前面几篇文章分别介绍了Tomcat的安装、优化和框架，本文主要用于分析Tomcat源码启动过程，研究一个框架最好的着手方式可能就是研究它的启动过程，因为在这过程中我们可以看到它内部的层次关系，模块关系等，对整个框架有一个高层次的掌握，后续研究就可以得心应手了。
研究Tomcat源码启动过程的最简单方式是可以通过Tomcat的启动类Bootstrap着手。

Tomcat主要类分析

1) Catalina
Catalina负责Server的加载、启动和关闭，Bootstrap启动后将Server的加载、启动和关闭的操作全部委托到Catalina类。

2) Server
Server是Service的生存环境。一个Server中可以有多个Service。StandardServer是Server的标准实现类，StandardServer可以持有多个Service。

2) Service
Service将Connector和Container包装在一起提供对外服务，一个Service可以有多个Connector，但是只有一个Container。StandardService是Service的标准实现，StandardService分别持有多个Connector和一个Engine，将两者联系起来对外工作。

3) Connector
Connector负责对外交流。它的主要任务是负责接收浏览器的发过来的tcp连接请求，创建一个Request和Response对象分别用于和请求端交换数据，然后会产生一个线程来处理这个请求并把产生的Request 和Response对象传给处理这个请求的线程，处理这个请求的线程就是Container组件要做的事了。
ProtocolHandler负责选择底层运行模式Bio或者Nio，监听端口，创建线程以及数据的处理等，它是对AbstractEndPoint的轻度封装。Http11NioProtocol代表Nio模式Protocol，Tomcat8以后默认采用此协议。
AbstractEndPoint代表具体的底层通讯，它负责运行模式Bio或者Nio，绑定端口，监听端口，端口的事件处理等关于Socket的事情。JIoEndPoint代表Bio模式，NioEndPoint代表Nio模式，Tomcat8以后默认都是使用Nio模式。

4) Engine
Engine是完整的容器，其下面拥有多个虚拟主机，它的责任就是将Connector请求分配给虚拟机处理。StandardEngine是Engine的标准实现类。

5) Executor
Executor工作线程，用于处理Connector传递的请求。
Tomcat启动过程
1) Bootstrap启动程序，Bootstrap是Tomcat的入口，负责加载和启动Server

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); // bootstrap加载Server daemon.load(args); // bootstrap启动Server 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); } }
2) Bootstrap类load方法加载Server，Bootstrap加载Server最终委托给Catalina的load方法

private void load(String[] arguments) throws Exception { // Call the load() method String methodName = "load"; Object param[]; Class paramTypes[]; if (arguments == null || arguments.length == 0) { paramTypes = null; param = null; } else { paramTypes = new Class[1]; paramTypes[0] = arguments.getClass(); param = new Object[1]; param[0] = arguments; } // Catalina load方法调用 Method method = catalinaDaemon.getClass().getMethod(methodName, paramTypes); if (log.isDebugEnabled()) log.debug("Calling startup class " + method); method.invoke(catalinaDaemon, param); }
3) Catalina load方法负责加载Server

public void load() { long t1 = System.nanoTime(); initDirs(); initNaming(); // Create and execute our Digester Digester digester = createStartDigester(); InputSource inputSource = null; InputStream inputStream = null; File file = null; try { try { file = configFile(); inputStream = new FileInputStream(file); inputSource = new InputSource(file.toURI().toURL().toString()); } catch (Exception e) { if (log.isDebugEnabled()) { log.debug(sm.getString("catalina.configFail", file), e); } } if (inputStream == null) { try { inputStream = getClass().getClassLoader().getResourceAsStream(getConfigFile()); inputSource = new InputSource(getClass().getClassLoader().getResource(getConfigFile()).toString()); } catch (Exception e) { if (log.isDebugEnabled()) { log.debug(sm.getString("catalina.configFail", getConfigFile()), e); } } } // This should be included in catalina.jar // Alternative: don\'t bother with xml, just create it manually. if (inputStream == null) { try { inputStream = getClass().getClassLoader().getResourceAsStream("server-embed.xml"); inputSource = new InputSource(getClass().getClassLoader().getResource("server-embed.xml").toString()); } catch (Exception e) { if (log.isDebugEnabled()) { log.debug(sm.getString("catalina.configFail", "server-embed.xml"), e); } } } if (inputStream == null || inputSource == null) { if (file == null) { log.warn(sm.getString("catalina.configFail", getConfigFile() + "] or [server-embed.xml]")); } else { log.warn(sm.getString("catalina.configFail", file.getAbsolutePath())); if (file.exists() && !file.canRead()) { log.warn("Permissions incorrect, read permission is not allowed on the file."); } } return; } try { inputSource.setByteStream(inputStream); digester.push(this); digester.parse(inputSource); } catch (SAXParseException spe) { log.warn("Catalina.start using " + getConfigFile() + ": " + spe.getMessage()); return; } catch (Exception e) { log.warn("Catalina.start using " + getConfigFile() + ": " , e); return; } } finally { if (inputStream != null) { try { inputStream.close(); } catch (IOException e) { // Ignore } } } // 设置Server属性 getServer().setCatalina(this); getServer().setCatalinaHome(Bootstrap.getCatalinaHomeFile()); getServer().setCatalinaBase(Bootstrap.getCatalinaBaseFile()); // Stream redirection initStreams(); // Start the new server try { // Server初始化 getServer().init(); } catch (LifecycleException e) { if (Boolean.getBoolean("org.apache.catalina.startup.EXIT_ON_INIT_FAILURE")) { throw new java.lang.Error(e); } else { log.error("Catalina.start", e); } } long t2 = System.nanoTime(); if(log.isInfoEnabled()) { log.info("Initialization processed in " + ((t2 - t1) / 1000000) + " ms"); } }
4) StandardServer初始化，StandardServer对应的多个Service也会被一一初始化

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(); } } // 初始化所有的service for (int i = 0; i < services.length; i++) { services[i].init(); } }
5) StandardService初始化，StandardService分别初始化Engine, Executor和Connector

protected void initInternal() throws LifecycleException { super.initInternal(); // 初始化Engine if (engine != null) { engine.init(); } // 初始化所有的Executor for (Executor executor : findExecutors()) { if (executor instanceof JmxEnabled) { ((JmxEnabled) executor).setDomain(getDomain()); } executor.init(); } // Initialize mapper listener mapperListener.init(); // 初始化所有的Connector synchronized (connectorsLock) { for (Connector connector : connectors) { try { connector.init(); } catch (Exception e) { String message = sm.getString("standardService.connector.initFailed", connector); log.error(message, e); if (Boolean.getBoolean("org.apache.catalina.startup.EXIT_ON_INIT_FAILURE")) throw new LifecycleException(message); } } } }
6) Engine初始化

protected void initInternal() throws LifecycleException { // Ensure that a Realm is present before any attempt is made to start // one. This will create the default NullRealm if necessary. getRealm(); super.initInternal(); }
7) Connector初始化

protected void initInternal() throws LifecycleException { super.initInternal(); // Initialize adapter adapter = new CoyoteAdapter(this); protocolHandler.setAdapter(adapter); // Make sure parseBodyMethodsSet has a default if( null == parseBodyMethodsSet ) { setParseBodyMethods(getParseBodyMethods()); } if (protocolHandler.isAprRequired() && !AprLifecycleListener.isAprAvailable()) { throw new LifecycleException(sm.getString("coyoteConnector.protocolHandlerNoApr", getProtocolHandlerClassName())); } if (AprLifecycleListener.isAprAvailable() && AprLifecycleListener.getUseOpenSSL() && protocolHandler instanceof AbstractHttp11JsseProtocol) { AbstractHttp11JsseProtocol jsseProtocolHandler = (AbstractHttp11JsseProtocol) protocolHandler; if (jsseProtocolHandler.isSSLEnabled() && jsseProtocolHandler.getSslImplementationName() == null) { // OpenSSL is compatible with the JSSE configuration, so use it if APR is available jsseProtocolHandler.setSslImplementationName(OpenSSLImplementation.class.getName()); } } try { // 初始化Protocol，Bio或者Nio，开启监听端口 protocolHandler.init(); } catch (Exception e) { throw new LifecycleException(sm.getString("coyoteConnector.protocolHandlerInitializationFailed"), e); } }
8) Bootstrap start方法启动Server，Bootstrap启动Server最终也委托给Catalina的start方法

public void start() throws Exception { if (catalinaDaemon == null) init(); // Cataline start方法调用 Method method = catalinaDaemon.getClass().getMethod("start", (Class[]) null); method.invoke(catalinaDaemon, (Object[]) null); }
9) Catalina start方法启动Server

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(); // 启动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(); } }
10) StandardServer启动

protected void startInternal() throws LifecycleException { fireLifecycleEvent(CONFIGURE_START_EVENT, null); setState(LifecycleState.STARTING); globalNamingResources.start(); // 启动所有的Service synchronized (servicesLock) { for (int i = 0; i < services.length; i++) { services[i].start(); } } }
11) StandardService启动

protected void startInternal() throws LifecycleException { if(log.isInfoEnabled()) log.info(sm.getString("standardService.start.name", this.name)); setState(LifecycleState.STARTING); // 启动Engine if (engine != null) { synchronized (engine) { engine.start(); } } // 启动所有的Executor synchronized (executors) { for (Executor executor: executors) { executor.start(); } } mapperListener.start(); // 启动所有的Connector synchronized (connectorsLock) { for (Connector connector: connectors) { try { // If it has already failed, don\'t try and start it if (connector.getState() != LifecycleState.FAILED) { connector.start(); } } catch (Exception e) { log.error(sm.getString( "standardService.connector.startFailed", connector), e); } } } }
12) StandardEngine启动

protected synchronized void startInternal() throws LifecycleException { // Log our server identification information if(log.isInfoEnabled()) log.info( "Starting Servlet Engine: " + ServerInfo.getServerInfo()); // Standard container startup super.startInternal(); }
13) Connector启动

protected void startInternal() throws LifecycleException { // Validate settings before starting if (getPort() < 0) { throw new LifecycleException(sm.getString( "coyoteConnector.invalidPort", Integer.valueOf(getPort()))); } setState(LifecycleState.STARTING); try { // 启动Protocol, Bio或者Nio，开始接受请求 protocolHandler.start(); } catch (Exception e) { String errPrefix = ""; if(this.service != null) { errPrefix += "service.getName(): "" + this.service.getName() + ""; "; } throw new LifecycleException (errPrefix + " " + sm.getString ("coyoteConnector.protocolHandlerStartFailed"), e); } }
总结
从Tomcat启动过程来看，Bootstrap是Tomcat的启动入口，而Tomcat只是对Catalina的调用，所有的加载、启动和关闭操作最终都由Catalina完成，Catalina负责控制Server的启动、加载和关闭，Server的加载、启动和关闭内部都是依次对Engine, Executor和Connector的调用。通过Tomcat启动过程，我们可以清楚地了解Tomcat内部架构，后面可以更好地了解Tomcat内部机制。