/*

 * Copyright (c) 1994, 2011, Oracle and/or its affiliates. All rights reserved.

 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.  Oracle designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Oracle in the LICENSE file that accompanied this code.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

 * or visit www.oracle.com if you need additional information or have any

 * questions.

 */

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 JDK1.0

 */

public class Throwable implements Serializable {

    /** use serialVersionUID from JDK 1.0.2 for interoperability */

    private static final long serialVersionUID = -3042686055658047285L;

    /**

     * Native code 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 sentinal 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 be no-ops.

     *

     * @serial

     * @since 1.4

     */

    private StackTraceElement[] stackTrace = UNASSIGNED_STACK;

    // Setting this static field introduces an acceptable

    // initialization dependency on a few java.util classes.

    private static final List<Throwable> SUPPRESSED_SENTINEL =

        Collections.unmodifiableList(new ArrayList<Throwable>(0));

    /**

     * 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   JDK1.1

     */

    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");

        if (cause == this)

            throw new IllegalArgumentException("Self-causation not permitted");

        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);

     *     }