tomat(16)关闭钩子

【0】REAMDE

0）本文部分文字描述转自：“how tomcat works”，旨在学习“tomat(16)关闭钩子”的相关知识；

1）problem+solution：

1.1）problem：在很多实际环境中，当用户关闭应用程序时，并不会按照推荐的方法关闭应用程序，很有可能不做清理工作；

1.2）solution：java 为程序员提供了一种优雅的方法可以在在关闭过程中执行一些代码，以确保那些负责善后处理的代码可能能够执行；

2）在java中，虚拟机会对两类事件（events）进行响应，然后执行关闭操作：

event1）当调用System.exit()方法或程序的最后一个非守护进程线程退出时，应用程序正常退出；

event2）用户突然强制虚拟机中断运行，例如用户按 CTRL+C 快捷键或在未关闭java程序的case下，从系统中退出；

3）虚拟机在执行关闭操作时，会经过以下两个阶段（stages）：

stage1）虚拟机启动所有已经注册的关闭钩子，如果有的话。关闭钩子是先前已经通过 Runtime 类注册的线程，所有的关闭钩子会并发执行，直到完成任务；（干货——关闭钩子是线程）

stage2）虚拟机根据case 调用所有没有被调用过的终结期（finalizer）；

Attention）本文重点说明第一个stage，因为该阶段允许程序员告诉虚拟机在应用程序中执行一些清理代码。

4）创建关闭钩子很简单（steps）：

step1）创建Thread类的一个子类；

step2）实现你自己的run()方法，当应用程序（正常或突然）关闭时，会调用此方法；

step3）在应用程序中，实例化关闭钩子类；

step4）使用当前 Runtime.addShutdownHook() 方法注册关闭钩子；

5）看个荔枝（关于创建钩子的测试用例）：

package com.tomcat.chapter16.shutdownhook;public class ShutdownHookDemo {public void start() {System.out.println("Demo");ShutdownHook shutdownHook = new ShutdownHook();Runtime.getRuntime().addShutdownHook(shutdownHook); // 添加一个关闭钩子.}public static void main(String[] args) {ShutdownHookDemo demo = new ShutdownHookDemo();demo.start();try {System.in.read();} catch (Exception e) {}System.out.println("Normal exit");}
}class ShutdownHook extends Thread {public void run() {System.out.println("Shutting down");}
}

对以上代码的分析（Analysis）：

A1）在实例化ShutdownHookDemo 类后，main()方法会调用start()方法；start()方法会创建一个关闭钩子，并通过当前运行时注册它：

ShutdownHook shutdownHook = new ShutdownHook();Runtime.getRuntime().addShutdownHook(shutdownHook); // 添加一个关闭钩子.

A2）然后，应用程序会等待用户输入： System.in.read();

A3）当用户按 enter 键后，应用程序会退出。虚拟机会执行关闭钩子，输入字符串“Shutting down”；

【1】关闭钩子的荔枝

1）源代码如下（该应用程序在启动时会创建一个临时文件，并在关闭时删除该临时文件）

public class MySwingAppWithoutShutdownHook extends JFrame { //不带有关闭钩子的swingJButton exitButton = new JButton();JTextArea jTextArea1 = new JTextArea();String dir = System.getProperty("user.dir");String filename = "temp.txt";public MySwingApp() {exitButton.setText("Exit");exitButton.setBounds(new Rectangle(304, 248, 76, 37));exitButton.addActionListener(new java.awt.event.ActionListener() {public void actionPerformed(ActionEvent e) {exitButton_actionPerformed(e);}});this.getContentPane().setLayout(null);jTextArea1.setText("Click the Exit button to quit");jTextArea1.setBounds(new Rectangle(9, 7, 371, 235));this.getContentPane().add(exitButton, null);this.getContentPane().add(jTextArea1, null);this.setDefaultCloseOperation(EXIT_ON_CLOSE);this.setBounds(0, 0, 400, 330);this.setVisible(true);initialize();}private void initialize() {// create a temp fileFile file = new File(dir, filename);try {System.out.println("Creating temporary file");file.createNewFile();} catch (IOException e) {System.out.println("Failed creating temporary file.");}}private void shutdown() {// delete the temp fileFile file = new File(dir, filename);if (file.exists()) {System.out.println("Deleting temporary file.");file.delete();}}void exitButton_actionPerformed(ActionEvent e) {shutdown();System.exit(0);}public static void main(String[] args) {MySwingApp mySwingApp = new MySwingApp();}
}

对以上代码的分析（Analysis）：

step1）应用程序会调用其 initialize()方法；

step2）initialize()方法会在用户目录中创建一个临时文件，名为 “temp.txt”；

step3）当用户关闭应用程序时，该程序会删除该临时文件；

2）problem+solution

2.1）problem：我们希望用户总是能够单击exit 来退出，这动作监听器就会调用shutdown()方法，可以删除临时文件了；但如果用户通过单击右上角的关闭按钮或是通过其他方式强制退出，则临时文件就无法删除了；

2.2）solution：使用关闭钩子来删除临时文件；关闭钩子是一个内部类，这样它就能够访问主类的所有方法；（干货——关闭钩子是一个内部类，这样它就能够访问主类的所有方法）

3）带有关闭钩子的源代码如下：

public class MySwingAppWithShutdownHook extends JFrame { //  带有关闭钩子的swing.JButton exitButton = new JButton();JTextArea jTextArea1 = new JTextArea();String dir = System.getProperty("user.dir");String filename = "temp.txt";public MySwingAppWithShutdownHook() {exitButton.setText("Exit");exitButton.setBounds(new Rectangle(304, 248, 76, 37));exitButton.addActionListener(new java.awt.event.ActionListener() {public void actionPerformed(ActionEvent e) {exitButton_actionPerformed(e);}});this.getContentPane().setLayout(null);jTextArea1.setText("Click the Exit button to quit");jTextArea1.setBounds(new Rectangle(9, 7, 371, 235));this.getContentPane().add(exitButton, null);this.getContentPane().add(jTextArea1, null);this.setDefaultCloseOperation(EXIT_ON_CLOSE);this.setBounds(0,0, 400, 330);this.setVisible(true);initialize();}private void initialize() {// add shutdown hookMyShutdownHook shutdownHook = new MyShutdownHook();Runtime.getRuntime().addShutdownHook(shutdownHook);// create a temp fileFile file = new File(dir, filename);try {System.out.println("Creating temporary file");file.createNewFile();}catch (IOException e) {System.out.println("Failed creating temporary file.");}}private void shutdown() { // highlight line.// delete the temp fileFile file = new File(dir, filename);if (file.exists()) {System.out.println("Deleting temporary file.");file.delete();}}void exitButton_actionPerformed(ActionEvent e) {shutdown();System.exit(0);}public static void main(String[] args) {MySwingAppWithShutdownHook mySwingApp = new MySwingAppWithShutdownHook();}private class MyShutdownHook extends Thread { // highlight line.public void run() {shutdown(); // highlight line.}}
}

对以上代码的分析（Analysis）：

step1）程序会首先创建一个内部类MyShutdownHook的一个实例，即钩子类实例，并将注册该钩子实例；

step2）其他的代码与MySwingAppWithoutShutdownHook的源代码类似；

Attention）唯一不同的是，当突然关闭应用程序时，该临时文件总是会被删除；

【2】tomcat中的关闭钩子

1）intro：tomcat也是通过关闭钩子来完成退出过程的；

2）org.apache.catalina.startup.Catalina类：负责启动管理其他组件的Server对象。一个名为 CatalinaShutdownHook的内部类继承自 java.lang.Thread类，提供了run()方法的实现，它会调用 Server.stop()方法，执行关闭操作；

Supplement）本文习惯性地给出 Catalina.main()方法的调用过程：

protected void start() { //org.apache.catalina.startup.Catalina.start().//.....       // Replace System.out and System.err with a custom PrintStreamSystemLogHandler log = new SystemLogHandler(System.out);System.setOut(log);System.setErr(log);Thread shutdownHook = new CatalinaShutdownHook(); // 创建关闭钩子// Start the new serverif (server instanceof Lifecycle) {try {server.initialize();((Lifecycle) server).start();try {// Register shutdown hookRuntime.getRuntime().addShutdownHook(shutdownHook); // 添加关闭钩子.} catch (Throwable t) {// This will fail on JDK 1.2. Ignoring, as Tomcat can run// fine without the shutdown hook.}// Wait for the server to be told to shut downserver.await();} //......}// Shut down the serverif (server instanceof Lifecycle) {try {try {// Remove the ShutdownHook first so that server.stop() // doesn't get invoked twiceRuntime.getRuntime().removeShutdownHook(shutdownHook);} catch (Throwable t) {// This will fail on JDK 1.2. Ignoring, as Tomcat can run// fine without the shutdown hook.}((Lifecycle) server).stop();} //......}}

 protected class CatalinaShutdownHook extends Thread { //org.apache.catalina.startup.Catalina.CatalinaShutdownHook
// an inner class defined in Catalinapublic void run() {if (server != null) {try {((Lifecycle) server).stop(); // highlight line.} catch (LifecycleException e) {System.out.println("Catalina.stop: " + e);e.printStackTrace(System.out);if (e.getThrowable() != null) {System.out.println("----- Root Cause -----");e.getThrowable().printStackTrace(System.out);}}}              }}

Supplement）

S1）在Catalina实例启动时，会实例化关闭钩子，并在一个阶段将其添加到 Runtime类中；

S2）org.apache.catalina.startup.Catalina 类的源代码如下所示：（其中Catalina.createStartDigester() 创建了很多规则，规则集参见 tomcat(15)Digester库）

 public class Catalina { protected String configFile = "conf/server.xml";   protected boolean debug = false; protected ClassLoader parentClassLoader =ClassLoader.getSystemClassLoader();   protected Server server = null; protected boolean starting = false; protected boolean stopping = false; protected boolean useNaming = true;  public static void main(String args[]) {(new Catalina()).process(args);}public void process(String args[]) {setCatalinaHome();setCatalinaBase();try {if (arguments(args))execute();} catch (Exception e) {e.printStackTrace(System.out);}}public void setParentClassLoader(ClassLoader parentClassLoader) {this.parentClassLoader = parentClassLoader;}public void setServer(Server server) {this.server = server;} protected boolean arguments(String args[]) {boolean isConfig = false;if (args.length < 1) {usage();return (false);}for (int i = 0; i < args.length; i++) {if (isConfig) {configFile = args[i];isConfig = false;} else if (args[i].equals("-config")) {isConfig = true;} else if (args[i].equals("-debug")) {debug = true;} else if (args[i].equals("-nonaming")) {useNaming = false;} else if (args[i].equals("-help")) {usage();return (false);} else if (args[i].equals("start")) {starting = true;} else if (args[i].equals("stop")) {stopping = true;} else {usage();return (false);}}return (true);}protected File configFile() {File file = new File(configFile);if (!file.isAbsolute())file = new File(System.getProperty("catalina.base"), configFile);return (file);}protected Digester createStartDigester() {// Initialize the digesterDigester digester = new Digester();digester.setClassLoader(StandardServer.class.getClassLoader());if (debug)digester.setDebug(999);digester.setValidating(false);// Configure the actions we will be usingdigester.addObjectCreate("Server","org.apache.catalina.core.StandardServer","className");digester.addSetProperties("Server");digester.addSetNext("Server","setServer","org.apache.catalina.Server");digester.addObjectCreate("Server/GlobalNamingResources","org.apache.catalina.deploy.NamingResources");digester.addSetProperties("Server/GlobalNamingResources");digester.addSetNext("Server/GlobalNamingResources","setGlobalNamingResources","org.apache.catalina.deploy.NamingResources");digester.addObjectCreate("Server/Listener",null, // MUST be specified in the element"className");digester.addSetProperties("Server/Listener");digester.addSetNext("Server/Listener","addLifecycleListener","org.apache.catalina.LifecycleListener");digester.addObjectCreate("Server/Service","org.apache.catalina.core.StandardService","className");digester.addSetProperties("Server/Service");digester.addSetNext("Server/Service","addService","org.apache.catalina.Service");digester.addObjectCreate("Server/Service/Listener",null, // MUST be specified in the element"className");digester.addSetProperties("Server/Service/Listener");digester.addSetNext("Server/Service/Listener","addLifecycleListener","org.apache.catalina.LifecycleListener");digester.addObjectCreate("Server/Service/Connector","org.apache.catalina.connector.http.HttpConnector","className");digester.addSetProperties("Server/Service/Connector");digester.addSetNext("Server/Service/Connector","addConnector","org.apache.catalina.Connector");digester.addObjectCreate("Server/Service/Connector/Factory","org.apache.catalina.net.DefaultServerSocketFactory","className");digester.addSetProperties("Server/Service/Connector/Factory");digester.addSetNext("Server/Service/Connector/Factory","setFactory","org.apache.catalina.net.ServerSocketFactory");digester.addObjectCreate("Server/Service/Connector/Listener",null, // MUST be specified in the element"className");digester.addSetProperties("Server/Service/Connector/Listener");digester.addSetNext("Server/Service/Connector/Listener","addLifecycleListener","org.apache.catalina.LifecycleListener");// Add RuleSets for nested elementsdigester.addRuleSet(new NamingRuleSet("Server/GlobalNamingResources/"));digester.addRuleSet(new EngineRuleSet("Server/Service/"));digester.addRuleSet(new HostRuleSet("Server/Service/Engine/"));digester.addRuleSet(new ContextRuleSet("Server/Service/Engine/Default"));digester.addRuleSet(new NamingRuleSet("Server/Service/Engine/DefaultContext/"));digester.addRuleSet(new ContextRuleSet("Server/Service/Engine/Host/Default"));digester.addRuleSet(new NamingRuleSet("Server/Service/Engine/Host/DefaultContext/"));digester.addRuleSet(new ContextRuleSet("Server/Service/Engine/Host/"));digester.addRuleSet(new NamingRuleSet("Server/Service/Engine/Host/Context/"));digester.addRule("Server/Service/Engine",new SetParentClassLoaderRule(digester,parentClassLoader));return (digester);}   protected Digester createStopDigester() {// Initialize the digesterDigester digester = new Digester();if (debug)digester.setDebug(999);// Configure the rules we need for shutting downdigester.addObjectCreate("Server","org.apache.catalina.core.StandardServer","className");digester.addSetProperties("Server");digester.addSetNext("Server","setServer","org.apache.catalina.Server");return (digester);}  protected void execute() throws Exception {if (starting)start();else if (stopping)stop();}protected void setCatalinaBase() {if (System.getProperty("catalina.base") != null)return;System.setProperty("catalina.base",System.getProperty("catalina.home"));}    protected void setCatalinaHome() {if (System.getProperty("catalina.home") != null)return;System.setProperty("catalina.home",System.getProperty("user.dir"));} protected void start() {// Setting additional variablesif (!useNaming) {System.setProperty("catalina.useNaming", "false");} else {System.setProperty("catalina.useNaming", "true");String value = "org.apache.naming";String oldValue =System.getProperty(javax.naming.Context.URL_PKG_PREFIXES);if (oldValue != null) {value = value + ":" + oldValue;}System.setProperty(javax.naming.Context.URL_PKG_PREFIXES, value);value = System.getProperty(javax.naming.Context.INITIAL_CONTEXT_FACTORY);if (value == null) {System.setProperty(javax.naming.Context.INITIAL_CONTEXT_FACTORY,"org.apache.naming.java.javaURLContextFactory");}}// Create and execute our DigesterDigester digester = createStartDigester();File file = configFile();try {InputSource is =new InputSource("file://" + file.getAbsolutePath());FileInputStream fis = new FileInputStream(file);is.setByteStream(fis);digester.push(this);digester.parse(is);fis.close();} catch (Exception e) {System.out.println("Catalina.start using "+ configFile() + ": " + e);e.printStackTrace(System.out);System.exit(1);}// If a SecurityManager is being used, set properties for// checkPackageAccess() and checkPackageDefinitionif( System.getSecurityManager() != null ) {String access = Security.getProperty("package.access");if( access != null && access.length() > 0 )access += ",";elseaccess = "sun.,";Security.setProperty("package.access",access + "org.apache.catalina.,org.apache.jasper.");String definition = Security.getProperty("package.definition");if( definition != null && definition.length() > 0 )definition += ",";elsedefinition = "sun.,";Security.setProperty("package.definition",// FIX ME package "javax." was removed to prevent HotSpot// fatal internal errorsdefinition + "java.,org.apache.catalina.,org.apache.jasper.,org.apache.coyote.");}// Replace System.out and System.err with a custom PrintStreamSystemLogHandler log = new SystemLogHandler(System.out);System.setOut(log);System.setErr(log);Thread shutdownHook = new CatalinaShutdownHook();// Start the new serverif (server instanceof Lifecycle) {try {server.initialize();((Lifecycle) server).start();try {// Register shutdown hookRuntime.getRuntime().addShutdownHook(shutdownHook);} catch (Throwable t) {// This will fail on JDK 1.2. Ignoring, as Tomcat can run// fine without the shutdown hook.}// Wait for the server to be told to shut downserver.await();} catch (LifecycleException e) {System.out.println("Catalina.start: " + e);e.printStackTrace(System.out);if (e.getThrowable() != null) {System.out.println("----- Root Cause -----");e.getThrowable().printStackTrace(System.out);}}}// Shut down the serverif (server instanceof Lifecycle) {try {try {// Remove the ShutdownHook first so that server.stop() // doesn't get invoked twiceRuntime.getRuntime().removeShutdownHook(shutdownHook);} catch (Throwable t) {// This will fail on JDK 1.2. Ignoring, as Tomcat can run// fine without the shutdown hook.}((Lifecycle) server).stop();} catch (LifecycleException e) {System.out.println("Catalina.stop: " + e);e.printStackTrace(System.out);if (e.getThrowable() != null) {System.out.println("----- Root Cause -----");e.getThrowable().printStackTrace(System.out);}}}}protected void stop() {// Create and execute our DigesterDigester digester = createStopDigester();File file = configFile();try {InputSource is =new InputSource("file://" + file.getAbsolutePath());FileInputStream fis = new FileInputStream(file);is.setByteStream(fis);digester.push(this);digester.parse(is);fis.close();} catch (Exception e) {System.out.println("Catalina.stop: " + e);e.printStackTrace(System.out);System.exit(1);}// Stop the existing servertry {Socket socket = new Socket("127.0.0.1", server.getPort());OutputStream stream = socket.getOutputStream();String shutdown = server.getShutdown();for (int i = 0; i < shutdown.length(); i++)stream.write(shutdown.charAt(i));stream.flush();stream.close();socket.close();} catch (IOException e) {System.out.println("Catalina.stop: " + e);e.printStackTrace(System.out);System.exit(1);}}    protected void usage() {System.out.println("usage: java org.apache.catalina.startup.Catalina"+ " [ -config {pathname} ] [ -debug ]"+ " [ -nonaming ] { start | stop }");}   protected class CatalinaShutdownHook extends Thread {public void run() {if (server != null) {try {((Lifecycle) server).stop();} catch (LifecycleException e) {System.out.println("Catalina.stop: " + e);e.printStackTrace(System.out);if (e.getThrowable() != null) {System.out.println("----- Root Cause -----");e.getThrowable().printStackTrace(System.out);}}}            }}
}
final class SetParentClassLoaderRule extends Rule {public SetParentClassLoaderRule(Digester digester,ClassLoader parentClassLoader) {super(digester);this.parentClassLoader = parentClassLoader;}ClassLoader parentClassLoader = null;public void begin(Attributes attributes) throws Exception {if (digester.getDebug() >= 1)digester.log("Setting parent class loader");Container top = (Container) digester.peek();top.setParentClassLoader(parentClassLoader);}
}

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