Throwable.java

/*
 * Copyright (c) 1994, 2016, 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 1.0
 */
/*
 * 异常体系的祖先
 *
 * 异常体系：
 *                           Throwable
 *                               |
 *                    +----------+-----------+
 *                    |                      |
 *                Exception                Error
 *                    |                      |
 *        +-----------+-----------+         ...
 *        |                       |
 * RuntimeException              ...
 *        |
 *       ...
 *
 * Throwable       : 所有异常的祖先类
 * RuntimeException: 运行时异常，或称非检查异常；这类异常意味着程序出现了难以恢复的错误，允许不进行捕获
 * Exception       : 除运行时异常之外的异常，或称检查异常；这类异常意味着程序出现错误时可能是允许被恢复的，需要在编译期就捕获，否则无法编译
 * Error           : 非常严重的异常，该类异常往往意味着JVM内部出现了问题
 */
public class Throwable implements Serializable {
    
    /** use serialVersionUID from JDK 1.0.2 for interoperability */
    private static final long serialVersionUID = -3042686055658047285L;
    
    /*
     * 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.
     */
    
    /** 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: ";
    
    private static final Throwable[] EMPTY_THROWABLE_ARRAY = new Throwable[0];
    
    /** 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";
    
    /**
     * A shared value for an empty stack.
     */
    // 共享的空栈
    private static final StackTraceElement[] UNASSIGNED_STACK = new StackTraceElement[0];
    
    /** 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 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;
    
    /**
     * 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
     */
    // 异常堆栈，如果是只读堆栈，则stackTrace为null
    private StackTraceElement[] stackTrace = UNASSIGNED_STACK;
    
    /**
     * The JVM code sets the depth of the backtrace for later retrieval
     */
    private transient int depth;
    
    /**
     * 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
     */
    // 抑制异常，如果禁止该功能，则suppressedExceptions为null
    private List<Throwable> suppressedExceptions = SUPPRESSED_SENTINEL;
    
    
    
    /*▼ 构造器 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * 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;
        }
    }
    
    /*▲ 构造器 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 异常原因 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * 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
     */
    /*
     * 为当前异常设置一个触发原因
     *
     * 注：每个异常最多只能有一个触发原因
     */
    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 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
     */
    // 返回触发当前异常的"原因"，这个"原因"也是一个异常
    public synchronized Throwable getCause() {
        return (cause == this ? null : cause);
    }
    
    /*▲ 异常原因 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 抑制异常 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * 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
     */
    /*
     * 为当前异常添加一个抑制(次要)异常
     *
     * 注：这种机制弥补了"Cause"的缺陷，当存在多个待抛异常时，可以用此种方式来区分主次
     */
    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);
        }
        
        // Suppressed exceptions not recorded
        if(suppressedExceptions == null) {
            return;
        }
        
        if(suppressedExceptions == SUPPRESSED_SENTINEL) {
            suppressedExceptions = new ArrayList<>(1);
        }
        
        suppressedExceptions.add(exception);
    }
    
    /**
     * 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
     */
    // 返回当前异常的"抑制(次要)异常"
    public final synchronized Throwable[] getSuppressed() {
        if(suppressedExceptions == SUPPRESSED_SENTINEL || suppressedExceptions == null) {
            return EMPTY_THROWABLE_ARRAY;
        }
        
        return suppressedExceptions.toArray(EMPTY_THROWABLE_ARRAY);
    }
    
    /*▲ 抑制异常 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 异常消息 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Returns the detail message string of this throwable.
     *
     * @return the detail message string of this {@code Throwable} instance
     * (which may be {@code null}).
     */
    // 返回异常消息
    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
     */
    // 返回本地化的异常消息
    public String getLocalizedMessage() {
        return getMessage();
    }
    
    /*▲ 异常消息 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 异常堆栈 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * 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()
     */
    // 初始化栈帧信息
    public synchronized Throwable fillInStackTrace() {
        if(stackTrace != null || backtrace != null /* Out of protocol state */) {
            fillInStackTrace(0);
            stackTrace = UNASSIGNED_STACK;
        }
        
        return this;
    }
    
    /**
     * 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
     */
    // 设置异常堆栈
    public void setStackTrace(StackTraceElement[] stackTrace) {
        // 获取异常堆栈信息
        StackTraceElement[] defensiveCopy = stackTrace.clone();
    
        // 遍历异常堆栈，确保其中没有null值
        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;
            }
        
            // 更新异常堆栈
            this.stackTrace = defensiveCopy;
        }
    }
    
    /**
     * 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
     */
    // 返回异常堆栈
    public StackTraceElement[] getStackTrace() {
        return getOurStackTrace().clone();
    }
    
    /**
     * 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>
     */
    // 将异常栈帧打印到标准输出流
    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
     */
    // 将异常栈帧打印到指定的输出流
    public void printStackTrace(PrintStream s) {
        printStackTrace(new WrappedPrintStream(s));
    }
    
    /**
     * 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));
    }
    
    /*▲ 异常堆栈 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    /**
     * 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;
    }
    
    
    
    /*▼ 序列化 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * 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;
        }
    }
    
    /**
     * 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();
        }
    }
    
    /*▲ 序列化 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    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);
            }
        }
    }
    
    private native Throwable fillInStackTrace(int dummy);
    
    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);
            return stackTrace;
        }
        
        if(stackTrace == null) {
            return UNASSIGNED_STACK;
        }
        
        return stackTrace;
    }
    
    
    /**
     * 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};
    }
    
    /**
     * 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);
        }
    }
}