Tomcat源碼分析三：Tomcat啟動加載過程（一）的源碼解析

Tomcat啟動加載過程（一）的源碼解析

今天，我將分享用源碼的方式講解Tomcat啟動的加載過程，關於Tomcat的架構請參閱《Tomcat源碼分析二：先看看Tomcat的整體架構》一文。

先看看應用情況

在《Servlet與Tomcat運行示例》一文中，我詳細的記錄了Tomcat是如何啟動一個Servlet的程序的步驟。其中，第6步驟是啟動Tomcat，也就是在windows系統上執行startup.bat， 在linux操作系統上執行startup.sh的腳本。那么，我們就從這個腳本出發，走進Tomcat，看看它是如何啟動的？這里，我們以startup.sh為例，windows端的startup.bat類似。

startup.sh的內容是什么？

我們先看看tomcat的啟動腳本startup.sh的內容是什么，先看看其腳本內容(省略部分注釋)，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 "$@"

提取其中主要的幾句：

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

簡而概之，該腳本的執行內容為：調用catalina.sh腳本。下面，我們繼續來看下catalina.sh腳本的內容

catalina.sh腳本

由於catalina.sh腳本內容比較多，這里提取一些重要的內容，然后解釋其用途：

再簡要的描述下在catalina.sh中作用：完成環境檢查、環境初始化、參數初始化、啟動操作步驟。注意一下上圖中被綠色框出來的內容，可以看到其調用執行的是org.apache.catalina.startup.Bootstrap類，並且傳輸過去的command指令為start。

回歸Java代碼

Bootstrap類進行了什么操作呢？

接下來，我們帶着這幾個問題來去探索一下Bootstrap類：

• Bootstrap類在接收到start指令后要去干什么？
• Bootstrap類在啟動過程中的職責是什么？

下面，我們帶着上面的幾個問題來具體的探討一下Tomcat的源碼。先來看看Bootstrap類的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);
        }
    }

從這段代碼中，可以看出，其主要實現了兩個功能：

• 初始化一個守護進程變量daemon
• 加載catalina.sh傳遞過來的參數，解析catalina.sh傳遞過來的指令，並按照指令執行程序，控制守護進程daemon的啟停等操作

bootstrap.init();有什么操作呢？

針對上面的兩個功能，我們進入到 init()方法看下有什么操作，先看下init()方法的代碼：

	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，首先執行的方法initClassLoaders()的作用是初始化三個類加載器，代碼如下：

    /**
     * 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: 根據common.loader屬性的配置(通過代碼CatalinaProperties.getProperty(name + ".loader");讀取：catalina.properties), 創建commonLoader類加載器, 默認情況下順序加載 ${catalina.base}/lib, ${catalina.base}/lib/.jar, ${catalina.home}/lib, ${catalina.home}/lib/.jar 四個目錄下的class和jar.
• catalinaLoader: 根據server.loader屬性的配置, 創建catalinaLoader類加載器，其父類加載其為commonLoader, 默認server.loader屬性為空, 直接使用commonLoader.
• sharedLoader:根據shared.loader屬性配置,創建sharedLoader類加載器，其父類加載其為commonLoader, 默認shared.loader屬性為空, 直接使用commonLoader.

當執行完initClassLoaders()方法之后，調用Thread.currentThread().setContextClassLoader(catalinaLoader);設置上下文類加載器為catalinaLoader，從上面解析的情況看，其實設置的上下文類加載器為catalinaLoader的父類commonLoader。

SecurityClassLoad.securityClassLoad(catalinaLoader) 的作用是如果有SecurityManager，提前加載部分類。

之后，通過使用catalinaLoader加載org.apache.catalina.startup.Catalina類，創建實例Catalina並利用反射調用方法setParentClassLoader()，設置Catalina實例的parentClassLoader屬性為sharedLoader類加載器（也就是commonLoader）。

最后，設置daemon為新創建的實例Bootstrap。接下來，看一下main()方法下的指令處理。

傳遞過來的command指令是如何處理的呢？

我們觀察一下main()方法的后半段，這里貼一下代碼：

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，其默認指令為start， 然后，其根據接收到的參數區分為startd、stopd、start、stop、configtest和其他6種指令情況。這里我們主要看一下start指令的執行邏輯。

• daemon.setAwait(true) ：這句代碼有什么含義呢，下面我們來具體的分析一下：

    /**
     * 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);
    }

這段代碼的主要作用是通過反射調用Catalina.setAwait(true)，主要目的是當啟動完成后， 阻塞main線程，等待stop命令到來。 如果不設置daemon.setAwait(true)， 則main線程執行完之后就 直接退出了。

• **daemon.load(args) **
  daemon.load(args);其實是最終執行的Catalina.load()，在Catalina.load()方法中，主要功能是首先初始化temp目錄，然后再初始化naming的一些系統屬性，然后獲取server.xml配置文件, 創建Digester實例, 開始解析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()： 啟動Tomcat

通過調用daemon.start()啟動Tomcat，其內容如下：

    /**
     * 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);
    }

程序通過反射的方式調用Catalina.start()方式啟動Tomcat，下面，我們看下Catalina.start()方法的實現邏輯：

    /**
     * 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，程序調用getServer().start()啟動，getServer()方法返回的是一個StandardServer類，繼而其調用的是StandardServer.startInternal()方法，在StandardServer中，又調用到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();
            }
        }
		// ......省略部分代碼
	}

    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，這里為什么不是start()方法，而是startInternal()方法呢？原因是StandardServer和StandService類都繼承了LifecycleMBeanBase類，而LifecycleMBeanBase類又繼承了LifecycleBase類。下面看下LifecycleBase類的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，調用start()方法，最終都會調用到startInternal()方法。在下篇文章中，我們將詳細看下StandardService.java中的engine.start()、executor.start()、connector.start()都分別啟動了什么？敬請期待！

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