spring mvc DispatcherServlet详解之三获取ModelAndView过程

整个spring mvc的架构如下图所示：

上篇文件讲解了DispatcherServlet通过request获取控制器Controller的过程，现在来讲解DispatcherServletDispatcherServlet的第二步：通过request从Controller获取ModelAndView。

DispatcherServlet调用Controller的过程：

DispatcherServlet.java

doService()--->doDispatch()--->handlerAdapter的handle()方法

 try {// Actually invoke the handler.
 mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
 }
 finally {
 if (asyncManager.isConcurrentHandlingStarted()) {
 return;
 }
 }

最常用的实现了HandlerAdapter接口是SimpleControllerHandlerAdapter类，该类将

两个不兼容的类：DispatcherServlet 和Controller 类连接到一起。

 Adapter to use the plain {@link Controller} workflow interface with
 the generic {@link org.springframework.web.servlet.DispatcherServlet}.
 Supports handlers that implement the {@link LastModified} interface.
 
 <p>This is an SPI class, not used directly by application code.

类之间的转换代码如下所示，调用了Controller类的handleRequest()方法来处理请求：

 @Override
 public ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)
 throws Exception {
 return ((Controller) handler).handleRequest(request, response);
 }

重量级人物控制器Controller开始闪亮登场，Controller是一个基本的接口，它接受request和response，从这点上来说，它有点像servlet，但不同之处在于它在mvc模式流程中起作用，它和struts中的Action作用类似。继承该接口的控制器或者类应该保证是线程安全的，可复用的，能够在一个应用生命周期中处理大量的request。为了使Controller的配置更便捷，通常使用javaBeans来继承Controller。

/**
 * Base Controller interface, representing a component that receives
 * {@code HttpServletRequest} and {@code HttpServletResponse}
 * instances just like a {@code HttpServlet} but is able to
 * participate in an MVC workflow. Controllers are comparable to the
 * notion of a Struts {@code Action}.
 *
 * <p>Any implementation of the Controller interface should be a
 * <i>reusable, thread-safe</i> class, capable of handling multiple
 * HTTP requests throughout the lifecycle of an application. To be able to
 * configure a Controller easily, Controller implementations are encouraged
 * to be (and usually are) JavaBeans.
 * </p>
 *
 * <p><b><a name="workflow">Workflow</a></b></p>
 *
 * <p>
 * After a <cde>DispatcherServlet</code> has received a request and has
 * done its work to resolve locales, themes and suchlike, it then tries
 * to resolve a Controller, using a
 * {@link org.springframework.web.servlet.HandlerMapping HandlerMapping}.
 * When a Controller has been found to handle the request, the
 * {@link #handleRequest(HttpServletRequest, HttpServletResponse) handleRequest}
 * method of the located Controller will be invoked; the located Controller
 * is then responsible for handling the actual request and - if applicable -
 * returning an appropriate
 * {@link org.springframework.web.servlet.ModelAndView ModelAndView}.
 * So actually, this method is the main entrypoint for the
 * {@link org.springframework.web.servlet.DispatcherServlet DispatcherServlet}
 * which delegates requests to controllers.</p>
 *
 * <p>So basically any <i>direct</i> implementation of the Controller interface
 * just handles HttpServletRequests and should return a ModelAndView, to be further
 * interpreted by the DispatcherServlet. Any additional functionality such as
 * optional validation, form handling, etc should be obtained through extending
 * one of the abstract controller classes mentioned above.</p>
 *
 * <p><b>Notes on design and testing</b></p>
 *
 * <p>The Controller interface is explicitly designed to operate on HttpServletRequest
 * and HttpServletResponse objects, just like an HttpServlet. It does not aim to
 * decouple itself from the Servlet API, in contrast to, for example, WebWork, JSF or Tapestry.
 * Instead, the full power of the Servlet API is available, allowing Controllers to be
 * general-purpose: a Controller is able to not only handle web user interface
 * requests but also to process remoting protocols or to generate reports on demand.</p>
 *
 * <p>Controllers can easily be tested by passing in mock objects for the
 * HttpServletRequest and HttpServletResponse objects as parameters to the
 * {@link #handleRequest(HttpServletRequest, HttpServletResponse) handleRequest}
 * method. As a convenience, Spring ships with a set of Servlet API mocks
 * that are suitable for testing any kind of web components, but are particularly
 * suitable for testing Spring web controllers. In contrast to a Struts Action,
 * there is no need to mock the ActionServlet or any other infrastructure;
 * HttpServletRequest and HttpServletResponse are sufficient.</p>
 *
 * <p>If Controllers need to be aware of specific environment references, they can
 * choose to implement specific awareness interfaces, just like any other bean in a
 * Spring (web) application context can do, for example:</p>
 * <ul>
 * <li>{@code org.springframework.context.ApplicationContextAware}</li>
 * <li>{@code org.springframework.context.ResourceLoaderAware}</li>
 * <li>{@code org.springframework.web.context.ServletContextAware}</li>
 * </ul>
 *
 * <p>Such environment references can easily be passed in testing environments,
 * through the corresponding setters defined in the respective awareness interfaces.
 * In general, it is recommended to keep the dependencies as minimal as possible:
 * for example, if all you need is resource loading, implement ResourceLoaderAware only.
 * Alternatively, derive from the WebApplicationObjectSupport base class, which gives
 * you all those references through convenient accessors - but requires an
 * ApplicationContext reference on initialization.
 *
 * <p>Controllers can optionally implement the {@link LastModified} interface.
*/

Controller的handleRequest()方法处理请求，并返回ModelAndView给DispatcherServlet去渲染render。

Controller接口的抽象实现类为：AbstractController，它通过互斥锁(mutex)来保证线程安全。

 /**
 * Set if controller execution should be synchronized on the session,
 * to serialize parallel invocations from the same client.
 * <p>More specifically, the execution of the {@code handleRequestInternal}
 * method will get synchronized if this flag is "true". The best available
 * session mutex will be used for the synchronization; ideally, this will
 * be a mutex exposed by HttpSessionMutexListener.
 * <p>The session mutex is guaranteed to be the same object during
 * the entire lifetime of the session, available under the key defined
 * by the {@code SESSION_MUTEX_ATTRIBUTE} constant. It serves as a
 * safe reference to synchronize on for locking on the current session.
 * <p>In many cases, the HttpSession reference itself is a safe mutex
 * as well, since it will always be the same object reference for the
 * same active logical session. However, this is not guaranteed across
 * different servlet containers; the only 100% safe way is a session mutex.
 * @see AbstractController#handleRequestInternal
 * @see org.springframework.web.util.HttpSessionMutexListener
 * @see org.springframework.web.util.WebUtils#getSessionMutex(javax.servlet.http.HttpSession)
 */

线程安全实现：

 public ModelAndView handleRequest(HttpServletRequest request, HttpServletResponse response)
 throws Exception {
 // Delegate to WebContentGenerator for checking and preparing.
 checkAndPrepare(request, response, this instanceof LastModified);
 // Execute handleRequestInternal in synchronized block if required.
 if (this.synchronizeOnSession) {
 HttpSession session = request.getSession(false);
 if (session != null) {
 Object mutex = WebUtils.getSessionMutex(session);
 synchronized (mutex) {
 return handleRequestInternal(request, response);
 }
 }
 }
 return handleRequestInternal(request, response);
 }

handleRequestInternal()为抽象方法，留待具体实现类来实现。它的直接子类有：

AbstractUrlViewController, MultiActionController, ParameterizableViewController, ServletForwardingController, ServletWrappingController

简单Controller实现

在web.xml中有时候定义节点<welcome-list>index.html</welcome-list>等，这种简单的请,Controller是如何实现的呢？我们来看看UrlFilenameViewController，它是Controller的一个间接实现，实现了AbstractUrlViewController。它把url的虚拟路径转换成一个view的名字，然后返回这个view。

 protected ModelAndView handleRequestInternal(HttpServletRequest request, HttpServletResponse response) {
 String lookupPath = getUrlPathHelper().getLookupPathForRequest(request);
 String viewName = getViewNameForRequest(request);
 if (logger.isDebugEnabled()) {
 logger.debug("Returning view name '" + viewName + "' for lookup path [" + lookupPath + "]");
 }
 return new ModelAndView(viewName, RequestContextUtils.getInputFlashMap(request));
 }

复杂Controller实现

一个可以处理多种请求类型的Controller实现：MultiActionController。它类似于struts中的DispatcherAction，但更灵活，而且支持代理。

/**
 * {@link org.springframework.web.servlet.mvc.Controller Controller}
 * implementation that allows multiple request types to be handled by the same
 * class. Subclasses of this class can handle several different types of
 * request with methods of the form
 *
 * <pre class="code">public (ModelAndView | Map | String | void) actionName(HttpServletRequest request, HttpServletResponse response, [,HttpSession] [,AnyObject]);</pre>
 *
 * A Map return value indicates a model that is supposed to be passed to a default view
 * (determined through a {@link org.springframework.web.servlet.RequestToViewNameTranslator}).
 * A String return value indicates the name of a view to be rendered without a specific model.
 *
 * <p>May take a third parameter (of type {@link HttpSession}) in which an
 * existing session will be required, or a third parameter of an arbitrary
 * class that gets treated as the command (that is, an instance of the class
 * gets created, and request parameters get bound to it)
 *
 * <p>These methods can throw any kind of exception, but should only let
 * propagate those that they consider fatal, or which their class or superclass
 * is prepared to catch by implementing an exception handler.
 *
 * <p>When returning just a {@link Map} instance view name translation will be
 * used to generate the view name. The configured
 * {@link org.springframework.web.servlet.RequestToViewNameTranslator} will be
 * used to determine the view name.
 *
 * <p>When returning {@code void} a return value of {@code null} is
 * assumed meaning that the handler method is responsible for writing the
 * response directly to the supplied {@link HttpServletResponse}.
 *
 * <p>This model allows for rapid coding, but loses the advantage of
 * compile-time checking. It is similar to a Struts {@code DispatchAction},
 * but more sophisticated. Also supports delegation to another object.
 *
 * <p>An implementation of the {@link MethodNameResolver} interface defined in
 * this package should return a method name for a given request, based on any
 * aspect of the request, such as its URL or an "action" parameter. The actual
 * strategy can be configured via the "methodNameResolver" bean property, for
 * each {@code MultiActionController}.
 *
 * <p>The default {@code MethodNameResolver} is
 * {@link InternalPathMethodNameResolver}; further included strategies are
 * {@link PropertiesMethodNameResolver} and {@link ParameterMethodNameResolver}.
 *
 * <p>Subclasses can implement custom exception handler methods with names such
 * as:
 *
 * <pre class="code">public ModelAndView anyMeaningfulName(HttpServletRequest request, HttpServletResponse response, ExceptionClass exception);</pre>
 *
 * The third parameter can be any subclass or {@link Exception} or
 * {@link RuntimeException}.
 *
 * <p>There can also be an optional {@code xxxLastModified} method for
 * handlers, of signature:
 *
 * <pre class="code">public long anyMeaningfulNameLastModified(HttpServletRequest request)</pre>
 *
 * If such a method is present, it will be invoked. Default return from
 * {@code getLastModified} is -1, meaning that the content must always be
 * regenerated.
 *
 * <p><b>Note that all handler methods need to be public and that
 * method overloading is <i>not</i> allowed.</b>
 *
 * <p>See also the description of the workflow performed by
 * {@link AbstractController the superclass} (in that section of the class
 * level Javadoc entitled 'workflow').
 *
 * <p><b>Note:</b> For maximum data binding flexibility, consider direct usage of a
 * {@link ServletRequestDataBinder} in your controller method, instead of relying
 * on a declared command argument. This allows for full control over the entire
 * binder setup and usage, including the invocation of {@link Validator Validators}
 * and the subsequent evaluation of binding/validation errors.*/

根据方法名决定处理的handler

 protected ModelAndView handleRequestInternal(HttpServletRequest request, HttpServletResponse response)
 throws Exception {
 try {
 String methodName = this.methodNameResolver.getHandlerMethodName(request);
 return invokeNamedMethod(methodName, request, response);
 }
 catch (NoSuchRequestHandlingMethodException ex) {
 return handleNoSuchRequestHandlingMethod(ex, request, response);
 }
 }

触发执行方法：

protected final ModelAndView invokeNamedMethod(
 String methodName, HttpServletRequest request, HttpServletResponse response) throws Exception {
 Method method = this.handlerMethodMap.get(methodName);
 if (method == null) {
 throw new NoSuchRequestHandlingMethodException(methodName, getClass());
 }
 try {
 Class<?>[] paramTypes = method.getParameterTypes();
 List<Object> params = new ArrayList<Object>(4);
 params.add(request);
 params.add(response);
 if (paramTypes.length >= 3 && paramTypes[2].equals(HttpSession.class)) {
 HttpSession session = request.getSession(false);
 if (session == null) {
 throw new HttpSessionRequiredException(
 "Pre-existing session required for handler method '" + methodName + "'");
 }
 params.add(session);
 }
 // If last parameter isn't of HttpSession type, it's a command.
 if (paramTypes.length >= 3 &&
 !paramTypes[paramTypes.length - 1].equals(HttpSession.class)) {
 Object command = newCommandObject(paramTypes[paramTypes.length - 1]);
 params.add(command);
 bind(request, command);
 }
 Object returnValue = method.invoke(this.delegate, params.toArray(new Object[params.size()]));
 return massageReturnValueIfNecessary(returnValue);
 }
 catch (InvocationTargetException ex) {
 // The handler method threw an exception.
 return handleException(request, response, ex.getTargetException());
 }
 catch (Exception ex) {
 // The binding process threw an exception.
 return handleException(request, response, ex);
 }

处理返回结果，要么返回null要么返回ModelAndView实例。当返回一个Map类型时，ModelAndView实例包装的Map类型。

/**
 * Processes the return value of a handler method to ensure that it either returns
 * {@code null} or an instance of {@link ModelAndView}. When returning a {@link Map},
 * the {@link Map} instance is wrapped in a new {@link ModelAndView} instance.
 */
 @SuppressWarnings("unchecked")
 private ModelAndView massageReturnValueIfNecessary(Object returnValue) {
 if (returnValue instanceof ModelAndView) {
 return (ModelAndView) returnValue;
 }
 else if (returnValue instanceof Map) {
 return new ModelAndView().addAllObjects((Map<String, ?>) returnValue);
 }
 else if (returnValue instanceof String) {
 return new ModelAndView((String) returnValue);
 }
 else {
 // Either returned null or was 'void' return.
 // We'll assume that the handle method already wrote the response.
 return null;
 }
 }

小结：

DispatcherServlet接受一个请求，然后解析完locales, themes等后，通过HadlerMapping解析控制器Controller去处理请求。

找到Controller后，出发当前controller的handleRequest()方法，此controller负责真正处理请求,然后一个ModelAndView实例。

DispatcherServlet 代理此Controller，接收返回结果，然后进行渲染。