前几天看到蚂蚁开源的sofa框架,其中提供了应用关闭后的回调方法,看了原理之后发现是利用了JAVA虚拟机关闭钩子(Shutdown Hook)来实现的。
Java程序经常也会遇到进程挂掉的情况,一些状态没有正确的保存下来,这时候就需要在JVM关掉的时候执行一些清理现场的代码。JAVA中的ShutdownHook提供了比较好的方案。
JDK提供了Java.Runtime.addShutdownHook(Thread hook)方法,可以注册一个JVM关闭的钩子,这个钩子可以在一下几种场景中被调用:
- 程序正常退出
- 使用System.exit()
- 终端使用Ctrl+C触发的中断
- 系统关闭
- OutOfMemory宕机
- 使用Kill pid命令干掉进程(注:在使用kill -9 pid时,是不会被调用的)
下面是JDK1.7中关于钩子的定义:1
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11public void addShutdownHook(Thread hook)
param:
hook - An initialized but unstarted Thread object
throw:
IllegalArgumentException - If the specified hook has already been registered, or if it can be determined that the hook is already running or has already been run
IllegalStateException - If the virtual machine is already in the process of shutting down
SecurityException - If a security manager is present and it denies RuntimePermission("shutdownHooks")
from:
1.3
see:
removeShutdownHook(java.lang.Thread), halt(int), exit(int)
直接上测试代码1
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52public class ShutdownHookTest {
private void addHook() {
Runtime.getRuntime().addShutdownHook(new Thread(() -> System.out.println("Execute Hook.....")));
}
public void case1() {
System.out.println("case1: The Application is doing something");
try {
TimeUnit.MILLISECONDS.sleep(5000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
public void case2() {
System.out.println("case2: The Application is doing something");
System.exit(0);
}
public void case3() {
Thread thread = new Thread(() -> {
while (true) {
System.out.println("thread is running....");
try {
TimeUnit.MILLISECONDS.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
});
thread.start();
}
public void case5() {
System.out.println("case5: The Application is doing something");
byte[] b = new byte[500 * 1024 * 1024];
System.out.println("init finish");
}
public static void main(String[] args) {
ShutdownHookTest hookTest = new ShutdownHookTest();
hookTest.addHook();
// hookTest.case1();
// hookTest.case2();
// hookTest.case3();
hookTest.case5();
}
}
首先来测试第一种,程序正常退出的情况:1
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4case1: The Application is doing something
Execute Hook.....
Process finished with exit code 0
如上可以看到,当main线程运行结束之后就会调用关闭钩子。
再看第二种情况:1
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4case2: The Application is doing something
Execute Hook.....
Process finished with exit code 0
当系统推出后,也会调用关闭钩子。
接着看第三种情况,程序启动之后过一会儿关闭程序。输出:1
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6thread is running....
thread is running....
thread is running....
Execute Hook.....
Process finished with exit code 130 (interrupted by signal 2: SIGINT)
最后我们看下第五种情况,运行参数设置为:-Xmx20M 这样可以保证会有OutOfMemoryError的发生。结果:1
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5case5: The Application is doing something
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
at com.winsky.logs.ShutdownHookTest.case5(ShutdownHookTest.java:47)
at com.winsky.logs.ShutdownHookTest.main(ShutdownHookTest.java:59)
Execute Hook.....
其他还有几种情况就不一一演示了,有兴趣可以自行撸代码尝试一下。