前言

Context容器是一个Web项目的代表，主要管理Servlet实例，在Tomcat中Servlet实例是以Wrapper出现的。如今问题是怎样才干通过Context容器找到详细的Servlet呢？在解决问题之前。Context容器须要先启动，启动的过程就是载入个类资源文件以及打开子容器以及Pipeline管道的过程。启动Context容器后。就能够处理详细的请求了，详细是通过Request对象，从代码清单4-3的Wrapper wrapper = request.getWrapper()就能够看出来。

那么Context调用invoke方法后又发生什么了呢？详细运行的是org.apache.catalina.core.StandardContextValve的invoke方法。相当于进入了Context管道中，要開始通过管道中一个个闸门了。

StandardContext的处理流程能够用以下的图简化：

代码清单4-3：

/*** Select the appropriate child Wrapper to process this request,* based on the specified request URI.  If no matching Wrapper can* be found, return an appropriate HTTP error.** @param request Request to be processed* @param response Response to be produced** @exception IOException if an input/output error occurred* @exception ServletException if a servlet error occurred*/
@Override
public final void invoke(Request request, Response response)throws IOException, ServletException {// Disallow any direct access to resources under WEB-INF or META-INFMessageBytes requestPathMB = request.getRequestPathMB();if ((requestPathMB.startsWithIgnoreCase("/META-INF/", 0))|| (requestPathMB.equalsIgnoreCase("/META-INF"))|| (requestPathMB.startsWithIgnoreCase("/WEB-INF/", 0))|| (requestPathMB.equalsIgnoreCase("/WEB-INF"))) {response.sendError(HttpServletResponse.SC_NOT_FOUND);return;}// Select the Wrapper to be used for this RequestWrapper wrapper = request.getWrapper();if (wrapper == null || wrapper.isUnavailable()) {response.sendError(HttpServletResponse.SC_NOT_FOUND);return;}// Acknowledge the requesttry {response.sendAcknowledgement();} catch (IOException ioe) {container.getLogger().error(sm.getString("standardContextValve.acknowledgeException"), ioe);request.setAttribute(RequestDispatcher.ERROR_EXCEPTION, ioe);response.sendError(HttpServletResponse.SC_INTERNAL_SERVER_ERROR);return;}if (request.isAsyncSupported()) {request.setAsyncSupported(wrapper.getPipeline().isAsyncSupported());}wrapper.getPipeline().getFirst().invoke(request, response);
}

1. 禁止直接訪问WEB-INF或者META-INF文件夹下的资源
2. 选择详细的Wrapper处理请求
3. 返回一个确认响应
4. 调用Wrapper容器的invoke方法，把处理请求交给StandardWrapperValve处理

Wrapper容器
Wrapper容器负责管理一个Servlet，包含Servlet的装载、初始化、资源回收。Wrapper是最底层的容器，其不能在加入子容器了。Wrapper是一个接口。其标准实现类是StandardWrapper，以下是这两个类的结构：

上面的图都仅仅是类的一部分。因为Wrapper与Servlet息息相关。当中的loadServlet方法负责装载Servlet，其源代码例如以下：

代码清单4-4：

/*** Load and initialize an instance of this servlet, if there is not already* at least one initialized instance.  This can be used, for example, to* load servlets that are marked in the deployment descriptor to be loaded* at server startup time.*/
public synchronized Servlet loadServlet() throws ServletException {if (unloading) {throw new ServletException(sm.getString("standardWrapper.unloading", getName()));}// Nothing to do if we already have an instance or an instance poolif (!singleThreadModel && (instance != null))return instance;PrintStream out = System.out;if (swallowOutput) {SystemLogHandler.startCapture();}Servlet servlet;try {long t1=System.currentTimeMillis();// Complain if no servlet class has been specifiedif (servletClass == null) {unavailable(null);throw new ServletException(sm.getString("standardWrapper.notClass", getName()));}InstanceManager instanceManager = ((StandardContext)getParent()).getInstanceManager();try {servlet = (Servlet) instanceManager.newInstance(servletClass);} catch (ClassCastException e) {unavailable(null);// Restore the context ClassLoaderthrow new ServletException(sm.getString("standardWrapper.notServlet", servletClass), e);} catch (Throwable e) {e = ExceptionUtils.unwrapInvocationTargetException(e);ExceptionUtils.handleThrowable(e);unavailable(null);// Added extra log statement for Bugzilla 36630:// http://issues.apache.org/bugzilla/show_bug.cgi?id=36630if(log.isDebugEnabled()) {log.debug(sm.getString("standardWrapper.instantiate", servletClass), e);}// Restore the context ClassLoaderthrow new ServletException(sm.getString("standardWrapper.instantiate", servletClass), e);}if (multipartConfigElement == null) {MultipartConfig annotation =servlet.getClass().getAnnotation(MultipartConfig.class);if (annotation != null) {multipartConfigElement =new MultipartConfigElement(annotation);}}processServletSecurityAnnotation(servlet.getClass());// Special handling for ContainerServlet instancesif ((servlet instanceof ContainerServlet) &&(isContainerProvidedServlet(servletClass) ||((Context) getParent()).getPrivileged() )) {((ContainerServlet) servlet).setWrapper(this);}classLoadTime=(int) (System.currentTimeMillis() -t1);if (servlet instanceof SingleThreadModel) {if (instancePool == null) {instancePool = new Stack<Servlet>();}singleThreadModel = true;}//init servlet instanceinitServlet(servlet);fireContainerEvent("load", this);loadTime=System.currentTimeMillis() -t1;} finally {if (swallowOutput) {String log = SystemLogHandler.stopCapture();if (log != null && log.length() > 0) {if (getServletContext() != null) {getServletContext().log(log);} else {out.println(log);}}}}return servlet;
}

该类主要负责初始化一个Servlet实例，并调用该实例的init方法，然后通知感兴趣的事件监听程序。代码清单4-3中调用了Wrapper的invoke方法，这种方法完毕什么呢？

代码清单4-5：

/*** Invoke the servlet we are managing, respecting the rules regarding* servlet lifecycle and SingleThreadModel support.** @param request Request to be processed* @param response Response to be produced** @exception IOException if an input/output error occurred* @exception ServletException if a servlet error occurred*/
@Override
public final void invoke(Request request, Response response)throws IOException, ServletException {// Initialize local variables we may needboolean 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 unavailableif (!context.getState().isAvailable()) {response.sendError(HttpServletResponse.SC_SERVICE_UNAVAILABLE,sm.getString("standardContext.isUnavailable"));unavailable = true;}// Check for the servlet being marked unavailableif (!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 requesttry {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 allocatedboolean 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 requestApplicationFilterFactory factory =ApplicationFilterFactory.getInstance();ApplicationFilterChain filterChain =factory.createFilterChain(request, wrapper, servlet);// Reset comet flag value after creating the filter chainrequest.setComet(false);// Call the filter chain for this request// NOTE: This also calls the servlet's service() methodtry {if ((servlet != null) && (filterChain != null)) {// Swallow output if neededif (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 requestif (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 instancetry {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 instancetry {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);}}
}

1. 初始化一些本地变量
2. 推断当前应用是否可用。就是推断是否确实有这个项目
3. 分配一个Servlet实例
4. 为请求创建一个过滤器链
5. 过滤器过滤请求
6. 关闭过滤器
7. 又一次委派原来委派的Servlet实例
8. 释放资源

这种方法与上面的loadServlet关系例如以下：

能够看出在调用loadServlet的allocate方法的时候调用了StandardWrapperValve的invoke方法，在Wrapper容器获得请求后，通过allocate方法从实例池栈中弹出一个servlet实例来处理这个请求，servlet实例被封装成filterChain对象，紧接着通过一系列的过滤器过滤到达servlet.service()方法。这个过程能够例如以下：