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。
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。
org.apache.catalina.startup.Catalina#start
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：
org.apache.catalina.util.LifecycleBase#start
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：
org.apache.catalina.core.StandardServer#startInternal
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：