tomcat源码 Container

1.Container的有四个子容器，分别是Engine，Host，Context，Wrapper,如下：

1、Engine：整个Catalina servlet引擎,标准实现为StandardEngine。
2、Host：表示包含一个或多个Context容器的虚拟主机，标准实现为StandardHost。
3、Context：表示一个web应用程序，对应着平时开发对应的一套程序，或者一个WEB-INF目录以及下面的web.xml文件一个Context可以有多个Wrapper，标准实现为StandardContext。
4、Wrapper：包装一个独立的Servlet容器，每个Wrapper封装一个Servlet，标准实现为StandardWrapper。

2.Container一个也有四个子接口Engine,Host,Context,Wrapper和一个默认实现类ContainerBase,另外这四个子容器都是对应一个StandardXXX实现类，都继承ContainerBase,并且Container还继承LifeCycle接口，所以这四个容器也是符合Tomcat的生命周期模式，结构图如下：

每个Container容器都有对应的阀Valve，多个Valve组成了Pipeline，这就是Container的具体实现过程，也可以在server.xml文件中配置Pipeline和Valve的集合实现。管道Pipe包含了容器中要执行的任务，而每一个阀Valve表示一个具体的任务，在每个管道中，都会有一个默认的阀，可以添加任意数量的阀，可通过server.xml文件配置。对过滤器熟悉的话就会发现，管道和阀的工作机制和过滤器工作机制相似，Pipeline相当于过滤器链FilterChain，Valve相当于每一个过滤器Filter。阀可以处理传给它的request对象和response对象，处理完一个Valve后接着处理下一个Valve，最后处理的阀是基础阀。下面就追踪每一个容器的管道，解析容器处理请求的流程。

首先是Engine容器，默认实现是StandardEngine，创建StandardEngine时实例化其基础阀，代码如下：

public StandardEngine() {

    super();
    //设置基础阀StandardEngineValve
    pipeline.setBasic(new StandardEngineValve());
    /* Set the jmvRoute using the system property jvmRoute */
    try {
        setJvmRoute(System.getProperty("jvmRoute"));
    } catch(Exception ex) {
        log.warn(sm.getString("standardEngine.jvmRouteFail"));
    }
    // By default, the engine will hold the reloading thread
    backgroundProcessorDelay = 10;

}

继续跟踪StandardEngineValve的invoke()方法，源码为：

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

    // Select the Host to be used for this Request
    // 选出和该request相关的Host，在MappingData中保存了请求和容器(Host,Context,Wrapper)之间的映射
    Host host = request.getHost();
    if (host == null) {
        response.sendError
            (HttpServletResponse.SC_BAD_REQUEST,
             sm.getString("standardEngine.noHost",
                          request.getServerName()));
        return;
    }
    if (request.isAsyncSupported()) {
        request.setAsyncSupported(host.getPipeline().isAsyncSupported());
    }

    // Ask this Host to process this request
    // host.getPipeline()得到Host对应的管道Pipeline，将request和response对象交给Host的阀去处理
    host.getPipeline().getFirst().invoke(request, response);

}

StandardEngineValve的invoke()方法是在CoyoteAdapter类中service方法中调用的，也就是Connector将请求交给Container的过程：

public void service(org.apache.coyote.Request req, org.apache.coyote.Response res)
            throws Exception {

        Request request = (Request) req.getNote(ADAPTER_NOTES);
        Response response = (Response) res.getNote(ADAPTER_NOTES);

        ......
        
            // Parse and set Catalina and configuration specific
            // request parameters
            //根据request得到对应的MappingData,里面保存了解析地址对应的Engine,Host,Context,Wrapper，在后面的管道中使用
            postParseSuccess = postParseRequest(req, request, res, response);
            if (postParseSuccess) {
                //check valves if we support async
                request.setAsyncSupported(
                        connector.getService().getContainer().getPipeline().isAsyncSupported());
                // Calling the container
                //得到Connector关联的Container，然后将request和response对象交给Engine的管道Pineline中的阀去处理。
                connector.getService().getContainer().getPipeline().getFirst().invoke(
                        request, response);
            }
            
        ......
    }

这个service方法就是SocketProcessor#doRun--->Http11Processor#service--->CoyoteAdapter#service

同样Host容器构造器中设置了其基础阀StandardHostValve：

public StandardHost() {

    super();
    pipeline.setBasic(new StandardHostValve());

}

同样跟踪StandardHostValve的invoke方法：

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

    // Select the Context to be used for this Request
    // 该request容器关联的Context，保存在MappingData中
    Context context = request.getContext();
    if (context == null) {
        response.sendError(HttpServletResponse.SC_INTERNAL_SERVER_ERROR,
             sm.getString("standardHost.noContext"));
        return;
    }

    //是否支持异步
    if (request.isAsyncSupported()) {
        request.setAsyncSupported(context.getPipeline().isAsyncSupported());
    }

    boolean asyncAtStart = request.isAsync();
    boolean asyncDispatching = request.isAsyncDispatching();

    try {
        //设置StandardHostValve的类加载器
        context.bind(Globals.IS_SECURITY_ENABLED, MY_CLASSLOADER);

        if (!asyncAtStart && !context.fireRequestInitEvent(request.getRequest())) {
            // Don't fire listeners during async processing (the listener
            // fired for the request that called startAsync()).
            // If a request init listener throws an exception, the request
            // is aborted.
            return;
        }

        // Ask this Context to process this request. Requests that are in
        // async mode and are not being dispatched to this resource must be
        // in error and have been routed here to check for application
        // defined error pages.
        try {
            // 将request传递给Context的阀去处理，有错误的页面必须在此处处理，不会继续向下传递到Context容器中
            if (!asyncAtStart || asyncDispatching) {
                context.getPipeline().getFirst().invoke(request, response);
            } else {
                // Make sure this request/response is here because an error
                // report is required.
                if (!response.isErrorReportRequired()) {
                    throw new IllegalStateException(sm.getString("standardHost.asyncStateError"));
                }
            }
        } catch (Throwable t) {
            ExceptionUtils.handleThrowable(t);
            container.getLogger().error("Exception Processing " + request.getRequestURI(), t);
            // If a new error occurred while trying to report a previous
            // error allow the original error to be reported.
            if (!response.isErrorReportRequired()) {
                request.setAttribute(RequestDispatcher.ERROR_EXCEPTION, t);
                throwable(request, response, t);
            }
        }

        // Now that the request/response pair is back under container
        // control lift the suspension so that the error handling can
        // complete and/or the container can flush any remaining data
        response.setSuspended(false);

        Throwable t = (Throwable) request.getAttribute(RequestDispatcher.ERROR_EXCEPTION);

        // Protect against NPEs if the context was destroyed during a
        // long running request.
        if (!context.getState().isAvailable()) {
            return;
        }

        // Look for (and render if found) an application level error page
         //设置错误页面
        if (response.isErrorReportRequired()) {
            if (t != null) {
                throwable(request, response, t);
            } else {
                status(request, response);
            }
        }

        if (!request.isAsync() && !asyncAtStart) {
            context.fireRequestDestroyEvent(request.getRequest());
        }
    } finally {
        // Access a session (if present) to update last accessed time, based
        // on a strict interpretation of the specification
        if (ACCESS_SESSION) {
            request.getSession(false);
        }

        context.unbind(Globals.IS_SECURITY_ENABLED, MY_CLASSLOADER);
    }
}

Context和Wrapper的管道和阀的实现过程与Engine和Host完全一样，不再继续分析。最后主要解析StandardWrapperValve的invoke()方法，看该方法如何将request交个一个servlet处理。鉴于该方法源码太长，只展示出了部分重要代码。

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

    ......
    //获取关联的StandardWrapper
    StandardWrapper wrapper = (StandardWrapper) getContainer();
    Servlet servlet = null;
    //wrapper的父容器Context
    Context context = (Context) wrapper.getParent();

    ......

    // Allocate a servlet instance to process this request
    // 分配一个servlet实例处理该request
    try {
        if (!unavailable) {
            //servlet可用时，分配servlet，接下来会跟踪allocate()方法
            servlet = wrapper.allocate();
        }
    } catch (UnavailableException e) {
        //分别设置了503错误和404 not found
        ......
    } catch (ServletException e) {
        ......
    } catch (Throwable e) {
        ......
    }

    ......
    // Create the filter chain for this request
    // 为该request设置过滤器
    ApplicationFilterChain filterChain =
            ApplicationFilterFactory.createFilterChain(request, wrapper, servlet);

    // Call the filter chain for this request
    // NOTE: This also calls the servlet's service() method
    // 过滤器作用于该request，并且此过程中调用了servlet的service()方法
    try {
        if ((servlet != null) && (filterChain != null)) {
            // Swallow output if needed
            if (context.getSwallowOutput()) {
                try {
                    SystemLogHandler.startCapture();
                    if (request.isAsyncDispatching()) {
                        request.getAsyncContextInternal().doInternalDispatch();
                    } 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()) {
                    request.getAsyncContextInternal().doInternalDispatch();
                } else {
                    filterChain.doFilter
                        (request.getRequest(), response.getResponse());
                }
            }

        }
    } catch (ClientAbortException e) {
        ......
    } catch (IOException e) {
        ......
    } catch (UnavailableException e) {
        ......
    } catch (ServletException e) {
        ......
    } catch (Throwable e) {
        ......
    }

    // Release the filter chain (if any) for this request
    // 释放该request的过滤链
    if (filterChain != null) {
        filterChain.release();
    }

    // Deallocate the allocated servlet instance
    try {
        // 回收servlet容器实例
        if (servlet != null) {
            wrapper.deallocate(servlet);
        }
    } catch (Throwable e) {
        ......
    }

    ......
}

接着跟踪Wrapper的allocate源码：该方法主要功能是分配一个初始化了的servlet实例，其service方法可以被调用。

public Servlet allocate() throws ServletException {

    // If we are currently unloading this servlet, throw an exception
    // servlet类没有加载时剖出异常
    if (unloading) {
        throw new ServletException(sm.getString("standardWrapper.unloading", getName()));
    }

    boolean newInstance = false;

    // If not SingleThreadedModel, return the same instance every time
    if (!singleThreadModel) {
        // Load and initialize our instance if necessary
        // servlet没有加载时要先载入该servlet
        if (instance == null || !instanceInitialized) {
            synchronized (this) {
                if (instance == null) {
                    try {
                        if (log.isDebugEnabled()) {
                            log.debug("Allocating non-STM instance");
                        }

                        // Note: We don't know if the Servlet implements
                        // SingleThreadModel until we have loaded it.
                        //加载servlet，接下来继续分析loadServlet()方法
                        instance = loadServlet();
                        newInstance = true;
                        //类加载之前并不知道该servlet是否为singleThreadModel，在loadServlet()中会改变singleThreadModel的值，所以此处要再判断一次
                        if (!singleThreadModel) {
                            // For non-STM, increment here to prevent a race
                            // condition with unload. Bug 43683, test case
                            // #3
                            countAllocated.incrementAndGet();
                        }
                    } catch (ServletException e) {
                        throw e;
                    } catch (Throwable e) {
                        ExceptionUtils.handleThrowable(e);
                        throw new ServletException(sm.getString("standardWrapper.allocate"), e);
                    }
                }
                if (!instanceInitialized) {
                    //初始化servlet
                    initServlet(instance);
                }
            }
        }

        //新加载的servlet实现singleThreadModel时将instance加入到instancePool中，否则直接返回instance
        if (singleThreadModel) {
            if (newInstance) {
                // Have to do this outside of the sync above to prevent a
                // possible deadlock
                synchronized (instancePool) {
                    instancePool.push(instance);
                    nInstances++;
                }
            }
        } else {
            if (log.isTraceEnabled()) {
                log.trace("  Returning non-STM instance");
            }
            // For new instances, count will have been incremented at the
            // time of creation
            if (!newInstance) {
                countAllocated.incrementAndGet();
            }
            return instance;
        }
    }

    //SingleThreadedModel类型的servlet时返回instancePool中的一个instance。
    synchronized (instancePool) {
        while (countAllocated.get() >= nInstances) {
            // Allocate a new instance if possible, or else wait
            if (nInstances < maxInstances) {
                try {
                    instancePool.push(loadServlet());
                    nInstances++;
                } catch (ServletException e) {
                    throw e;
                } catch (Throwable e) {
                    ExceptionUtils.handleThrowable(e);
                    throw new ServletException(sm.getString("standardWrapper.allocate"), e);
                }
            } else {
                try {
                    instancePool.wait();
                } catch (InterruptedException e) {
                    // Ignore
                }
            }
        }
        if (log.isTraceEnabled()) {
            log.trace("  Returning allocated STM instance");
        }
        countAllocated.incrementAndGet();
        return instancePool.pop();
    }
}

接下来看一下servlet的load过程loadServlet：

public synchronized Servlet loadServlet() throws ServletException {

    // Nothing to do if we already have an instance or an instance pool
    // 如果不是SingleThreadModel类型的servlet，并且已经存在一个instance实例时，不需要加载。
    if (!singleThreadModel && (instance != null))
        return instance;

    ......

    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()));
        }

        //Context容器中的instanceManager，是一个类加载器，其newInstance方法根据class路径加载servlet
        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) {
            ......
        }

        ......

        if (servlet instanceof SingleThreadModel) {
            if (instancePool == null) {
                instancePool = new Stack<>();
            }
            //此处修改了singleThreadModel值，所以allocate方法中新加载servlet类后要重新判断这个值
            singleThreadModel = true;
        }

        //初始化刚加载的servlet
        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;

}

通过以上分析，我们知道了一个request请求是如何从Engine容器一路流动到了具体处理容器Wrapper中的，就是通过管道和阀的工作机制实现的，每一个容器都会对应一个管道，可以向管道中添加任意数量的阀valve，但必须要有一个基础阀，上一层的容器通过调用下一次容器的管道的阀的invoke方法实现request对象的传递。