tomcat源码之类加载机制

Dear 丶 2023-06-07 12:23 13阅读 0赞

1.  tomcat类加载思维导向图

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTA2Mjc4NDA_size_16_color_FFFFFF_t_70][]

2. java 类加载思维导向图

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTA2Mjc4NDA_size_16_color_FFFFFF_t_70 1][]

3.  tomcat类加载顺序

**   当应用需要到某个类时，则会按照下面的顺序进行类加载**：

> 　    1 使用bootstrap引导类加载器加载，加载jvm所需的基础类
> 
> 　　2 使用system系统类加载器加载，加载tomcat启动类等相关
> 
> 　　3 使用应用类加载器在WEB-INF/classes中加载，加载部署的项目
> 
> 　　4 使用应用类加载器在WEB-INF/lib中加载，加载部署的项目依赖的jar
> 
> 　　5 使用common类加载器在CATALINA\_HOME/lib中加载, 加载tomcat依赖的一些通用jar，例如servlet.jart

4. tomcat所有的加载器

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTA2Mjc4NDA_size_16_color_FFFFFF_t_70 2][]

5. 源码分析

tomcat启动类 Bootstrap

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTA2Mjc4NDA_size_16_color_FFFFFF_t_70 3][]

涉及类加载器

>     ClassLoader commonLoader = null;
>     ClassLoader catalinaLoader = null;
>     ClassLoader sharedLoader = null;

类加载器初始化

从main方法开始

>     public static void main(String args[]) {
>     
>         synchronized (daemonLock) {
>             if (daemon == null) {
>                 // Don't set daemon until init() has completed
>                 Bootstrap bootstrap = new Bootstrap();
>                 try {
>                     // 初始化
>                     bootstrap.init();
>                 } catch (Throwable t) {
>                     handleThrowable(t);
>                     t.printStackTrace();
>                     return;
>                 }
>                 daemon = bootstrap;
>             } else {
>                 // When running as a service the call to stop will be on a new
>                 // thread so make sure the correct class loader is used to
>                 // prevent a range of class not found exceptions.
>                 Thread.currentThread().setContextClassLoader(daemon.catalinaLoader);
>             }
>         }
>     
>         try {
>             String command = "start";
>             if (args.length > 0) {
>                 command = args[args.length - 1];
>             }
>             
>             if (command.equals("startd")) {
>                 args[args.length - 1] = "start";
>                 daemon.load(args);
>                 daemon.start();
>             } else if (command.equals("stopd")) {
>                 args[args.length - 1] = "stop";
>                 daemon.stop();
>              // 启动
>             } else if (command.equals("start")) {
>                 daemon.setAwait(true);
>                 daemon.load(args);
>                 daemon.start();
>                 if (null == daemon.getServer()) {
>                     System.exit(1);
>                 }
>             } else if (command.equals("stop")) {
>                 daemon.stopServer(args);
>             } else if (command.equals("configtest")) {
>                 daemon.load(args);
>                 if (null == daemon.getServer()) {
>                     System.exit(1);
>                 }
>                 System.exit(0);
>             } else {
>                 log.warn("Bootstrap: command \"" + command + "\" does not exist.");
>             }
>         } catch (Throwable t) {
>             // Unwrap the Exception for clearer error reporting
>             if (t instanceof InvocationTargetException &&
>                     t.getCause() != null) {
>                 t = t.getCause();
>             }
>             handleThrowable(t);
>             t.printStackTrace();
>             System.exit(1);
>         }
>     }

start启动之后，调用start()方法

/**
         * Start the Catalina daemon.
         * @throws Exception Fatal start error
         */
        public void start() throws Exception {
            if (catalinaDaemon == null) {
                init();
            }
    
            Method method = catalinaDaemon.getClass().getMethod("start", (Class [])null);
            method.invoke(catalinaDaemon, (Object [])null);
        }

调用Init()方法

>     /**
>      * Initialize daemon.
>      * @throws Exception Fatal initialization error
>      */
>     public void init() throws Exception {
>     
>         initClassLoaders();
>         // 打破双亲委托加载机制
>         Thread.currentThread().setContextClassLoader(catalinaLoader);
>     
>         SecurityClassLoad.securityClassLoad(catalinaLoader);
>     
>         // Load our startup class and call its process() method
>         if (log.isDebugEnabled())
>             log.debug("Loading startup class");
>        // 初始化Catalina
>         Class<?> startupClass = catalinaLoader.loadClass("org.apache.catalina.startup.Catalina");
>         Object startupInstance = startupClass.getConstructor().newInstance();
>     
>         // Set the shared extensions class loader
>         if (log.isDebugEnabled())
>             log.debug("Setting startup class properties");
>         String methodName = "setParentClassLoader";
>         Class<?> paramTypes[] = new Class[1];
>         paramTypes[0] = Class.forName("java.lang.ClassLoader");
>         Object paramValues[] = new Object[1];
>         paramValues[0] = sharedLoader;
>         Method method =
>             startupInstance.getClass().getMethod(methodName, paramTypes);
>         method.invoke(startupInstance, paramValues);
>     
>         catalinaDaemon = startupInstance;
>     }

>     private void initClassLoaders() {
>         try {
>             // 先初始化CommonLoader
>             commonLoader = createClassLoader("common", null);
>             if (commonLoader == null) {
>                 // no config file, default to this loader - we might be in a 'single' env.
>                 commonLoader = this.getClass().getClassLoader();
>             }
>             // CommonLoader为parent类加载器
>             catalinaLoader = createClassLoader("server", commonLoader);
>              // CommonLoader为parent类加载器
>             sharedLoader = createClassLoader("shared", commonLoader);
>         } catch (Throwable t) {
>             handleThrowable(t);
>             log.error("Class loader creation threw exception", t);
>             System.exit(1);
>         }
>     }

此处给出类加载器的关系

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTA2Mjc4NDA_size_16_color_FFFFFF_t_70 4][]

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