/*
 * Copyright (c) 1994, 2016, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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 */

package java.lang;

import java.io.*;
import java.util.*;

/**
 * The {@code Throwable} class is the superclass of all errors and
 * exceptions in the Java language. Only objects that are instances of this
 * class (or one of its subclasses) are thrown by the Java Virtual Machine or
 * can be thrown by the Java {@code throw} statement. Similarly, only
 * this class or one of its subclasses can be the argument type in a
 * {@code catch} clause.
 *
 * For the purposes of compile-time checking of exceptions, {@code
 * Throwable} and any subclass of {@code Throwable} that is not also a
 * subclass of either {@link RuntimeException} or {@link Error} are
 * regarded as checked exceptions.
 *
 * <p>Instances of two subclasses, {@link java.lang.Error} and
 * {@link java.lang.Exception}, are conventionally used to indicate
 * that exceptional situations have occurred. Typically, these instances
 * are freshly created in the context of the exceptional situation so
 * as to include relevant information (such as stack trace data).
 *
 * <p>A throwable contains a snapshot of the execution stack of its
 * thread at the time it was created. It can also contain a message
 * string that gives more information about the error. Over time, a
 * throwable can {@linkplain Throwable#addSuppressed suppress} other
 * throwables from being propagated. Finally, the throwable can also
 * contain a <i>cause</i>: another throwable that caused this
 * throwable to be constructed. The recording of this causal information
 * is referred to as the <i>chained exception</i> facility, as the
 * cause can, itself, have a cause, and so on, leading to a "chain" of
 * exceptions, each caused by another.
 *
 * <p>One reason that a throwable may have a cause is that the class that
 * throws it is built atop a lower layered abstraction, and an operation on
 * the upper layer fails due to a failure in the lower layer. It would be bad
 * design to let the throwable thrown by the lower layer propagate outward, as
 * it is generally unrelated to the abstraction provided by the upper layer.
 * Further, doing so would tie the API of the upper layer to the details of
 * its implementation, assuming the lower layer's exception was a checked
 * exception. Throwing a "wrapped exception" (i.e., an exception containing a
 * cause) allows the upper layer to communicate the details of the failure to
 * its caller without incurring either of these shortcomings. It preserves
 * the flexibility to change the implementation of the upper layer without
 * changing its API (in particular, the set of exceptions thrown by its
 * methods).
 *
 * <p>A second reason that a throwable may have a cause is that the method
 * that throws it must conform to a general-purpose interface that does not
 * permit the method to throw the cause directly. For example, suppose
 * a persistent collection conforms to the {@link java.util.Collection
 * Collection} interface, and that its persistence is implemented atop
 * {@code java.io}. Suppose the internals of the {@code add} method
 * can throw an {@link java.io.IOException IOException}. The implementation
 * can communicate the details of the {@code IOException} to its caller
 * while conforming to the {@code Collection} interface by wrapping the
 * {@code IOException} in an appropriate unchecked exception. (The
 * specification for the persistent collection should indicate that it is
 * capable of throwing such exceptions.)
 *
 * <p>A cause can be associated with a throwable in two ways: via a
 * constructor that takes the cause as an argument, or via the
 * {@link #initCause(Throwable)} method. New throwable classes that
 * wish to allow causes to be associated with them should provide constructors
 * that take a cause and delegate (perhaps indirectly) to one of the
 * {@code Throwable} constructors that takes a cause.
 *
 * Because the {@code initCause} method is public, it allows a cause to be
 * associated with any throwable, even a "legacy throwable" whose
 * implementation predates the addition of the exception chaining mechanism to
 * {@code Throwable}.
 *
 * <p>By convention, class {@code Throwable} and its subclasses have two
 * constructors, one that takes no arguments and one that takes a
 * {@code String} argument that can be used to produce a detail message.
 * Further, those subclasses that might likely have a cause associated with
 * them should have two more constructors, one that takes a
 * {@code Throwable} (the cause), and one that takes a
 * {@code String} (the detail message) and a {@code Throwable} (the
 * cause).
 *
 * @author unascribed
 * @author Josh Bloch (Added exception chaining and programmatic access to
 * stack trace in 1.4.)
 * @jls 11.2 Compile-Time Checking of Exceptions
 * @since 1.0
 */
public class Throwable implements Serializable {
 /** use serialVersionUID from JDK 1.0.2 for interoperability */
 private static final long serialVersionUID = -3042686055658047285L;

 /**
 * The JVM saves some indication of the stack backtrace in this slot.
 */
 private transient Object backtrace;

 /**
 * Specific details about the Throwable. For example, for
 * {@code FileNotFoundException}, this contains the name of
 * the file that could not be found.
 *
 * @serial
 */
 private String detailMessage;


 /**
 * Holder class to defer initializing sentinel objects only used
 * for serialization.
 */
 private static class SentinelHolder {
 /**
 * {@linkplain #setStackTrace(StackTraceElement[]) Setting the
 * stack trace} to a one-element array containing this sentinel
 * value indicates future attempts to set the stack trace will be
 * ignored. The sentinel is equal to the result of calling:<br>
 * {@code new StackTraceElement("", "", null, Integer.MIN_VALUE)}
 */
 public static final StackTraceElement STACK_TRACE_ELEMENT_SENTINEL =
 new StackTraceElement("", "", null, Integer.MIN_VALUE);

 /**
 * Sentinel value used in the serial form to indicate an immutable
 * stack trace.
 */
 public static final StackTraceElement[] STACK_TRACE_SENTINEL =
 new StackTraceElement[] {STACK_TRACE_ELEMENT_SENTINEL};
 }

 /**
 * A shared value for an empty stack.
 */
 private static final StackTraceElement[] UNASSIGNED_STACK = new StackTraceElement[0];

 /*
 * To allow Throwable objects to be made immutable and safely
 * reused by the JVM, such as OutOfMemoryErrors, fields of
 * Throwable that are writable in response to user actions, cause,
 * stackTrace, and suppressedExceptions obey the following
 * protocol:
 *
 * 1) The fields are initialized to a non-null sentinel value
 * which indicates the value has logically not been set.
 *
 * 2) Writing a null to the field indicates further writes
 * are forbidden
 *
 * 3) The sentinel value may be replaced with another non-null
 * value.
 *
 * For example, implementations of the HotSpot JVM have
 * preallocated OutOfMemoryError objects to provide for better
 * diagnosability of that situation. These objects are created
 * without calling the constructor for that class and the fields
 * in question are initialized to null. To support this
 * capability, any new fields added to Throwable that require
 * being initialized to a non-null value require a coordinated JVM
 * change.
 */

 /**
 * The throwable that caused this throwable to get thrown, or null if this
 * throwable was not caused by another throwable, or if the causative
 * throwable is unknown. If this field is equal to this throwable itself,
 * it indicates that the cause of this throwable has not yet been
 * initialized.
 *
 * @serial
 * @since 1.4
 */
 private Throwable cause = this;

 /**
 * The stack trace, as returned by {@link #getStackTrace()}.
 *
 * The field is initialized to a zero-length array. A {@code
 * null} value of this field indicates subsequent calls to {@link
 * #setStackTrace(StackTraceElement[])} and {@link
 * #fillInStackTrace()} will be no-ops.
 *
 * @serial
 * @since 1.4
 */
 private StackTraceElement[] stackTrace = UNASSIGNED_STACK;

 /**
 * The JVM code sets the depth of the backtrace for later retrieval
 */
 private transient int depth;

 // Setting this static field introduces an acceptable
 // initialization dependency on a few java.util classes.
 private static final List<Throwable> SUPPRESSED_SENTINEL = Collections.emptyList();

 /**
 * The list of suppressed exceptions, as returned by {@link
 * #getSuppressed()}. The list is initialized to a zero-element
 * unmodifiable sentinel list. When a serialized Throwable is
 * read in, if the {@code suppressedExceptions} field points to a
 * zero-element list, the field is reset to the sentinel value.
 *
 * @serial
 * @since 1.7
 */
 private List<Throwable> suppressedExceptions = SUPPRESSED_SENTINEL;

 /** Message for trying to suppress a null exception. */
 private static final String NULL_CAUSE_MESSAGE = "Cannot suppress a null exception.";

 /** Message for trying to suppress oneself. */
 private static final String SELF_SUPPRESSION_MESSAGE = "Self-suppression not permitted";

 /** Caption for labeling causative exception stack traces */
 private static final String CAUSE_CAPTION = "Caused by: ";

 /** Caption for labeling suppressed exception stack traces */
 private static final String SUPPRESSED_CAPTION = "Suppressed: ";

 /**
 * Constructs a new throwable with {@code null} as its detail message.
 * The cause is not initialized, and may subsequently be initialized by a
 * call to {@link #initCause}.
 *
 * <p>The {@link #fillInStackTrace()} method is called to initialize
 * the stack trace data in the newly created throwable.
 */
 public Throwable() {
 fillInStackTrace();
 }

 /**
 * Constructs a new throwable with the specified detail message. The
 * cause is not initialized, and may subsequently be initialized by
 * a call to {@link #initCause}.
 *
 * <p>The {@link #fillInStackTrace()} method is called to initialize
 * the stack trace data in the newly created throwable.
 *
 * @param message the detail message. The detail message is saved for
 * later retrieval by the {@link #getMessage()} method.
 */
 public Throwable(String message) {
 fillInStackTrace();
 detailMessage = message;
 }

 /**
 * Constructs a new throwable with the specified detail message and
 * cause. <p>Note that the detail message associated with
 * {@code cause} is <i>not</i> automatically incorporated in
 * this throwable's detail message.
 *
 * <p>The {@link #fillInStackTrace()} method is called to initialize
 * the stack trace data in the newly created throwable.
 *
 * @param message the detail message (which is saved for later retrieval
 * by the {@link #getMessage()} method).
 * @param cause the cause (which is saved for later retrieval by the
 * {@link #getCause()} method). (A {@code null} value is
 * permitted, and indicates that the cause is nonexistent or
 * unknown.)
 * @since 1.4
 */
 public Throwable(String message, Throwable cause) {
 fillInStackTrace();
 detailMessage = message;
 this.cause = cause;
 }

 /**
 * Constructs a new throwable with the specified cause and a detail
 * message of {@code (cause==null ? null : cause.toString())} (which
 * typically contains the class and detail message of {@code cause}).
 * This constructor is useful for throwables that are little more than
 * wrappers for other throwables (for example, {@link
 * java.security.PrivilegedActionException}).
 *
 * <p>The {@link #fillInStackTrace()} method is called to initialize
 * the stack trace data in the newly created throwable.
 *
 * @param cause the cause (which is saved for later retrieval by the
 * {@link #getCause()} method). (A {@code null} value is
 * permitted, and indicates that the cause is nonexistent or
 * unknown.)
 * @since 1.4
 */
 public Throwable(Throwable cause) {
 fillInStackTrace();
 detailMessage = (cause==null ? null : cause.toString());
 this.cause = cause;
 }

 /**
 * Constructs a new throwable with the specified detail message,
 * cause, {@linkplain #addSuppressed suppression} enabled or
 * disabled, and writable stack trace enabled or disabled. If
 * suppression is disabled, {@link #getSuppressed} for this object
 * will return a zero-length array and calls to {@link
 * #addSuppressed} that would otherwise append an exception to the
 * suppressed list will have no effect. If the writable stack
 * trace is false, this constructor will not call {@link
 * #fillInStackTrace()}, a {@code null} will be written to the
 * {@code stackTrace} field, and subsequent calls to {@code
 * fillInStackTrace} and {@link
 * #setStackTrace(StackTraceElement[])} will not set the stack
 * trace. If the writable stack trace is false, {@link
 * #getStackTrace} will return a zero length array.
 *
 * <p>Note that the other constructors of {@code Throwable} treat
 * suppression as being enabled and the stack trace as being
 * writable. Subclasses of {@code Throwable} should document any
 * conditions under which suppression is disabled and document
 * conditions under which the stack trace is not writable.
 * Disabling of suppression should only occur in exceptional
 * circumstances where special requirements exist, such as a
 * virtual machine reusing exception objects under low-memory
 * situations. Circumstances where a given exception object is
 * repeatedly caught and rethrown, such as to implement control
 * flow between two sub-systems, is another situation where
 * immutable throwable objects would be appropriate.
 *
 * @param message the detail message.
 * @param cause the cause. (A {@code null} value is permitted,
 * and indicates that the cause is nonexistent or unknown.)
 * @param enableSuppression whether or not suppression is enabled or disabled
 * @param writableStackTrace whether or not the stack trace should be
 * writable
 *
 * @see OutOfMemoryError
 * @see NullPointerException
 * @see ArithmeticException
 * @since 1.7
 */
 protected Throwable(String message, Throwable cause,
 boolean enableSuppression,
 boolean writableStackTrace) {
 if (writableStackTrace) {
 fillInStackTrace();
 } else {
 stackTrace = null;
 }
 detailMessage = message;
 this.cause = cause;
 if (!enableSuppression)
 suppressedExceptions = null;
 }

 /**
 * Returns the detail message string of this throwable.
 *
 * @return the detail message string of this {@code Throwable} instance
 * (which may be {@code null}).
 */
 public String getMessage() {
 return detailMessage;
 }

 /**
 * Creates a localized description of this throwable.
 * Subclasses may override this method in order to produce a
 * locale-specific message. For subclasses that do not override this
 * method, the default implementation returns the same result as
 * {@code getMessage()}.
 *
 * @return The localized description of this throwable.
 * @since 1.1
 */
 public String getLocalizedMessage() {
 return getMessage();
 }

 /**
 * Returns the cause of this throwable or {@code null} if the
 * cause is nonexistent or unknown. (The cause is the throwable that
 * caused this throwable to get thrown.)
 *
 * <p>This implementation returns the cause that was supplied via one of
 * the constructors requiring a {@code Throwable}, or that was set after
 * creation with the {@link #initCause(Throwable)} method. While it is
 * typically unnecessary to override this method, a subclass can override
 * it to return a cause set by some other means. This is appropriate for
 * a "legacy chained throwable" that predates the addition of chained
 * exceptions to {@code Throwable}. Note that it is <i>not</i>
 * necessary to override any of the {@code PrintStackTrace} methods,
 * all of which invoke the {@code getCause} method to determine the
 * cause of a throwable.
 *
 * @return the cause of this throwable or {@code null} if the
 * cause is nonexistent or unknown.
 * @since 1.4
 */
 public synchronized Throwable getCause() {
 return (cause==this ? null : cause);
 }

 /**
 * Initializes the <i>cause</i> of this throwable to the specified value.
 * (The cause is the throwable that caused this throwable to get thrown.)
 *
 * <p>This method can be called at most once. It is generally called from
 * within the constructor, or immediately after creating the
 * throwable. If this throwable was created
 * with {@link #Throwable(Throwable)} or
 * {@link #Throwable(String,Throwable)}, this method cannot be called
 * even once.
 *
 * <p>An example of using this method on a legacy throwable type
 * without other support for setting the cause is:
 *
 * <pre>
 * try {
 * lowLevelOp();
 * } catch (LowLevelException le) {
 * throw (HighLevelException)
 * new HighLevelException().initCause(le); // Legacy constructor
 * }
 * </pre>
 *
 * @param cause the cause (which is saved for later retrieval by the
 * {@link #getCause()} method). (A {@code null} value is
 * permitted, and indicates that the cause is nonexistent or
 * unknown.)
 * @return a reference to this {@code Throwable} instance.
 * @throws IllegalArgumentException if {@code cause} is this
 * throwable. (A throwable cannot be its own cause.)
 * @throws IllegalStateException if this throwable was
 * created with {@link #Throwable(Throwable)} or
 * {@link #Throwable(String,Throwable)}, or this method has already
 * been called on this throwable.
 * @since 1.4
 */
 public synchronized Throwable initCause(Throwable cause) {
 if (this.cause != this)
 throw new IllegalStateException("Can't overwrite cause with " +
 Objects.toString(cause, "a null"), this);
 if (cause == this)
 throw new IllegalArgumentException("Self-causation not permitted", this);
 this.cause = cause;
 return this;
 }

 /**
 * Returns a short description of this throwable.
 * The result is the concatenation of:
 * <ul>
 * <li> the {@linkplain Class#getName() name} of the class of this object
 * <li> ": " (a colon and a space)
 * <li> the result of invoking this object's {@link #getLocalizedMessage}
 * method
 * </ul>
 * If {@code getLocalizedMessage} returns {@code null}, then just
 * the class name is returned.
 *
 * @return a string representation of this throwable.
 */
 public String toString() {
 String s = getClass().getName();
 String message = getLocalizedMessage();
 return (message != null) ? (s + ": " + message) : s;
 }

 /**
 * Prints this throwable and its backtrace to the
 * standard error stream. This method prints a stack trace for this
 * {@code Throwable} object on the error output stream that is
 * the value of the field {@code System.err}. The first line of
 * output contains the result of the {@link #toString()} method for
 * this object. Remaining lines represent data previously recorded by
 * the method {@link #fillInStackTrace()}. The format of this
 * information depends on the implementation, but the following
 * example may be regarded as typical:
 * <blockquote><pre>
 * java.lang.NullPointerException
 * at MyClass.mash(MyClass.java:9)
 * at MyClass.crunch(MyClass.java:6)
 * at MyClass.main(MyClass.java:3)
 * </pre></blockquote>
 * This example was produced by running the program:
 * <pre>
 * class MyClass {
 * public static void main(String[] args) {
 * crunch(null);
 * }
 * static void crunch(int[] a) {
 * mash(a);
 * }
 * static void mash(int[] b) {
 * System.out.println(b[0]);
 * }
 * }
 * </pre>
 * The backtrace for a throwable with an initialized, non-null cause
 * should generally include the backtrace for the cause. The format
 * of this information depends on the implementation, but the following
 * example may be regarded as typical:
 * <pre>
 * HighLevelException: MidLevelException: LowLevelException
 * at Junk.a(Junk.java:13)
 * at Junk.main(Junk.java:4)
 * Caused by: MidLevelException: LowLevelException
 * at Junk.c(Junk.java:23)
 * at Junk.b(Junk.java:17)
 * at Junk.a(Junk.java:11)
 * ... 1 more
 * Caused by: LowLevelException
 * at Junk.e(Junk.java:30)
 * at Junk.d(Junk.java:27)
 * at Junk.c(Junk.java:21)
 * ... 3 more
 * </pre>
 * Note the presence of lines containing the characters {@code "..."}.
 * These lines indicate that the remainder of the stack trace for this
 * exception matches the indicated number of frames from the bottom of the
 * stack trace of the exception that was caused by this exception (the
 * "enclosing" exception). This shorthand can greatly reduce the length
 * of the output in the common case where a wrapped exception is thrown
 * from same method as the "causative exception" is caught. The above
 * example was produced by running the program:
 * <pre>
 * public class Junk {
 * public static void main(String args[]) {
 * try {
 * a();
 * } catch(HighLevelException e) {
 * e.printStackTrace();
 * }
 * }
 * static void a() throws HighLevelException {
 * try {
 * b();
 * } catch(MidLevelException e) {
 * throw new HighLevelException(e);
 * }
 * }
 * static void b() throws MidLevelException {
 * c();
 * }
 * static void c() throws MidLevelException {
 * try {
 * d();
 * } catch(LowLevelException e) {
 * throw new MidLevelException(e);
 * }
 * }
 * static void d() throws LowLevelException {
 * e();
 * }
 * static void e() throws LowLevelException {
 * throw new LowLevelException();
 * }
 * }
 *
 * class HighLevelException extends Exception {
 * HighLevelException(Throwable cause) { super(cause); }
 * }
 *
 * class MidLevelException extends Exception {
 * MidLevelException(Throwable cause) { super(cause); }
 * }
 *
 * class LowLevelException extends Exception {
 * }
 * </pre>
 * As of release 7, the platform supports the notion of
 * <i>suppressed exceptions</i> (in conjunction with the {@code
 * try}-with-resources statement). Any exceptions that were
 * suppressed in order to deliver an exception are printed out
 * beneath the stack trace. The format of this information
 * depends on the implementation, but the following example may be
 * regarded as typical:
 *
 * <pre>
 * Exception in thread "main" java.lang.Exception: Something happened
 * at Foo.bar(Foo.java:10)
 * at Foo.main(Foo.java:5)
 * Suppressed: Resource$CloseFailException: Resource ID = 0
 * at Resource.close(Resource.java:26)
 * at Foo.bar(Foo.java:9)
 * ... 1 more
 * </pre>
 * Note that the "... n more" notation is used on suppressed exceptions
 * just at it is used on causes. Unlike causes, suppressed exceptions are
 * indented beyond their "containing exceptions."
 *
 * <p>An exception can have both a cause and one or more suppressed
 * exceptions:
 * <pre>
 * Exception in thread "main" java.lang.Exception: Main block
 * at Foo3.main(Foo3.java:7)
 * Suppressed: Resource$CloseFailException: Resource ID = 2
 * at Resource.close(Resource.java:26)
 * at Foo3.main(Foo3.java:5)
 * Suppressed: Resource$CloseFailException: Resource ID = 1
 * at Resource.close(Resource.java:26)
 * at Foo3.main(Foo3.java:5)
 * Caused by: java.lang.Exception: I did it
 * at Foo3.main(Foo3.java:8)
 * </pre>
 * Likewise, a suppressed exception can have a cause:
 * <pre>
 * Exception in thread "main" java.lang.Exception: Main block
 * at Foo4.main(Foo4.java:6)
 * Suppressed: Resource2$CloseFailException: Resource ID = 1
 * at Resource2.close(Resource2.java:20)
 * at Foo4.main(Foo4.java:5)
 * Caused by: java.lang.Exception: Rats, you caught me
 * at Resource2$CloseFailException.&lt;init&gt;(Resource2.java:45)
 * ... 2 more
 * </pre>
 */
 public void printStackTrace() {
 printStackTrace(System.err);
 }

 /**
 * Prints this throwable and its backtrace to the specified print stream.
 *
 * @param s {@code PrintStream} to use for output
 */
 public void printStackTrace(PrintStream s) {
 printStackTrace(new WrappedPrintStream(s));
 }

 private void printStackTrace(PrintStreamOrWriter s) {
 // Guard against malicious overrides of Throwable.equals by
 // using a Set with identity equality semantics.
 Set<Throwable> dejaVu = Collections.newSetFromMap(new IdentityHashMap<>());
 dejaVu.add(this);

 synchronized (s.lock()) {
 // Print our stack trace
 s.println(this);
 StackTraceElement[] trace = getOurStackTrace();
 for (StackTraceElement traceElement : trace)
 s.println("\tat " + traceElement);

 // Print suppressed exceptions, if any
 for (Throwable se : getSuppressed())
 se.printEnclosedStackTrace(s, trace, SUPPRESSED_CAPTION, "\t", dejaVu);

 // Print cause, if any
 Throwable ourCause = getCause();
 if (ourCause != null)
 ourCause.printEnclosedStackTrace(s, trace, CAUSE_CAPTION, "", dejaVu);
 }
 }

 /**
 * Print our stack trace as an enclosed exception for the specified
 * stack trace.
 */
 private void printEnclosedStackTrace(PrintStreamOrWriter s,
 StackTraceElement[] enclosingTrace,
 String caption,
 String prefix,
 Set<Throwable> dejaVu) {
 assert Thread.holdsLock(s.lock());
 if (dejaVu.contains(this)) {
 s.println("\t[CIRCULAR REFERENCE:" + this + "]");
 } else {
 dejaVu.add(this);
 // Compute number of frames in common between this and enclosing trace
 StackTraceElement[] trace = getOurStackTrace();
 int m = trace.length - 1;
 int n = enclosingTrace.length - 1;
 while (m >= 0 && n >=0 && trace[m].equals(enclosingTrace[n])) {
 m--; n--;
 }
 int framesInCommon = trace.length - 1 - m;

 // Print our stack trace
 s.println(prefix + caption + this);
 for (int i = 0; i <= m; i++)
 s.println(prefix + "\tat " + trace[i]);
 if (framesInCommon != 0)
 s.println(prefix + "\t... " + framesInCommon + " more");

 // Print suppressed exceptions, if any
 for (Throwable se : getSuppressed())
 se.printEnclosedStackTrace(s, trace, SUPPRESSED_CAPTION,
 prefix +"\t", dejaVu);

 // Print cause, if any
 Throwable ourCause = getCause();
 if (ourCause != null)
 ourCause.printEnclosedStackTrace(s, trace, CAUSE_CAPTION, prefix, dejaVu);
 }
 }

 /**
 * Prints this throwable and its backtrace to the specified
 * print writer.
 *
 * @param s {@code PrintWriter} to use for output
 * @since 1.1
 */
 public void printStackTrace(PrintWriter s) {
 printStackTrace(new WrappedPrintWriter(s));
 }

 /**
 * Wrapper class for PrintStream and PrintWriter to enable a single
 * implementation of printStackTrace.
 */
 private abstract static class PrintStreamOrWriter {
 /** Returns the object to be locked when using this StreamOrWriter */
 abstract Object lock();

 /** Prints the specified string as a line on this StreamOrWriter */
 abstract void println(Object o);
 }

 private static class WrappedPrintStream extends PrintStreamOrWriter {
 private final PrintStream printStream;

 WrappedPrintStream(PrintStream printStream) {
 this.printStream = printStream;
 }

 Object lock() {
 return printStream;
 }

 void println(Object o) {
 printStream.println(o);
 }
 }

 private static class WrappedPrintWriter extends PrintStreamOrWriter {
 private final PrintWriter printWriter;

 WrappedPrintWriter(PrintWriter printWriter) {
 this.printWriter = printWriter;
 }

 Object lock() {
 return printWriter;
 }

 void println(Object o) {
 printWriter.println(o);
 }
 }

 /**
 * Fills in the execution stack trace. This method records within this
 * {@code Throwable} object information about the current state of
 * the stack frames for the current thread.
 *
 * <p>If the stack trace of this {@code Throwable} {@linkplain
 * Throwable#Throwable(String, Throwable, boolean, boolean) is not
 * writable}, calling this method has no effect.
 *
 * @return a reference to this {@code Throwable} instance.
 * @see java.lang.Throwable#printStackTrace()
 */
 public synchronized Throwable fillInStackTrace() {
 if (stackTrace != null ||
 backtrace != null /* Out of protocol state */ ) {
 fillInStackTrace(0);
 stackTrace = UNASSIGNED_STACK;
 }
 return this;
 }

 private native Throwable fillInStackTrace(int dummy);

 /**
 * Provides programmatic access to the stack trace information printed by
 * {@link #printStackTrace()}. Returns an array of stack trace elements,
 * each representing one stack frame. The zeroth element of the array
 * (assuming the array's length is non-zero) represents the top of the
 * stack, which is the last method invocation in the sequence. Typically,
 * this is the point at which this throwable was created and thrown.
 * The last element of the array (assuming the array's length is non-zero)
 * represents the bottom of the stack, which is the first method invocation
 * in the sequence.
 *
 * <p>Some virtual machines may, under some circumstances, omit one
 * or more stack frames from the stack trace. In the extreme case,
 * a virtual machine that has no stack trace information concerning
 * this throwable is permitted to return a zero-length array from this
 * method. Generally speaking, the array returned by this method will
 * contain one element for every frame that would be printed by
 * {@code printStackTrace}. Writes to the returned array do not
 * affect future calls to this method.
 *
 * @return an array of stack trace elements representing the stack trace
 * pertaining to this throwable.
 * @since 1.4
 */
 public StackTraceElement[] getStackTrace() {
 return getOurStackTrace().clone();
 }

 private synchronized StackTraceElement[] getOurStackTrace() {
 // Initialize stack trace field with information from
 // backtrace if this is the first call to this method
 if (stackTrace == UNASSIGNED_STACK ||
 (stackTrace == null && backtrace != null) /* Out of protocol state */) {
 stackTrace = StackTraceElement.of(this, depth);
 } else if (stackTrace == null) {
 return UNASSIGNED_STACK;
 }
 return stackTrace;
 }

 /**
 * Sets the stack trace elements that will be returned by
 * {@link #getStackTrace()} and printed by {@link #printStackTrace()}
 * and related methods.
 *
 * This method, which is designed for use by RPC frameworks and other
 * advanced systems, allows the client to override the default
 * stack trace that is either generated by {@link #fillInStackTrace()}
 * when a throwable is constructed or deserialized when a throwable is
 * read from a serialization stream.
 *
 * <p>If the stack trace of this {@code Throwable} {@linkplain
 * Throwable#Throwable(String, Throwable, boolean, boolean) is not
 * writable}, calling this method has no effect other than
 * validating its argument.
 *
 * @param stackTrace the stack trace elements to be associated with
 * this {@code Throwable}. The specified array is copied by this
 * call; changes in the specified array after the method invocation
 * returns will have no affect on this {@code Throwable}'s stack
 * trace.
 *
 * @throws NullPointerException if {@code stackTrace} is
 * {@code null} or if any of the elements of
 * {@code stackTrace} are {@code null}
 *
 * @since 1.4
 */
 public void setStackTrace(StackTraceElement[] stackTrace) {
 // Validate argument
 StackTraceElement[] defensiveCopy = stackTrace.clone();
 for (int i = 0; i < defensiveCopy.length; i++) {
 if (defensiveCopy[i] == null)
 throw new NullPointerException("stackTrace[" + i + "]");
 }

 synchronized (this) {
 if (this.stackTrace == null && // Immutable stack
 backtrace == null) // Test for out of protocol state
 return;
 this.stackTrace = defensiveCopy;
 }
 }

 /**
 * Reads a {@code Throwable} from a stream, enforcing
 * well-formedness constraints on fields. Null entries and
 * self-pointers are not allowed in the list of {@code
 * suppressedExceptions}. Null entries are not allowed for stack
 * trace elements. A null stack trace in the serial form results
 * in a zero-length stack element array. A single-element stack
 * trace whose entry is equal to {@code new StackTraceElement("",
 * "", null, Integer.MIN_VALUE)} results in a {@code null} {@code
 * stackTrace} field.
 *
 * Note that there are no constraints on the value the {@code
 * cause} field can hold; both {@code null} and {@code this} are
 * valid values for the field.
 */
 private void readObject(ObjectInputStream s)
 throws IOException, ClassNotFoundException {
 s.defaultReadObject(); // read in all fields
 if (suppressedExceptions != null) {
 List<Throwable> suppressed = null;
 if (suppressedExceptions.isEmpty()) {
 // Use the sentinel for a zero-length list
 suppressed = SUPPRESSED_SENTINEL;
 } else { // Copy Throwables to new list
 suppressed = new ArrayList<>(1);
 for (Throwable t : suppressedExceptions) {
 // Enforce constraints on suppressed exceptions in
 // case of corrupt or malicious stream.
 if (t == null)
 throw new NullPointerException(NULL_CAUSE_MESSAGE);
 if (t == this)
 throw new IllegalArgumentException(SELF_SUPPRESSION_MESSAGE);
 suppressed.add(t);
 }
 }
 suppressedExceptions = suppressed;
 } // else a null suppressedExceptions field remains null

 /*
 * For zero-length stack traces, use a clone of
 * UNASSIGNED_STACK rather than UNASSIGNED_STACK itself to
 * allow identity comparison against UNASSIGNED_STACK in
 * getOurStackTrace. The identity of UNASSIGNED_STACK in
 * stackTrace indicates to the getOurStackTrace method that
 * the stackTrace needs to be constructed from the information
 * in backtrace.
 */
 if (stackTrace != null) {
 if (stackTrace.length == 0) {
 stackTrace = UNASSIGNED_STACK.clone();
 } else if (stackTrace.length == 1 &&
 // Check for the marker of an immutable stack trace
 SentinelHolder.STACK_TRACE_ELEMENT_SENTINEL.equals(stackTrace[0])) {
 stackTrace = null;
 } else { // Verify stack trace elements are non-null.
 for(StackTraceElement ste : stackTrace) {
 if (ste == null)
 throw new NullPointerException("null StackTraceElement in serial stream. ");
 }
 }
 } else {
 // A null stackTrace field in the serial form can result
 // from an exception serialized without that field in
 // older JDK releases; treat such exceptions as having
 // empty stack traces.
 stackTrace = UNASSIGNED_STACK.clone();
 }
 }

 /**
 * Write a {@code Throwable} object to a stream.
 *
 * A {@code null} stack trace field is represented in the serial
 * form as a one-element array whose element is equal to {@code
 * new StackTraceElement("", "", null, Integer.MIN_VALUE)}.
 */
 private synchronized void writeObject(ObjectOutputStream s)
 throws IOException {
 // Ensure that the stackTrace field is initialized to a
 // non-null value, if appropriate. As of JDK 7, a null stack
 // trace field is a valid value indicating the stack trace
 // should not be set.
 getOurStackTrace();

 StackTraceElement[] oldStackTrace = stackTrace;
 try {
 if (stackTrace == null)
 stackTrace = SentinelHolder.STACK_TRACE_SENTINEL;
 s.defaultWriteObject();
 } finally {
 stackTrace = oldStackTrace;
 }
 }

 /**
 * Appends the specified exception to the exceptions that were
 * suppressed in order to deliver this exception. This method is
 * thread-safe and typically called (automatically and implicitly)
 * by the {@code try}-with-resources statement.
 *
 * <p>The suppression behavior is enabled <em>unless</em> disabled
 * {@linkplain #Throwable(String, Throwable, boolean, boolean) via
 * a constructor}. When suppression is disabled, this method does
 * nothing other than to validate its argument.
 *
 * <p>Note that when one exception {@linkplain
 * #initCause(Throwable) causes} another exception, the first
 * exception is usually caught and then the second exception is
 * thrown in response. In other words, there is a causal
 * connection between the two exceptions.
 *
 * In contrast, there are situations where two independent
 * exceptions can be thrown in sibling code blocks, in particular
 * in the {@code try} block of a {@code try}-with-resources
 * statement and the compiler-generated {@code finally} block
 * which closes the resource.
 *
 * In these situations, only one of the thrown exceptions can be
 * propagated. In the {@code try}-with-resources statement, when
 * there are two such exceptions, the exception originating from
 * the {@code try} block is propagated and the exception from the
 * {@code finally} block is added to the list of exceptions
 * suppressed by the exception from the {@code try} block. As an
 * exception unwinds the stack, it can accumulate multiple
 * suppressed exceptions.
 *
 * <p>An exception may have suppressed exceptions while also being
 * caused by another exception. Whether or not an exception has a
 * cause is semantically known at the time of its creation, unlike
 * whether or not an exception will suppress other exceptions
 * which is typically only determined after an exception is
 * thrown.
 *
 * <p>Note that programmer written code is also able to take
 * advantage of calling this method in situations where there are
 * multiple sibling exceptions and only one can be propagated.
 *
 * @param exception the exception to be added to the list of
 * suppressed exceptions
 * @throws IllegalArgumentException if {@code exception} is this
 * throwable; a throwable cannot suppress itself.
 * @throws NullPointerException if {@code exception} is {@code null}
 * @since 1.7
 */
 public final synchronized void addSuppressed(Throwable exception) {
 if (exception == this)
 throw new IllegalArgumentException(SELF_SUPPRESSION_MESSAGE, exception);

 if (exception == null)
 throw new NullPointerException(NULL_CAUSE_MESSAGE);

 if (suppressedExceptions == null) // Suppressed exceptions not recorded
 return;

 if (suppressedExceptions == SUPPRESSED_SENTINEL)
 suppressedExceptions = new ArrayList<>(1);

 suppressedExceptions.add(exception);
 }

 private static final Throwable[] EMPTY_THROWABLE_ARRAY = new Throwable[0];

 /**
 * Returns an array containing all of the exceptions that were
 * suppressed, typically by the {@code try}-with-resources
 * statement, in order to deliver this exception.
 *
 * If no exceptions were suppressed or {@linkplain
 * #Throwable(String, Throwable, boolean, boolean) suppression is
 * disabled}, an empty array is returned. This method is
 * thread-safe. Writes to the returned array do not affect future
 * calls to this method.
 *
 * @return an array containing all of the exceptions that were
 * suppressed to deliver this exception.
 * @since 1.7
 */
 public final synchronized Throwable[] getSuppressed() {
 if (suppressedExceptions == SUPPRESSED_SENTINEL ||
 suppressedExceptions == null)
 return EMPTY_THROWABLE_ARRAY;
 else
 return suppressedExceptions.toArray(EMPTY_THROWABLE_ARRAY);
 }
}