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Tomcat源码分析三：Tomcat启动加载过程（一）的源码解析

Tomcat启动加载过程（一）的源码解析

今天，我将分享用源码的方式讲解Tomcat启动的加载过程，关于Tomcat的架构请参阅《Tomcat源码分析二：先看看Tomcat的整体架构》一文。

先看看应用情况

在《Servlet与Tomcat运行示例》一文中，我详细的记录了Tomcat是如何启动一个Servlet的程序的步骤。其中，第6步骤是启动Tomcat，也就是在windows系统上执行startup.bat，在linux操作系统上执行startup.sh的脚本。那么，我们就从这个脚本出发，走进Tomcat，看看它是如何启动的？这里，我们以startup.sh为例，windows端的startup.bat类似。

startup.sh的内容是什么？

我们先看看tomcat的启动脚本startup.sh的内容是什么，先看看其脚本内容(省略部分注释)，如下：

#!/bin/sh

# -----------------------------------------------------------------------------
# Start Script for the CATALINA Server
# -----------------------------------------------------------------------------

# Better OS/400 detection: see Bugzilla 31132
os400=false
case "`uname`" in
OS400*) os400=true;;
esac

# resolve links - $0 may be a softlink
PRG="$0"

while [ -h "$PRG" ] ; do
  ls=`ls -ld "$PRG"`
  link=`expr "$ls" : '.*-> /(.*/)$'`
  if expr "$link" : '/.*' > /dev/null; then
    PRG="$link"
  else
    PRG=`dirname "$PRG"`/"$link"
  fi
done

PRGDIR=`dirname "$PRG"`
EXECUTABLE=catalina.sh

# Check that target executable exists
if $os400; then
  # -x will Only work on the os400 if the files are:
  # 1. owned by the user
  # 2. owned by the PRIMARY group of the user
  # this will not work if the user belongs in secondary groups
  eval
else
  if [ ! -x "$PRGDIR"/"$EXECUTABLE" ]; then
    echo "Cannot find $PRGDIR/$EXECUTABLE"
    echo "The file is absent or does not have execute permission"
    echo "This file is needed to run this program"
    exit 1
  fi
fi

exec "$PRGDIR"/"$EXECUTABLE" start "$@"

提取其中主要的几句：

PRGDIR=`dirname "$PRG"`
EXECUTABLE=catalina.sh
exec "$PRGDIR"/"$EXECUTABLE" start "$@"

简而概之，该脚本的执行内容为：调用catalina.sh脚本。下面，我们继续来看下catalina.sh脚本的内容

catalina.sh脚本

由于catalina.sh脚本内容比较多，这里提取一些重要的内容，然后解释其用途：

Tomcat源码分析三：Tomcat启动加载过程（一）的源码解析

再简要的描述下在catalina.sh中作用：完成环境检查、环境初始化、参数初始化、启动操作步骤。注意一下上图中被绿色框出来的内容，可以看到其调用执行的是org.apache.catalina.startup.Bootstrap类，并且传输过去的command指令为start。

回归Java代码

Bootstrap类进行了什么操作呢？

接下来，我们带着这几个问题来去探索一下Bootstrap类：

Bootstrap类在接收到start指令后要去干什么？
Bootstrap类在启动过程中的职责是什么？

下面，我们带着上面的几个问题来具体的探讨一下Tomcat的源码。先来看看Bootstrap类的main方法：

public static void main(String args[]) {

        synchronized (daemonLock) {
            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 {
                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();
                if (null == daemon.getServer()) {
                    System.exit(1);
                }
            } 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);
        }
    }

从这段代码中，可以看出，其主要实现了两个功能：

初始化一个守护进程变量daemon
加载catalina.sh传递过来的参数，解析catalina.sh传递过来的指令，并按照指令执行程序，控制守护进程daemon的启停等操作

bootstrap.init();有什么操作呢？

针对上面的两个功能，我们进入到 init()方法看下有什么操作，先看下init()方法的代码：

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;
    }

在init()方法中，首先执行的方法initClassLoaders()的作用是初始化三个类加载器，代码如下：

/**
     * Daemon reference.
     */
    private Object catalinaDaemon = null;

    ClassLoader commonLoader = null;
    ClassLoader catalinaLoader = null;
    ClassLoader sharedLoader = null;

    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);
        }
    }
        
    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);
    }

//  catalina.properties
common.loader="${catalina.base}/lib","${catalina.base}/lib/*.jar","${catalina.home}/lib","${catalina.home}/lib/*.jar"

commonLoader : 根据common.loader属性的配置(通过代码 CatalinaProperties.getProperty(name + ".loader"); 读取：catalina.properties), 创建commonLoader类加载器, 默认情况下顺序加载 ${catalina.base}/lib, ${catalina.base}/lib/ .jar, ${catalina.home}/lib, ${catalina.home}/lib/ .jar 四个目录下的class和jar.
catalinaLoader : 根据server.loader属性的配置, 创建catalinaLoader类加载器，其父类加载其为commonLoader, 默认server.loader属性为空, 直接使用commonLoader.
sharedLoader :根据shared.loader属性配置,创建sharedLoader类加载器，其父类加载其为commonLoader, 默认shared.loader属性为空, 直接使用commonLoader.

当执行完initClassLoaders()方法之后，调用Thread.currentThread().setContextClassLoader(catalinaLoader);设置上下文类加载器为catalinaLoader，从上面解析的情况看，其实设置的上下文类加载器为catalinaLoader的父类commonLoader。

SecurityClassLoad.securityClassLoad(catalinaLoader) 的作用是如果有SecurityManager，提前加载部分类。

之后，通过使用catalinaLoader加载org.apache.catalina.startup.Catalina类，创建实例Catalina并利用反射调用方法setParentClassLoader()，设置Catalina实例的parentClassLoader属性为sharedLoader类加载器（也就是commonLoader）。

最后，设置daemon为新创建的实例Bootstrap。接下来，看一下main()方法下的指令处理。

传递过来的command指令是如何处理的呢？

我们观察一下main()方法的后半段，这里贴一下代码：

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();
            if (null == daemon.getServer()) {
                    System.exit(1);
            }
    } 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) {
    // ...... 省略
}

可以看到，其默认指令为 start ，然后，其根据接收到的参数区分为startd、stopd、start、stop、configtest和其他6种指令情况。这里我们主要看一下start指令的执行逻辑。

daemon.setAwait(true) ：这句代码有什么含义呢，下面我们来具体的分析一下：

/**
     * Set flag.
     * @param await <code>true</code> if the daemon should block
     * @throws Exception Reflection error
     */
    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);
    }

这段代码的主要作用是通过反射调用Catalina.setAwait(true)，主要目的是当启动完成后，阻塞main线程，等待stop命令到来。如果不设置daemon.setAwait(true)，则main线程执行完之后就直接退出了。

daemon.load(args)

daemon.load(args);其实是最终执行的Catalina.load()，在Catalina.load()方法中，主要功能是首先初始化temp目录，然后再初始化naming的一些系统属性，然后获取server.xml配置文件, 创建Digester实例, 开始解析server.xml的操作。

/**
     * Start a new server instance.
     */
    public void load() {

        if (loaded) {
            return;
        }
        loaded = true;

        long t1 = System.nanoTime();

        initDirs();

        // Before digester - it may be needed
        initNaming();

        // Set configuration source
        ConfigFileLoader.setSource(new CatalinaBaseConfigurationSource(Bootstrap.getCatalinaBaseFile(), getConfigFile()));
        File file = configFile();

        // Create and execute our Digester
        Digester digester = createStartDigester();

        try (ConfigurationSource.Resource resource = ConfigFileLoader.getSource().getServerXml()) {
            InputStream inputStream = resource.getInputStream();
            InputSource inputSource = new InputSource(resource.getURI().toURL().toString());
            inputSource.setByteStream(inputStream);
            digester.push(this);
            digester.parse(inputSource);
        } catch (Exception e) {
            log.warn(sm.getString("catalina.configFail", file.getAbsolutePath()), e);
            if (file.exists() && !file.canRead()) {
                log.warn(sm.getString("catalina.incorrectPermissions"));
            }
            return;
        }

        getServer().setCatalina(this);
        getServer().setCatalinaHome(Bootstrap.getCatalinaHomeFile());
        getServer().setCatalinaBase(Bootstrap.getCatalinaBaseFile());

        // Stream redirection
        initStreams();

        // Start the new server
        try {
            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(sm.getString("catalina.initError"), e);
            }
        }

        long t2 = System.nanoTime();
        if(log.isInfoEnabled()) {
            log.info(sm.getString("catalina.init", Long.valueOf((t2 - t1) / 1000000)));
        }
    }

daemon.start() ：启动Tomcat

通过调用daemon.start()启动Tomcat，其内容如下：

/**
     * Start the Catalina daemon.
     * @throws Exception Fatal start error
     */
    public void start() throws Exception {
        if (catalinaDaemon == null) {
            init();
        }

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

程序通过反射的方式调用Catalina.start()方式启动Tomcat，下面，我们看下Catalina.start()方法的实现逻辑：

/**
     * Start a new server instance.
     */
    public void start() {

        if (getServer() == null) {
            load();
        }

        if (getServer() == null) {
            log.fatal(sm.getString("catalina.noServer"));
            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(sm.getString("catalina.startup", Long.valueOf((t2 - t1) / 1000000)));
        }

        // 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();
        }
    }

可以看出，程序调用getServer().start()启动，getServer()方法返回的是一个StandardServer类，继而其调用的是StandardServer.startInternal()方法，在StandardServer中，又调用到StandardService.startInternal()方法。

// StandardServer.java
    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();
            }
        }
        // ......省略部分代码
    }

    protected void startInternal() throws LifecycleException {

        if(log.isInfoEnabled())
            log.info(sm.getString("standardService.start.name", this.name));
        setState(LifecycleState.STARTING);

        // Start our defined Container first
        if (engine != null) {
            synchronized (engine) {
                engine.start();
            }
        }

        synchronized (executors) {
            for (Executor executor: executors) {
                executor.start();
            }
        }

        mapperListener.start();

        // Start our defined Connectors second
        synchronized (connectorsLock) {
            for (Connector connector: connectors) {
                // If it has already failed, don't try and start it
                if (connector.getState() != LifecycleState.FAILED) {
                    connector.start();
                }
            }
        }
    }

注意，这里为什么不是start()方法，而是startInternal()方法呢？原因是StandardServer和StandService类都继承了LifecycleMBeanBase类，而LifecycleMBeanBase类又继承了LifecycleBase类。下面看下LifecycleBase类的start()方法：

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)) {
                // 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());
        }
    }

可以看出，调用start()方法，最终都会调用到startInternal()方法。在下篇文章中，我们将详细看下StandardService.java中的engine.start()、executor.start()、connector.start()都分别启动了什么？敬请期待！