异常处理类-Throwable源码详解

package java.lang;
import java.io.*;
/**
 *
 * Throwable是所有Error和Exceptiong的父类
 * 注意它有四个构造函数:
 * Throwable()
 * Throwable(String message)
 * Throwable(Throwable cause)
 * Throwable(String message, Throwable cause)
 *
 */
public class Throwable implements Serializable {
    private static final long serialVersionUID = -3042686055658047285L;

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

    /**
     * 描述此异常的信息
     */
    private String detailMessage;

    /**
     * 表示当前异常由那个Throwable引起
     * 如果为null表示此异常不是由其他Throwable引起的
     * 如果此对象与自己相同,表明此异常的起因对象还没有被初始化
     */
    private Throwable cause = this;

    /**
     * 描述异常轨迹的数组
     */
    private StackTraceElement[] stackTrace;

    /**
     * 构造函数,起因对象没有被初始化可以在以后使用initCause进行初始化
     * fillInStackTrace可以用来初始化它的异常轨迹的数组
     */
    public Throwable() {
        fillInStackTrace();
    }

    /**
     * 构造函数
     */
    public Throwable(String message) {
        //填充异常轨迹数组
        fillInStackTrace();
        //初始化异常描述信息
        detailMessage = message;
    }

    /**
     * 构造函数,cause表示起因对象
     */
    public Throwable(String message, Throwable cause) {
        fillInStackTrace();
        detailMessage = message;
        this.cause = cause;
    }

    /**
     * 构造函数
     */
    public Throwable(Throwable cause) {
        fillInStackTrace();
        detailMessage = (cause==null ? null : cause.toString());
        this.cause = cause;
    }

    /**
     * 获取详细信息
     */
    public String getMessage() {
        return detailMessage;
    }

    /**
     * 获取详细信息
     */
    public String getLocalizedMessage() {
        return getMessage();
    }

    /**
     * 获取起因对象
     */
    public Throwable getCause() {
        return (cause==this ? null : cause);
    }

    /**
     * 初始化起因对象,这个方法只能在未被初始化的情况下调用一次
     */
    public synchronized Throwable initCause(Throwable cause) {
        //如果不是未初始化状态则抛出异常
        if (this.cause != this)
            throw new IllegalStateException("Can't overwrite cause");

        //要设置的起因对象与自身相等则抛出异常
        if (cause == this)
            throw new IllegalArgumentException("Self-causation not permitted");

        //设置起因对象
        this.cause = cause;
        //返回设置的起因的对象
        return this;
    }

    /**
     * 字符串表示形式
     */
    public String toString() {
        String s = getClass().getName();
        String message = getLocalizedMessage();
        return (message != null) ? (s + ": " + message) : s;
    }

    /**
     * 打印出错误轨迹
     */
    public void printStackTrace() {
        printStackTrace(System.err);
    }

    /**
     * 打印出错误轨迹
     */
    public void printStackTrace(PrintStream s) {
        synchronized (s) {
            //调用当前对象的toString方法
            s.println(this);
            //获取异常轨迹数组
            StackTraceElement[] trace = getOurStackTrace();

            //打印出每个元素的字符串表示
            for (int i=0; i < trace.length; i++)
                s.println("\tat " + trace[i]);

            //获取起因对象
            Throwable ourCause = getCause();

            //递归的打印出起因对象的信息
            if (ourCause != null)
                ourCause.printStackTraceAsCause(s, trace);
        }
    }

    /**
     * 打印起因对象的信息
     * @param s 打印的流
     * @param causedTrace 有此对象引起的异常的异常轨迹
     */
    private void printStackTraceAsCause(PrintStream s,
                                        StackTraceElement[] causedTrace)
    {
        //获得当前的异常轨迹
        StackTraceElement[] trace = getOurStackTrace();
        //m为当前异常轨迹数组的最后一个元素位置,
        //n为当前对象引起的异常的异常轨迹数组的最后一个元素
        int m = trace.length-1, n = causedTrace.length-1;
        //分别从两个数组的后面做循环,如果相等则一直循环,直到不等或数组到头
        while (m >= 0 && n >=0 && trace[m].equals(causedTrace[n])) {
            m--; n--;
        }

        //相同的个数
        int framesInCommon = trace.length - 1 - m;

        //打印出不同的错误轨迹
        s.println("Caused by: " + this);
        for (int i=0; i <= m; i++)
            s.println("\tat " + trace[i]);
        //如果有相同的则打印出相同的个数
        if (framesInCommon != 0)
            s.println("\t... " + framesInCommon + " more");

        //获得此对象的起因对象,并递归打印出信息
        Throwable ourCause = getCause();
        if (ourCause != null)
            ourCause.printStackTraceAsCause(s, trace);
    }

    /**
     * 打印出错误轨迹
     */
    public void printStackTrace(PrintWriter s) {
        synchronized (s) {
            s.println(this);
            StackTraceElement[] trace = getOurStackTrace();
            for (int i=0; i < trace.length; i++)
                s.println("\tat " + trace[i]);

            Throwable ourCause = getCause();
            if (ourCause != null)
                ourCause.printStackTraceAsCause(s, trace);
        }
    }

    /**
     * 打印起因对象的信息
     */
    private void printStackTraceAsCause(PrintWriter s,
                                        StackTraceElement[] causedTrace)
    {
        // assert Thread.holdsLock(s);  

        // Compute number of frames in common between this and caused
        StackTraceElement[] trace = getOurStackTrace();
        int m = trace.length-1, n = causedTrace.length-1;
        while (m >= 0 && n >=0 && trace[m].equals(causedTrace[n])) {
            m--; n--;
        }
        int framesInCommon = trace.length - 1 - m;

        s.println("Caused by: " + this);
        for (int i=0; i <= m; i++)
            s.println("\tat " + trace[i]);
        if (framesInCommon != 0)
            s.println("\t... " + framesInCommon + " more");

        // Recurse if we have a cause
        Throwable ourCause = getCause();
        if (ourCause != null)
            ourCause.printStackTraceAsCause(s, trace);
    }

    /**
     * 填充异常轨迹
     */
    public synchronized native Throwable fillInStackTrace();

    /**
     * 返回当前的异常轨迹的拷贝
     */
    public StackTraceElement[] getStackTrace() {
        return (StackTraceElement[]) getOurStackTrace().clone();
    }

    /**
     * 获取当前的异常轨迹
     */
    private synchronized StackTraceElement[] getOurStackTrace() {
        //如果第一次调用此方法则初始化异常轨迹数组
        if (stackTrace == null) {
            //获得异常轨迹深度
            int depth = getStackTraceDepth();
            //创建新的异常轨迹数组,并填充它
            stackTrace = new StackTraceElement[depth];

            for (int i=0; i < depth; i++)
                stackTrace[i] = getStackTraceElement(i);//获取指定位标的异常轨迹
        }

        return stackTrace;
    }

    /**
     * 设置异常轨迹
     */
    public void setStackTrace(StackTraceElement[] stackTrace) {
        //拷贝设置参数
        StackTraceElement[] defensiveCopy =
                (StackTraceElement[]) stackTrace.clone();

        //如果设置参数有空元素则抛出异常
        for (int i = 0; i < defensiveCopy.length; i++)
            if (defensiveCopy[i] == null)
                throw new NullPointerException("stackTrace[" + i + "]");

        //设置当前对象的异常轨迹
        this.stackTrace = defensiveCopy;
    }

    /**
     * 异常轨迹的深度,0表示无法获得
     */
    private native int getStackTraceDepth();

    /**
     * 获取指定位标的异常轨迹
     */
    private native StackTraceElement getStackTraceElement(int index);

    private synchronized void writeObject(java.io.ObjectOutputStream s)
            throws IOException
    {
        getOurStackTrace();
        s.defaultWriteObject();
    }
}