JVM之shutdown hook虚拟机关闭钩子分析

Myth丶恋晨 2022-10-21 13:54 233阅读 0赞

## 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 ...