Tomcat启动过程源码解读

根据Tomcat源码来看一下Tomcat启动过程都做了什么

部分代码为主要流程代码，删去了try-catch以及一些校验逻辑，方便理解主流程

先来一张启动过程时序图，了解一下启动顺序

Tomcat启动的入口类：org.apache.catalina.startup.Bootstrap#main

main方法是整个tomcat启动时的入口。在main方法中，使用bootstrap.init()来初始化类加载器和创建Catalina实例，然后再启动Catalina线程。

 public static void main(String args[]) {
 
     if (daemon == null) {
         // Don't set daemon until init() has completed
         Bootstrap bootstrap = new Bootstrap();
         try {
             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();
         } else if (command.equals("start")) {
             daemon.setAwait(true);
             daemon.load(args);
             daemon.start();
         } 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);
     }
 
 }

bootstrap.init()方法，用于初始化容器相关，首先创建类加载器，然后通过反射创建org.apache.catalina.startup.Catalina实例：

 public void init() throws Exception {
 
     initClassLoaders();
 
     Thread.currentThread().setContextClassLoader(catalinaLoader);
 
     SecurityClassLoad.securityClassLoad(catalinaLoader);
 
     // Load our startup class and call its process() method
     if (log.isDebugEnabled())
         log.debug("Loading startup class");
     Class<?> startupClass =
         catalinaLoader.loadClass
         ("org.apache.catalina.startup.Catalina");
     Object startupInstance = startupClass.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;
 
 }

之后Bootstrap的demon.start()方法就会调用Catalina的start方法。

Catalina实例执行start方法。这里有两个点，一个是load()加载server.xml配置、初始化Server的过程，一个是getServer().start()开启服务、初始化并开启一系列组件、子容器的过程。

org.apache.catalina.startup.Catalina#start

 public void start() {
 
     if (getServer() == null) {
         load();
     }
 
     if (getServer() == null) {
         log.fatal("Cannot start server. Server instance is not configured.");
         return;
     }
 
     long t1 = System.nanoTime();
 
     // Start the new server
     try {
         getServer().start();
     } catch (LifecycleException e) {
         log.fatal(sm.getString("catalina.serverStartFail"), e);
         try {
             getServer().destroy();
         } catch (LifecycleException e1) {
             log.debug("destroy() failed for failed Server ", e1);
         }
         return;
     }
 
     long t2 = System.nanoTime();
     if(log.isInfoEnabled()) {
         log.info("Server startup in " + ((t2 - t1) / 1000000) + " ms");
     }
 
     // Register shutdown hook
     if (useShutdownHook) {
         if (shutdownHook == null) {
             shutdownHook = new CatalinaShutdownHook();
         }
         Runtime.getRuntime().addShutdownHook(shutdownHook);
 
         // If JULI is being used, disable JULI's shutdown hook since
         // shutdown hooks run in parallel and log messages may be lost
         // if JULI's hook completes before the CatalinaShutdownHook()
         LogManager logManager = LogManager.getLogManager();
         if (logManager instanceof ClassLoaderLogManager) {
             ((ClassLoaderLogManager) logManager).setUseShutdownHook(
                     false);
         }
     }
 
     if (await) {
         await();
         stop();
     }
 }

load方法解析server.xml配置文件，并加载Server、Service、Connector、Container、Engine、Host、Context、Wrapper一系列的容器。加载完成后，调用getServer().start()来开启一个新的Server。

下面先看load方法怎么加载组件和容器的：

 /**
   * Start a new server instance.
   */
  public void load() {
 
      long t1 = System.nanoTime();
 
      initDirs();
 
      // Before digester - it may be needed
      initNaming();
 
      // Create and execute our Digester
      Digester digester = createStartDigester();
 
      InputSource inputSource = null;
      InputStream inputStream = null;
      File file = null;
      file = configFile();
      inputStream = new FileInputStream(file);
      inputSource = new InputSource(file.toURI().toURL().toString());
      inputSource.setByteStream(inputStream);
      digester.push(this);
      digester.parse(inputSource);
 
      getServer().setCatalina(this);
      getServer().setCatalinaHome(Bootstrap.getCatalinaHomeFile());
      getServer().setCatalinaBase(Bootstrap.getCatalinaBaseFile());
 
      // Stream redirection
      initStreams();
 
      // Start the new server
      getServer().init();
  }

首先利用Digester类解析server.xml文件，得到容器的配置，并创建相应的对象，并关联父子容器。依次创建的是StandardServer、StandardService、StandardEngine、StandardHost。

然后拿到StandardServer实例调用init()方法初始化Tomcat容器的一系列组件。一些容器初始化的的时候，都会调用其子容器的init()方法，初始化它的子容器。顺序是StandardServer、StandardService、StandardEngine、Connector。每个容器都在初始化自身相关设置的同时，将子容器初始化。

这里插入一个Tomcat中生命周期的概念。在初始化、开启一系列组件、容器的过程中，由tomcat'管理的组件和容器，都有一个共同的特点，都实现了org.apache.catalina.Lifecycle接口，由Tomcat管理其生命周期。Lifecycle提供一种统一的管理对象生命周期的接口。通过Lifecycle、LifecycleListener、LifecycleEvent，Catalina实现了对tomcat各种组件、容器统一的启动和停止的方式。

在Tomcat服务开启过程中启动的一些列组件、容器，都继承了org.apache.catalina.util.LifecycleBase这个抽象类，其中的init()、start() 方法、stop() 方法，为其子类实现了统一的start和stop管理。方法中具体的initInternal()、startInternal() 和stopInternal() 方法，交由子类自己实现。

看一下LifecycleBase的init()和start()的实现吧：

org.apache.catalina.util.LifecycleBase#start

 public final synchronized void init() throws LifecycleException {
     if (!state.equals(LifecycleState.NEW)) {
         invalidTransition(Lifecycle.BEFORE_INIT_EVENT);
     }
 
     try {
         setStateInternal(LifecycleState.INITIALIZING, null, false);
         initInternal();
         setStateInternal(LifecycleState.INITIALIZED, null, false);
     } catch (Throwable t) {
         ExceptionUtils.handleThrowable(t);
         setStateInternal(LifecycleState.FAILED, null, false);
         throw new LifecycleException(
                 sm.getString("lifecycleBase.initFail",toString()), t);
     }
 }
 
 public final synchronized void start() throws LifecycleException {
 
     if (LifecycleState.STARTING_PREP.equals(state) || LifecycleState.STARTING.equals(state) ||
             LifecycleState.STARTED.equals(state)) {
 
         if (log.isDebugEnabled()) {
             Exception e = new LifecycleException();
             log.debug(sm.getString("lifecycleBase.alreadyStarted", toString()), e);
         } else if (log.isInfoEnabled()) {
             log.info(sm.getString("lifecycleBase.alreadyStarted", toString()));
         }
 
         return;
     }
 
     if (state.equals(LifecycleState.NEW)) {
         init();
     } else if (state.equals(LifecycleState.FAILED)) {
         stop();
     } else if (!state.equals(LifecycleState.INITIALIZED) &&
             !state.equals(LifecycleState.STOPPED)) {
         invalidTransition(Lifecycle.BEFORE_START_EVENT);
     }
 
     try {
         setStateInternal(LifecycleState.STARTING_PREP, null, false);
         startInternal();
         if (state.equals(LifecycleState.FAILED)) {
             stop();
         } else if (!state.equals(LifecycleState.STARTING)) {
             invalidTransition(Lifecycle.AFTER_START_EVENT);
         } else {
             setStateInternal(LifecycleState.STARTED, null, false);
         }
     } catch (Throwable t) {
         ExceptionUtils.handleThrowable(t);
         setStateInternal(LifecycleState.FAILED, null, false);
         throw new LifecycleException(sm.getString("lifecycleBase.startFail", toString()), t);
     }
 }

可以看到，init()和start()方法里，调用了initInternal()方法、startInternal()方法和stop()方法，这三者最终会走子类的具体实现。

上面的StandardServer的初始化过程就是一个活生生的例子。在Catalina的load过程中，getServer().init()方法就是LifecycleBase中的init()方法，调用initInternal()时是走的StandardServer的实现，StandardServer的initInternal()中会调用StandardServer的init()方法，进行子容器的初始化。然后依次初始化。

看一下代码，了解一下StandardServer中的initInternal()实现。

 /**
  * Invoke a pre-startup initialization. This is used to allow connectors
  * to bind to restricted ports under Unix operating environments.
  */
 @Override
 protected void initInternal() throws LifecycleException {
 
     super.initInternal();
 
     // Register global String cache
     // Note although the cache is global, if there are multiple Servers
     // present in the JVM (may happen when embedding) then the same cache
     // will be registered under multiple names
     onameStringCache = register(new StringCache(), "type=StringCache");
 
     // Register the MBeanFactory
     MBeanFactory factory = new MBeanFactory();
     factory.setContainer(this);
     onameMBeanFactory = register(factory, "type=MBeanFactory");
 
     // Register the naming resources
     globalNamingResources.init();
 
     // Populate the extension validator with JARs from common and shared
     // class loaders
     if (getCatalina() != null) {
         ClassLoader cl = getCatalina().getParentClassLoader();
         // Walk the class loader hierarchy. Stop at the system class loader.
         // This will add the shared (if present) and common class loaders
         while (cl != null && cl != ClassLoader.getSystemClassLoader()) {
             if (cl instanceof URLClassLoader) {
                 URL[] urls = ((URLClassLoader) cl).getURLs();
                 for (URL url : urls) {
                     if (url.getProtocol().equals("file")) {
                         try {
                             File f = new File (url.toURI());
                             if (f.isFile() &&
                                     f.getName().endsWith(".jar")) {
                                 ExtensionValidator.addSystemResource(f);
                             }
                         } catch (URISyntaxException e) {
                             // Ignore
                         } catch (IOException e) {
                             // Ignore
                         }
                     }
                 }
             }
             cl = cl.getParent();
         }
     }
     // Initialize our defined Services
     for (int i = 0; i < services.length; i++) {
         services[i].init();
     }
 }

再举一个具体的例子:

回到刚才的启动过程中，getServer().start()开启服务的方法，实际就是上面提到的LifecycleBase中的start()方法。其中，会调用org.apache.catalina.core.StandardServer#initInternal方法，初始化Server并调用Service的init方法。org.apache.catalina.core.StandardServer在其实现的startInternal() 中，开启naming resources和services，调用service的start方法，开启所有service，调用其service的startInternal()方法。

下面看一下StandardServer中的startInternal()的实现：

org.apache.catalina.core.StandardServer#startInternal

 protected void startInternal() throws LifecycleException {
 
     fireLifecycleEvent(CONFIGURE_START_EVENT, null);
     setState(LifecycleState.STARTING);
 
     globalNamingResources.start();
 
     // Start our defined Services
     synchronized (servicesLock) {
         for (int i = 0; i < services.length; i++) {
             services[i].start();
         }
     }
 }

这里的service，是org.apache.catalina.core.StandardService的实例。

总结一下启动的Tomcat启动的过程

在Catalina的load方法里，就已经调用了StandardServer里的init方法，一层一层初始化了globalNamingResources，StandardService--》StandardEngine，executors，MapperListener，Connector--》CoyoteAdapter，protocolHandler。至此就将tomcat的catalina中的组件、容器初始化完成。 接下来就是调用start方法一层一层开启，StandardServer的startInternal方法，按层次start：globalNamingResources，StandardService--》StandardEngine，executors，MapperListener，Connector--》StandardHost，StandardContext，protocolHandler。顺序基本同init过程。StandardEngine在start时，会init子容器，并调用子容器的start方法。子容器依次这样init、start，就开启了StandardHost和StandardContext。

 

参考文章：

tomcat源码分析-Connector初始化与启动

tomcat源码分析-Container初始化与加载

tomcat源码分析-http请求在Container中的执行路线

tomcat源码解析(一)--启动与Server.xml文件的解析