Tomcat7中web应用加载原理（三）Listener、Filter、Servlet的加载和调用

前一篇文章分析到了org.apache.catalina.deploy.WebXml类的configureContext方法，可以看到在这个方法中通过各种setXXX、addXXX方法的调用，使得每个应用中的web.xml文件的解析后将应用内部的表示Servlet、Listener、Filter的配置信息与表示一个web应用的Context对象关联起来。

这里列出configureContext方法中与Servlet、Listener、Filter的配置信息设置相关的调用代码：

        for (FilterDef filter : filters.values()) {
            if (filter.getAsyncSupported() == null) {
                filter.setAsyncSupported("false");
            }
            context.addFilterDef(filter);
        }
        for (FilterMap filterMap : filterMaps) {
            context.addFilterMap(filterMap);
        }

这是设置Filter相关配置信息的。

 

        for (String listener : listeners) {
            context.addApplicationListener(
                    new ApplicationListener(listener, false));
        }

这是给应用添加Listener的。

 

        for (ServletDef servlet : servlets.values()) {
            Wrapper wrapper = context.createWrapper();
            // Description is ignored
            // Display name is ignored
            // Icons are ignored

            // jsp-file gets passed to the JSP Servlet as an init-param

            if (servlet.getLoadOnStartup() != null) {
                wrapper.setLoadOnStartup(servlet.getLoadOnStartup().intValue());
            }
            if (servlet.getEnabled() != null) {
                wrapper.setEnabled(servlet.getEnabled().booleanValue());
            }
            wrapper.setName(servlet.getServletName());
            Map<String,String> params = servlet.getParameterMap();
            for (Entry<String, String> entry : params.entrySet()) {
                wrapper.addInitParameter(entry.getKey(), entry.getValue());
            }
            wrapper.setRunAs(servlet.getRunAs());
            Set<SecurityRoleRef> roleRefs = servlet.getSecurityRoleRefs();
            for (SecurityRoleRef roleRef : roleRefs) {
                wrapper.addSecurityReference(
                        roleRef.getName(), roleRef.getLink());
            }
            wrapper.setServletClass(servlet.getServletClass());
            MultipartDef multipartdef = servlet.getMultipartDef();
            if (multipartdef != null) {
                if (multipartdef.getMaxFileSize() != null &&
                        multipartdef.getMaxRequestSize()!= null &&
                        multipartdef.getFileSizeThreshold() != null) {
                    wrapper.setMultipartConfigElement(new MultipartConfigElement(
                            multipartdef.getLocation(),
                            Long.parseLong(multipartdef.getMaxFileSize()),
                            Long.parseLong(multipartdef.getMaxRequestSize()),
                            Integer.parseInt(
                                    multipartdef.getFileSizeThreshold())));
                } else {
                    wrapper.setMultipartConfigElement(new MultipartConfigElement(
                            multipartdef.getLocation()));
                }
            }
            if (servlet.getAsyncSupported() != null) {
                wrapper.setAsyncSupported(
                        servlet.getAsyncSupported().booleanValue());
            }
            wrapper.setOverridable(servlet.isOverridable());
            context.addChild(wrapper);
        }
        for (Entry<String, String> entry : servletMappings.entrySet()) {
            context.addServletMapping(entry.getKey(), entry.getValue());
        }

这段代码是设置Servlet的相关配置信息的。

 

以上是在各个web应用的web.xml文件中（如果是servlet3，还会包括将这些配置信息放在类的注解中，所以解析web.xml文件之前可能会存在各个web.xml文件信息的合并步骤，这些动作的代码在前一篇文章中讲ContextConfig类的webConfig方法中）的相关配置信息的设置，但需要注意的是，这里仅仅是将这些配置信息保存到了StandardContext的相应实例变量中，真正在一次请求访问中用到的Servlet、Listener、Filter的实例并没有构造出来，以上方法调用仅构造了代表这些实例的封装类的实例，如StandardWrapper、ApplicationListener、FilterDef、FilterMap。

 

那么一个web应用中的Servlet、Listener、Filter的实例究竟在什么时候构造出来的呢？答案在org.apache.catalina.core.StandardContext类的startInternal方法中：

    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();
        }
        
        // Add missing components as necessary
        if (webappResources == null) {   // (1) Required by Loader
            if (log.isDebugEnabled())
                log.debug("Configuring default Resources");
            try {
                if ((getDocBase() != null) && (getDocBase().endsWith(".war")) &&
                        (!(new File(getBasePath())).isDirectory()))
                    setResources(new WARDirContext());
                else
                    setResources(new FileDirContext());
            } catch (IllegalArgumentException e) {
                log.error("Error initializing resources: " + e.getMessage());
                ok = false;
            }
        }
        if (ok) {
            if (!resourcesStart()) {
                log.error( "Error in resourceStart()");
                ok = false;
            }
        }

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

        // Initialize character set mapper
        getCharsetMapper();

        // Post work directory
        postWorkDirectory();

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

        if (!dependencyCheck) {
            // do not make application available if depency 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
                if ((loader != null) && (loader instanceof Lifecycle))
                    ((Lifecycle) loader).start();

                // since the loader just started, the webapp classloader is now
                // created.
                // 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();
                
                if ((cluster != null) && (cluster instanceof Lifecycle))
                    ((Lifecycle) cluster).start();
                Realm realm = getRealmInternal();
                if ((realm != null) && (realm instanceof Lifecycle))
                    ((Lifecycle) realm).start();
                if ((resources != null) && (resources instanceof Lifecycle))
                    ((Lifecycle) resources).start();

                // 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;
                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("standardContext.clusterFail", 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);
                }
            }

        } finally {
            // Unbinding thread
            unbindThread(oldCCL);
        }

        if (!getConfigured()) {
            log.error( "Error getConfigured");
            ok = false;
        }

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

        // Initialize associated mapper
        mapper.setContext(getPath(), welcomeFiles, resources);

        // Binding thread
        oldCCL = bindThread();

        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 NamingResources(): getNamingResources());
                setInstanceManager(new DefaultInstanceManager(context,
                        injectionMap, this, this.getClass().getClassLoader()));
                getServletContext().setAttribute(
                        InstanceManager.class.getName(), getInstanceManager());
            }
        }

        try {
            // 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( "Error listenerStart");
                    ok = false;
                }
            }
            
            try {
                // Start manager
                if ((manager != null) && (manager instanceof Lifecycle)) {
                    ((Lifecycle) getManager()).start();
                }
            } catch(Exception e) {
                log.error("Error manager.start()", e);
                ok = false;
            }

            // Configure and call application filters
            if (ok) {
                if (!filterStart()) {
                    log.error("Error filterStart");
                    ok = false;
                }
            }
            
            // Load and initialize all "load on startup" servlets
            if (ok) {
                loadOnStartup(findChildren());
            }
            
            // 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);
        }

        // Close all JARs right away to avoid always opening a peak number 
        // of files on startup
        if (getLoader() instanceof WebappLoader) {
            ((WebappLoader) getLoader()).closeJARs(true);
        }

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

这303行可以讲的东西有很多，为了不偏离本文主题只抽出与现在要讨论的问题相关的代码来分析。

 

第125行会发布一个CONFIGURE_START_EVENT事件，按前一篇博文所述，这里即会触发对web.xml的解析。第205、206行设置实例管理器为DefaultInstanceManager（这个类在后面谈实例构造时会用到）。第237行会调用listenerStart方法，第255行调用了filterStart方法，第263行调用了loadOnStartup方法，这三处调用即触发Listener、Filter、Servlet真正对象的构造，下面逐个分析这些方法。

 

listenerStart方法的完整代码较长，这里仅列出与Listenner对象构造相关的代码：

        // Instantiate the required listeners
        ApplicationListener listeners[] = applicationListeners;
        Object results[] = new Object[listeners.length];
        boolean ok = true;
        Set<Object> noPluggabilityListeners = new HashSet<Object>();
        for (int i = 0; i < results.length; i++) {
            if (getLogger().isDebugEnabled())
                getLogger().debug(" Configuring event listener class '" +
                    listeners[i] + "'");
            try {
                ApplicationListener listener = listeners[i];
                results[i] = instanceManager.newInstance(
                        listener.getClassName());
                if (listener.isPluggabilityBlocked()) {
                    noPluggabilityListeners.add(results[i]);
                }
            } catch (Throwable t) {
                t = ExceptionUtils.unwrapInvocationTargetException(t);
                ExceptionUtils.handleThrowable(t);
                getLogger().error
                    (sm.getString("standardContext.applicationListener",
                                  listeners[i]), t);
                ok = false;
            }
        }

先从Context对象中取出实例变量applicationListeners（该变量的值在web.xml解析时设置），第12行通过调用instanceManager.newInstance(listener.getClassName())，前面在看StandardContext的startInternal方法第205行时看到instanceManager被设置为DefaultInstanceManager对象，所以这里实际会执行DefaultInstanceManager类的newInstance方法：

    public Object newInstance(String className) throws IllegalAccessException, InvocationTargetException, NamingException, InstantiationException, ClassNotFoundException {
        Class<?> clazz = loadClassMaybePrivileged(className, classLoader);
        return newInstance(clazz.newInstance(), clazz);
    }

所以instanceManager.newInstance(listener.getClassName())这段代码的作用是取出web.xml中配置的Listener的class配置信息，从而构造实际配置的Listener对象。

 

看下filterStart方法：

    public boolean filterStart() {

        if (getLogger().isDebugEnabled())
            getLogger().debug("Starting filters");
        // Instantiate and record a FilterConfig for each defined filter
        boolean ok = true;
        synchronized (filterConfigs) {
            filterConfigs.clear();
            Iterator<String> names = filterDefs.keySet().iterator();
            while (names.hasNext()) {
                String name = names.next();
                if (getLogger().isDebugEnabled())
                    getLogger().debug(" Starting filter '" + name + "'");
                ApplicationFilterConfig filterConfig = null;
                try {
                    filterConfig =
                        new ApplicationFilterConfig(this, filterDefs.get(name));
                    filterConfigs.put(name, filterConfig);
                } catch (Throwable t) {
                    t = ExceptionUtils.unwrapInvocationTargetException(t);
                    ExceptionUtils.handleThrowable(t);
                    getLogger().error
                        (sm.getString("standardContext.filterStart", name), t);
                    ok = false;
                }
            }
        }

        return (ok);

    }

这段代码看起来很简单，取出web.xml解析时读到的filter配置信息，在第17行调用ApplicationFilterConfig了构造方法：

    ApplicationFilterConfig(Context context, FilterDef filterDef)
        throws ClassCastException, ClassNotFoundException,
               IllegalAccessException, InstantiationException,
               ServletException, InvocationTargetException, NamingException {

        super();

        this.context = context;
        this.filterDef = filterDef;
        // Allocate a new filter instance if necessary
        if (filterDef.getFilter() == null) {
            getFilter();
        } else {
            this.filter = filterDef.getFilter();
            getInstanceManager().newInstance(filter);
            initFilter();
        }
    }

默认情况下filterDef中是没有Filter对象的，所以会调用第12行getFilter方法：

    Filter getFilter() throws ClassCastException, ClassNotFoundException,
        IllegalAccessException, InstantiationException, ServletException,
        InvocationTargetException, NamingException {

        // Return the existing filter instance, if any
        if (this.filter != null)
            return (this.filter);

        // Identify the class loader we will be using
        String filterClass = filterDef.getFilterClass();
        this.filter = (Filter) getInstanceManager().newInstance(filterClass);
        
        initFilter();
        
        return (this.filter);

    }

与Listener的对象构造类似，都是通过调用getInstanceManager().newInstance方法。当然，按照Servlet规范，第13行还会调用Filter的init方法。

 

看下loadOnStartup方法： 

    public void loadOnStartup(Container children[]) {

        // Collect "load on startup" servlets that need to be initialized
        TreeMap<Integer, ArrayList<Wrapper>> map =
            new TreeMap<Integer, ArrayList<Wrapper>>();
        for (int i = 0; i < children.length; i++) {
            Wrapper wrapper = (Wrapper) children[i];
            int loadOnStartup = wrapper.getLoadOnStartup();
            if (loadOnStartup < 0)
                continue;
            Integer key = Integer.valueOf(loadOnStartup);
            ArrayList<Wrapper> list = map.get(key);
            if (list == null) {
                list = new ArrayList<Wrapper>();
                map.put(key, list);
            }
            list.add(wrapper);
        }

        // Load the collected "load on startup" servlets
        for (ArrayList<Wrapper> list : map.values()) {
            for (Wrapper wrapper : list) {
                try {
                    wrapper.load();
                } catch (ServletException e) {
                    getLogger().error(sm.getString("standardWrapper.loadException",
                                      getName()), StandardWrapper.getRootCause(e));
                    // NOTE: load errors (including a servlet that throws
                    // UnavailableException from tht init() method) are NOT
                    // fatal to application startup
                }
            }
        }

    }

在web应用启动时将会加载配置了load-on-startup属性的Servlet。第24行，调用了StandardWrapper类的load方法：

    public synchronized void load() throws ServletException {
        instance = loadServlet();
        
        if (!instanceInitialized) {
            initServlet(instance);
        }

        if (isJspServlet) {
            StringBuilder oname =
                new StringBuilder(MBeanUtils.getDomain(getParent()));
            
            oname.append(":type=JspMonitor,name=");
            oname.append(getName());
            
            oname.append(getWebModuleKeyProperties());
            
            try {
                jspMonitorON = new ObjectName(oname.toString());
                Registry.getRegistry(null, null)
                    .registerComponent(instance, jspMonitorON, null);
            } catch( Exception ex ) {
                log.info("Error registering JSP monitoring with jmx " +
                         instance);
            }
        }
    }

在第2行loadServlet方法中与上面的Listener和Filter对象构造一样调用instanceManager.newInstance来构造Servlet对象，与Filter类似在第5行调用Servlet的init方法。

当然，这种加载只是针对配置了load-on-startup属性的Servlet而言，其它一般Servlet的加载和初始化会推迟到真正请求访问web应用而第一次调用该Servlet时，下面会看到这种情况下代码分析。

 

以上分析的web应用启动后这些对象的加载情况，接下来分析一下一次请求访问时，相关的Filter、Servlet对象的调用。

 

在本博前面的《Tomcat7中一次请求处理的前世今生》系列文章中曾经分析了一次请求如何与容器中的Engine、Host、Context、Wrapper各级组件匹配，并在这些容器组件内部的管道中流转的。在该系列第四篇文章最后提到，一次请求最终会执行与它最适配的一个StandardWrapper的基础阀org.apache.catalina.core.StandardWrapperValve的invoke方法。当时限于篇幅没继续往下分析，这里接着这段来看看请求的流转。看下invoke方法的代码：

    public final void invoke(Request request, Response response)
        throws IOException, ServletException {

        // Initialize local variables we may need
        boolean unavailable = false;
        Throwable throwable = null;
        // This should be a Request attribute...
        long t1=System.currentTimeMillis();
        requestCount++;
        StandardWrapper wrapper = (StandardWrapper) getContainer();
        Servlet servlet = null;
        Context context = (Context) wrapper.getParent();
        
        // Check for the application being marked unavailable
        if (!context.getState().isAvailable()) {
            response.sendError(HttpServletResponse.SC_SERVICE_UNAVAILABLE,
                           sm.getString("standardContext.isUnavailable"));
            unavailable = true;
        }

        // Check for the servlet being marked unavailable
        if (!unavailable && wrapper.isUnavailable()) {
            container.getLogger().info(sm.getString("standardWrapper.isUnavailable",
                    wrapper.getName()));
            long available = wrapper.getAvailable();
            if ((available > 0L) && (available < Long.MAX_VALUE)) {
                response.setDateHeader("Retry-After", available);
                response.sendError(HttpServletResponse.SC_SERVICE_UNAVAILABLE,
                        sm.getString("standardWrapper.isUnavailable",
                                wrapper.getName()));
            } else if (available == Long.MAX_VALUE) {
                response.sendError(HttpServletResponse.SC_NOT_FOUND,
                        sm.getString("standardWrapper.notFound",
                                wrapper.getName()));
            }
            unavailable = true;
        }

        // Allocate a servlet instance to process this request
        try {
            if (!unavailable) {
                servlet = wrapper.allocate();
            }
        } catch (UnavailableException e) {
            container.getLogger().error(
                    sm.getString("standardWrapper.allocateException",
                            wrapper.getName()), e);
            long available = wrapper.getAvailable();
            if ((available > 0L) && (available < Long.MAX_VALUE)) {
                response.setDateHeader("Retry-After", available);
                response.sendError(HttpServletResponse.SC_SERVICE_UNAVAILABLE,
                           sm.getString("standardWrapper.isUnavailable",
                                        wrapper.getName()));
            } else if (available == Long.MAX_VALUE) {
                response.sendError(HttpServletResponse.SC_NOT_FOUND,
                           sm.getString("standardWrapper.notFound",
                                        wrapper.getName()));
            }
        } catch (ServletException e) {
            container.getLogger().error(sm.getString("standardWrapper.allocateException",
                             wrapper.getName()), StandardWrapper.getRootCause(e));
            throwable = e;
            exception(request, response, e);
        } catch (Throwable e) {
            ExceptionUtils.handleThrowable(e);
            container.getLogger().error(sm.getString("standardWrapper.allocateException",
                             wrapper.getName()), e);
            throwable = e;
            exception(request, response, e);
            servlet = null;
        }

        // Identify if the request is Comet related now that the servlet has been allocated
        boolean comet = false;
        if (servlet instanceof CometProcessor && request.getAttribute(
                Globals.COMET_SUPPORTED_ATTR) == Boolean.TRUE) {
            comet = true;
            request.setComet(true);
        }
        
        MessageBytes requestPathMB = request.getRequestPathMB();
        DispatcherType dispatcherType = DispatcherType.REQUEST;
        if (request.getDispatcherType()==DispatcherType.ASYNC) dispatcherType = DispatcherType.ASYNC; 
        request.setAttribute(Globals.DISPATCHER_TYPE_ATTR,dispatcherType);
        request.setAttribute(Globals.DISPATCHER_REQUEST_PATH_ATTR,
                requestPathMB);
        // Create the filter chain for this request
        ApplicationFilterFactory factory =
            ApplicationFilterFactory.getInstance();
        ApplicationFilterChain filterChain =
            factory.createFilterChain(request, wrapper, servlet);
        
        // Reset comet flag value after creating the filter chain
        request.setComet(false);

        // Call the filter chain for this request
        // NOTE: This also calls the servlet's service() method
        try {
            if ((servlet != null) && (filterChain != null)) {
                // Swallow output if needed
                if (context.getSwallowOutput()) {
                    try {
                        SystemLogHandler.startCapture();
                        if (request.isAsyncDispatching()) {
                            //TODO SERVLET3 - async
                            ((AsyncContextImpl)request.getAsyncContext()).doInternalDispatch(); 
                        } else if (comet) {
                            filterChain.doFilterEvent(request.getEvent());
                            request.setComet(true);
                        } else {
                            filterChain.doFilter(request.getRequest(), 
                                    response.getResponse());
                        }
                    } finally {
                        String log = SystemLogHandler.stopCapture();
                        if (log != null && log.length() > 0) {
                            context.getLogger().info(log);
                        }
                    }
                } else {
                    if (request.isAsyncDispatching()) {
                        //TODO SERVLET3 - async
                        ((AsyncContextImpl)request.getAsyncContext()).doInternalDispatch();
                    } else if (comet) {
                        request.setComet(true);
                        filterChain.doFilterEvent(request.getEvent());
                    } else {
                        filterChain.doFilter
                            (request.getRequest(), response.getResponse());
                    }
                }

            }
        } catch (ClientAbortException e) {
            throwable = e;
            exception(request, response, e);
        } catch (IOException e) {
            container.getLogger().error(sm.getString(
                    "standardWrapper.serviceException", wrapper.getName(),
                    context.getName()), e);
            throwable = e;
            exception(request, response, e);
        } catch (UnavailableException e) {
            container.getLogger().error(sm.getString(
                    "standardWrapper.serviceException", wrapper.getName(),
                    context.getName()), e);
            //            throwable = e;
            //            exception(request, response, e);
            wrapper.unavailable(e);
            long available = wrapper.getAvailable();
            if ((available > 0L) && (available < Long.MAX_VALUE)) {
                response.setDateHeader("Retry-After", available);
                response.sendError(HttpServletResponse.SC_SERVICE_UNAVAILABLE,
                           sm.getString("standardWrapper.isUnavailable",
                                        wrapper.getName()));
            } else if (available == Long.MAX_VALUE) {
                response.sendError(HttpServletResponse.SC_NOT_FOUND,
                            sm.getString("standardWrapper.notFound",
                                        wrapper.getName()));
            }
            // Do not save exception in 'throwable', because we
            // do not want to do exception(request, response, e) processing
        } catch (ServletException e) {
            Throwable rootCause = StandardWrapper.getRootCause(e);
            if (!(rootCause instanceof ClientAbortException)) {
                container.getLogger().error(sm.getString(
                        "standardWrapper.serviceExceptionRoot",
                        wrapper.getName(), context.getName(), e.getMessage()),
                        rootCause);
            }
            throwable = e;
            exception(request, response, e);
        } catch (Throwable e) {
            ExceptionUtils.handleThrowable(e);
            container.getLogger().error(sm.getString(
                    "standardWrapper.serviceException", wrapper.getName(),
                    context.getName()), e);
            throwable = e;
            exception(request, response, e);
        }

        // Release the filter chain (if any) for this request
        if (filterChain != null) {
            if (request.isComet()) {
                // If this is a Comet request, then the same chain will be used for the
                // processing of all subsequent events.
                filterChain.reuse();
            } else {
                filterChain.release();
            }
        }

        // Deallocate the allocated servlet instance
        try {
            if (servlet != null) {
                wrapper.deallocate(servlet);
            }
        } catch (Throwable e) {
            ExceptionUtils.handleThrowable(e);
            container.getLogger().error(sm.getString("standardWrapper.deallocateException",
                             wrapper.getName()), e);
            if (throwable == null) {
                throwable = e;
                exception(request, response, e);
            }
        }

        // If this servlet has been marked permanently unavailable,
        // unload it and release this instance
        try {
            if ((servlet != null) &&
                (wrapper.getAvailable() == Long.MAX_VALUE)) {
                wrapper.unload();
            }
        } catch (Throwable e) {
            ExceptionUtils.handleThrowable(e);
            container.getLogger().error(sm.getString("standardWrapper.unloadException",
                             wrapper.getName()), e);
            if (throwable == null) {
                throwable = e;
                exception(request, response, e);
            }
        }
        long t2=System.currentTimeMillis();

        long time=t2-t1;
        processingTime += time;
        if( time > maxTime) maxTime=time;
        if( time < minTime) minTime=time;

    }

因为要支持Servlet3的新特性及各种异常处理，这段代码显得比较长。关注重点第42行，这里会调用StandardWrapper的allocate方法，不再贴出这个方法的代码，需要提醒的是在allocate方法中可能会调用loadServlet()方法，这就是上一段提到的请求访问web应用而第一次调用该Servlet时再加载并初始化Servlet。

第87到91行会构造一个过滤器链（filterChain）用于执行这一次请求所经过的相应Filter，第111和第128行会调用该filterChain的doFilter方法：

    public void doFilter(ServletRequest request, ServletResponse response)
        throws IOException, ServletException {

        if( Globals.IS_SECURITY_ENABLED ) {
            final ServletRequest req = request;
            final ServletResponse res = response;
            try {
                java.security.AccessController.doPrivileged(
                    new java.security.PrivilegedExceptionAction<Void>() {
                        @Override
                        public Void run() 
                            throws ServletException, IOException {
                            internalDoFilter(req,res);
                            return null;
                        }
                    }
                );
            } catch( PrivilegedActionException pe) {
                Exception e = pe.getException();
                if (e instanceof ServletException)
                    throw (ServletException) e;
                else if (e instanceof IOException)
                    throw (IOException) e;
                else if (e instanceof RuntimeException)
                    throw (RuntimeException) e;
                else
                    throw new ServletException(e.getMessage(), e);
            }
        } else {
            internalDoFilter(request,response);
        }
    }

在该方法最后调用了internalDoFilter方法：

    private void internalDoFilter(ServletRequest request, 
                                  ServletResponse response)
        throws IOException, ServletException {

        // Call the next filter if there is one
        if (pos < n) {
            ApplicationFilterConfig filterConfig = filters[pos++];
            Filter filter = null;
            try {
                filter = filterConfig.getFilter();
                support.fireInstanceEvent(InstanceEvent.BEFORE_FILTER_EVENT,
                                          filter, request, response);
                
                if (request.isAsyncSupported() && "false".equalsIgnoreCase(
                        filterConfig.getFilterDef().getAsyncSupported())) {
                    request.setAttribute(Globals.ASYNC_SUPPORTED_ATTR,
                            Boolean.FALSE);
                }
                if( Globals.IS_SECURITY_ENABLED ) {
                    final ServletRequest req = request;
                    final ServletResponse res = response;
                    Principal principal = 
                        ((HttpServletRequest) req).getUserPrincipal();

                    Object[] args = new Object[]{req, res, this};
                    SecurityUtil.doAsPrivilege
                        ("doFilter", filter, classType, args, principal);
                    
                } else {  
                    filter.doFilter(request, response, this);
                }

                support.fireInstanceEvent(InstanceEvent.AFTER_FILTER_EVENT,
                                          filter, request, response);
            } catch (IOException e) {
                if (filter != null)
                    support.fireInstanceEvent(InstanceEvent.AFTER_FILTER_EVENT,
                                              filter, request, response, e);
                throw e;
            } catch (ServletException e) {
                if (filter != null)
                    support.fireInstanceEvent(InstanceEvent.AFTER_FILTER_EVENT,
                                              filter, request, response, e);
                throw e;
            } catch (RuntimeException e) {
                if (filter != null)
                    support.fireInstanceEvent(InstanceEvent.AFTER_FILTER_EVENT,
                                              filter, request, response, e);
                throw e;
            } catch (Throwable e) {
                e = ExceptionUtils.unwrapInvocationTargetException(e);
                ExceptionUtils.handleThrowable(e);
                if (filter != null)
                    support.fireInstanceEvent(InstanceEvent.AFTER_FILTER_EVENT,
                                              filter, request, response, e);
                throw new ServletException
                  (sm.getString("filterChain.filter"), e);
            }
            return;
        }

        // We fell off the end of the chain -- call the servlet instance
        try {
            if (ApplicationDispatcher.WRAP_SAME_OBJECT) {
                lastServicedRequest.set(request);
                lastServicedResponse.set(response);
            }

            support.fireInstanceEvent(InstanceEvent.BEFORE_SERVICE_EVENT,
                                      servlet, request, response);
            if (request.isAsyncSupported()
                    && !support.getWrapper().isAsyncSupported()) {
                request.setAttribute(Globals.ASYNC_SUPPORTED_ATTR,
                        Boolean.FALSE);
            }
            // Use potentially wrapped request from this point
            if ((request instanceof HttpServletRequest) &&
                (response instanceof HttpServletResponse)) {
                    
                if( Globals.IS_SECURITY_ENABLED ) {
                    final ServletRequest req = request;
                    final ServletResponse res = response;
                    Principal principal = 
                        ((HttpServletRequest) req).getUserPrincipal();
                    Object[] args = new Object[]{req, res};
                    SecurityUtil.doAsPrivilege("service",
                                               servlet,
                                               classTypeUsedInService, 
                                               args,
                                               principal);   
                } else {  
                    servlet.service(request, response);
                }
            } else {
                servlet.service(request, response);
            }
            support.fireInstanceEvent(InstanceEvent.AFTER_SERVICE_EVENT,
                                      servlet, request, response);
        } catch (IOException e) {
            support.fireInstanceEvent(InstanceEvent.AFTER_SERVICE_EVENT,
                                      servlet, request, response, e);
            throw e;
        } catch (ServletException e) {
            support.fireInstanceEvent(InstanceEvent.AFTER_SERVICE_EVENT,
                                      servlet, request, response, e);
            throw e;
        } catch (RuntimeException e) {
            support.fireInstanceEvent(InstanceEvent.AFTER_SERVICE_EVENT,
                                      servlet, request, response, e);
            throw e;
        } catch (Throwable e) {
            ExceptionUtils.handleThrowable(e);
            support.fireInstanceEvent(InstanceEvent.AFTER_SERVICE_EVENT,
                                      servlet, request, response, e);
            throw new ServletException
              (sm.getString("filterChain.servlet"), e);
        } finally {
            if (ApplicationDispatcher.WRAP_SAME_OBJECT) {
                lastServicedRequest.set(null);
                lastServicedResponse.set(null);
            }
        }

    }

概述一下这段代码，第6到60行是执行过滤器链中的各个过滤器的doFilter方法，实例变量n表示过滤器链中所有的过滤器，pos表示当前要执行的过滤器。其中第7行取出当前要执行的Filter，之后将pos加1，接着第30行执行Filter的doFilter方法。一般的过滤器实现中在最后都会有这一句：

FilterChain.doFilter(request, response);

这样就又回到了filterChain的doFilter方法，形成了一个递归调用。要注意的是，filterChain对象内部的pos是不断加的，所以假如过滤器链中的各个Filter的doFilter方法都执行完之后将会执行到第63行，在接下来的第92行、第95行即调用Servlet的service方法。