System.exit()源码分析

  最近代码中常用的System.exit()，就来看看源码。

  首先位于java.lang.System中，源码如下：

/**
     * Terminates the currently running Java Virtual Machine. The
     * argument serves as a status code; by convention, a nonzero status
     * code indicates abnormal termination.
       用来终止当前正在运行的JVM。参数用作状态码；根据惯例：非零状态码表示非正常终止

     * <p>
     * This method calls the <code>exit</code> method in class
     * <code>Runtime</code>. This method never returns normally.
      该方法调用Runtime类的exit方法，该方法永远不会正常返回

     * <p>
     * The call <code>System.exit(n)</code> is effectively equivalent to
     * the call:
     * <blockquote><pre>
     * Runtime.getRuntime().exit(n)
     * </pre></blockquote>
     *
     * @param      status   exit status.
     * @throws  SecurityException
     *        if a security manager exists and its <code>checkExit</code>
     *        method doesn't allow exit with the specified status.
     * @see        java.lang.Runtime#exit(int)
     */
    public static void exit(int status) {
        Runtime.getRuntime().exit(status);
    }

  是用来终止JVM的，也就是说整个程序都停止了，占用的内存也释放了。

  继续往下找Runtime.getRuntime():返回与当前java应用程序相关的运行时对象

/**
     * Returns the runtime object associated with the current Java application.
     * Most of the methods of class <code>Runtime</code> are instance
     * methods and must be invoked with respect to the current runtime object.
     *
     * @return  the <code>Runtime</code> object associated with the current
     *          Java application.
     */
    public static Runtime getRuntime() {
        return currentRuntime;
    }

  再来看Runtime类的exit方法：

    /**
     * Terminates the currently running Java virtual machine by initiating its
     * shutdown sequence.  This method never returns normally.  The argument
     * serves as a status code; by convention, a nonzero status code indicates
     * abnormal termination.
      通过启动虚拟机的关闭序列，终止当前正在运行的 Java 虚拟机。此方法从不正常返回。
     可以将变量作为一个状态码；根据惯例，非零的状态码表示非正常终止

     *
     * <p> The virtual machine's shutdown sequence consists of two phases.  In
     * the first phase all registered {@link #addShutdownHook shutdown hooks},
     * if any, are started in some unspecified order and allowed to run
     * concurrently until they finish.  In the second phase all uninvoked
     * finalizers are run if {@link #runFinalizersOnExit finalization-on-exit}
     * has been enabled.  Once this is done the virtual machine {@link #halt
     * halts}.
     虚拟机的关闭序列包含两个阶段。
     在第一个阶段中，会以某种未指定的顺序启动所有已注册的关闭钩子 (hook)（如果有的话），
     并且允许它们同时运行直至结束。
     在第二个阶段中，如果已启用退出终结，则运行所有未调用的终结方法。一旦完成这个阶段，虚拟机就会暂停

     *
     * <p> If this method is invoked after the virtual machine has begun its
     * shutdown sequence then if shutdown hooks are being run this method will
     * block indefinitely.  If shutdown hooks have already been run and on-exit
     * finalization has been enabled then this method halts the virtual machine
     * with the given status code if the status is nonzero; otherwise, it
     * blocks indefinitely.
      如果在虚拟机已开始其关闭序列后才调用此方法，那么若正在运行关闭钩子，则将无限期地阻断此方法。
      如果已经运行完关闭钩子，并且已启用退出终结 (on-exit finalization)，那么此方法将利用给定的状态码
    （如果状态码是非零值）暂停虚拟机；否则将无限期地阻断虚拟机。

     *
     * <p> The <tt>{@link System#exit(int) System.exit}</tt> method is the
     * conventional and convenient means of invoking this method. <p>
     *
     * @param  status
     *         Termination status.  By convention, a nonzero status code
     *         indicates abnormal termination.
     *
     * @throws SecurityException
     *         If a security manager is present and its <tt>{@link
     *         SecurityManager#checkExit checkExit}</tt> method does not permit
     *         exiting with the specified status
     *
     * @see java.lang.SecurityException
     * @see java.lang.SecurityManager#checkExit(int)
     * @see #addShutdownHook
     * @see #removeShutdownHook
     * @see #runFinalizersOnExit
     * @see #halt(int)
     */
    public void exit(int status) {
　　
        SecurityManager security = System.getSecurityManager();
        // 如果已经为当前应用程序建立了安全管理器，则返回此安全管理器；否则，返回null。

　　　　 if (security != null) {
            security.checkExit(status);
            // 如果不允许调用线程使用特定的状态码暂停java虚拟机，则抛出SecurityException
　　　　 }
　　　　 // 安全检查完毕，下来是正式终止，status的非0与0状态得到体现
        Shutdown.exit(status);

    }

  接下来看看Shutdown.exit(status)的源码：在此区分0与非0

    /* Invoked by Runtime.exit, which does all the security checks.
     * Also invoked by handlers for system-provided termination events,
     * which should pass a nonzero status code.
     */
    static void exit(int status) {
        boolean runMoreFinalizers = false;
        synchronized (lock) {
            if (status != 0) runFinalizersOnExit = false;
            switch (state) {
            case RUNNING:       /* Initiate shutdown */
                state = HOOKS;
                break;
            case HOOKS:         /* Stall and halt */
                break;
            case FINALIZERS:
                if (status != 0) {
                    /* Halt immediately on nonzero status */
                    halt(status);
                } else {
                    /* Compatibility with old behavior:
                     * Run more finalizers and then halt
                     */
                    runMoreFinalizers = runFinalizersOnExit;
                }
                break;
            }
        }
        if (runMoreFinalizers) {
            runAllFinalizers();
            halt(status);
        }
        synchronized (Shutdown.class) {
            /* Synchronize on the class object, causing any other thread
             * that attempts to initiate shutdown to stall indefinitely
             */
            sequence();
            halt(status);
        }
    }