深入理解Tomcat系列之五：Context容器和Wrapper容器

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-INF
    MessageBytes 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 Request
    Wrapper wrapper = request.getWrapper();
    if (wrapper == null || wrapper.isUnavailable()) {
        response.sendError(HttpServletResponse.SC_NOT_FOUND);
        return;
    }
    // Acknowledge the request
    try {
        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 pool
    if (!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 specified
        if (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 ClassLoader
            throw 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=36630
            if(log.isDebugEnabled()) {
                log.debug(sm.getString("standardWrapper.instantiate", servletClass), e);
            }
            // Restore the context ClassLoader
            throw 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 instances
        if ((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 instance
        initServlet(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 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);
        }
    }
}

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：