Tomcat源代码阅读#1：classloader初始化

Bootstrap

通过Tomcat的启动脚本能够看到启动的入口是在Bootstrap，来看下Bootstrap的main方法，

    /**
     * Main method and entry point when starting Tomcat via the provided
     * scripts.
     *
     * @param args Command line arguments to be processed
     */
    public static void main(String args[]) {

        if (daemon == null) {
            ////////////////////////////////////////
            // 1. Bootstrap初始化
            ////////////////////////////////////////
            // 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);
        }

        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);
                ////////////////////////////////////////
                // 2. org.apache.catalina.startup.Catalina#load
                ////////////////////////////////////////
                daemon.load(args);
                ////////////////////////////////////////
                // 3. org.apache.catalina.startup.Catalina#start
                ////////////////////////////////////////
                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);
        }

    }

来看下init方法，

    /**
     * Initialize daemon.
     */
    public void init()
        throws Exception
    {
        ////////////////////////////////////////
        // 1. 设置catalina.home与catalina.base
        ////////////////////////////////////////
        // Set Catalina path
        setCatalinaHome();
        setCatalinaBase();

        ////////////////////////////////////////
        // 2. classloader初始化
        ////////////////////////////////////////
        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");
        ////////////////////////////////////////
        // 3. 使用catalinaLoader载入Catalina
        ////////////////////////////////////////
        Class<?> startupClass =
            catalinaLoader.loadClass
            ("org.apache.catalina.startup.Catalina");
        Object startupInstance = startupClass.newInstance();

        // Set the shared extensions class loader
        if (log.isDebugEnabled())
            log.debug("Setting startup class properties");
        ////////////////////////////////////////
        // 4. 将Catalina的parentClassLoader设置为sharedLoader
        ////////////////////////////////////////
        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;

    }

能够看到须要先初始化ClassLoader。然后才去载入最重要的服务器类。也就是org.apache.catalina.startup.Catalina。有三个ClassLoader，

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

以下来看下ClassLoader初始化的代码，

    private void initClassLoaders() {
        try {
            ////////////////////////////////////////
            // commonLoader的parent置为null
            ////////////////////////////////////////
            commonLoader = createClassLoader("common", null);
            ////////////////////////////////////////
            // 假设没有配置common.loader，则commonLoader就是AppClassLoader
            ////////////////////////////////////////
            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 {
        ////////////////////////////////////////
        // 从catalina.base/conf/catalina.properties读取配置
        ////////////////////////////////////////
        String value = CatalinaProperties.getProperty(name + ".loader");

        ////////////////////////////////////////
        // 默认情况下catalinaLoader与sharedLoader配置为空
        // 所以三个ClassLoader事实上是同一个引用
        ////////////////////////////////////////
        if ((value == null) || (value.equals("")))
            return parent;

        value = replace(value);

        ////////////////////////////////////////
        // 加入配置的classpath
        ////////////////////////////////////////
        List<Repository> repositories = new ArrayList<Repository>();

        StringTokenizer tokenizer = new StringTokenizer(value, ",");
        while (tokenizer.hasMoreElements()) {
            String repository = tokenizer.nextToken().trim();
            if (repository.length() == 0) {
                continue;
            }

            // 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));
            }
        }

        ////////////////////////////////////////
        // 差点儿相同是new URLClassLoader
        ////////////////////////////////////////
        ClassLoader classLoader = ClassLoaderFactory.createClassLoader
            (repositories, parent);

        // Retrieving MBean server
        MBeanServer mBeanServer = null;
        if (MBeanServerFactory.findMBeanServer(null).size() > 0) {
            mBeanServer = MBeanServerFactory.findMBeanServer(null).get(0);
        } else {
            mBeanServer = ManagementFactory.getPlatformMBeanServer();
        }

        // Register the server classloader
        ObjectName objectName =
            new ObjectName("Catalina:type=ServerClassLoader,name=" + name);
        mBeanServer.registerMBean(classLoader, objectName);

        return classLoader;

    }

ClassLoader的配置放在catalina.properties，

#
#
# List of comma-separated paths defining the contents of the "common"
# classloader. Prefixes should be used to define what is the repository type.
# Path may be relative to the CATALINA_HOME or CATALINA_BASE path or absolute.
# If left as blank,the JVM system loader will be used as Catalina's "common"
# loader.
# Examples:
#     "foo": Add this folder as a class repository
#     "foo/*.jar": Add all the JARs of the specified folder as class
#                  repositories
#     "foo/bar.jar": Add bar.jar as a class repository
common.loader=${catalina.base}/lib,${catalina.base}/lib/*.jar,${catalina.home}/lib,${catalina.home}/lib/*.jar

#
# List of comma-separated paths defining the contents of the "server"
# classloader. Prefixes should be used to define what is the repository type.
# Path may be relative to the CATALINA_HOME or CATALINA_BASE path or absolute.
# If left as blank, the "common" loader will be used as Catalina's "server"
# loader.
# Examples:
#     "foo": Add this folder as a class repository
#     "foo/*.jar": Add all the JARs of the specified folder as class
#                  repositories
#     "foo/bar.jar": Add bar.jar as a class repository
server.loader=

#
# List of comma-separated paths defining the contents of the "shared"
# classloader. Prefixes should be used to define what is the repository type.
# Path may be relative to the CATALINA_BASE path or absolute. If left as blank,
# the "common" loader will be used as Catalina's "shared" loader.
# Examples:
#     "foo": Add this folder as a class repository
#     "foo/*.jar": Add all the JARs of the specified folder as class
#                  repositories
#     "foo/bar.jar": Add bar.jar as a class repository
# Please note that for single jars, e.g. bar.jar, you need the URL form
# starting with file:.
shared.loader=

能够看到仅仅有commonLoader才有配置，所以createClassLoader("server", commonLoader);与createClassLoader("shared", commonLoader);返回的都会是commonLoader。

另一点值得注意的是，commonLoader的parent是null，也就是说当commonLoader进行类载入要托付给父类载入器时。将会绕过AppClassLoader和ExtClassLoader，直接交给BootstrapClassLoader（不明确的同学能够參考这里）。

Catalina

使用catalinaLoader载入了Catalina之后。会调用Catalina#load方法，

    public void load() {

        long t1 = System.nanoTime();

        ////////////////////////////////////////
        // 设置catalina.home与catalina.base
        ////////////////////////////////////////
        initDirs();

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

        ////////////////////////////////////////
        // 创建server.xml的解析器
        ////////////////////////////////////////
        // Create and execute our Digester
        Digester digester = createStartDigester();

        ////////////////////////////////////////
        // 读取catalina.base/conf/server.xml
        ////////////////////////////////////////

        InputSource inputSource = null;
        InputStream inputStream = null;
        File file = null;
        try {
            file = configFile();
            inputStream = new FileInputStream(file);
            inputSource = new InputSource(file.toURI().toURL().toString());
        } catch (Exception e) {
            if (log.isDebugEnabled()) {
                log.debug(sm.getString("catalina.configFail", file), e);
            }
        }
        if (inputStream == null) {
            try {
                inputStream = getClass().getClassLoader()
                    .getResourceAsStream(getConfigFile());
                inputSource = new InputSource
                    (getClass().getClassLoader()
                     .getResource(getConfigFile()).toString());
            } catch (Exception e) {
                if (log.isDebugEnabled()) {
                    log.debug(sm.getString("catalina.configFail",
                            getConfigFile()), e);
                }
            }
        }

        // This should be included in catalina.jar
        // Alternative: don't bother with xml, just create it manually.
        if( inputStream==null ) {
            try {
                inputStream = getClass().getClassLoader()
                        .getResourceAsStream("server-embed.xml");
                inputSource = new InputSource
                (getClass().getClassLoader()
                        .getResource("server-embed.xml").toString());
            } catch (Exception e) {
                if (log.isDebugEnabled()) {
                    log.debug(sm.getString("catalina.configFail",
                            "server-embed.xml"), e);
                }
            }
        }

        if (inputStream == null || inputSource == null) {
            if  (file == null) {
                log.warn(sm.getString("catalina.configFail",
                        getConfigFile() + "] or [server-embed.xml]"));
            } else {
                log.warn(sm.getString("catalina.configFail",
                        file.getAbsolutePath()));
                if (file.exists() && !file.canRead()) {
                    log.warn("Permissions incorrect, read permission is not allowed on the file.");
                }
            }
            return;
        }

        ////////////////////////////////////////
        // 解析server.xml
        ////////////////////////////////////////
        try {
            inputSource.setByteStream(inputStream);
            digester.push(this);
            digester.parse(inputSource);
        } catch (SAXParseException spe) {
            log.warn("Catalina.start using " + getConfigFile() + ": " +
                    spe.getMessage());
            return;
        } catch (Exception e) {
            log.warn("Catalina.start using " + getConfigFile() + ": " , e);
            return;
        } finally {
            try {
                inputStream.close();
            } catch (IOException e) {
                // Ignore
            }
        }

        getServer().setCatalina(this);

        // Stream redirection
        initStreams();

        ////////////////////////////////////////
        // org.apache.catalina.Server初始化
        ////////////////////////////////////////
        // 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("Catalina.start", e);
            }

        }

        long t2 = System.nanoTime();
        if(log.isInfoEnabled()) {
            log.info("Initialization processed in " + ((t2 - t1) / 1000000) + " ms");
        }

    }

load方法做的事情就是通过server.xml的配置去初始化Server。随后的Catalina#start事实上就是去启动Server。

    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();

        ////////////////////////////////////////
        // 启动org.apache.catalina.Server
        ////////////////////////////////////////
        // 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();
        }
    }

alright，今天就先到这吧。

參考资料

• https://tomcat.apache.org/tomcat-7.0-doc/building.html
• https://tomcat.apache.org/tomcat-7.0-doc/class-loader-howto.html
• https://tomcat.apache.org/tomcat-7.0-doc/architecture/overview.html
• https://tomcat.apache.org/tomcat-7.0-doc/architecture/startup.html
• https://tomcat.apache.org/tomcat-7.0-doc/architecture/requestProcess.html