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。