JVM之shutdown hook虚拟机关闭钩子分析

1：写在前面

jvm shutdown hook，即虚拟机关闭钩子，是允许在虚拟机退出前执行一些操作的机制，可以用于进行一些资源的清理等善后工作，对于包括但不限于以下的情况注册的虚拟机关闭钩子会被执行。

1：程序的正常退出
2：System.exit()
3：Ctrl+C
4：OutOfMemoryError
5：kill pid(注意非“-9”)

下面我们来测试下这些中的部分情况。

2：程序正常退出

2.1：测试代码

public class ShutdownHookTest {

    public void registerShutdownHook() {
        System.out.println("register shutdown hook begin...");
        // 注册停止运行钩子
        Runtime.getRuntime().addShutdownHook(new Thread(() -> {
            System.out.println("shut downhook run ...");
        }));
        System.out.println("register shutdown hook end...");
    }

    public static void main(String[] args) throws Exception {
        // 先注册停止运行钩子
        new ShutdownHookTest().registerShutdownHook();
        // 模拟正常的应用程序运行
        System.out.println("application running...");
        Thread.sleep(3000);
        System.out.println("application run over...");
    }
}

2.2：运行

register shutdown hook begin...
register shutdown hook end...
application running...
application run over...
shut downhook run ...

3：OutOfMemoryError

3.1：测试代码

public class ShutdownHookOutOfMemoryTest {

    public void registerOutOfMemoryShutdownHook() {
        System.out.println("register out of memory shut down hook begin...");
        Runtime.getRuntime().addShutdownHook(new Thread(() -> {
            System.out.println("out of memory shutdown hook run ...");
        }));
        System.out.println("register out of memory shut down hook end...");
    }

    public static void main(String[] args) throws Exception {
        // 注册虚拟机关闭钩子
        new ShutdownHookOutOfMemoryTest().registerOutOfMemoryShutdownHook();
        // 因为配置-Xmx20M，所以这里会发生OutOfMemoryError
        int FIFTY_M = 1024 * 1024 * 50;
        byte[] b = new byte[FIFTY_M];
        System.out.println("application running...");
        Thread.sleep(3000);
    }
}

3.2：运行

注意配置-Xmx20M:

register out of memory shut down hook begin...
register out of memory shut down hook end...
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
out of memory shutdown hook run ...