Spring MVC初始化过程源码分析

素颜马尾好姑娘i 2021-12-21 04:59 293阅读 0赞

### 文章目录 ###

*  1. 引言
 *  2. HttpServletBean
 *  3. FrameworkServlet
 *  4. DispatcherServlet
 *  5. 小结

# 1. 引言 #

Spring MVC核心结构图如下所示  
![DispatcherServlet结构图][DispatcherServlet]  
其中aware意为感知，实现XXXAware接口就会通过接口的唯一方法setXXX得到ApplicationContext或Environment等。EnvironmentCapable有惟一方法getEnvironment()。同时需要注意的是ApplicationContext为应用上下文，Environment封装了servletContext、servletConfig、JndiProperty、系统环境变量和系统属性等。

# 2. HttpServletBean #

HttpServlet的简单扩展，它将配置参数(web.xml中servlet标记中的init-param条目)视为bean属性。  
对于任何类型的servlet都是一个方便的超类。他实现了EnvironmentAware和EnvironmentCapable可以获取environment属性。这里主要关注其init()方法。

/**
    	 * Map config parameters onto bean properties of this servlet, and
    	 * invoke subclass initialization.
    	 * @throws ServletException if bean properties are invalid (or required
    	 * properties are missing), or if subclass initialization fails.
    	 */
    	@Override
    	public final void init() throws ServletException {
    
    		// Set bean properties from init parameters.
    		PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
    		if (!pvs.isEmpty()) {
    			try {
    				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) {
    				if (logger.isErrorEnabled()) {
    					logger.error("Failed to set bean properties on servlet '" + getServletName() + "'", ex);
    				}
    				throw ex;
    			}
    		}
    
    		// 由子类实现初始化
    		initServletBean();
    	}

可以看到子类通过重写initServletBean()方法来进一步初始化。

# 3. FrameworkServlet #

/**
    	 * Overridden method of {@link HttpServletBean}, invoked after any bean properties
    	 * have been set. Creates this servlet's WebApplicationContext.
    	 */
    	@Override
    	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WebApplicationContext
    			this.webApplicationContext = initWebApplicationContext();
    			//空方法，子类可以重写
    			initFrameworkServlet();
    		}
    		catch (ServletException ex) {
    			...

在FrameworkServlet中重写了initServletBean()方法，其主要做了两件事

//初始化webApplicationContext
    this.webApplicationContext = initWebApplicationContext();
    //初始化FrameworkServlet，模板方法
    initFrameworkServlet();

让我们来了解一下initWebApplicationContext方法

/**
    	 * Initialize and publish the WebApplicationContext for this servlet.
    	 * <p>Delegates to {@link #createWebApplicationContext} for actual creation
    	 * of the context. Can be overridden in subclasses.
    	 */
    	protected WebApplicationContext initWebApplicationContext() {
    		//获取rootContext
    		WebApplicationContext rootContext =
    				WebApplicationContextUtils.getWebApplicationContext(getServletContext());
    		WebApplicationContext wac = null;
    		//如果已经通过构造方法设置了WebApplicationContext
    		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()) {
    					// 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方法用于封装ApplicationContext数据并且初始化所有相关Bean对象。
    					//它会从web.xml中读取名为contextConfigLocation的配置，这就是spring xml数据源设置，然后放到ApplicationContext中，
    					//最后调用传说中的refresh方法执行所有Java对象的创建
    					configureAndRefreshWebApplicationContext(cwac);
    				}
    			}
    		}
    		if (wac == null) {
    			//从servlet context中获取webApplicationContext
    			// 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
    			wac = findWebApplicationContext();
    		}
    		if (wac == null) {
    			//如果从servlet context中找不到就创建一个新的
    			// No context instance is defined for this servlet -> create a local one
    			wac = createWebApplicationContext(rootContext);
    		}
    
    		if (!this.refreshEventReceived) {
    			//如果没有refresh，在这里刷新
    			// 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);
    		}
    
    		if (this.publishContext) {
    			//将ApplicationContext保存到ServletContext中
    			// 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;
    	}

initWebApplicationContext方法做了三件事：

1.  获取spring的根容器rootContext
2.  设置webApplicationContext并根据情况调用onRefresh方法
3.  将webApplicationContext设置到ServletContext中

整个FrameworkServlet中重要的事情就是将创建出来的WebApplicationContext设置到ServletContext中，方便获取。

# 4. DispatcherServlet #

onRefresh方法是DispatcherServlet的入口方法，onrefresh调用了initStrategies，在initStrategies中调用了9个初始化方法

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);
    	}

# 5. 小结 #

简单分析了一个Spring MVC的创建过程，Spring MVC中Servlet一共有三个层次，分别是HttpServletBean,FrameworkServlet和DispatcherServlet。HttpServletBean直接继承自Java的HttpServlet，其作用是将Servlet中配置的参数设置到对应的属性；FrameworkServlet初始化了WebApplciationContext，DispatcherServlet初始化了自身的9个组件。

[DispatcherServlet]: /images/20211221/794b7832b24a493c99248132c5b05e36.png