/*

 * Copyright (c) 1994, 2006, 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

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 * questions.

 */

package java.lang;

import  java.io.*;

/**

 * The <code>Throwable</code> 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</code> statement. Similarly, only

 * this class or one of its subclasses can be the argument type in a

 * <code>catch</code> 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. Finally, it can contain a <i>cause</i>:

 * another throwable that caused this throwable to get thrown.  The cause

 * facility is new in release 1.4.  It is also known 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

 * <tt>java.io</tt>.  Suppose the internals of the <tt>add</tt> method

 * can throw an {@link java.io.IOException IOException}.  The implementation

 * can communicate the details of the <tt>IOException</tt> to its caller

 * while conforming to the <tt>Collection</tt> interface by wrapping the

 * <tt>IOException</tt> 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

 * <tt>Throwable</tt> constructors that takes a cause.  For example:

 * <pre>

 *     try {

 *         lowLevelOp();

 *     } catch (LowLevelException le) {

 *         throw new HighLevelException(le);  // Chaining-aware constructor

 *     }

 * </pre>

 * Because the <tt>initCause</tt> 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

 * <tt>Throwable</tt>. For example:

 * <pre>

 *     try {

 *         lowLevelOp();

 *     } catch (LowLevelException le) {

 *         throw (HighLevelException)

 *     }

 * </pre>

 *

 * <p>Prior to release 1.4, there were many throwables that had their own

 * non-standard exception chaining mechanisms (

 * {@link ExceptionInInitializerError}, {@link ClassNotFoundException},

 * {@link java.lang.reflect.UndeclaredThrowableException},

 * {@link java.lang.reflect.InvocationTargetException},

 * {@link java.io.WriteAbortedException},

 * {@link java.security.PrivilegedActionException},

 * {@link java.awt.print.PrinterIOException},

 * {@link java.rmi.RemoteException} and

 * {@link javax.naming.NamingException}).

 * All of these throwables have been retrofitted to

 * use the standard exception chaining mechanism, while continuing to

 * implement their "legacy" chaining mechanisms for compatibility.

 *

 * <p>Further, as of release 1.4, many general purpose <tt>Throwable</tt>

 * classes (for example {@link Exception}, {@link RuntimeException},

 * {@link Error}) have been retrofitted with constructors that take

 * a cause.  This was not strictly necessary, due to the existence of the

 * <tt>initCause</tt> method, but it is more convenient and expressive to

 * delegate to a constructor that takes a cause.

 *

 * <p>By convention, class <code>Throwable</code> and its subclasses have two

 * constructors, one that takes no arguments and one that takes a

 * <code>String</code> 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</code> (the cause), and one that takes a

 * <code>String</code> (the detail message) and a <code>Throwable</code> (the

 * cause).

 *

 * <p>Also introduced in release 1.4 is the {@link #getStackTrace()} method,

 * which allows programmatic access to the stack trace information that was

 * previously available only in text form, via the various forms of the

 * {@link #printStackTrace()} method.  This information has been added to the

 * <i>serialized representation</i> of this class so <tt>getStackTrace</tt>

 * and <tt>printStackTrace</tt> will operate properly on a throwable that

 * was obtained by deserialization.

 *

 * @author  unascribed

 * @author  Josh Bloch (Added exception chaining and programmatic access to

 *          stack trace in 1.4.)

 * @jls3 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

     * <tt>FileNotFoundException</tt>, this contains the name of

     * the file that could not be found.

     *

     * @serial

     */

    private String detailMessage;

    /**

     * 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()}.

     *

     * @serial

     * @since 1.4

     */

    private StackTraceElement[] stackTrace;

    /*

     * This field is lazily initialized on first use or serialization and

     * nulled out when fillInStackTrace is called.

     */

    /**

     * Constructs a new throwable with <code>null</code> 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</code> 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 <tt>null</tt> 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 <tt>(cause==null ? null : cause.toString())</tt> (which

     * typically contains the class and detail message of <tt>cause</tt>).

     * 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 <tt>null</tt> 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;

    }

    /**

     * Returns the detail message string of this throwable.

     *

     * @return  the detail message string of this <tt>Throwable</tt> instance

     *          (which may be <tt>null</tt>).

     */

    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()</code>.

     *

     * @return  The localized description of this throwable.

     * @since   JDK1.1

     */

    public String getLocalizedMessage() {

        return getMessage();

    }

    /**

     * Returns the cause of this throwable or <code>null</code> 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 <tt>Throwable</tt>, 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 <tt>Throwable</tt>.  Note that it is <i>not</i>

     * necessary to override any of the <tt>PrintStackTrace</tt> methods,

     * all of which invoke the <tt>getCause</tt> method to determine the

     * cause of a throwable.

     *

     * @return  the cause of this throwable or <code>null</code> if the

     *          cause is nonexistent or unknown.

     * @since 1.4

     */

    public 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.

     *

     * @param  cause the cause (which is saved for later retrieval by the

     *         {@link #getCause()} method).  (A <tt>null</tt> value is

     *         permitted, and indicates that the cause is nonexistent or

     *         unknown.)

     * @return  a reference to this <code>Throwable</code> instance.

     * @throws IllegalArgumentException if <code>cause</code> 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 <tt>getLocalizedMessage</tt> returns <tt>null</tt>, 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</code> object on the error output stream that is

     * the value of the field <code>System.err</code>. 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 <tt>"..."</tt>.

     * 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>