Tomcat源码解析系列（七）Context

前言
上篇文章简单介绍了 Host，这篇文章里讲到了Context的启动时机，就是在 Engine 的 start 方法中调用了子容器的 start 方法（在ContainerBase#startInternal），子容器又调用子容器的方法，以此类推，所有子容器的start 方法都会被调用。

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1. StandardContext#initInternal

@Override
protected void initInternal() throws LifecycleException {
    super.initInternal();

    // Register the naming resources
    if (namingResources != null) {
        namingResources.init();
    }

    // Send j2ee.object.created notification
    if (this.getObjectName() != null) {
        Notification notification = new Notification("j2ee.object.created",
                this.getObjectName(), sequenceNumber.getAndIncrement());
        broadcaster.sendNotification(notification);
    }
}

首先调用了父类的 initInternal 方法，也就是 ContainerBase#initInternal。
然后调用了 namingResources（NamingResourcesImpl类型的属性） 的 init 方法，这个 namingResources 跟 StandardServer 里的 globalNamingResources 类似，只不过 globalNamingResources 是全局的，而这里的 namingResources 只是这个 StandardContext 的。
最后构造一个 Notification 对象并调用 broadcaster.sendNotification 方法来广播这个通知。
broadcaster 是在 StandardContext 对象构造函数里初始化的。

broadcaster = new NotificationBroadcasterSupport();

NotificationBroadcasterSupport 这个类是在javax.management 包下，不是本文重点，先略过。

2. StandardContext#startInternal

protected synchronized void startInternal() throws LifecycleException {

    if(log.isDebugEnabled())
        log.debug("Starting " + getBaseName());

    // Send j2ee.state.starting notification
    if (this.getObjectName() != null) {
        Notification notification = new Notification("j2ee.state.starting",
                this.getObjectName(), sequenceNumber.getAndIncrement());
        broadcaster.sendNotification(notification);
    }

    setConfigured(false);
    boolean ok = true;

    // Currently this is effectively a NO-OP but needs to be called to
    // ensure the NamingResources follows the correct lifecycle
    if (namingResources != null) {
        namingResources.start();
    }

    // Post work directory
    postWorkDirectory();

    // Add missing components as necessary
    if (getResources() == null) {   // (1) Required by Loader
        if (log.isDebugEnabled())
            log.debug("Configuring default Resources");

        try {
            setResources(new StandardRoot(this));
        } catch (IllegalArgumentException e) {
            log.error(sm.getString("standardContext.resourcesInit"), e);
            ok = false;
        }
    }
    if (ok) {
        resourcesStart();
    }

    if (getLoader() == null) {
        WebappLoader webappLoader = new WebappLoader(getParentClassLoader());
        webappLoader.setDelegate(getDelegate());
        setLoader(webappLoader);
    }

    // An explicit cookie processor hasn't been specified; use the default
    if (cookieProcessor == null) {
        cookieProcessor = new Rfc6265CookieProcessor();
    }

    // Initialize character set mapper
    getCharsetMapper();

    // Validate required extensions
    boolean dependencyCheck = true;
    try {
        dependencyCheck = ExtensionValidator.validateApplication
            (getResources(), this);
    } catch (IOException ioe) {
    log.error(sm.getString("standardContext.extensionValidationError"), ioe);
        dependencyCheck = false;
    }

    if (!dependencyCheck) {
        // do not make application available if dependency check fails
        ok = false;
    }

    // Reading the "catalina.useNaming" environment variable
    String useNamingProperty = System.getProperty("catalina.useNaming");
    if ((useNamingProperty != null)
        && (useNamingProperty.equals("false"))) {
        useNaming = false;
    }

    if (ok && isUseNaming()) {
        if (getNamingContextListener() == null) {
            NamingContextListener ncl = new NamingContextListener();
            ncl.setName(getNamingContextName());
            ncl.setExceptionOnFailedWrite(getJndiExceptionOnFailedWrite());
            addLifecycleListener(ncl);
            setNamingContextListener(ncl);
        }
    }

    // Standard container startup
    if (log.isDebugEnabled())
        log.debug("Processing standard container startup");


    // Binding thread
    ClassLoader oldCCL = bindThread();

    try {
        if (ok) {
            // Start our subordinate components, if any
            Loader loader = getLoader();
            if (loader instanceof Lifecycle) {
                ((Lifecycle) loader).start();
            }

            // since the loader just started, the webapp classloader is now
            // created.
            setClassLoaderProperty("clearReferencesRmiTargets",
                    getClearReferencesRmiTargets());
            setClassLoaderProperty("clearReferencesStopThreads",
                    getClearReferencesStopThreads());
            setClassLoaderProperty("clearReferencesStopTimerThreads",
                    getClearReferencesStopTimerThreads());
            setClassLoaderProperty("clearReferencesHttpClientKeepAliveThread",
                    getClearReferencesHttpClientKeepAliveThread());
            setClassLoaderProperty("clearReferencesObjectStreamClassCaches",
                    getClearReferencesObjectStreamClassCaches());
            setClassLoaderProperty("clearReferencesObjectStreamClassCaches",
                    getClearReferencesObjectStreamClassCaches());
            setClassLoaderProperty("clearReferencesThreadLocals",
                    getClearReferencesThreadLocals());

            // By calling unbindThread and bindThread in a row, we setup the
            // current Thread CCL to be the webapp classloader
            unbindThread(oldCCL);
            oldCCL = bindThread();

            // Initialize logger again. Other components might have used it
            // too early, so it should be reset.
            logger = null;
            getLogger();

            Realm realm = getRealmInternal();
            if(null != realm) {
                if (realm instanceof Lifecycle) {
                    ((Lifecycle) realm).start();
                }

                // Place the CredentialHandler into the ServletContext so
                // applications can have access to it. Wrap it in a "safe"
                // handler so application's can't modify it.
                CredentialHandler safeHandler = new CredentialHandler() {
                    @Override
                    public boolean matches(String inputCredentials, String storedCredentials) {
                        return getRealmInternal().getCredentialHandler().matches(inputCredentials, storedCredentials);
                    }

                    @Override
                    public String mutate(String inputCredentials) {
                        return getRealmInternal().getCredentialHandler().mutate(inputCredentials);
                    }
                };
                context.setAttribute(Globals.CREDENTIAL_HANDLER, safeHandler);
            }

            // Notify our interested LifecycleListeners
            fireLifecycleEvent(Lifecycle.CONFIGURE_START_EVENT, null);

            // Start our child containers, if not already started
            for (Container child : findChildren()) {
                if (!child.getState().isAvailable()) {
                    child.start();
                }
            }

            // Start the Valves in our pipeline (including the basic),
            // if any
            if (pipeline instanceof Lifecycle) {
                ((Lifecycle) pipeline).start();
            }

            // Acquire clustered manager
            Manager contextManager = null;
            Manager manager = getManager();
            if (manager == null) {
                if (log.isDebugEnabled()) {
                    log.debug(sm.getString("standardContext.cluster.noManager",
                            Boolean.valueOf((getCluster() != null)),
                            Boolean.valueOf(distributable)));
                }
                if ((getCluster() != null) && distributable) {
                    try {
                        contextManager = getCluster().createManager(getName());
                    } catch (Exception ex) {
                        log.error(sm.getString("standardContext.cluster.managerError"), ex);
                        ok = false;
                    }
                } else {
                    contextManager = new StandardManager();
                }
            }

            // Configure default manager if none was specified
            if (contextManager != null) {
                if (log.isDebugEnabled()) {
                    log.debug(sm.getString("standardContext.manager",
                            contextManager.getClass().getName()));
                }
                setManager(contextManager);
            }

            if (manager!=null && (getCluster() != null) && distributable) {
                //let the cluster know that there is a context that is distributable
                //and that it has its own manager
                getCluster().registerManager(manager);
            }
        }

        if (!getConfigured()) {
            log.error(sm.getString("standardContext.configurationFail"));
            ok = false;
        }

        // We put the resources into the servlet context
        if (ok)
            getServletContext().setAttribute
                (Globals.RESOURCES_ATTR, getResources());

        if (ok ) {
            if (getInstanceManager() == null) {
                javax.naming.Context context = null;
                if (isUseNaming() && getNamingContextListener() != null) {
                    context = getNamingContextListener().getEnvContext();
                }
                Map<String, Map<String, String>> injectionMap = buildInjectionMap(
                        getIgnoreAnnotations() ? new NamingResourcesImpl(): getNamingResources());
                setInstanceManager(new DefaultInstanceManager(context,
                        injectionMap, this, this.getClass().getClassLoader()));
            }
            getServletContext().setAttribute(
                    InstanceManager.class.getName(), getInstanceManager());
            InstanceManagerBindings.bind(getLoader().getClassLoader(), getInstanceManager());
        }

        // Create context attributes that will be required
        if (ok) {
            getServletContext().setAttribute(
                    JarScanner.class.getName(), getJarScanner());
        }

        // Set up the context init params
        mergeParameters();

        // Call ServletContainerInitializers
        for (Map.Entry<ServletContainerInitializer, Set<Class<?>>> entry :
            initializers.entrySet()) {
            try {
                entry.getKey().onStartup(entry.getValue(),
                        getServletContext());
            } catch (ServletException e) {
                log.error(sm.getString("standardContext.sciFail"), e);
                ok = false;
                break;
            }
        }

        // Configure and call application event listeners
        if (ok) {
            if (!listenerStart()) {
                log.error(sm.getString("standardContext.listenerFail"));
                ok = false;
            }
        }

        // Check constraints for uncovered HTTP methods
        // Needs to be after SCIs and listeners as they may programmatically
        // change constraints
        if (ok) {
            checkConstraintsForUncoveredMethods(findConstraints());
        }

        try {
            // Start manager
            Manager manager = getManager();
            if (manager instanceof Lifecycle) {
                ((Lifecycle) manager).start();
            }
        } catch(Exception e) {
            log.error(sm.getString("standardContext.managerFail"), e);
            ok = false;
        }

        // Configure and call application filters
        if (ok) {
            if (!filterStart()) {
                log.error(sm.getString("standardContext.filterFail"));
                ok = false;
            }
        }

        // Load and initialize all "load on startup" servlets
        if (ok) {
            if (!loadOnStartup(findChildren())){
                log.error(sm.getString("standardContext.servletFail"));
                ok = false;
            }
        }

        // Start ContainerBackgroundProcessor thread
        super.threadStart();
    } finally {
        // Unbinding thread
        unbindThread(oldCCL);
    }

    // Set available status depending upon startup success
    if (ok) {
        if (log.isDebugEnabled())
            log.debug("Starting completed");
    } else {
        log.error(sm.getString("standardContext.startFailed", getName()));
    }

    startTime=System.currentTimeMillis();

    // Send j2ee.state.running notification
    if (ok && (this.getObjectName() != null)) {
        Notification notification =
            new Notification("j2ee.state.running", this.getObjectName(),
                             sequenceNumber.getAndIncrement());
        broadcaster.sendNotification(notification);
    }

    // The WebResources implementation caches references to JAR files. On
    // some platforms these references may lock the JAR files. Since web
    // application start is likely to have read from lots of JARs, trigger
    // a clean-up now.
    getResources().gc();

    // Reinitializing if something went wrong
    if (!ok) {
        setState(LifecycleState.FAILED);
    } else {
        setState(LifecycleState.STARTING);
    }
}

StandardContext 的 startInternal 方法比较长，做了很多事情，是 tomcat 启动过程中非常重要的环节。最重要的是做了这几件事：

1、生成工作目录，postWorkDirectory()。
2、设置并启动解析资源，并加入到Context 的 Attribute 中，setResources(...)、resourcesStart()，
3、设置并启动 WebappLoader
4、设置 coockieProcessor 和 chasetMapper
5、如果 Context 的 Realm 类型属性不为空，启动 Realm
6、创建 CredentialHandler 对象，并加入到 Context 的 Attribute 中
7、启动子容器
8、启动自己的 Pipeline（实际上也是启动 Pipeline 的 Value）
9、创建并设置一个 Manager 对象，然后启动，然后加入到 Context 的 Attribute 中
10、创建并设置一个 JarScanner 对象，然后加入到 Context 的 Attribute 中
11、合并初始化参数，mergeParameters();
12、调用 ServletContainerInitializer 的 onStartUp 方法
13、创建一个或多个 ApplicationFilterConfig 对象，并初始化 Filter（调用Filter#init），并存放在 StandardContext 的一个Map里
14、初始化 loadOnStartup 的 Servlet，loadOnStartup(findChildren())，findChildren() 返回的是 Wrapper，Wrapper 包含了 Servlet 类型的属性，调用 Wrapper#load 方法，在这个方法里调用 Servlet 对象的 init 方法。
15、调用父类的 threadStart 方法来执行 backgroudProcess 方法
16、调用resource的 gc() 方法（也就是 StandardRoot#gc() ）来释放资源，

StandardContext 的 startInternal 方法准备了 Web 应用程序所需要的一切资源，不过并没有调用父类ContainerBase 的 startInternal 方法。

3. StandardContext#backgroundProcess

@Override
public void backgroundProcess() {

    if (!getState().isAvailable())
        return;

    Loader loader = getLoader();
    if (loader != null) {
        try {
            loader.backgroundProcess();
        } catch (Exception e) {
            log.warn(sm.getString(
                    "standardContext.backgroundProcess.loader", loader), e);
        }
    }
    Manager manager = getManager();
    if (manager != null) {
        try {
            manager.backgroundProcess();
        } catch (Exception e) {
            log.warn(sm.getString(
                    "standardContext.backgroundProcess.manager", manager),
                    e);
        }
    }
    WebResourceRoot resources = getResources();
    if (resources != null) {
        try {
            resources.backgroundProcess();
        } catch (Exception e) {
            log.warn(sm.getString(
                    "standardContext.backgroundProcess.resources",
                    resources), e);
        }
    }
    InstanceManager instanceManager = getInstanceManager();
    if (instanceManager != null) {
        try {
            instanceManager.backgroundProcess();
        } catch (Exception e) {
            log.warn(sm.getString(
                    "standardContext.backgroundProcess.instanceManager",
                    resources), e);
        }
    }
    super.backgroundProcess();
}

StandardContext#backgroundProcess 方法里分别调用 Loader、Mananger、WebResourceRoot(StandardRoot)、InstanceManager 的 backgroundProcess 方法，最后还调用父类 ContainerBase 的 backgroundProcess()。

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小结
本文分析了 Context 的 start 和 init 方法，其中 startInternal 方法做了很多事情，其中最重要的是加载了用户的Web程序代码并初始化了 Filter 和 Servlet。