System.exit()源码分析
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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); } }
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