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    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    * @jls 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 shared value for an empty stack.
  158        */
  159       private static final StackTraceElement[] UNASSIGNED_STACK = new StackTraceElement[0];
  160   
  161       /*
  162        * To allow Throwable objects to be made immutable and safely
  163        * reused by the JVM, such as OutOfMemoryErrors, fields of
  164        * Throwable that are writable in response to user actions, cause,
  165        * stackTrace, and suppressedExceptions obey the following
  166        * protocol:
  167        *
  168        * 1) The fields are initialized to a non-null sentinel value
  169        * which indicates the value has logically not been set.
  170        *
  171        * 2) Writing a null to the field indicates further writes
  172        * are forbidden
  173        *
  174        * 3) The sentinel value may be replaced with another non-null
  175        * value.
  176        *
  177        * For example, implementations of the HotSpot JVM have
  178        * preallocated OutOfMemoryError objects to provide for better
  179        * diagnosability of that situation.  These objects are created
  180        * without calling the constructor for that class and the fields
  181        * in question are initialized to null.  To support this
  182        * capability, any new fields added to Throwable that require
  183        * being initialized to a non-null value require a coordinated JVM
  184        * change.
  185        */
  186   
  187       /**
  188        * The throwable that caused this throwable to get thrown, or null if this
  189        * throwable was not caused by another throwable, or if the causative
  190        * throwable is unknown.  If this field is equal to this throwable itself,
  191        * it indicates that the cause of this throwable has not yet been
  192        * initialized.
  193        *
  194        * @serial
  195        * @since 1.4
  196        */
  197       private Throwable cause = this;
  198   
  199       /**
  200        * The stack trace, as returned by {@link #getStackTrace()}.
  201        *
  202        * The field is initialized to a zero-length array.  A {@code
  203        * null} value of this field indicates subsequent calls to {@link
  204        * #setStackTrace(StackTraceElement[])} and {@link
  205        * #fillInStackTrace()} will be be no-ops.
  206        *
  207        * @serial
  208        * @since 1.4
  209        */
  210       private StackTraceElement[] stackTrace = UNASSIGNED_STACK;
  211   
  212       // Setting this static field introduces an acceptable
  213       // initialization dependency on a few java.util classes.
  214       private static final List<Throwable> SUPPRESSED_SENTINEL =
  215           Collections.unmodifiableList(new ArrayList<Throwable>(0));
  216   
  217       /**
  218        * The list of suppressed exceptions, as returned by {@link
  219        * #getSuppressed()}.  The list is initialized to a zero-element
  220        * unmodifiable sentinel list.  When a serialized Throwable is
  221        * read in, if the {@code suppressedExceptions} field points to a
  222        * zero-element list, the field is reset to the sentinel value.
  223        *
  224        * @serial
  225        * @since 1.7
  226        */
  227       private List<Throwable> suppressedExceptions = SUPPRESSED_SENTINEL;
  228   
  229       /** Message for trying to suppress a null exception. */
  230       private static final String NULL_CAUSE_MESSAGE = "Cannot suppress a null exception.";
  231   
  232       /** Message for trying to suppress oneself. */
  233       private static final String SELF_SUPPRESSION_MESSAGE = "Self-suppression not permitted";
  234   
  235       /** Caption  for labeling causative exception stack traces */
  236       private static final String CAUSE_CAPTION = "Caused by: ";
  237   
  238       /** Caption for labeling suppressed exception stack traces */
  239       private static final String SUPPRESSED_CAPTION = "Suppressed: ";
  240   
  241       /**
  242        * Constructs a new throwable with {@code null} as its detail message.
  243        * The cause is not initialized, and may subsequently be initialized by a
  244        * call to {@link #initCause}.
  245        *
  246        * <p>The {@link #fillInStackTrace()} method is called to initialize
  247        * the stack trace data in the newly created throwable.
  248        */
  249       public Throwable() {
  250           fillInStackTrace();
  251       }
  252   
  253       /**
  254        * Constructs a new throwable with the specified detail message.  The
  255        * cause is not initialized, and may subsequently be initialized by
  256        * a call to {@link #initCause}.
  257        *
  258        * <p>The {@link #fillInStackTrace()} method is called to initialize
  259        * the stack trace data in the newly created throwable.
  260        *
  261        * @param   message   the detail message. The detail message is saved for
  262        *          later retrieval by the {@link #getMessage()} method.
  263        */
  264       public Throwable(String message) {
  265           fillInStackTrace();
  266           detailMessage = message;
  267       }
  268   
  269       /**
  270        * Constructs a new throwable with the specified detail message and
  271        * cause.  <p>Note that the detail message associated with
  272        * {@code cause} is <i>not</i> automatically incorporated in
  273        * this throwable's detail message.
  274        *
  275        * <p>The {@link #fillInStackTrace()} method is called to initialize
  276        * the stack trace data in the newly created throwable.
  277        *
  278        * @param  message the detail message (which is saved for later retrieval
  279        *         by the {@link #getMessage()} method).
  280        * @param  cause the cause (which is saved for later retrieval by the
  281        *         {@link #getCause()} method).  (A {@code null} value is
  282        *         permitted, and indicates that the cause is nonexistent or
  283        *         unknown.)
  284        * @since  1.4
  285        */
  286       public Throwable(String message, Throwable cause) {
  287           fillInStackTrace();
  288           detailMessage = message;
  289           this.cause = cause;
  290       }
  291   
  292       /**
  293        * Constructs a new throwable with the specified cause and a detail
  294        * message of {@code (cause==null ? null : cause.toString())} (which
  295        * typically contains the class and detail message of {@code cause}).
  296        * This constructor is useful for throwables that are little more than
  297        * wrappers for other throwables (for example, {@link
  298        * java.security.PrivilegedActionException}).
  299        *
  300        * <p>The {@link #fillInStackTrace()} method is called to initialize
  301        * the stack trace data in the newly created throwable.
  302        *
  303        * @param  cause the cause (which is saved for later retrieval by the
  304        *         {@link #getCause()} method).  (A {@code null} value is
  305        *         permitted, and indicates that the cause is nonexistent or
  306        *         unknown.)
  307        * @since  1.4
  308        */
  309       public Throwable(Throwable cause) {
  310           fillInStackTrace();
  311           detailMessage = (cause==null ? null : cause.toString());
  312           this.cause = cause;
  313       }
  314   
  315       /**
  316        * Constructs a new throwable with the specified detail message,
  317        * cause, {@linkplain #addSuppressed suppression} enabled or
  318        * disabled, and writable stack trace enabled or disabled.  If
  319        * suppression is disabled, {@link #getSuppressed} for this object
  320        * will return a zero-length array and calls to {@link
  321        * #addSuppressed} that would otherwise append an exception to the
  322        * suppressed list will have no effect.  If the writable stack
  323        * trace is false, this constructor will not call {@link
  324        * #fillInStackTrace()}, a {@code null} will be written to the
  325        * {@code stackTrace} field, and subsequent calls to {@code
  326        * fillInStackTrace} and {@link
  327        * #setStackTrace(StackTraceElement[])} will not set the stack
  328        * trace.  If the writable stack trace is false, {@link
  329        * #getStackTrace} will return a zero length array.
  330        *
  331        * <p>Note that the other constructors of {@code Throwable} treat
  332        * suppression as being enabled and the stack trace as being
  333        * writable.  Subclasses of {@code Throwable} should document any
  334        * conditions under which suppression is disabled and document
  335        * conditions under which the stack trace is not writable.
  336        * Disabling of suppression should only occur in exceptional
  337        * circumstances where special requirements exist, such as a
  338        * virtual machine reusing exception objects under low-memory
  339        * situations.  Circumstances where a given exception object is
  340        * repeatedly caught and rethrown, such as to implement control
  341        * flow between two sub-systems, is another situation where
  342        * immutable throwable objects would be appropriate.
  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        * <p>An example of using this method on a legacy throwable type
  430        * without other support for setting the cause is:
  431        *
  432        * <pre>
  433        * try {
  434        *     lowLevelOp();
  435        * } catch (LowLevelException le) {
  436        *     throw (HighLevelException)
  437        *           new HighLevelException().initCause(le); // Legacy constructor
  438        * }
  439        * </pre>
  440        *
  441        * @param  cause the cause (which is saved for later retrieval by the
  442        *         {@link #getCause()} method).  (A {@code null} value is
  443        *         permitted, and indicates that the cause is nonexistent or
  444        *         unknown.)
  445        * @return  a reference to this {@code Throwable} instance.
  446        * @throws IllegalArgumentException if {@code cause} is this
  447        *         throwable.  (A throwable cannot be its own cause.)
  448        * @throws IllegalStateException if this throwable was
  449        *         created with {@link #Throwable(Throwable)} or
  450        *         {@link #Throwable(String,Throwable)}, or this method has already
  451        *         been called on this throwable.
  452        * @since  1.4
  453        */
  454       public synchronized Throwable initCause(Throwable cause) {
  455           if (this.cause != this)
  456               throw new IllegalStateException("Can't overwrite cause");
  457           if (cause == this)
  458               throw new IllegalArgumentException("Self-causation not permitted");
  459           this.cause = cause;
  460           return this;
  461       }
  462   
  463       /**
  464        * Returns a short description of this throwable.
  465        * The result is the concatenation of:
  466        * <ul>
  467        * <li> the {@linkplain Class#getName() name} of the class of this object
  468        * <li> ": " (a colon and a space)
  469        * <li> the result of invoking this object's {@link #getLocalizedMessage}
  470        *      method
  471        * </ul>
  472        * If {@code getLocalizedMessage} returns {@code null}, then just
  473        * the class name is returned.
  474        *
  475        * @return a string representation of this throwable.
  476        */
  477       public String toString() {
  478           String s = getClass().getName();
  479           String message = getLocalizedMessage();
  480           return (message != null) ? (s + ": " + message) : s;
  481       }
  482   
  483       /**
  484        * Prints this throwable and its backtrace to the
  485        * standard error stream. This method prints a stack trace for this
  486        * {@code Throwable} object on the error output stream that is
  487        * the value of the field {@code System.err}. The first line of
  488        * output contains the result of the {@link #toString()} method for
  489        * this object.  Remaining lines represent data previously recorded by
  490        * the method {@link #fillInStackTrace()}. The format of this
  491        * information depends on the implementation, but the following
  492        * example may be regarded as typical:
  493        * <blockquote><pre>
  494        * java.lang.NullPointerException
  495        *         at MyClass.mash(MyClass.java:9)
  496        *         at MyClass.crunch(MyClass.java:6)
  497        *         at MyClass.main(MyClass.java:3)
  498        * </pre></blockquote>
  499        * This example was produced by running the program:
  500        * <pre>
  501        * class MyClass {
  502        *     public static void main(String[] args) {
  503        *         crunch(null);
  504        *     }
  505        *     static void crunch(int[] a) {
  506        *         mash(a);
  507        *     }
  508        *     static void mash(int[] b) {
  509        *         System.out.println(b[0]);
  510        *     }
  511        * }
  512        * </pre>
  513        * The backtrace for a throwable with an initialized, non-null cause
  514        * should generally include the backtrace for the cause.  The format
  515        * of this information depends on the implementation, but the following
  516        * example may be regarded as typical:
  517        * <pre>
  518        * HighLevelException: MidLevelException: LowLevelException
  519        *         at Junk.a(Junk.java:13)
  520        *         at Junk.main(Junk.java:4)
  521        * Caused by: MidLevelException: LowLevelException
  522        *         at Junk.c(Junk.java:23)
  523        *         at Junk.b(Junk.java:17)
  524        *         at Junk.a(Junk.java:11)
  525        *         ... 1 more
  526        * Caused by: LowLevelException
  527        *         at Junk.e(Junk.java:30)
  528        *         at Junk.d(Junk.java:27)
  529        *         at Junk.c(Junk.java:21)
  530        *         ... 3 more
  531        * </pre>
  532        * Note the presence of lines containing the characters {@code "..."}.
  533        * These lines indicate that the remainder of the stack trace for this
  534        * exception matches the indicated number of frames from the bottom of the
  535        * stack trace of the exception that was caused by this exception (the
  536        * "enclosing" exception).  This shorthand can greatly reduce the length
  537        * of the output in the common case where a wrapped exception is thrown
  538        * from same method as the "causative exception" is caught.  The above
  539        * example was produced by running the program:
  540        * <pre>
  541        * public class Junk {
  542        *     public static void main(String args[]) {
  543        *         try {
  544        *             a();
  545        *         } catch(HighLevelException e) {
  546        *             e.printStackTrace();
  547        *         }
  548        *     }
  549        *     static void a() throws HighLevelException {
  550        *         try {
  551        *             b();
  552        *         } catch(MidLevelException e) {
  553        *             throw new HighLevelException(e);
  554        *         }
  555        *     }
  556        *     static void b() throws MidLevelException {
  557        *         c();
  558        *     }
  559        *     static void c() throws MidLevelException {
  560        *         try {
  561        *             d();
  562        *         } catch(LowLevelException e) {
  563        *             throw new MidLevelException(e);
  564        *         }
  565        *     }
  566        *     static void d() throws LowLevelException {
  567        *        e();
  568        *     }
  569        *     static void e() throws LowLevelException {
  570        *         throw new LowLevelException();
  571        *     }
  572        * }
  573        *
  574        * class HighLevelException extends Exception {
  575        *     HighLevelException(Throwable cause) { super(cause); }
  576        * }
  577        *
  578        * class MidLevelException extends Exception {
  579        *     MidLevelException(Throwable cause)  { super(cause); }
  580        * }
  581        *
  582        * class LowLevelException extends Exception {
  583        * }
  584        * </pre>
  585        * As of release 7, the platform supports the notion of
  586        * <i>suppressed exceptions</i> (in conjunction with the {@code
  587        * try}-with-resources statement). Any exceptions that were
  588        * suppressed in order to deliver an exception are printed out
  589        * beneath the stack trace.  The format of this information
  590        * depends on the implementation, but the following example may be
  591        * regarded as typical:
  592        *
  593        * <pre>
  594        * Exception in thread "main" java.lang.Exception: Something happened
  595        *  at Foo.bar(Foo.java:10)
  596        *  at Foo.main(Foo.java:5)
  597        *  Suppressed: Resource$CloseFailException: Resource ID = 0
  598        *          at Resource.close(Resource.java:26)
  599        *          at Foo.bar(Foo.java:9)
  600        *          ... 1 more
  601        * </pre>
  602        * Note that the "... n more" notation is used on suppressed exceptions
  603        * just at it is used on causes. Unlike causes, suppressed exceptions are
  604        * indented beyond their "containing exceptions."
  605        *
  606        * <p>An exception can have both a cause and one or more suppressed
  607        * exceptions:
  608        * <pre>
  609        * Exception in thread "main" java.lang.Exception: Main block
  610        *  at Foo3.main(Foo3.java:7)
  611        *  Suppressed: Resource$CloseFailException: Resource ID = 2
  612        *          at Resource.close(Resource.java:26)
  613        *          at Foo3.main(Foo3.java:5)
  614        *  Suppressed: Resource$CloseFailException: Resource ID = 1
  615        *          at Resource.close(Resource.java:26)
  616        *          at Foo3.main(Foo3.java:5)
  617        * Caused by: java.lang.Exception: I did it
  618        *  at Foo3.main(Foo3.java:8)
  619        * </pre>
  620        * Likewise, a suppressed exception can have a cause:
  621        * <pre>
  622        * Exception in thread "main" java.lang.Exception: Main block
  623        *  at Foo4.main(Foo4.java:6)
  624        *  Suppressed: Resource2$CloseFailException: Resource ID = 1
  625        *          at Resource2.close(Resource2.java:20)
  626        *          at Foo4.main(Foo4.java:5)
  627        *  Caused by: java.lang.Exception: Rats, you caught me
  628        *          at Resource2$CloseFailException.<init>(Resource2.java:45)
  629        *          ... 2 more
  630        * </pre>
  631        */
  632       public void printStackTrace() {
  633           printStackTrace(System.err);
  634       }
  635   
  636       /**
  637        * Prints this throwable and its backtrace to the specified print stream.
  638        *
  639        * @param s {@code PrintStream} to use for output
  640        */
  641       public void printStackTrace(PrintStream s) {
  642           printStackTrace(new WrappedPrintStream(s));
  643       }
  644   
  645       private void printStackTrace(PrintStreamOrWriter s) {
  646           // Guard against malicious overrides of Throwable.equals by
  647           // using a Set with identity equality semantics.
  648           Set<Throwable> dejaVu =
  649               Collections.newSetFromMap(new IdentityHashMap<Throwable, Boolean>());
  650           dejaVu.add(this);
  651   
  652           synchronized (s.lock()) {
  653               // Print our stack trace
  654               s.println(this);
  655               StackTraceElement[] trace = getOurStackTrace();
  656               for (StackTraceElement traceElement : trace)
  657                   s.println("\tat " + traceElement);
  658   
  659               // Print suppressed exceptions, if any
  660               for (Throwable se : getSuppressed())
  661                   se.printEnclosedStackTrace(s, trace, SUPPRESSED_CAPTION, "\t", dejaVu);
  662   
  663               // Print cause, if any
  664               Throwable ourCause = getCause();
  665               if (ourCause != null)
  666                   ourCause.printEnclosedStackTrace(s, trace, CAUSE_CAPTION, "", dejaVu);
  667           }
  668       }
  669   
  670       /**
  671        * Print our stack trace as an enclosed exception for the specified
  672        * stack trace.
  673        */
  674       private void printEnclosedStackTrace(PrintStreamOrWriter s,
  675                                            StackTraceElement[] enclosingTrace,
  676                                            String caption,
  677                                            String prefix,
  678                                            Set<Throwable> dejaVu) {
  679           assert Thread.holdsLock(s.lock());
  680           if (dejaVu.contains(this)) {
  681               s.println("\t[CIRCULAR REFERENCE:" + this + "]");
  682           } else {
  683               dejaVu.add(this);
  684               // Compute number of frames in common between this and enclosing trace
  685               StackTraceElement[] trace = getOurStackTrace();
  686               int m = trace.length - 1;
  687               int n = enclosingTrace.length - 1;
  688               while (m >= 0 && n >=0 && trace[m].equals(enclosingTrace[n])) {
  689                   m--; n--;
  690               }
  691               int framesInCommon = trace.length - 1 - m;
  692   
  693               // Print our stack trace
  694               s.println(prefix + caption + this);
  695               for (int i = 0; i <= m; i++)
  696                   s.println(prefix + "\tat " + trace[i]);
  697               if (framesInCommon != 0)
  698                   s.println(prefix + "\t... " + framesInCommon + " more");
  699   
  700               // Print suppressed exceptions, if any
  701               for (Throwable se : getSuppressed())
  702                   se.printEnclosedStackTrace(s, trace, SUPPRESSED_CAPTION,
  703                                              prefix +"\t", dejaVu);
  704   
  705               // Print cause, if any
  706               Throwable ourCause = getCause();
  707               if (ourCause != null)
  708                   ourCause.printEnclosedStackTrace(s, trace, CAUSE_CAPTION, prefix, dejaVu);
  709           }
  710       }
  711   
  712       /**
  713        * Prints this throwable and its backtrace to the specified
  714        * print writer.
  715        *
  716        * @param s {@code PrintWriter} to use for output
  717        * @since   JDK1.1
  718        */
  719       public void printStackTrace(PrintWriter s) {
  720           printStackTrace(new WrappedPrintWriter(s));
  721       }
  722   
  723       /**
  724        * Wrapper class for PrintStream and PrintWriter to enable a single
  725        * implementation of printStackTrace.
  726        */
  727       private abstract static class PrintStreamOrWriter {
  728           /** Returns the object to be locked when using this StreamOrWriter */
  729           abstract Object lock();
  730   
  731           /** Prints the specified string as a line on this StreamOrWriter */
  732           abstract void println(Object o);
  733       }
  734   
  735       private static class WrappedPrintStream extends PrintStreamOrWriter {
  736           private final PrintStream printStream;
  737   
  738           WrappedPrintStream(PrintStream printStream) {
  739               this.printStream = printStream;
  740           }
  741   
  742           Object lock() {
  743               return printStream;
  744           }
  745   
  746           void println(Object o) {
  747               printStream.println(o);
  748           }
  749       }
  750   
  751       private static class WrappedPrintWriter extends PrintStreamOrWriter {
  752           private final PrintWriter printWriter;
  753   
  754           WrappedPrintWriter(PrintWriter printWriter) {
  755               this.printWriter = printWriter;
  756           }
  757   
  758           Object lock() {
  759               return printWriter;
  760           }
  761   
  762           void println(Object o) {
  763               printWriter.println(o);
  764           }
  765       }
  766   
  767       /**
  768        * Fills in the execution stack trace. This method records within this
  769        * {@code Throwable} object information about the current state of
  770        * the stack frames for the current thread.
  771        *
  772        * <p>If the stack trace of this {@code Throwable} {@linkplain
  773        * Throwable#Throwable(String, Throwable, boolean, boolean) is not
  774        * writable}, calling this method has no effect.
  775        *
  776        * @return  a reference to this {@code Throwable} instance.
  777        * @see     java.lang.Throwable#printStackTrace()
  778        */
  779       public synchronized Throwable fillInStackTrace() {
  780           if (stackTrace != null ||
  781               backtrace != null /* Out of protocol state */ ) {
  782               fillInStackTrace(0);
  783               stackTrace = UNASSIGNED_STACK;
  784           }
  785           return this;
  786       }
  787   
  788       private native Throwable fillInStackTrace(int dummy);
  789   
  790       /**
  791        * Provides programmatic access to the stack trace information printed by
  792        * {@link #printStackTrace()}.  Returns an array of stack trace elements,
  793        * each representing one stack frame.  The zeroth element of the array
  794        * (assuming the array's length is non-zero) represents the top of the
  795        * stack, which is the last method invocation in the sequence.  Typically,
  796        * this is the point at which this throwable was created and thrown.
  797        * The last element of the array (assuming the array's length is non-zero)
  798        * represents the bottom of the stack, which is the first method invocation
  799        * in the sequence.
  800        *
  801        * <p>Some virtual machines may, under some circumstances, omit one
  802        * or more stack frames from the stack trace.  In the extreme case,
  803        * a virtual machine that has no stack trace information concerning
  804        * this throwable is permitted to return a zero-length array from this
  805        * method.  Generally speaking, the array returned by this method will
  806        * contain one element for every frame that would be printed by
  807        * {@code printStackTrace}.  Writes to the returned array do not
  808        * affect future calls to this method.
  809        *
  810        * @return an array of stack trace elements representing the stack trace
  811        *         pertaining to this throwable.
  812        * @since  1.4
  813        */
  814       public StackTraceElement[] getStackTrace() {
  815           return getOurStackTrace().clone();
  816       }
  817   
  818       private synchronized StackTraceElement[] getOurStackTrace() {
  819           // Initialize stack trace field with information from
  820           // backtrace if this is the first call to this method
  821           if (stackTrace == UNASSIGNED_STACK ||
  822               (stackTrace == null && backtrace != null) /* Out of protocol state */) {
  823               int depth = getStackTraceDepth();
  824               stackTrace = new StackTraceElement[depth];
  825               for (int i=0; i < depth; i++)
  826                   stackTrace[i] = getStackTraceElement(i);
  827           } else if (stackTrace == null) {
  828               return UNASSIGNED_STACK;
  829           }
  830           return stackTrace;
  831       }
  832   
  833       /**
  834        * Sets the stack trace elements that will be returned by
  835        * {@link #getStackTrace()} and printed by {@link #printStackTrace()}
  836        * and related methods.
  837        *
  838        * This method, which is designed for use by RPC frameworks and other
  839        * advanced systems, allows the client to override the default
  840        * stack trace that is either generated by {@link #fillInStackTrace()}
  841        * when a throwable is constructed or deserialized when a throwable is
  842        * read from a serialization stream.
  843        *
  844        * <p>If the stack trace of this {@code Throwable} {@linkplain
  845        * Throwable#Throwable(String, Throwable, boolean, boolean) is not
  846        * writable}, calling this method has no effect other than
  847        * validating its argument.
  848        *
  849        * @param   stackTrace the stack trace elements to be associated with
  850        * this {@code Throwable}.  The specified array is copied by this
  851        * call; changes in the specified array after the method invocation
  852        * returns will have no affect on this {@code Throwable}'s stack
  853        * trace.
  854        *
  855        * @throws NullPointerException if {@code stackTrace} is
  856        *         {@code null} or if any of the elements of
  857        *         {@code stackTrace} are {@code null}
  858        *
  859        * @since  1.4
  860        */
  861       public void setStackTrace(StackTraceElement[] stackTrace) {
  862           // Validate argument
  863           StackTraceElement[] defensiveCopy = stackTrace.clone();
  864           for (int i = 0; i < defensiveCopy.length; i++) {
  865               if (defensiveCopy[i] == null)
  866                   throw new NullPointerException("stackTrace[" + i + "]");
  867           }
  868   
  869           synchronized (this) {
  870               if (this.stackTrace == null && // Immutable stack
  871                   backtrace == null) // Test for out of protocol state
  872                   return;
  873               this.stackTrace = defensiveCopy;
  874           }
  875       }
  876   
  877       /**
  878        * Returns the number of elements in the stack trace (or 0 if the stack
  879        * trace is unavailable).
  880        *
  881        * package-protection for use by SharedSecrets.
  882        */
  883       native int getStackTraceDepth();
  884   
  885       /**
  886        * Returns the specified element of the stack trace.
  887        *
  888        * package-protection for use by SharedSecrets.
  889        *
  890        * @param index index of the element to return.
  891        * @throws IndexOutOfBoundsException if {@code index < 0 ||
  892        *         index >= getStackTraceDepth() }
  893        */
  894       native StackTraceElement getStackTraceElement(int index);
  895   
  896       /**
  897        * Reads a {@code Throwable} from a stream, enforcing
  898        * well-formedness constraints on fields.  Null entries and
  899        * self-pointers are not allowed in the list of {@code
  900        * suppressedExceptions}.  Null entries are not allowed for stack
  901        * trace elements.  A null stack trace in the serial form results
  902        * in a zero-length stack element array. A single-element stack
  903        * trace whose entry is equal to {@code new StackTraceElement("",
  904        * "", null, Integer.MIN_VALUE)} results in a {@code null} {@code
  905        * stackTrace} field.
  906        *
  907        * Note that there are no constraints on the value the {@code
  908        * cause} field can hold; both {@code null} and {@code this} are
  909        * valid values for the field.
  910        */
  911       private void readObject(ObjectInputStream s)
  912           throws IOException, ClassNotFoundException {
  913           s.defaultReadObject();     // read in all fields
  914           if (suppressedExceptions != null) {
  915               List<Throwable> suppressed = null;
  916               if (suppressedExceptions.isEmpty()) {
  917                   // Use the sentinel for a zero-length list
  918                   suppressed = SUPPRESSED_SENTINEL;
  919               } else { // Copy Throwables to new list
  920                   suppressed = new ArrayList<>(1);
  921                   for (Throwable t : suppressedExceptions) {
  922                       // Enforce constraints on suppressed exceptions in
  923                       // case of corrupt or malicious stream.
  924                       if (t == null)
  925                           throw new NullPointerException(NULL_CAUSE_MESSAGE);
  926                       if (t == this)
  927                           throw new IllegalArgumentException(SELF_SUPPRESSION_MESSAGE);
  928                       suppressed.add(t);
  929                   }
  930               }
  931               suppressedExceptions = suppressed;
  932           } // else a null suppressedExceptions field remains null
  933   
  934           /*
  935            * For zero-length stack traces, use a clone of
  936            * UNASSIGNED_STACK rather than UNASSIGNED_STACK itself to
  937            * allow identity comparison against UNASSIGNED_STACK in
  938            * getOurStackTrace.  The identity of UNASSIGNED_STACK in
  939            * stackTrace indicates to the getOurStackTrace method that
  940            * the stackTrace needs to be constructed from the information
  941            * in backtrace.
  942            */
  943           if (stackTrace != null) {
  944               if (stackTrace.length == 0) {
  945                   stackTrace = UNASSIGNED_STACK.clone();
  946               }  else if (stackTrace.length == 1 &&
  947                           // Check for the marker of an immutable stack trace
  948                           SentinelHolder.STACK_TRACE_ELEMENT_SENTINEL.equals(stackTrace[0])) {
  949                   stackTrace = null;
  950               } else { // Verify stack trace elements are non-null.
  951                   for(StackTraceElement ste : stackTrace) {
  952                       if (ste == null)
  953                           throw new NullPointerException("null StackTraceElement in serial stream. ");
  954                   }
  955               }
  956           } else {
  957               // A null stackTrace field in the serial form can result
  958               // from an exception serialized without that field in
  959               // older JDK releases; treat such exceptions as having
  960               // empty stack traces.
  961               stackTrace = UNASSIGNED_STACK.clone();
  962           }
  963       }
  964   
  965       /**
  966        * Write a {@code Throwable} object to a stream.
  967        *
  968        * A {@code null} stack trace field is represented in the serial
  969        * form as a one-element array whose element is equal to {@code
  970        * new StackTraceElement("", "", null, Integer.MIN_VALUE)}.
  971        */
  972       private synchronized void writeObject(ObjectOutputStream s)
  973           throws IOException {
  974           // Ensure that the stackTrace field is initialized to a
  975           // non-null value, if appropriate.  As of JDK 7, a null stack
  976           // trace field is a valid value indicating the stack trace
  977           // should not be set.
  978           getOurStackTrace();
  979   
  980           StackTraceElement[] oldStackTrace = stackTrace;
  981           try {
  982               if (stackTrace == null)
  983                   stackTrace = SentinelHolder.STACK_TRACE_SENTINEL;
  984               s.defaultWriteObject();
  985           } finally {
  986               stackTrace = oldStackTrace;
  987           }
  988       }
  989   
  990       /**
  991        * Appends the specified exception to the exceptions that were
  992        * suppressed in order to deliver this exception. This method is
  993        * thread-safe and typically called (automatically and implicitly)
  994        * by the {@code try}-with-resources statement.
  995        *
  996        * <p>The suppression behavior is enabled <em>unless</em> disabled
  997        * {@linkplain #Throwable(String, Throwable, boolean, boolean) via
  998        * a constructor}.  When suppression is disabled, this method does
  999        * nothing other than to validate its argument.
 1000        *
 1001        * <p>Note that when one exception {@linkplain
 1002        * #initCause(Throwable) causes} another exception, the first
 1003        * exception is usually caught and then the second exception is
 1004        * thrown in response.  In other words, there is a causal
 1005        * connection between the two exceptions.
 1006        *
 1007        * In contrast, there are situations where two independent
 1008        * exceptions can be thrown in sibling code blocks, in particular
 1009        * in the {@code try} block of a {@code try}-with-resources
 1010        * statement and the compiler-generated {@code finally} block
 1011        * which closes the resource.
 1012        *
 1013        * In these situations, only one of the thrown exceptions can be
 1014        * propagated.  In the {@code try}-with-resources statement, when
 1015        * there are two such exceptions, the exception originating from
 1016        * the {@code try} block is propagated and the exception from the
 1017        * {@code finally} block is added to the list of exceptions
 1018        * suppressed by the exception from the {@code try} block.  As an
 1019        * exception unwinds the stack, it can accumulate multiple
 1020        * suppressed exceptions.
 1021        *
 1022        * <p>An exception may have suppressed exceptions while also being
 1023        * caused by another exception.  Whether or not an exception has a
 1024        * cause is semantically known at the time of its creation, unlike
 1025        * whether or not an exception will suppress other exceptions
 1026        * which is typically only determined after an exception is
 1027        * thrown.
 1028        *
 1029        * <p>Note that programmer written code is also able to take
 1030        * advantage of calling this method in situations where there are
 1031        * multiple sibling exceptions and only one can be propagated.
 1032        *
 1033        * @param exception the exception to be added to the list of
 1034        *        suppressed exceptions
 1035        * @throws IllegalArgumentException if {@code exception} is this
 1036        *         throwable; a throwable cannot suppress itself.
 1037        * @throws NullPointerException if {@code exception} is {@code null}
 1038        * @since 1.7
 1039        */
 1040       public final synchronized void addSuppressed(Throwable exception) {
 1041           if (exception == this)
 1042               throw new IllegalArgumentException(SELF_SUPPRESSION_MESSAGE);
 1043   
 1044           if (exception == null)
 1045               throw new NullPointerException(NULL_CAUSE_MESSAGE);
 1046   
 1047           if (suppressedExceptions == null) // Suppressed exceptions not recorded
 1048               return;
 1049   
 1050           if (suppressedExceptions == SUPPRESSED_SENTINEL)
 1051               suppressedExceptions = new ArrayList<>(1);
 1052   
 1053           suppressedExceptions.add(exception);
 1054       }
 1055   
 1056       private static final Throwable[] EMPTY_THROWABLE_ARRAY = new Throwable[0];
 1057   
 1058       /**
 1059        * Returns an array containing all of the exceptions that were
 1060        * suppressed, typically by the {@code try}-with-resources
 1061        * statement, in order to deliver this exception.
 1062        *
 1063        * If no exceptions were suppressed or {@linkplain
 1064        * #Throwable(String, Throwable, boolean, boolean) suppression is
 1065        * disabled}, an empty array is returned.  This method is
 1066        * thread-safe.  Writes to the returned array do not affect future
 1067        * calls to this method.
 1068        *
 1069        * @return an array containing all of the exceptions that were
 1070        *         suppressed to deliver this exception.
 1071        * @since 1.7
 1072        */
 1073       public final synchronized Throwable[] getSuppressed() {
 1074           if (suppressedExceptions == SUPPRESSED_SENTINEL ||
 1075               suppressedExceptions == null)
 1076               return EMPTY_THROWABLE_ARRAY;
 1077           else
 1078               return suppressedExceptions.toArray(EMPTY_THROWABLE_ARRAY);
 1079       }
 1080   }

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