SpringMVC源码情操陶冶-DispatcherServlet父类简析

阅读源码有助于陶冶情操，本文对springmvc作个简单的向导

springmvc-web.xml配置

<servlet>
        <servlet-name>dispatch</servlet-name>
        <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
        <!--springmvc配置文件加载路径-->
        <init-param>
            <param-name>contextConfigLocation</param-name>
            <param-value>classpath:spring/mvc/*.xml</param-value>
        </init-param>
        <load-on-startup>1</load-on-startup>
        <async-supported>true</async-supported>
    </servlet>
    <servlet-mapping>
        <servlet-name>dispatch</servlet-name>
        <url-pattern>/</url-pattern>
    </servlet-mapping>

DispatcherServlet类结构

查看源码可以得知类结构如下

• DispatcherServlet extends FrameworkServlet
• FrameworkServlet extends HttpServletBean implements ApplicationContextAware
• HttpServletBean extends HttpServlet implements EnvironmentCapable, EnvironmentAware

本文将对Dispatcher的父类作下向导，方便读者以及博主自我查阅

HttpServletBean-web服务抽象类

具体的介绍可以详看官方注释，这里我们只关注它的init方法，代码如下

//开始初始化springmvc servlet，并实例化相应的PropertyValues供子类调用
public final void init() throws ServletException {
		if (logger.isDebugEnabled()) {
			logger.debug("Initializing servlet '" + getServletName() + "'");
		}

		// Set bean properties from init parameters.
		try {
		//此处便是读取<servlet>节点中的<init-param>参数保存至MutablePropertyValues#propertyValueList集合中
			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()));
			//默认为空
			initBeanWrapper(bw);
			//将bean与属性关联
			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.供子类复写
		initServletBean();

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

小结：
从以上代码可见只是简单的获取servlet中的<init-param>参数并绑定到BeanWrapper对象中，并通过initServletBean()方法供子类完善其余的功能。

FrameworkServlet-结合spring的web抽象类

官方简单的解释为：Spring的web基本servlet框架，通过基于javaBean解决方式来集合Spring的application context上下文。

• ApplicationContextAware接口
  该接口的使用主要是为当前类可以拥有spring application context上下文类，可方便的获取bean对象，内部只有一个接口方法setApplicationContext(ApplicationContext app)，在FrameworkServlet中的实现

public void setApplicationContext(ApplicationContext applicationContext) {
		//由spring调用并进行相应的applicationContext的注入
		if (this.webApplicationContext == null && applicationContext instanceof WebApplicationContext) {
			this.webApplicationContext = (WebApplicationContext) applicationContext;
			this.webApplicationContextInjected = true;
		}
	}

• doService()-抽象接口
  主要是供子类去实现处理servlet的请求

• 复写initServletBean()方法，完善功能

@Override
protected final void initServletBean() throws ServletException {
		//打印我们熟悉的日志
		getServletContext().log("Initializing Spring FrameworkServlet '" + getServletName() + "'");
		if (this.logger.isInfoEnabled()) {
			this.logger.info("FrameworkServlet '" + getServletName() + "': initialization started");
		}
		//对该方法的初始化时间作下统计
		long startTime = System.currentTimeMillis();

		try {
			//此处的初始化操作类似于ContextLoader#initWebApplicationContext
			this.webApplicationContext = initWebApplicationContext();
			//供子类调用去初始化另外的功能
			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");
		}
	}

• 承接initServletBean()方法的initWebApplicationContext()，代码如下

protected WebApplicationContext initWebApplicationContext() {
		//一般来说,spring初始化时间比springmvc要早，所以rootContext一般都存在
		WebApplicationContext rootContext =				WebApplicationContextUtils.getWebApplicationContext(getServletContext());
		WebApplicationContext wac = null;
		//先检查spring是否已经注入了webApplicationContext，其中的刷新操作类似于`ContextLoader#initWebApplicationContext`，初次调用此处为空
		if (this.webApplicationContext != null) {
			// A context instance was injected at construction time -> use it
			wac = this.webApplicationContext;
			if (wac instanceof ConfigurableWebApplicationContext) {
				ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) wac;
				if (!cwac.isActive()) {
					if (cwac.getParent() == null) {
						//springmvc的父类为spring上下文
						cwac.setParent(rootContext);
					}
					//此处方法不同于`ContextLoader`的相同方法
					configureAndRefreshWebApplicationContext(cwac);
				}
			}
		}
		//再而尝试从ServletContext的`contextAttribute`对应的值去获取
		if (wac == null) {
			wac = findWebApplicationContext();
		}
		//不然则创建新的webApplicationContext
		if (wac == null) {
			wac = createWebApplicationContext(rootContext);
		}
		//springmvc的第一次刷新
		if (!this.refreshEventReceived) {
			//调用子类的onRefresh(wac)方法初始化springmvc
			onRefresh(wac);
		}
		//保存至servletContext属性中
		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;
	}

• FrameworkServlet#createWebApplicationContext()-springmvc创建上下文对象

protected WebApplicationContext createWebApplicationContext(ApplicationContext parent) {
		//默认为XmlWebApplicationContext.class也可指定contextClass属性在web.xml
		Class<?> contextClass = getContextClass();
		if (this.logger.isDebugEnabled()) {
			this.logger.debug("Servlet with name '" + getServletName() +
					"' will try to create custom WebApplicationContext context of class '" +
					contextClass.getName() + "'" + ", using parent context [" + parent + "]");
		}
		if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
			throw new ApplicationContextException(
					"Fatal initialization error in servlet with name '" + getServletName() +
					"': custom WebApplicationContext class [" + contextClass.getName() +
					"] is not of type ConfigurableWebApplicationContext");
		}
		ConfigurableWebApplicationContext wac =
				(ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);

		wac.setEnvironment(getEnvironment());
		wac.setParent(parent);
		//设置springmvc的配置文件即 contextConfigLocation属性
		wac.setConfigLocation(getContextConfigLocation());
		//此处与ContextLoader#configureAndRefreshWebApplicationContext()类似
		configureAndRefreshWebApplicationContext(wac);

		return wac;
	}

• 承接createWebApplicationContext方法的FrameworkServlet#configureAndRefreshWebApplicationContext()-springmvc的刷新处理，不同于spring

protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac) {
		//设置id属性
		if (ObjectUtils.identityToString(wac).equals(wac.getId())) {
			if (this.contextId != null) {
				wac.setId(this.contextId);
			}
			else {				wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX + ObjectUtils.getDisplayString(getServletContext().getContextPath()) + "/" + getServletName());
			}
		}

		wac.setServletContext(getServletContext());
		wac.setServletConfig(getServletConfig());
		//默认的namespace为getServletName()+"-servlet"
		wac.setNamespace(getNamespace());
		wac.addApplicationListener(new SourceFilteringListener(wac, new ContextRefreshListener()));

		ConfigurableEnvironment env = wac.getEnvironment();
		if (env instanceof ConfigurableWebEnvironment) {
			((ConfigurableWebEnvironment) env).initPropertySources(getServletContext(), getServletConfig());
		}
		//空方法
		postProcessWebApplicationContext(wac);
		//读取web.xml中的`context-param`节点中的globalInitializerClasses、contextInitializerClasses并初始化
		applyInitializers(wac);
		//刷新，此处可见Spring源码情操陶冶-AbstractApplicationContext博文
		wac.refresh();
	}

小结：

• FrameworkServlet实例化springmvc上下文环境，与spring初始化上下文环境类似

• onRefresh()方法开放出来供子类DispatherServlet调用完善springmvc的相关初始化操作

• 读取init-param为contextConfigLocation属性的值用于加载springmvc的配置文件，其中的bean文件的读取操作可参照Spring源码情操陶冶-AbstractApplicationContext博文

• springmvc的namespace为getServletName()+'-servlet'，比如dispatcher-servlet，用于未指定contextConfigLocation属性的话，便会去加载WEB-INF${namespace}.xml，比如WEB-INFdispatcher-servlet.xml，而对于spring，默认加载的为WEB-INFapplicationContext.xml

• applicationContext.xml和dispatcher-servlet配置文件的加载，既支持classpath方式的加载，也支持WEB-INF方式的加载。后者是通过ServletContextResourceLoader加载的，实质是通过ServletContext.getRealPath()方法加载web环境下的web-inf的资源，其中参数必须以"/"为开头

下节预告

SpringMVC源码情操陶冶-DispatcherServlet类简析