System.exit(status)和Runtime.getR

System.exit(status);
Runtime.getRuntime().halt(status)

System.exit(status)解释如下：

Terminates the currently running Java Virtual Machine.
The  argument serves as a status code; 
by convention, a nonzero status code indicates abnormal termination 

终止当前正在运行的Java虚拟机。参数作为状态代码，按照惯例，一个非零状态码表示异常终止。
用线程描述，在多线程情况下，可能更准确一些。

1. 调用方法后，线程会退出。
2. 未捕获的异常被线程抛出，但如果有其他非守护线程，程序将继续运行。
3. 反馈状态码，一般在脚本中有用。
4. 线程退出，还是做一些清理动作。

源码：

/**
     * 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.
     *
     * <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}.
     *
     * <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.
     *
     * <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) {
        // Make sure we don't try this several times
        synchronized(this) {
            if (!shuttingDown) {
                shuttingDown = true;

                Thread[] hooks;
                synchronized (shutdownHooks) {
                    // create a copy of the hooks
                    hooks = new Thread[shutdownHooks.size()];
                    shutdownHooks.toArray(hooks);
                }

                // Start all shutdown hooks concurrently
                for (Thread hook : hooks) {
                    hook.start();
                }

                // Wait for all shutdown hooks to finish
                for (Thread hook : hooks) {
                    try {
                        hook.join();
                    } catch (InterruptedException ex) {
                        // Ignore, since we are at VM shutdown.
                    }
                }

                // Ensure finalization on exit, if requested
                if (finalizeOnExit) {
                    runFinalization();
                }

                // Get out of here finally...
                nativeExit(status);
            }
        }
    }

Java虚拟机退出包括两个阶段：

第一个阶段：会以某种未指定的顺序启动所有已注册钩子，并且允许它们同时运行直至结束。
第二个阶段：如果已启用runFinalizersOnExit设置为true，则运行所有未调用的终结方法（finalizer方法）。

Runtime.getRuntime().halt(status)解释如下：

Forcibly terminates the currently running Java virtual machine.  
This method never returns normally. 

现在runtime的halt比较好理解了，他不会执行钩子函数和finalizer方法，而是直接退出。

源码

 /**
     * Forcibly terminates the currently running Java virtual machine.  This
     * method never returns normally.
     *
     * <p> This method should be used with extreme caution.  Unlike the
     * <tt>{@link #exit exit}</tt> method, this method does not cause shutdown
     * hooks to be started and does not run uninvoked finalizers if
     * finalization-on-exit has been enabled.  If the shutdown sequence has
     * already been initiated then this method does not wait for any running
     * shutdown hooks or finalizers to finish their work. <p>
     *
     * @param  status
     *         Termination status.  By convention, a nonzero status code
     *         indicates abnormal termination.  If the <tt>{@link Runtime#exit
     *         exit}</tt> (equivalently, <tt>{@link System#exit(int)
     *         System.exit}</tt>) method has already been invoked then this
     *         status code will override the status code passed to that method.
     *
     * @throws SecurityException
     *         If a security manager is present and its <tt>{@link
     *         SecurityManager#checkExit checkExit}</tt> method does not permit
     *         an exit with the specified status
     *
     * @see #exit
     * @see #addShutdownHook
     * @see #removeShutdownHook
     * @since 1.3
     */
    public void halt(int status) {
        nativeExit(status);
    }