本篇结构：

• 前言
• 总览一下main方法
• init
• setAwait
• load
• start

一、前言

上一篇中介绍了startup.bat和catalina.bat脚本。了解到日常双击startup.bat启动tomcat，其实是来到catalina.bat脚本中，由catalina.bat脚本去执行org.apache.catalina.startup.Bootstrap这个类中的main方法。

这里就来看看Bootstrap类的main方法做了些什么。

二、总览一下main方法

public static void main(String args[]) {

    //上部分
    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);
    }

    //下部分
    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);
    }

}

可以看出main方法大致可以分为两部分。

上部分是实例化一个Bootstrap对象，并调用init方法，然后赋值给daemon变量，当然如果daemon已经不是空了，说明已经初始化过了，就将daemon.catalinaLoader直接设置到当前线程（daemon.catalinaLoader是用来加载tomcat内部服务器所需类的类加载器）。

下部分是根据传递进来的参数决定走哪一步，当双击startup.bat时，传进来的是start，所以会来到这段：

else if (command.equals("start")) {
    daemon.setAwait(true);
    daemon.load(args);
    daemon.start();
}

这里主要是调用三个方法，setAwait，load和start。

所以对于Bootstrap重要关注的就是init，setAwait，load和start这四个方法。

三、init

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.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;

}

init方法调用initClassLoaders初始化类加载器，然后将初始化好的catalinaLoader设置到当前线程，接着通过反射调用org.apache.catalina.startup.Catalina类的setParentClassLoader，将sharedLoader传入。

3.1、首先看initClassLoaders方法：

private void initClassLoaders() {
    try {
        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();
        }
        catalinaLoader = createClassLoader("server", commonLoader);
        sharedLoader = createClassLoader("shared", commonLoader);
    } catch (Throwable t) {
        handleThrowable(t);
        log.error("Class loader creation threw exception", t);
        System.exit(1);
    }
}

在类加载器篇中有提到，这里初始化三个类加载器，分别是commonLoader，catalinaLoader，sharedLoader并建立他们之间的关系，catalinaLoader和sharedLoader的parent是commonLoader。

这三个类加载器都是通过createClassLoader建立的：

private ClassLoader createClassLoader(String name, ClassLoader parent)
        throws Exception {

    String value = CatalinaProperties.getProperty(name + ".loader");
    if ((value == null) || (value.equals("")))
        return parent;

    value = replace(value);

    List<Repository> repositories = new ArrayList<>();

    String[] repositoryPaths = getPaths(value);

    for (String repository : repositoryPaths) {
        // Check for a JAR URL repository
        try {
            @SuppressWarnings("unused")
            URL url = new URL(repository);
            repositories.add(
                    new Repository(repository, RepositoryType.URL));
            continue;
        } catch (MalformedURLException e) {
            // Ignore
        }

        // Local repository
        if (repository.endsWith("*.jar")) {
            repository = repository.substring
                (0, repository.length() - "*.jar".length());
            repositories.add(
                    new Repository(repository, RepositoryType.GLOB));
        } else if (repository.endsWith(".jar")) {
            repositories.add(
                    new Repository(repository, RepositoryType.JAR));
        } else {
            repositories.add(
                    new Repository(repository, RepositoryType.DIR));
        }
    }

    return ClassLoaderFactory.createClassLoader(repositories, parent);
}

createClassLoader首先回去读取CatalinaProperties中的common.loader，server.loader，shared.loader三个属性，进入CatalinaProperties类中会发现这三个属性来自conf/catalina.properties文件。

接着往下createClassLoader会将common.loader，server.loader，shared.loader三个属性中的值获取然后解析成Repository，然后交给ClassLoaderFactory.createClassLoader方法去创建类加载器，最后可以实现三个不同的类加载器分别加载不同目录下的类。

当然要说清楚的是，默认情况下，catalina.properties中server.loader，shared.loader并没有配置值，三个类加载是同一个，默认加载{catalina.home}/lib目录下的类和jar包。

如果想配置对所有web应用都可见但对tomcat内部服务器不可见的类，此时应该在catalina.properties文件中的shared.loader下进行配置。

Thread.currentThread().setContextClassLoader(catalinaLoader)将catalinaLoader设置为Tomcat主线程的线程上下文类加载器。

3.2、SecurityClassLoad.securityClassLoad(catalinaLoader)

SecurityClassLoad.securityClassLoad用于线程安全的加载tomcat容器所需的class。

当然，要使这个方法真正起作用，需要启动tomcat安全管理器，由代码可知：

public static void securityClassLoad(ClassLoader loader) throws Exception {
    securityClassLoad(loader, true);
}


static void securityClassLoad(ClassLoader loader, boolean requireSecurityManager)
        throws Exception {

    if (requireSecurityManager && System.getSecurityManager() == null) {
        return;
    }

    loadCorePackage(loader);
    loadCoyotePackage(loader);
    loadLoaderPackage(loader);
    loadRealmPackage(loader);
    loadServletsPackage(loader);
    loadSessionPackage(loader);
    loadUtilPackage(loader);
    loadJavaxPackage(loader);
    loadConnectorPackage(loader);
    loadTomcatPackage(loader);
}

如果没启用安全管理器，System.getSecurityManager()=null，直接return。

可以通过命令行的方式启功安全管理器：
catalina.bat run -security或者startup.bat -security

一旦启动了安全管理器，就会根据conf/catalina.policy文件定义的提供默认的安全策略，securityClassLoad方法中System.getSecurityManager()不再等于null，于是就会去执行一系列加载方法，将tomcat的class加载进来。

想了解to吗tomcat的安全策略，可以参考下这篇博文：

你很少使用的安全管理SecurityManager

3.3、通过反射实例化catalina

Class<?> startupClass = catalinaLoader.loadClass("org.apache.catalina.startup.Catalina");
Object startupInstance = startupClass.getConstructor().newInstance();

// Set the shared extensions class loader
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;

可以看到这部分代码是通过反射来实例化catalina，然后反射调用setParentClassLoader将sharedLoader传入到catalina实例中。

为什么不直接通过Bootstrap类直接启动tomcat，而是通过反射生成Catalina实例启动？

再查看tomcat目录结构时应该发现，Bootstrap并不在$CATALINA_HOME/lib目录下，而是在$CATALINA_HOME/bin目录中，Bootstrap和Catalina松耦合（通过反射调用Catalina），它直接依赖JRE运行并为Tomcat应用服务器创建共享类加载器，用于构造Catalina和整个tomcat服务器。实现了启动入口和核心环境的解耦，简化了启动。

四、setAwait

来看setAwait：

public void setAwait(boolean await)
        throws Exception {

    Class<?> paramTypes[] = new Class[1];
    paramTypes[0] = Boolean.TYPE;
    Object paramValues[] = new Object[1];
    paramValues[0] = Boolean.valueOf(await);
    Method method =
        catalinaDaemon.getClass().getMethod("setAwait", paramTypes);
    method.invoke(catalinaDaemon, paramValues);

}

通过反射调用catalina的setAwait方法，其设置的值是留给后面用的，当Catalina将Tomcat的所有组件启动之后，会检查await属性，如果为true，会调用Catalina.await()，而Catalina.await()又会调用其内部的Server的await()。

public void start() {

    ...
        
    if (await) {
        await();
        stop();
    }
}

Server.await()包含一个while循环，此循环用于监听指定socket端口(默认为8005)的连接，当某个连接传入的参数为”SHUTDOWN”(默认为”SHUTDOWN”)时，终止此while循环(端口号和终止while循环的参数，在server.xml的Server标签设置)。

Server.await()用来维持Bootstrap的main方法(main thread)处于运行状态，而线程池中监听http请求的线程是守护线程(daemon thread)。

当Tomcat的指定端口接收到关闭命令时，Server.await()内的while循环终止，然后Catalina会调用stop()方法，关闭Tomcat的所有组件，最终Bootstrap的main thread终止，Tomcat关闭。

五、load

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;
    }
    Method method =
        catalinaDaemon.getClass().getMethod(methodName, paramTypes);
    if (log.isDebugEnabled())
        log.debug("Calling startup class " + method);
    method.invoke(catalinaDaemon, param);

}

该方法通过反射调用catalina的load方法。

catalina的load方法首先初始化目录（initDirs）和初始化命名服务（initNaming），然后是createStartDigester（要了解这个方法，应该先了解一下Digester，它是apache的一个开源组件，通过它可以很方便的从xml文件生成java对象），该方法初始化Digester，为Xml的标签即解析模式增加处理规则rule。

关于Digester，可以参考这篇博文：

利用Digester解析xml文件

来看createStartDigester方法：

protected Digester createStartDigester() {
    long t1=System.currentTimeMillis();
    // Initialize the digester
    Digester digester = new Digester();
    digester.setValidating(false);
    digester.setRulesValidation(true);
    Map<Class<?>, List<String>> fakeAttributes = new HashMap<>();
    List<String> attrs = new ArrayList<>();
    attrs.add("className");
    fakeAttributes.put(Object.class, attrs);
    digester.setFakeAttributes(fakeAttributes);
    digester.setUseContextClassLoader(true);

    // Configure the actions we will be using
    digester.addObjectCreate("Server",
                             "org.apache.catalina.core.StandardServer",
                             "className");
    digester.addSetProperties("Server");
    digester.addSetNext("Server",
                        "setServer",
                        "org.apache.catalina.Server");

    digester.addObjectCreate("Server/GlobalNamingResources",
                             "org.apache.catalina.deploy.NamingResourcesImpl");
    digester.addSetProperties("Server/GlobalNamingResources");
    digester.addSetNext("Server/GlobalNamingResources",
                        "setGlobalNamingResources",
                        "org.apache.catalina.deploy.NamingResourcesImpl");

    digester.addObjectCreate("Server/Listener",
                             null, // MUST be specified in the element
                             "className");
    digester.addSetProperties("Server/Listener");
    digester.addSetNext("Server/Listener",
                        "addLifecycleListener",
                        "org.apache.catalina.LifecycleListener");

    digester.addObjectCreate("Server/Service",
                             "org.apache.catalina.core.StandardService",
                             "className");
    digester.addSetProperties("Server/Service");
    digester.addSetNext("Server/Service",
                        "addService",
                        "org.apache.catalina.Service");

    digester.addObjectCreate("Server/Service/Listener",
                             null, // MUST be specified in the element
                             "className");
    digester.addSetProperties("Server/Service/Listener");
    digester.addSetNext("Server/Service/Listener",
                        "addLifecycleListener",
                        "org.apache.catalina.LifecycleListener");

    //Executor
    digester.addObjectCreate("Server/Service/Executor",
                     "org.apache.catalina.core.StandardThreadExecutor",
                     "className");
    digester.addSetProperties("Server/Service/Executor");

    digester.addSetNext("Server/Service/Executor",
                        "addExecutor",
                        "org.apache.catalina.Executor");


    digester.addRule("Server/Service/Connector",
                     new ConnectorCreateRule());
    digester.addRule("Server/Service/Connector", new SetAllPropertiesRule(
            new String[]{"executor", "sslImplementationName", "protocol"}));
    digester.addSetNext("Server/Service/Connector",
                        "addConnector",
                        "org.apache.catalina.connector.Connector");

    digester.addObjectCreate("Server/Service/Connector/SSLHostConfig",
                             "org.apache.tomcat.util.net.SSLHostConfig");
    digester.addSetProperties("Server/Service/Connector/SSLHostConfig");
    digester.addSetNext("Server/Service/Connector/SSLHostConfig",
            "addSslHostConfig",
            "org.apache.tomcat.util.net.SSLHostConfig");

    digester.addRule("Server/Service/Connector/SSLHostConfig/Certificate",
                     new CertificateCreateRule());
    digester.addRule("Server/Service/Connector/SSLHostConfig/Certificate",
                     new SetAllPropertiesRule(new String[]{"type"}));
    digester.addSetNext("Server/Service/Connector/SSLHostConfig/Certificate",
                        "addCertificate",
                        "org.apache.tomcat.util.net.SSLHostConfigCertificate");

    digester.addObjectCreate("Server/Service/Connector/SSLHostConfig/OpenSSLConf",
                             "org.apache.tomcat.util.net.openssl.OpenSSLConf");
    digester.addSetProperties("Server/Service/Connector/SSLHostConfig/OpenSSLConf");
    digester.addSetNext("Server/Service/Connector/SSLHostConfig/OpenSSLConf",
                        "setOpenSslConf",
                        "org.apache.tomcat.util.net.openssl.OpenSSLConf");

    digester.addObjectCreate("Server/Service/Connector/SSLHostConfig/OpenSSLConf/OpenSSLConfCmd",
                             "org.apache.tomcat.util.net.openssl.OpenSSLConfCmd");
    digester.addSetProperties("Server/Service/Connector/SSLHostConfig/OpenSSLConf/OpenSSLConfCmd");
    digester.addSetNext("Server/Service/Connector/SSLHostConfig/OpenSSLConf/OpenSSLConfCmd",
                        "addCmd",
                        "org.apache.tomcat.util.net.openssl.OpenSSLConfCmd");

    digester.addObjectCreate("Server/Service/Connector/Listener",
                             null, // MUST be specified in the element
                             "className");
    digester.addSetProperties("Server/Service/Connector/Listener");
    digester.addSetNext("Server/Service/Connector/Listener",
                        "addLifecycleListener",
                        "org.apache.catalina.LifecycleListener");

    digester.addObjectCreate("Server/Service/Connector/UpgradeProtocol",
                              null, // MUST be specified in the element
                              "className");
    digester.addSetProperties("Server/Service/Connector/UpgradeProtocol");
    digester.addSetNext("Server/Service/Connector/UpgradeProtocol",
                        "addUpgradeProtocol",
                        "org.apache.coyote.UpgradeProtocol");

    // Add RuleSets for nested elements
    digester.addRuleSet(new NamingRuleSet("Server/GlobalNamingResources/"));
    digester.addRuleSet(new EngineRuleSet("Server/Service/"));
    digester.addRuleSet(new HostRuleSet("Server/Service/Engine/"));
    digester.addRuleSet(new ContextRuleSet("Server/Service/Engine/Host/"));
    addClusterRuleSet(digester, "Server/Service/Engine/Host/Cluster/");
    digester.addRuleSet(new NamingRuleSet("Server/Service/Engine/Host/Context/"));

    // When the 'engine' is found, set the parentClassLoader.
    digester.addRule("Server/Service/Engine",
                     new SetParentClassLoaderRule(parentClassLoader));
    addClusterRuleSet(digester, "Server/Service/Engine/Cluster/");

    long t2=System.currentTimeMillis();
    if (log.isDebugEnabled()) {
        log.debug("Digester for server.xml created " + ( t2-t1 ));
    }
    return digester;

}

该方法初始化digester，创建一系列解析规则，然后在load方法中会调用：

digester.parse(inputSource);

可见digester解析的源是inputSource，而inputSource是来自于conf/server.xml：

file = configFile();
inputStream = new FileInputStream(file);
inputSource = new InputSource(file.toURI().toURL().toString());

configFile：

 protected File configFile() {

    //protected String configFile = "conf/server.xml";
    File file = new File(configFile);
    if (!file.isAbsolute()) {
        file = new File(Bootstrap.getCatalinaBase(), configFile);
    }
    return file;

}

这样便创建出了StandardServer对象，接着便调用getServer().init();

init方法来自StandardServer的父类LifecycleBase：

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) {
        handleSubClassException(t, "lifecycleBase.initFail", toString());
    }
}

具体实现是在子类的initInternal方法中，在调用initInternal方法前后都会设置状态，LifecycleState.INITIALIZING代表正在初始化，LifecycleState.INITIALIZED表示初始化完成，相应会触发生命周期事件。

在StandardServer的initInternal方法中会调用子组件Services的init方法，并依次传递下去，完成所有组件的init。

可见catalina的load方法主要是根据conf/server.xml配置文件利用Digester创建服务器组件，然后调用Server的init方法，逐层次的实现所有组件的初始化。

六、start

最后看下start方法：

public void start()
        throws Exception {
    if( catalinaDaemon==null ) init();

    Method method = catalinaDaemon.getClass().getMethod("start", (Class [] )null);
    method.invoke(catalinaDaemon, (Object [])null);

}

同样也是通过反射调用catalina的start方法：

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();
    }
}

该方法主要触发StandardServer的start方法，StandardServer的start方法同init方法一样来自LifecycleBase，主要是改变生命周期的状态，同时触发相应的生命周期时间，具体的执行逻辑交由具体的子类startInternal方法实现：

public final synchronized void start() throws LifecycleException {

    if (LifecycleState.STARTING_PREP.equals(state) || LifecycleState.STARTING.equals(state) ||
            LifecycleState.STARTED.equals(state)) {
        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)) {
            // This is a 'controlled' failure. The component put itself into the
            // FAILED state so call stop() to complete the clean-up.
            stop();
        } else if (!state.equals(LifecycleState.STARTING)) {
            // Shouldn't be necessary but acts as a check that sub-classes are
            // doing what they are supposed to.
            invalidTransition(Lifecycle.AFTER_START_EVENT);
        } else {
            setStateInternal(LifecycleState.STARTED, null, false);
        }
    } catch (Throwable t) {
        // This is an 'uncontrolled' failure so put the component into the
        // FAILED state and throw an exception.
        handleSubClassException(t, "lifecycleBase.startFail", toString());
    }
}

在StandardServer的startInternal方法中会调用子组件service的start方法，并依次调用其他组件的start方法。

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();
        }
    }
}

所以同load方法很相似，start方法主要是实现各组件的start方法依次调用，可以用一张图来理解：

还应该看到Catalina的start方法会使用前面的setAwait方法传递的值，为true时，会在8005端口监听，保证主线程一直在运行，直到收到SHUTDOWN命令。