1.Spring 源码学习—web.xml 之 ClassLoaderListener

不念不忘少年蓝@ 2021-09-24 03:38 190阅读 0赞

### web.xml 的 ClassLoaderListener 初始化 Spring 容器 ###

**1.web.xml**

<listener>
      	<listener-class>org.Springframework.web.context.ContextLoaderListener</listener-class>
      </listener>
      <context-param>
     	
      	<param-name>contextConfigLocation</param-name>
      	<param-value>classpath:applicationContext.xml</param-value>
      </context-param>
      
      
      
      <servlet>
      	<servlet-name>Springmvc</servlet-name>
      	<servlet-class>org.Springframework.web.servlet.DispatcherServlet</servlet-class>
      	
      	<load-on-startup>1</load-on-startup>
      </servlet>
      <servlet-mapping>
      	<servlet-name>Springmvc</servlet-name>
      	<url-pattern>*.html</url-pattern>
      </servlet-mapping>

**2.ContextLoaderListener**

ContextLoaderListener 源码

*  ContextLoaderListener 实现了 ServletContextListener 接口，可以在 web 容器启动时自动装配 ApplicationContext 的配置信息。容器的上下文即是 ServletContext。
 *  启动 web 容器时系统会调用 ServletContextListener 接口的实现类的 contextInitialized 方法。在 ContextLoaderListener 里此方法里便是 initWebApplicationContext(event.getServletContext())。
 *  initWebApplicationContext 是继承于 ContextLoader 的方法。event.getServletContext() 即是上下文 ServletContext。

public class ContextLoaderListener extends ContextLoader implements ServletContextListener {
    
    	public ContextLoaderListener() {
    	}
    
    	public ContextLoaderListener(WebApplicationContext context) {
    		super(context);
    	}
    
    	/**
    	 * Initialize the root web application context.
    	 */
    	@Override
    	public void contextInitialized(ServletContextEvent event) {
         // initWebApplicationContext 是 ContextLoader 的方法
    		initWebApplicationContext(event.getServletContext());
    	}
    
    	@Override
    	public void contextDestroyed(ServletContextEvent event) {
    		closeWebApplicationContext(event.getServletContext());
    		ContextCleanupListener.cleanupAttributes(event.getServletContext());
    	}
    }

ContextLoader 部分源码

public class ContextLoader {
        ...
        private static final String DEFAULT_STRATEGIES_PATH = "ContextLoader.properties";
    	private static final Properties defaultStrategies;
    
    	static {
    		// Load default strategy implementations from properties file.
    		// This is currently strictly internal and not meant to be customized
    		// by application developers.
    		try {
    			ClassPathResource resource = new ClassPathResource(DEFAULT_STRATEGIES_PATH, ContextLoader.class);
    			defaultStrategies = PropertiesLoaderUtils.loadProperties(resource);
    		}
    		catch (IOException ex) {
    			throw new IllegalStateException("Could not load 'ContextLoader.properties': " + ex.getMessage());
    		}
    	}
        ...
     }

*  java 加载此 ContextLoader 类时会读取与此类同目录的一个配置文件文件 ContextLoader.properties，即 ContextLoader 的成员变量 defaultStrategies 得到了值是 “org.springframework.web.context.support.XmlWebApplicationContext”。此值在下面讲解中有用处。

// ContextLoader.properties 内容
    org.springframework.web.context.WebApplicationContext=org.springframework.web.context.support.XmlWebApplicationContext

**3.ContextLoder 类的多个方法**

initWebApplicationContext 方法源码

*  查询上下文 ServletContext 中是否存在 key=WebApplicationContext.ROOT 的 value 值，只允许一个。第一次调用此方法肯定没有 value 值。

String ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE = WebApplicationContext.class.getName() + ".ROOT";
    
    public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {
    		if (servletContext.getAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE) != null) {
    			throw new IllegalStateException(
    					"Cannot initialize context because there is already a root application context present - " +
    					"check whether you have multiple ContextLoader* definitions in your web.xml!");
    		}
    
    		Log logger = LogFactory.getLog(ContextLoader.class);
    		servletContext.log("Initializing Spring root WebApplicationContext");
    		if (logger.isInfoEnabled()) {
    			logger.info("Root WebApplicationContext: initialization started");
    		}
    		long startTime = System.currentTimeMillis();
    
    		try {
    			// Store context in local instance variable, to guarantee that
    			// it is available on ServletContext shutdown.
    			if (this.context == null) {
    				this.context = createWebApplicationContext(servletContext);
    			}
    			if (this.context instanceof ConfigurableWebApplicationContext) {
    				ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) this.context;
    				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 ->
    						// determine parent for root web application context, if any.
    						ApplicationContext parent = loadParentContext(servletContext);
    						cwac.setParent(parent);
    					}
    					configureAndRefreshWebApplicationContext(cwac, servletContext);
    				}
    			}
    			servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);
    
    			ClassLoader ccl = Thread.currentThread().getContextClassLoader();
    			if (ccl == ContextLoader.class.getClassLoader()) {
    				currentContext = this.context;
    			}
    			else if (ccl != null) {
    				currentContextPerThread.put(ccl, this.context);
    			}
    
    			if (logger.isDebugEnabled()) {
    				logger.debug("Published root WebApplicationContext as ServletContext attribute with name [" +
    						WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE + "]");
    			}
    			if (logger.isInfoEnabled()) {
    				long elapsedTime = System.currentTimeMillis() - startTime;
    				logger.info("Root WebApplicationContext: initialization completed in " + elapsedTime + " ms");
    			}
    
    			return this.context;
    		}
    		catch (RuntimeException ex) {
    			logger.error("Context initialization failed", ex);
    			servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, ex);
    			throw ex;
    		}
    		catch (Error err) {
    			logger.error("Context initialization failed", err);
    			servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, err);
    			throw err;
    		}
    	}

createWebApplicationContext 方法源码

*  调用 createWebApplicationContext 方法创建 WebApplicationContext 实例（根据下面源码来说，其实就是 WebApplicationContext contextClass =(ConfigurableWebApplicationContext) (new XmlWebApplicationContext())，但实际是通过反射生成 ）。

protected WebApplicationContext createWebApplicationContext(ServletContext sc) {
    		Class<?> contextClass = determineContextClass(sc);
    		if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
    			throw new ApplicationContextException("Custom context class [" + contextClass.getName() +
    					"] is not of type [" + ConfigurableWebApplicationContext.class.getName() + "]");
    		}
    		return (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
    	}

determineContextClass 方法源码

*  determineContextClass 方法里默认策略获取类的名字便是上面说的 ContextLoader.properties 里的 org.springframework.web.context.support.XmlWebApplicationContext，返回类类型。
 *  先查找 web.xml 中<context-param> 定义的 contextClass 的值作为 Spring 上下文的初始化类，但没有找到。便会默认使用 XmlWebApplicationContext。

protected Class<?> determineContextClass(ServletContext servletContext) {
             // 成员变量 CONTEXT_CLASS_PARAM="contextClass"，web.xml 中<context-param> 没有这个定义，故不会加载
    		String contextClassName = servletContext.getInitParameter(CONTEXT_CLASS_PARAM);
    		if (contextClassName != null) {
    			try {
    				return ClassUtils.forName(contextClassName, ClassUtils.getDefaultClassLoader());
    			}
    			catch (ClassNotFoundException ex) {
    				throw new ApplicationContextException(
    						"Failed to load custom context class [" + contextClassName + "]", ex);
    			}
    		}
    		else {
    			contextClassName = defaultStrategies.getProperty(WebApplicationContext.class.getName());
    			try {
    				return ClassUtils.forName(contextClassName, ContextLoader.class.getClassLoader());
    			}
    			catch (ClassNotFoundException ex) {
    				throw new ApplicationContextException(
    						"Failed to load default context class [" + contextClassName + "]", ex);
    			}
    		}
    	}

*  接下来就是 Spring 容器的初始化工作了，涉及到 configureAndRefreshWebApplicationContext 方法
 *  成员变量 CONFIG\_LOCATION\_PARAM 便是 web.xml 中的定义在 <context-param> 中的 contextConfigLocation 变量，从而得到 Spring 初始化实例的配置文件的路径。
 *  servletContext.getInitParameter 是用来获取 web.xml 中配置的 <context-param> 的值。

// 成员变量 CONFIG_LOCATION_PARAM = "contextConfigLocation"
         protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac, ServletContext sc) {
    		...
    		wac.setServletContext(sc);
    		String configLocationParam = sc.getInitParameter(CONFIG_LOCATION_PARAM);
    		if (configLocationParam != null) {
    			wac.setConfigLocation(configLocationParam);
    		}
                ...
    	}