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Spring源码分析之SpringMVC的DispatcherServlet是如何处理Http请求的

2017-07-02 17:43 543 查看
一般我们会在web.xml文件中配置DispatcherServlet,比如如下配置方式:

<servlet>
<servlet-name>dispatcherServlet</servlet-name>
<servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
<init-param>
<param-name>contextConfigLocation</param-name>
<param-value>classpath:dispatcher-servlet.xml</param-value>
</init-param>
<load-on-startup>1</load-on-startup>
</servlet>

<servlet-mapping>
<servlet-name>dispatcherServlet</servlet-name>
<url-pattern>/</url-pattern>
</servlet-mapping>


上述代码配置了所有路径的http请求都会交给DispatcherServlet来处理

那么我们来看下DispatcherServlet的具体代码:

@SuppressWarnings("serial")
public class DispatcherServlet extends FrameworkServlet {
public DispatcherServlet(WebApplicationContext webApplicationContext) {
super(webApplicationContext);
setDispatchOptionsRequest(true);
}

@Override
protected void onRefresh(ApplicationContext context) {
initStrategies(context);
}

/**
* Initialize the strategy objects that this servlet uses.
* <p>May be overridden in subclasses in order to initialize further strategy objects.
*/
protected void initStrategies(ApplicationContext context) {
initMultipartResolver(context);
initLocaleResolver(context);
initThemeResolver(context);
initHandlerMappings(context);
initHandlerAdapters(context);
initHandlerExceptionResolvers(context);
initRequestToViewNameTranslator(context);
initViewResolvers(context);
initFlashMapManager(context);
}

@Override
protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {

}

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {

}


可以看到这个类继承了FrameworkServlet类,再来看下这个类的代码:

@SuppressWarnings("serial")
public abstract class FrameworkServlet extends HttpServletBean implements ApplicationContextAware {

@Override
protected final void initServletBean() throws ServletException {

}

protected WebApplicationContext initWebApplicationContext() {

if (!this.refreshEventReceived) {
// Either the context is not a ConfigurableApplicationContext with refresh
// support or the context injected at construction time had already been
// refreshed -> trigger initial onRefresh manually here.
onRefresh(wac);
}

}
@Override
public void destroy() {
getServletContext().log("Destroying Spring FrameworkServlet '" + getServletName() + "'");
// Only call close() on WebApplicationContext if locally managed...
if (this.webApplicationContext instanceof ConfigurableApplicationContext && !this.webApplicationContextInjected) {
((ConfigurableApplicationContext) this.webApplicationContext).close();
}
}

/**
* Override the parent class implementation in order to intercept PATCH requests.
*/
@Override
protected void service(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {

HttpMethod httpMethod = HttpMethod.resolve(request.getMethod());
if (HttpMethod.PATCH == httpMethod || httpMethod == null) {
processRequest(request, response);
}
else {
super.service(request, response);
}
}

@Override
protected final void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {

processRequest(request, response);
}

@Override
protected final void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {

processRequest(request, response);
}

protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
}

}
可以看到这个类继承了HttpServletBean类,而HttpServletBean类又继承了HttpServlet类,其中HttpServlet类的init,destroy,service三个方法负责管理Servlet的生命周期。

init方法:负责初始化Servlet对象

destroy方法:负责销毁Servlet对象

service方法:负责处理http请求

由于HttpServletBean实现了HttpServlet的init方法,可以看到该方法的代码如下:

public final void init() throws ServletException {
if (logger.isDebugEnabled()) {
logger.debug("Initializing servlet '" + getServletName() + "'");
}

// Set bean properties from init parameters.
// 获取servlet初始化的参数,对Bean属性进行配置
try {
PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);
ResourceLoader resourceLoader = new ServletContextResourceLoader(getServletContext());
bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, getEnvironment()));
initBeanWrapper(bw);
bw.setPropertyValues(pvs, true);
}
catch (BeansException ex) {
logger.error("Failed to set bean properties on servlet '" + getServletName() + "'", ex);
throw ex;
}

// Let subclasses do whatever initialization they like.
// 调用子类的initServletBean进行具体的初始化
initServletBean();

if (logger.isDebugEnabled()) {
logger.debug("Servlet '" + getServletName() + "' configured successfully");
}
}


这里会读取配置在ServletContext中的Bean属性参数,这里可以看到对PropertyValues和BeanWrapper的使用,这里就完成了对Servlet的初始化

接着调用了initServletBean方法来初始化DispatcherServlet持有的IOC容器,当然底层其实是调用initWebApplicationContext方法来初始化,其代码如下:

protected WebApplicationContext initWebApplicationContext() {
// 得到根上下文,这个根上下文是保存在ServletContext中的,使用这个根上下文作为当前MVC上下文的双亲上下文,当前的上下文就是wac
// cwac.setParent(rootContext)就是设置当前上下文的根上下文
WebApplicationContext rootContext =
WebApplicationContextUtils.getWebApplicationContext(getServletContext());
WebApplicationContext wac = null;

if (wac == null) {
// No context instance is defined for this servlet -> create a local one
wac = createWebApplicationContext(rootContext);
}

if (!this.refreshEventReceived) {
// Either the context is not a ConfigurableApplicationContext with refresh
// support or the context injected at construction time had already been
// refreshed -> trigger initial onRefresh manually here.
onRefresh(wac);
}
}


可以看到createWebApplicationContext方法就是在创建并初始化IOC容器,代码如下:

protected WebApplicationContext createWebApplicationContext(ApplicationContext parent) {

//设置IOC容器的各个参数
wac.setEnvironment(getEnvironment());
wac.setParent(parent);
wac.setConfigLocation(getContextConfigLocation());

configureAndRefreshWebApplicationContext(wac);

return wac;
}


这里设置了当前IOC容器上下文的双亲上下文,双亲上下文是如下方法获取到的:
WebApplicationContext rootContext =
WebApplicationContextUtils.getWebApplicationContext(getServletContext());
这样当从IOC容器中getBean的时候会先从其双亲上下文中获取bean

configureAndRefreshWebApplicationContext方法里面调用了wac.refresh(),也就是refresh方法来真正启动对IOC容器的初始化,这里就跟IOC容器的初始化保持一致了
到这里,对DispatcherServlet所持有的IOC容器的初始化就完成了,初始化完成后,DispatcherServlet就持有一个以自己的Servlet名称命名的IOC容器了,这个IOC容器是一个WebApplicationContext对象。

我们还可以看到initWebApplicationContext方法中还调用了onrefresh方法,该方法是由DispatcherServlet来实现的,并最终调用了DispatcherServlet的如下方法来进行初始化:

protected void initStrategies(ApplicationContext context) {
initMultipartResolver(context);
initLocaleResolver(context);
initThemeResolver(context);
initHandlerMappings(context);
initHandlerAdapters(context);
initHandlerExceptionResolvers(context);
initRequestToViewNameTranslator(context);
initViewResolvers(context);
initFlashMapManager(context);
}


在这个方法里,DispatcherServlet对MVC模块的其他部分进行了初始化,比如handlerMapping,ViewResolver等,它是启动整个Spring MVC框架的初始化

比如,支持国际化的LocalResolver,支持request映射的HandlerMapping,以及视图生成的ViewResolver等的初始化

这里的initHandlerMapping是为HTTP请求找到相应的Controller控制器。

DispatcherServlet的整个初始化过程就到这里结束了,下面说下DispatcherServlet是如何处理http请求的

首先http请求是由doService来处理的,那么我们可以看下HttpServlet类的该方法:

protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
String method = req.getMethod();
long errMsg;
if(method.equals("GET")) {
errMsg = this.getLastModified(req);
if(errMsg == -1L) {
this.doGet(req, resp);
} else {
long ifModifiedSince = req.getDateHeader("If-Modified-Since");
if(ifModifiedSince < errMsg) {
this.maybeSetLastModified(resp, errMsg);
this.doGet(req, resp);
} else {
resp.setStatus(304);
}
}
} else if(method.equals("HEAD")) {
errMsg = this.getLastModified(req);
this.maybeSetLastModified(resp, errMsg);
this.doHead(req, resp);
} else if(method.equals("POST")) {
this.doPost(req, resp);
} else if(method.equals("PUT")) {
this.doPut(req, resp);
} else if(method.equals("DELETE")) {
this.doDelete(req, resp);
} else if(method.equals("OPTIONS")) {
this.doOptions(req, resp);
} else if(method.equals("TRACE")) {
this.doTrace(req, resp);
} else {
String errMsg1 = lStrings.getString("http.method_not_implemented");
Object[] errArgs = new Object[]{method};
errMsg1 = MessageFormat.format(errMsg1, errArgs);
resp.sendError(501, errMsg1);
}

}


这个方法里面会根据不同的请求类型,来调用不同的处理方法,比如会调用doGet或者doPost方法来分别处理get请求和post请求,跟踪这些方法,可以发现,最终是调用了DispatcherServlet的doService方法,代码如下:

protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
try {
this.doDispatch(request, response);
} finally {
if(!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted() && attributesSnapshot1 != null) {
this.restoreAttributesAfterInclude(request, attributesSnapshot1);
}

}

}


可以看到该方法调用了doDispatch来进行处理,doDispatch是MVC模式的主要部分
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