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);
}
}