【原创】遨游springmvc之DispatcherServlet

1.机制

Dispatcher是springmvc前端控制器模式的实现，它提供一个集中的请求处理机制，所有的请求都将由一个单一的处理程序处理，Dispatcher负责请求的派遣，它与spring ioc完美继承，从而可以拥有spring的所有好处。

2.原理图

图2.1 原理图



通过原理图我们可知我们在用springmvc一直在强调的，它是由两个容器组成，即非web层容器和web层容器，一般配置在我们的spring.xml和springmvc.xml,非web层组件一般是整个应用都共享的，如经常用到的dao层，service层等，而DispatcherServlet则初始化springmvc上下文加载的bean如Controller、HandlerMapping、HandlerAdapter等，它只加载web层的bean

3.源代码解剖

3.1 依赖关系

直接上图(实线是继承extend、虚线是实现(implement))



3.2 源码解刨

在初始化web项目的时候，首先需要初始化servlet(HttpServlet),而servlet的初始化过程则转交给了HttpServletBean的init方法

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

// Set bean properties from init parameters.(给组件设置初始化参数)
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.（让子类实现扩展，由FrameworkServlet实现）
initServletBean();

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

FrameworkServlet实现HttpServletBean的initServletBean方法

protected final void initServletBean() throws ServletException {
getServletContext().log("Initializing Spring FrameworkServlet '" + getServletName() + "'");
if (this.logger.isInfoEnabled()) {
this.logger.info("FrameworkServlet '" + getServletName() + "': initialization started");
}
long startTime = System.currentTimeMillis();

try {
//初始化上下文
this.webApplicationContext = initWebApplicationContext();
//提供给子类初始化的扩展点
initFrameworkServlet();
}
catch (ServletException ex) {
this.logger.error("Context initialization failed", ex);
throw ex;
}
catch (RuntimeException ex) {
this.logger.error("Context initialization failed", ex);
throw ex;
}

if (this.logger.isInfoEnabled()) {
long elapsedTime = System.currentTimeMillis() - startTime;
this.logger.info("FrameworkServlet '" + getServletName() + "': initialization completed in " +
elapsedTime + " ms");
}
}

初始化上下文

//初始化上下文，并且将之赋给servlet
protected WebApplicationContext initWebApplicationContext() {
//ROOT上下文（ContextLoaderListener加载的）
WebApplicationContext rootContext =
WebApplicationContextUtils.getWebApplicationContext(getServletContext());
WebApplicationContext wac = null;

if (this.webApplicationContext != null) {
// A context instance was injected at construction time -> use it
// 1、创建该Servlet注入的上下文
wac = this.webApplicationContext;
if (wac instanceof ConfigurableWebApplicationContext) {
ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) wac;
if (!cwac.isActive()) {
// The context has not yet been refreshed -> provide services such as
// setting the parent context, setting the application context id, etc
if (cwac.getParent() == null) {
// The context instance was injected without an explicit parent -> set
// the root application context (if any; may be null) as the parent
cwac.setParent(rootContext);
}
configureAndRefreshWebApplicationContext(cwac);
}
}
}
if (wac == null) {
// No context instance was injected at construction time -> see if one
// has been registered in the servlet context. If one exists, it is assumed
// that the parent context (if any) has already been set and that the
// user has performed any initialization such as setting the context id
//2、查找已经绑定的上下文
wac = findWebApplicationContext();
}
if (wac == null) {
// No context instance is defined for this servlet -> create a local one
//3、如果没有找到相应的上下文，并指定父亲为ContextLoaderListener
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.
//4、刷新上下文初始化web层组件
onRefresh(wac);
}

if (this.publishContext) {
// Publish the context as a servlet context attribute.
String attrName = getServletContextAttributeName();
getServletContext().setAttribute(attrName, wac);
if (this.logger.isDebugEnabled()) {
this.logger.debug("Published WebApplicationContext of servlet '" + getServletName() +
"' as ServletContext attribute with name [" + attrName + "]");
}
}

return wac;
}

在DispatcherServlet中有一处比较核心的初始化代码：它初始化了许多核心web组件，而且我们下面的许多篇章都会围绕这这些组件

/**
* 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);
}

4.配置

用过springmvc web的人都知道，我们项目中都会有一个web.xml文件，而需要启动springmvc我们需要加入以下配置

<listener>
<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>
<context-param>
<param-name>contextConfigLocation</param-name>
<param-value>classpath:config/spring.xml</param-value>
</context-param>

<servlet>
<servlet-name>springmvc</servlet-name>
<servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
<init-param>
<param-name>contextConfigLocation</param-name>
<param-value>classpath:config/spring-mvc.xml</param-value>
</init-param>
<load-on-startup>1</load-on-startup>
</servlet>
<servlet-mapping>
<servlet-name>springmvc</servlet-name>
<url-pattern>/</url-pattern>
</servlet-mapping>

load-on-startup:表示容器启动时初始化该servlet

url-pattern:表示springmvc拦截的url路径，/表示全拦截，还有比如是*.do表示拦截.do结尾的请求

DispatcherServlet可以配置自己的初始化参数

contextClass	实现WebApplicationContext接口的类，当前的servlet用它来创建上下文。如果这个参数没有指定， 默认使用XmlWebApplicationContext。
contextConfigLocation	传给上下文实例（由contextClass指定）的字符串，用来指定上下文的位置。这个字符串可以被分成多个字符串（使用逗号作为分隔符） 来支持多个上下文（在多上下文的情况下，如果同一个bean被定义两次，后面一个优先）
namespace	WebApplicationContext命名空间。默认值是[server-name]-servlet
contextAttribute	ServletContext的一些属性

发现一个机智的导航😳