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。
1 public static void main(String args[]) { 2 3 if (daemon == null) { 4 // Don‘t set daemon until init() has completed 5 Bootstrap bootstrap = new Bootstrap(); 6 try { 7 bootstrap.init(); 8 } catch (Throwable t) { 9 handleThrowable(t); 10 t.printStackTrace(); 11 return; 12 } 13 daemon = bootstrap; 14 } else { 15 // When running as a service the call to stop will be on a new 16 // thread so make sure the correct class loader is used to prevent 17 // a range of class not found exceptions. 18 Thread.currentThread().setContextClassLoader(daemon.catalinaLoader); 19 } 20 21 try { 22 String command = "start"; 23 if (args.length > 0) { 24 command = args[args.length - 1]; 25 } 26 27 if (command.equals("startd")) { 28 args[args.length - 1] = "start"; 29 daemon.load(args); 30 daemon.start(); 31 } else if (command.equals("stopd")) { 32 args[args.length - 1] = "stop"; 33 daemon.stop(); 34 } else if (command.equals("start")) { 35 daemon.setAwait(true); 36 daemon.load(args); 37 daemon.start(); 38 } else if (command.equals("stop")) { 39 daemon.stopServer(args); 40 } else if (command.equals("configtest")) { 41 daemon.load(args); 42 if (null==daemon.getServer()) { 43 System.exit(1); 44 } 45 System.exit(0); 46 } else { 47 log.warn("Bootstrap: command \"" + command + "\" does not exist."); 48 } 49 } catch (Throwable t) { 50 // Unwrap the Exception for clearer error reporting 51 if (t instanceof InvocationTargetException && 52 t.getCause() != null) { 53 t = t.getCause(); 54 } 55 handleThrowable(t); 56 t.printStackTrace(); 57 System.exit(1); 58 } 59 60 }
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：
1 public void init() throws Exception { 2 3 initClassLoaders(); 4 5 Thread.currentThread().setContextClassLoader(catalinaLoader); 6 7 SecurityClassLoad.securityClassLoad(catalinaLoader); 8 9 // Load our startup class and call its process() method 10 if (log.isDebugEnabled()) 11 log.debug("Loading startup class"); 12 Class<?> startupClass = 13 catalinaLoader.loadClass 14 ("org.apache.catalina.startup.Catalina"); 15 Object startupInstance = startupClass.newInstance(); 16 17 // Set the shared extensions class loader 18 if (log.isDebugEnabled()) 19 log.debug("Setting startup class properties"); 20 String methodName = "setParentClassLoader"; 21 Class<?> paramTypes[] = new Class[1]; 22 paramTypes[0] = Class.forName("java.lang.ClassLoader"); 23 Object paramValues[] = new Object[1]; 24 paramValues[0] = sharedLoader; 25 Method method = 26 startupInstance.getClass().getMethod(methodName, paramTypes); 27 method.invoke(startupInstance, paramValues); 28 29 catalinaDaemon = startupInstance; 30 31 }
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。
1 public void start() { 2 3 if (getServer() == null) { 4 load(); 5 } 6 7 if (getServer() == null) { 8 log.fatal("Cannot start server. Server instance is not configured."); 9 return; 10 } 11 12 long t1 = System.nanoTime(); 13 14 // Start the new server 15 try { 16 getServer().start(); 17 } catch (LifecycleException e) { 18 log.fatal(sm.getString("catalina.serverStartFail"), e); 19 try { 20 getServer().destroy(); 21 } catch (LifecycleException e1) { 22 log.debug("destroy() failed for failed Server ", e1); 23 } 24 return; 25 } 26 27 long t2 = System.nanoTime(); 28 if(log.isInfoEnabled()) { 29 log.info("Server startup in " + ((t2 - t1) / 1000000) + " ms"); 30 } 31 32 // Register shutdown hook 33 if (useShutdownHook) { 34 if (shutdownHook == null) { 35 shutdownHook = new CatalinaShutdownHook(); 36 } 37 Runtime.getRuntime().addShutdownHook(shutdownHook); 38 39 // If JULI is being used, disable JULI‘s shutdown hook since 40 // shutdown hooks run in parallel and log messages may be lost 41 // if JULI‘s hook completes before the CatalinaShutdownHook() 42 LogManager logManager = LogManager.getLogManager(); 43 if (logManager instanceof ClassLoaderLogManager) { 44 ((ClassLoaderLogManager) logManager).setUseShutdownHook( 45 false); 46 } 47 } 48 49 if (await) { 50 await(); 51 stop(); 52 } 53 }
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：
1 /** 2 * Start a new server instance. 3 */ 4 public void load() { 5 6 long t1 = System.nanoTime(); 7 8 initDirs(); 9 10 // Before digester - it may be needed 11 initNaming(); 12 13 // Create and execute our Digester 14 Digester digester = createStartDigester(); 15 16 InputSource inputSource = null; 17 InputStream inputStream = null; 18 File file = null; 19 file = configFile(); 20 inputStream = new FileInputStream(file); 21 inputSource = new InputSource(file.toURI().toURL().toString()); 22 inputSource.setByteStream(inputStream); 23 digester.push(this); 24 digester.parse(inputSource); 25 26 27 getServer().setCatalina(this); 28 getServer().setCatalinaHome(Bootstrap.getCatalinaHomeFile()); 29 getServer().setCatalinaBase(Bootstrap.getCatalinaBaseFile()); 30 31 // Stream redirection 32 initStreams(); 33 34 // Start the new server 35 getServer().init(); 36 }
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：
1 public final synchronized void init() throws LifecycleException { 2 if (!state.equals(LifecycleState.NEW)) { 3 invalidTransition(Lifecycle.BEFORE_INIT_EVENT); 4 } 5 6 try { 7 setStateInternal(LifecycleState.INITIALIZING, null, false); 8 initInternal(); 9 setStateInternal(LifecycleState.INITIALIZED, null, false); 10 } catch (Throwable t) { 11 ExceptionUtils.handleThrowable(t); 12 setStateInternal(LifecycleState.FAILED, null, false); 13 throw new LifecycleException( 14 sm.getString("lifecycleBase.initFail",toString()), t); 15 } 16 } 17 18 19 public final synchronized void start() throws LifecycleException { 20 21 if (LifecycleState.STARTING_PREP.equals(state) || LifecycleState.STARTING.equals(state) || 22 LifecycleState.STARTED.equals(state)) { 23 24 if (log.isDebugEnabled()) { 25 Exception e = new LifecycleException(); 26 log.debug(sm.getString("lifecycleBase.alreadyStarted", toString()), e); 27 } else if (log.isInfoEnabled()) { 28 log.info(sm.getString("lifecycleBase.alreadyStarted", toString())); 29 } 30 31 return; 32 } 33 34 if (state.equals(LifecycleState.NEW)) { 35 init(); 36 } else if (state.equals(LifecycleState.FAILED)) { 37 stop(); 38 } else if (!state.equals(LifecycleState.INITIALIZED) && 39 !state.equals(LifecycleState.STOPPED)) { 40 invalidTransition(Lifecycle.BEFORE_START_EVENT); 41 } 42 43 try { 44 setStateInternal(LifecycleState.STARTING_PREP, null, false); 45 startInternal(); 46 if (state.equals(LifecycleState.FAILED)) { 47 stop(); 48 } else if (!state.equals(LifecycleState.STARTING)) { 49 invalidTransition(Lifecycle.AFTER_START_EVENT); 50 } else { 51 setStateInternal(LifecycleState.STARTED, null, false); 52 } 53 } catch (Throwable t) { 54 ExceptionUtils.handleThrowable(t); 55 setStateInternal(LifecycleState.FAILED, null, false); 56 throw new LifecycleException(sm.getString("lifecycleBase.startFail", toString()), t); 57 } 58 }
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。
1 /** 2 * Invoke a pre-startup initialization. This is used to allow connectors 3 * to bind to restricted ports under Unix operating environments. 4 */ 5 @Override 6 protected void initInternal() throws LifecycleException { 7 8 super.initInternal(); 9 10 // Register global String cache 11 // Note although the cache is global, if there are multiple Servers 12 // present in the JVM (may happen when embedding) then the same cache 13 // will be registered under multiple names 14 onameStringCache = register(new StringCache(), "type=StringCache"); 15 16 // Register the MBeanFactory 17 MBeanFactory factory = new MBeanFactory(); 18 factory.setContainer(this); 19 onameMBeanFactory = register(factory, "type=MBeanFactory"); 20 21 // Register the naming resources 22 globalNamingResources.init(); 23 24 // Populate the extension validator with JARs from common and shared 25 // class loaders 26 if (getCatalina() != null) { 27 ClassLoader cl = getCatalina().getParentClassLoader(); 28 // Walk the class loader hierarchy. Stop at the system class loader. 29 // This will add the shared (if present) and common class loaders 30 while (cl != null && cl != ClassLoader.getSystemClassLoader()) { 31 if (cl instanceof URLClassLoader) { 32 URL[] urls = ((URLClassLoader) cl).getURLs(); 33 for (URL url : urls) { 34 if (url.getProtocol().equals("file")) { 35 try { 36 File f = new File (url.toURI()); 37 if (f.isFile() && 38 f.getName().endsWith(".jar")) { 39 ExtensionValidator.addSystemResource(f); 40 } 41 } catch (URISyntaxException e) { 42 // Ignore 43 } catch (IOException e) { 44 // Ignore 45 } 46 } 47 } 48 } 49 cl = cl.getParent(); 50 } 51 } 52 // Initialize our defined Services 53 for (int i = 0; i < services.length; i++) { 54 services[i].init(); 55 } 56 }
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：
1 protected void startInternal() throws LifecycleException { 2 3 fireLifecycleEvent(CONFIGURE_START_EVENT, null); 4 setState(LifecycleState.STARTING); 5 6 globalNamingResources.start(); 7 8 // Start our defined Services 9 synchronized (servicesLock) { 10 for (int i = 0; i < services.length; i++) { 11 services[i].start(); 12 } 13 } 14 }
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-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
