【SpringMVC框架】前端控制器源代码分析

前端控制器源代码分析

虽然前面讲了一些springmvc的入门程序和配置文件中映射器和适配器的配置，但是我们作为编程人员，了解框架的部分源码还是有必要的，比如前端控制器，它是如何通过Servlet的web.xml配置文件实现拦截并跳转至DispatcherServlet的呢？下面我们详细探讨

众多周知我们的入门程序的web.xml是这么配置的

<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.4"
xmlns="http://java.sun.com/xml/ns/j2ee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd"> <!-- SpringMvc前端控制器 -->
<servlet>
<servlet-name>springmvc</servlet-name>
<servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
<!-- contextConfigLocation配置springmvc加载的配置文件(配置处理器映射器，适配器等)
如果不配置contextConfigLoaction，默认加载的是/WEB-INF/servlet名称-servlet.xml(springmvc-servlet.xml)
-->
<init-param>
<param-name>contextConfigLocation</param-name>
<param-value>classpath:springmvc.xml</param-value>
</init-param>
</servlet>

<servlet-mapping>
<servlet-name>springmvc</servlet-name>
<!-- 第一种:*.action。访问以.action结尾由DispatcherServlet进行解析；
第二种:/，所有访问的地址都由DispatcherServlet进行解析，对于静态文件的解析，
我们要配置不让DispatcherServlet进行解析。使用此种方法可以实现RESTful风格的url；
第三种:/*，这样配置不对，使用这种配置，最终要转发到一个jsp页面时，仍然会由
DispatcherServlet进行解析jsp地址，它不能根据jsp页面找到Handler，会报错-->
<url-pattern>*.action</url-pattern>
</servlet-mapping>

<welcome-file-list>
<welcome-file>index.jsp</welcome-file>
</welcome-file-list>
</web-app>

还记不记得springmvc的执行过程:
图-1.c.springmvc框架



通过前端控制器源码分析springmvc的执行过程。

我们点开org.springframework.web.servlet.DispatcherServlet看看，里面有一个doDiapatch的方法:

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
HttpServletRequest processedRequest = request;
HandlerExecutionChain mappedHandler = null;
boolean multipartRequestParsed = false;

WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);

try {
ModelAndView mv = null;
Exception dispatchException = null;

try {
processedRequest = checkMultipart(request);
multipartRequestParsed = processedRequest != request;

// Determine handler for the current request.
mappedHandler = getHandler(processedRequest, false);
if (mappedHandler == null || mappedHandler.getHandler() == null) {
noHandlerFound(processedRequest, response);
return;
}

// Determine handler adapter for the current request.
HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

// Process last-modified header, if supported by the handler.
String method = request.getMethod();
boolean isGet = "GET".equals(method);
if (isGet || "HEAD".equals(method)) {
long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
if (logger.isDebugEnabled()) {
String requestUri = urlPathHelper.getRequestUri(request);
logger.debug("Last-Modified value for [" + requestUri + "] is: " + lastModified);
}
if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
return;
}
}

if (!mappedHandler.applyPreHandle(processedRequest, response)) {
return;
}

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

applyDefaultViewName(request, mv);
mappedHandler.applyPostHandle(processedRequest, response, mv);
}
catch (Exception ex) {
dispatchException = ex;
}
processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
}
catch (Exception ex) {
triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
}
catch (Error err) {
triggerAfterCompletionWithError(processedRequest, response, mappedHandler, err);
}
finally {
if (asyncManager.isConcurrentHandlingStarted()) {
// Instead of postHandle and afterCompletion
mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
return;
}
// Clean up any resources used by a multipart request.
if (multipartRequestParsed) {
cleanupMultipart(processedRequest);
}
}
}

我们来分析这个方法

第一步：前端控制器接收请求

.action类型的URL通过过滤器进入DispatcherServlet类，调用其doDiapatch()方法

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
HttpServletRequest processedRequest = request;
HandlerExecutionChain mappedHandler = null;
boolean multipartRequestParsed = false;

WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
......
}

第二步：前端控制器调用处理器映射器查找 Handler

在doDiapatch()方法中调用了DispatcherServlet类的getHandler方法

HandlerExecutionChain mappedHandler = null;
......
mappedHandler = getHandler(processedRequest, false);

其中getHandler方法:

@Deprecated
protected HandlerExecutionChain getHandler(HttpServletRequest request, boolean cache) throws Exception {
return getHandler(request);
}

protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
for (HandlerMapping hm : this.handlerMappings) {
if (logger.isTraceEnabled()) {
logger.trace(
"Testing handler map [" + hm + "] in DispatcherServlet with name '" + getServletName() + "'");
}
HandlerExecutionChain handler = hm.getHandler(request);
if (handler != null) {
return handler;
}
}
return null;
}

说明映射器根据request当中的URL，找到了Handler，最终返回一个执行器的链(HandlerExecutionChain)。这个链里面有Handler。

第三步：调用处理器适配器执行Handler，得到执行结果ModelAndView

ModelAndView mv = null;
......
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

第四步：视图渲染，将model数据填充到request域。

视图解析，得到view:

在doDiapatch()方法中有这一句

processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);

其中processDispatchResult方法

private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
HandlerExecutionChain mappedHandler, ModelAndView mv, Exception exception) throws Exception {

boolean errorView = false;

if (exception != null) {
if (exception instanceof ModelAndViewDefiningException) {
logger.debug("ModelAndViewDefiningException encountered", exception);
mv = ((ModelAndViewDefiningException) exception).getModelAndView();
}
else {
Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
mv = processHandlerException(request, response, handler, exception);
errorView = (mv != null);
}
}

// Did the handler return a view to render?
if (mv != null && !mv.wasCleared()) {
render(mv, request, response);
if (errorView) {
WebUtils.clearErrorRequestAttributes(request);
}
}
else {
if (logger.isDebugEnabled()) {
logger.debug("Null ModelAndView returned to DispatcherServlet with name '" + getServletName() +
"': assuming HandlerAdapter completed request handling");
}
}

if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
// Concurrent handling started during a forward
return;
}

if (mappedHandler != null) {
mappedHandler.triggerAfterCompletion(request, response, null);
}
}

其中render(mv, request, response);方法中有

view = resolveViewName(mv.getViewName(), mv.getModelInternal(), locale, request);

渲染方法：

view.render(mv.getModelInternal(), request, response);

调用view的渲染方法，将model数据填充到request域

protected void exposeModelAsRequestAttributes(Map<String, Object> model, HttpServletRequest request) throws Exception {
//遍历model里面的数据，填充到request域
for (Map.Entry<String, Object> entry : model.entrySet()) {
String modelName = entry.getKey();
Object modelValue = entry.getValue();
if (modelValue != null) {
request.setAttribute(modelName, modelValue);
if (logger.isDebugEnabled()) {
logger.debug("Added model object '" + modelName + "' of type [" + modelValue.getClass().getName() +
"] to request in view with name '" + getBeanName() + "'");
}
}
else {
request.removeAttribute(modelName);
if (logger.isDebugEnabled()) {
logger.debug("Removed model object '" + modelName +
"' from request in view with name '" + getBeanName() + "'");
}
}
}
}

大致了解了源码，便于更好的理解框架。
转载请注明出处：http://blog.csdn.net/acmman/article/details/46980497