
 313      * @param  cause the cause (which is saved for later retrieval by the
 314      *         {@link #getCause()} method).  (A {@code null} value is
 315      *         permitted, and indicates that the cause is nonexistent or
 316      *         unknown.)
 317      * @since  1.4
 318      */
 319     public Throwable(Throwable cause) {
 320         fillInStackTrace();
 321         detailMessage = (cause==null ? null : cause.toString());
 322         this.cause = cause;
 323     }
 324 
 325     /**
 326      * Returns the detail message string of this throwable.
 327      *
 328      * @return  the detail message string of this {@code Throwable} instance
 329      *          (which may be {@code null}).
 330      */
 331     public String getMessage() {
 332         return detailMessage;
 333     }
 334 
 335     /**
 336      * Creates a localized description of this throwable.
 337      * Subclasses may override this method in order to produce a
 338      * locale-specific message.  For subclasses that do not override this
 339      * method, the default implementation returns the same result as
 340      * {@code getMessage()}.
 341      *
 342      * @return  The localized description of this throwable.
 343      * @since   JDK1.1
 344      */
 345     public String getLocalizedMessage() {
 346         return getMessage();
 347     }
 348 
 349     /**
 350      * Returns the cause of this throwable or {@code null} if the
 351      * cause is nonexistent or unknown.  (The cause is the throwable that
 352      * caused this throwable to get thrown.)
 353      *
 354      * <p>This implementation returns the cause that was supplied via one of
 355      * the constructors requiring a {@code Throwable}, or that was set after
 356      * creation with the {@link #initCause(Throwable)} method.  While it is
 357      * typically unnecessary to override this method, a subclass can override
 358      * it to return a cause set by some other means.  This is appropriate for
 359      * a "legacy chained throwable" that predates the addition of chained
 360      * exceptions to {@code Throwable}.  Note that it is <i>not</i>
 361      * necessary to override any of the {@code PrintStackTrace} methods,
 362      * all of which invoke the {@code getCause} method to determine the
 363      * cause of a throwable.
 364      *
 365      * @return  the cause of this throwable or {@code null} if the
 366      *          cause is nonexistent or unknown.
 367      * @since 1.4
 368      */
 369     public synchronized Throwable getCause() {
 370         return (cause==this ? null : cause);
 371     }
 372 
 373     /**
 374      * Initializes the <i>cause</i> of this throwable to the specified value.
 375      * (The cause is the throwable that caused this throwable to get thrown.)
 376      *
 377      * <p>This method can be called at most once.  It is generally called from
 378      * within the constructor, or immediately after creating the
 379      * throwable.  If this throwable was created
 380      * with {@link #Throwable(Throwable)} or
 381      * {@link #Throwable(String,Throwable)}, this method cannot be called
 382      * even once.
 383      *
 384      * @param  cause the cause (which is saved for later retrieval by the
 385      *         {@link #getCause()} method).  (A {@code null} value is
 386      *         permitted, and indicates that the cause is nonexistent or
 387      *         unknown.)
 388      * @return  a reference to this {@code Throwable} instance.
 389      * @throws IllegalArgumentException if {@code cause} is this
 390      *         throwable.  (A throwable cannot be its own cause.)
 391      * @throws IllegalStateException if this throwable was
 392      *         created with {@link #Throwable(Throwable)} or
 393      *         {@link #Throwable(String,Throwable)}, or this method has already
 394      *         been called on this throwable.
 395      * @since  1.4
 396      */
 397     public synchronized Throwable initCause(Throwable cause) {
 398         if (this.cause != this)
 399             throw new IllegalStateException("Can't overwrite cause");
 400         if (cause == this)
 401             throw new IllegalArgumentException("Self-causation not permitted");
 402         this.cause = cause;
 403         return this;
 404     }
 405 
 406     /**
 407      * Returns a short description of this throwable.
 408      * The result is the concatenation of:
 409      * <ul>
 410      * <li> the {@linkplain Class#getName() name} of the class of this object
 411      * <li> ": " (a colon and a space)
 412      * <li> the result of invoking this object's {@link #getLocalizedMessage}
 413      *      method
 414      * </ul>
 415      * If {@code getLocalizedMessage} returns {@code null}, then just
 416      * the class name is returned.
 417      *
 418      * @return a string representation of this throwable.
 419      */
 420     public String toString() {
 421         String s = getClass().getName();
 422         String message = getLocalizedMessage();
 423         return (message != null) ? (s + ": " + message) : s;
 424     }
 425 
 426     /**
 427      * Prints this throwable and its backtrace to the
 428      * standard error stream. This method prints a stack trace for this
 429      * {@code Throwable} object on the error output stream that is
 430      * the value of the field {@code System.err}. The first line of
 431      * output contains the result of the {@link #toString()} method for
 432      * this object.  Remaining lines represent data previously recorded by
 433      * the method {@link #fillInStackTrace()}. The format of this
 434      * information depends on the implementation, but the following
 435      * example may be regarded as typical:
 436      * <blockquote><pre>
 437      * java.lang.NullPointerException
 438      *         at MyClass.mash(MyClass.java:9)
 439      *         at MyClass.crunch(MyClass.java:6)
 440      *         at MyClass.main(MyClass.java:3)
 441      * </pre></blockquote>
 442      * This example was produced by running the program:
 443      * <pre>
 444      * class MyClass {
 445      *     public static void main(String[] args) {
 446      *         crunch(null);
 447      *     }
 448      *     static void crunch(int[] a) {
 449      *         mash(a);
 450      *     }
 451      *     static void mash(int[] b) {
 452      *         System.out.println(b[0]);
 453      *     }
 454      * }
 455      * </pre>
 456      * The backtrace for a throwable with an initialized, non-null cause
 457      * should generally include the backtrace for the cause.  The format
 458      * of this information depends on the implementation, but the following
 459      * example may be regarded as typical:
 460      * <pre>
 461      * HighLevelException: MidLevelException: LowLevelException
 462      *         at Junk.a(Junk.java:13)
 463      *         at Junk.main(Junk.java:4)
 464      * Caused by: MidLevelException: LowLevelException
 465      *         at Junk.c(Junk.java:23)
 466      *         at Junk.b(Junk.java:17)
 467      *         at Junk.a(Junk.java:11)
 468      *         ... 1 more
 469      * Caused by: LowLevelException
 470      *         at Junk.e(Junk.java:30)
 471      *         at Junk.d(Junk.java:27)
 472      *         at Junk.c(Junk.java:21)
 473      *         ... 3 more
 474      * </pre>
 475      * Note the presence of lines containing the characters {@code "..."}.
 476      * These lines indicate that the remainder of the stack trace for this
 477      * exception matches the indicated number of frames from the bottom of the
 478      * stack trace of the exception that was caused by this exception (the
 479      * "enclosing" exception).  This shorthand can greatly reduce the length
 480      * of the output in the common case where a wrapped exception is thrown
 481      * from same method as the "causative exception" is caught.  The above
 482      * example was produced by running the program:
 483      * <pre>
 484      * public class Junk {
 485      *     public static void main(String args[]) {
 486      *         try {
 487      *             a();
 488      *         } catch(HighLevelException e) {
 489      *             e.printStackTrace();
 490      *         }
 491      *     }
 492      *     static void a() throws HighLevelException {
 493      *         try {
 494      *             b();
 495      *         } catch(MidLevelException e) {
 496      *             throw new HighLevelException(e);
 497      *         }
 498      *     }
 499      *     static void b() throws MidLevelException {
 500      *         c();
 501      *     }
 502      *     static void c() throws MidLevelException {
 503      *         try {
 504      *             d();
 505      *         } catch(LowLevelException e) {
 506      *             throw new MidLevelException(e);
 507      *         }
 508      *     }
 509      *     static void d() throws LowLevelException {
 510      *        e();
 511      *     }
 512      *     static void e() throws LowLevelException {
 513      *         throw new LowLevelException();
 514      *     }
 515      * }
 516      *
 517      * class HighLevelException extends Exception {
 518      *     HighLevelException(Throwable cause) { super(cause); }
 519      * }
 520      *
 521      * class MidLevelException extends Exception {
 522      *     MidLevelException(Throwable cause)  { super(cause); }
 523      * }
 524      *
 525      * class LowLevelException extends Exception {
 526      * }
 527      * </pre>
 528      * As of release 7, the platform supports the notion of
 529      * <i>suppressed exceptions</i> (in conjunction with the {@code
 530      * try}-with-resources statement). Any exceptions that were
 531      * suppressed in order to deliver an exception are printed out
 532      * beneath the stack trace.  The format of this information
 533      * depends on the implementation, but the following example may be
 534      * regarded as typical:
 535      *
 536      * <pre>
 537      * Exception in thread "main" java.lang.Exception: Something happened
 538      *  at Foo.bar(Foo.java:10)
 539      *  at Foo.main(Foo.java:5)
 540      *  Suppressed: Resource$CloseFailException: Resource ID = 0
 541      *          at Resource.close(Resource.java:26)
 542      *          at Foo.bar(Foo.java:9)
 543      *          ... 1 more
 544      * </pre>
 545      * Note that the "... n more" notation is used on suppressed exceptions
 546      * just at it is used on causes. Unlike causes, suppressed exceptions are
 547      * indented beyond their "containing exceptions."
 548      *
 549      * <p>An exception can have both a cause and one or more suppressed
 550      * exceptions:
 551      * <pre>
 552      * Exception in thread "main" java.lang.Exception: Main block
 553      *  at Foo3.main(Foo3.java:7)
 554      *  Suppressed: Resource$CloseFailException: Resource ID = 2
 555      *          at Resource.close(Resource.java:26)
 556      *          at Foo3.main(Foo3.java:5)
 557      *  Suppressed: Resource$CloseFailException: Resource ID = 1
 558      *          at Resource.close(Resource.java:26)
 559      *          at Foo3.main(Foo3.java:5)
 560      * Caused by: java.lang.Exception: I did it
 561      *  at Foo3.main(Foo3.java:8)
 562      * </pre>
 563      * Likewise, a suppressed exception can have a cause:
 564      * <pre>
 565      * Exception in thread "main" java.lang.Exception: Main block
 566      *  at Foo4.main(Foo4.java:6)
 567      *  Suppressed: Resource2$CloseFailException: Resource ID = 1
 568      *          at Resource2.close(Resource2.java:20)
 569      *          at Foo4.main(Foo4.java:5)
 570      *  Caused by: java.lang.Exception: Rats, you caught me
 571      *          at Resource2$CloseFailException.<init>(Resource2.java:45)
 572      *          ... 2 more
 573      * </pre>
 574      */
 575     public void printStackTrace() {
 576         printStackTrace(System.err);
 577     }
 578 
 579     /**
 580      * Prints this throwable and its backtrace to the specified print stream.
 581      *
 582      * @param s {@code PrintStream} to use for output
 583      */
 584     public void printStackTrace(PrintStream s) {
 585         printStackTrace(new WrappedPrintStream(s));
 586     }
 587 
 588     private void printStackTrace(PrintStreamOrWriter s) {
 589         // Guard against malicious overrides of Throwable.equals by
 590         // using a Set with identity equality semantics.
 591         Set<Throwable> dejaVu =
 592             Collections.newSetFromMap(new IdentityHashMap<Throwable, Boolean>());
 593         dejaVu.add(this);
 594 
 595         synchronized (s.lock()) {
 596             // Print our stack trace
 597             s.println(this);
 598             StackTraceElement[] trace = getOurStackTrace();
 599             for (StackTraceElement traceElement : trace)
 600                 s.println("\tat " + traceElement);
 601 
 602             // Print suppressed exceptions, if any
 603             for (Throwable se : getSuppressed())
 604                 se.printEnclosedStackTrace(s, trace, SUPPRESSED_CAPTION, "\t", dejaVu);
 605 
 606             // Print cause, if any
 607             Throwable ourCause = getCause();
 608             if (ourCause != null)
 609                 ourCause.printEnclosedStackTrace(s, trace, CAUSE_CAPTION, "", dejaVu);
 610         }
 611     }
 612 
 613     /**
 614      * Print our stack trace as an enclosed exception for the specified
 615      * stack trace.
 616      */
 617     private void printEnclosedStackTrace(PrintStreamOrWriter s,
 618                                          StackTraceElement[] enclosingTrace,
 619                                          String caption,
 620                                          String prefix,
 621                                          Set<Throwable> dejaVu) {
 622         assert Thread.holdsLock(s.lock());
 623         if (dejaVu.contains(this)) {
 624             s.println("\t[CIRCULAR REFERENCE:" + this + "]");
 625         } else {
 626             dejaVu.add(this);
 627             // Compute number of frames in common between this and enclosing trace
 628             StackTraceElement[] trace = getOurStackTrace();
 629             int m = trace.length - 1;
 630             int n = enclosingTrace.length - 1;
 631             while (m >= 0 && n >=0 && trace[m].equals(enclosingTrace[n])) {
 632                 m--; n--;
 633             }
 634             int framesInCommon = trace.length - 1 - m;
 635 
 636             // Print our stack trace
 637             s.println(prefix + caption + this);
 638             for (int i = 0; i <= m; i++)
 639                 s.println(prefix + "\tat " + trace[i]);
 640             if (framesInCommon != 0)
 641                 s.println(prefix + "\t... " + framesInCommon + " more");
 642 
 643             // Print suppressed exceptions, if any
 644             for (Throwable se : getSuppressed())
 645                 se.printEnclosedStackTrace(s, trace, SUPPRESSED_CAPTION,
 646                                            prefix +"\t", dejaVu);
 647 
 648             // Print cause, if any
 649             Throwable ourCause = getCause();
 650             if (ourCause != null)
 651                 ourCause.printEnclosedStackTrace(s, trace, CAUSE_CAPTION, prefix, dejaVu);
 652         }
 653     }
 654 
 655     /**
 656      * Prints this throwable and its backtrace to the specified
 657      * print writer.
 658      *
 659      * @param s {@code PrintWriter} to use for output
 660      * @since   JDK1.1
 661      */
 662     public void printStackTrace(PrintWriter s) {
 663         printStackTrace(new WrappedPrintWriter(s));
 664     }
 665 
 666     /**
 667      * Wrapper class for PrintStream and PrintWriter to enable a single
 668      * implementation of printStackTrace.
 669      */
 670     private abstract static class PrintStreamOrWriter {
 671         /** Returns the object to be locked when using this StreamOrWriter */
 672         abstract Object lock();
 673 
 674         /** Prints the specified string as a line on this StreamOrWriter */
 675         abstract void println(Object o);
 676     }
 677 
 678     private static class WrappedPrintStream extends PrintStreamOrWriter {
 679         private final PrintStream printStream;
 680 
 681         WrappedPrintStream(PrintStream printStream) {
 682             this.printStream = printStream;
 683         }
 684 
 685         Object lock() {
 686             return printStream;
 687         }
 688 
 689         void println(Object o) {
 690             printStream.println(o);
 691         }
 692     }
 693 
 694     private static class WrappedPrintWriter extends PrintStreamOrWriter {
 695         private final PrintWriter printWriter;
 696 
 697         WrappedPrintWriter(PrintWriter printWriter) {
 698             this.printWriter = printWriter;
 699         }
 700 
 701         Object lock() {
 702             return printWriter;
 703         }
 704 
 705         void println(Object o) {
 706             printWriter.println(o);
 707         }
 708     }
 709 
 710     /**
 711      * Fills in the execution stack trace. This method records within this
 712      * {@code Throwable} object information about the current state of
 713      * the stack frames for the current thread.
 714      *
 715      * @return  a reference to this {@code Throwable} instance.
 716      * @see     java.lang.Throwable#printStackTrace()
 717      */
 718     public synchronized native Throwable fillInStackTrace();
 719 
 720     /**
 721      * Provides programmatic access to the stack trace information printed by
 722      * {@link #printStackTrace()}.  Returns an array of stack trace elements,
 723      * each representing one stack frame.  The zeroth element of the array
 724      * (assuming the array's length is non-zero) represents the top of the
 725      * stack, which is the last method invocation in the sequence.  Typically,
 726      * this is the point at which this throwable was created and thrown.
 727      * The last element of the array (assuming the array's length is non-zero)
 728      * represents the bottom of the stack, which is the first method invocation
 729      * in the sequence.
 730      *
 731      * <p>Some virtual machines may, under some circumstances, omit one
 732      * or more stack frames from the stack trace.  In the extreme case,
 733      * a virtual machine that has no stack trace information concerning
 734      * this throwable is permitted to return a zero-length array from this
 735      * method.  Generally speaking, the array returned by this method will
 736      * contain one element for every frame that would be printed by
 737      * {@code printStackTrace}.
 738      *
 739      * @return an array of stack trace elements representing the stack trace
 740      *         pertaining to this throwable.
 741      * @since  1.4
 742      */
 743     public StackTraceElement[] getStackTrace() {
 744         return getOurStackTrace().clone();
 745     }
 746 
 747     private synchronized StackTraceElement[] getOurStackTrace() {
 748         // Initialize stack trace if this is the first call to this method
 749         if (stackTrace == null) {
 750             int depth = getStackTraceDepth();
 751             stackTrace = new StackTraceElement[depth];
 752             for (int i=0; i < depth; i++)
 753                 stackTrace[i] = getStackTraceElement(i);
 754         }
 755         return stackTrace;
 756     }
 757 
 758     /**
 759      * Sets the stack trace elements that will be returned by
 760      * {@link #getStackTrace()} and printed by {@link #printStackTrace()}
 761      * and related methods.
 762      *
 763      * This method, which is designed for use by RPC frameworks and other
 764      * advanced systems, allows the client to override the default
 765      * stack trace that is either generated by {@link #fillInStackTrace()}
 766      * when a throwable is constructed or deserialized when a throwable is
 767      * read from a serialization stream.
 768      *
 769      * @param   stackTrace the stack trace elements to be associated with
 770      * this {@code Throwable}.  The specified array is copied by this
 771      * call; changes in the specified array after the method invocation
 772      * returns will have no affect on this {@code Throwable}'s stack
 773      * trace.
 774      *
 775      * @throws NullPointerException if {@code stackTrace} is
 776      *         {@code null}, or if any of the elements of
 777      *         {@code stackTrace} are {@code null}
 778      *
 779      * @since  1.4
 780      */
 781     public void setStackTrace(StackTraceElement[] stackTrace) {
 782         StackTraceElement[] defensiveCopy = stackTrace.clone();
 783         for (int i = 0; i < defensiveCopy.length; i++)
 784             if (defensiveCopy[i] == null)
 785                 throw new NullPointerException("stackTrace[" + i + "]");
 786 
 787         synchronized (this) {
 788             this.stackTrace = defensiveCopy;
 789         }
 790     }
 791 
 792     /**
 793      * Returns the number of elements in the stack trace (or 0 if the stack
 794      * trace is unavailable).
 795      *
 796      * package-protection for use by SharedSecrets.
 797      */
 798     native int getStackTraceDepth();
 799 
 800     /**
 801      * Returns the specified element of the stack trace.
 802      *
 803      * package-protection for use by SharedSecrets.
 804      *
 805      * @param index index of the element to return.
 806      * @throws IndexOutOfBoundsException if {@code index < 0 ||
 807      *         index >= getStackTraceDepth() }
 808      */
 809     native StackTraceElement getStackTraceElement(int index);
 810 
 811     /**
 812      * Read a {@code Throwable} from a stream, enforcing
 813      * well-formedness constraints on fields.  Null entries and
 814      * self-pointers are not allowed in the list of {@code
 815      * suppressedExceptions}.  Null entries are not allowed for stack
 816      * trace elements.
 817      *
 818      * Note that there are no constraints on the value the {@code
 819      * cause} field can hold; both {@code null} and {@code this} are
 820      * valid values for the field.
 821      */
 822     private void readObject(ObjectInputStream s)
 823         throws IOException, ClassNotFoundException {
 824         s.defaultReadObject();     // read in all fields
 825         if (suppressedExceptions != null) {
 826             List<Throwable> suppressed = null;
 827             if (suppressedExceptions.isEmpty()) {
 828                 // Use the sentinel for a zero-length list
 829                 suppressed = SUPPRESSED_SENTINEL;
 830             } else { // Copy Throwables to new list
 831                 suppressed = new ArrayList<>(1);
 832                 for (Throwable t : suppressedExceptions) {
 833                     // Enforce constraints on suppressed exceptions in
 834                     // case of corrupt or malicious stream.
 835                     if (t == null)
 836                         throw new NullPointerException(NULL_CAUSE_MESSAGE);
 837                     if (t == this)
 838                         throw new IllegalArgumentException(SELF_SUPPRESSION_MESSAGE);
 839                     suppressed.add(t);
 840                 }
 841             }
 842             suppressedExceptions = suppressed;
 843         } // else a null suppressedExceptions field remains null
 844 
 845         if (stackTrace != null) {
 846             for (StackTraceElement ste : stackTrace) {
 847                 if (ste == null)
 848                     throw new NullPointerException("null StackTraceElement in serial stream. ");
 849             }
 850         } else {
 851             // A null stackTrace field in the serial form can result from
 852             // an exception serialized without that field in older JDK releases.
 853             stackTrace = EMPTY_STACK;
 854         }
 855 
 856     }
 857 
 858     /**
 859      * Write a {@code Throwable} object to a stream.
 860      */
 861     private synchronized void writeObject(ObjectOutputStream s)
 862         throws IOException {
 863         getOurStackTrace();  // Ensure that stackTrace field is initialized.
 864         s.defaultWriteObject();
 865     }
 866 
 867     /**
 868      * Adds the specified exception to the list of exceptions that
 869      * were suppressed, typically by the {@code try}-with-resources
 870      * statement, in order to deliver this exception.
 871      *
 872      * If the first exception to be suppressed is {@code null}, that
 873      * indicates suppressed exception information will <em>not</em> be
 874      * recorded for this exception.  Subsequent calls to this method
 875      * will not record any suppressed exceptions.  Otherwise,
 876      * attempting to suppress {@code null} after an exception has
 877      * already been successfully suppressed results in a {@code
 878      * NullPointerException}.
 879      *
 880      * <p>Note that when one exception {@linkplain
 881      * #initCause(Throwable) causes} another exception, the first
 882      * exception is usually caught and then the second exception is
 883      * thrown in response.  In contrast, when one exception suppresses
 884      * another, two exceptions are thrown in sibling code blocks, such
 885      * as in a {@code try} block and in its {@code finally} block, and
 886      * control flow can only continue with one exception so the second
 887      * is recorded as a suppressed exception of the first.
 888      *
 889      * @param exception the exception to be added to the list of
 890      *        suppressed exceptions
 891      * @throws IllegalArgumentException if {@code exception} is this
 892      *         throwable; a throwable cannot suppress itself.
 893      * @throws NullPointerException if {@code exception} is null and
 894      *         an exception has already been suppressed by this exception
 895      * @since 1.7
 896      */
 897     public final synchronized void addSuppressed(Throwable exception) {
 898         if (exception == this)
 899             throw new IllegalArgumentException(SELF_SUPPRESSION_MESSAGE);
 900 
 901         if (exception == null) {
 902             if (suppressedExceptions == SUPPRESSED_SENTINEL) {
 903                 suppressedExceptions = null; // No suppression information recorded
 904                 return;
 905             } else
 906                 throw new NullPointerException(NULL_CAUSE_MESSAGE);
 907         } else {
 908             assert exception != null && exception != this;
 909 
 910             if (suppressedExceptions == null) // Suppressed exceptions not recorded
 911                 return;
 912 
 913             if (suppressedExceptions == SUPPRESSED_SENTINEL)
 914                 suppressedExceptions = new ArrayList<>(1);
 915 
 916             assert suppressedExceptions != SUPPRESSED_SENTINEL;
 917 
 918             suppressedExceptions.add(exception);
 919         }
 920     }
 921 
 922     private static final Throwable[] EMPTY_THROWABLE_ARRAY = new Throwable[0];
 923 
 924     /**
 925      * Returns an array containing all of the exceptions that were
 926      * suppressed, typically by the {@code try}-with-resources
 927      * statement, in order to deliver this exception.
 928      *
 929      * If no exceptions were suppressed, an empty array is returned.
 930      *
 931      * @return an array containing all of the exceptions that were
 932      *         suppressed to deliver this exception.
 933      * @since 1.7
 934      */
 935     public final synchronized Throwable[] getSuppressed() {
 936         if (suppressedExceptions == SUPPRESSED_SENTINEL ||
 937             suppressedExceptions == null)
 938             return EMPTY_THROWABLE_ARRAY;
 939         else
 940             return suppressedExceptions.toArray(EMPTY_THROWABLE_ARRAY);
 941     }
 942 }