你的位置:首页 > Java教程

[Java教程]ContextLoaderListener

基本概念:

servletContext > 

1、  使用ContextLoaderListener监听器的意义:

a) 为了在servlet容器启动时,就加载applicationContext.

b)避免在使用spring容器获取bean时,重复加载配置文件。

2、  spring版本:4.2.4

3、  配置方式:

a)在web.

<listener>

<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>

</listener>

<context-param>

<param-name>contextConfigLocation</param-name>

<param-value>classpath:applicationContext.

</context-param>

b) 如果配置了监听器,那么获取bean的方式就要相应的改变一下,代码如下:

1、ServletContext servletContext = ServletActionContext.getServletContext();

2、WebApplicationContext context = WebApplicationContextUtils.getWebApplicationContext(servletContext);

3、XXXService cs = (XXXService) context.getBean("XXXService");

4、cs.save();

c)我们先不管为什么WebApplicationContext对象与ApplicationContext对象一样能获取bean对象,我们此处探究的是,为什么配置了监听器,我们就能从servletcontext中取得WebApplicationContext对象。

4、  源码分析:

a)首先进入org.springframework.web.context.ContextLoaderListener 源码:代码如下:

//: org.springframework.web.context.ContextLoaderListener.java

 

package org.springframework.web.context;

 

import javax.servlet.ServletContextEvent;

import javax.servlet.ServletContextListener;

 

public class ContextLoaderListener extends ContextLoader implements ServletContextListener {

 

         public ContextLoaderListener() {

         }

 

         public ContextLoaderListener(WebApplicationContext context) {

                   super(context);

         }

 

         @Override

         public void contextInitialized(ServletContextEvent event) {

                   initWebApplicationContext(event.getServletContext());

         }

 

         @Override

         public void contextDestroyed(ServletContextEvent event) {

                   closeWebApplicationContext(event.getServletContext());

                   ContextCleanupListener.cleanupAttributes(event.getServletContext());

         }

}

b)我们可以看到此处继承了 ServletContextListener 接口,实现两个方法(contextInitialized、contextDestroyed),在servlet容器启动和销毁的时候做一些事情。

c) 在ContextLoaderListener监听器这个场景下,contextInitialized方法中会调用ContextLoader 类中的initWebApplicationContext方法,实例化出一个WebApplicationContext对象,此方法接收一个servletContext对象,此对象由Servlet容器传入。

5、  然后我们转到initWebApplicationContext方法中:代码如下:

//: org.springframework.web.context.ContextLoader.java

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.

         }

 

         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;

         }

}

 

a)  this.context = createWebApplicationContext(servletContext);首先我们看这句代码,同样我们先不谈就它是怎么创建的,我们见名知意,就是根据servletContext参数,创建一个WebApplicationContext对象,并赋值给ContextLoader类中的内部变量 context。

b) servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context); 紧接着将已经赋值的context变量,通过一个的标识名称放入servletContext对象中。

c) 看到这里,我想大家都知道为什么可以从ServletContex中取出WebApplicationContext对象了吧?

6、  接着我们分析在客户端获取WebApplicationContext对象的代码,再copy一份到这里:

1、ServletContext servletContext = ServletActionContext.getServletContext();

2、WebApplicationContext context = WebApplicationContextUtils.getWebApplicationContext(servletContext);

3、XXXService cs = (XXXService) context.getBean("XXXService");

4、cs.save();

a) 首先,我们从struts2框架提供的类中,获取servletContext对象。

b) 之后通过WebApplicationContextUtil类中的getWebApplicationContext方法,从servletContext对象中,将WebApplicationContext对象取出:我们进入方法:

public static WebApplicationContext getWebApplicationContext(ServletContext sc) {

         return getWebApplicationContext(sc, WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE);  // 此处标识与存入时的一致

}

c) 进而转入如下方法:

public static WebApplicationContext getWebApplicationContext(ServletContext sc, String attrName) {

         Assert.notNull(sc, "ServletContext must not be null");

         Object attr = sc.getAttribute(attrName);

         if (attr == null) {

                   return null;

         }

         if (attr instanceof RuntimeException) {

                   throw (RuntimeException) attr;

         }

         if (attr instanceof Error) {

                   throw (Error) attr;

         }

         if (attr instanceof Exception) {

                   throw new IllegalStateException((Exception) attr);

         }

         if (!(attr instanceof WebApplicationContext)) {

                   throw new IllegalStateException("Context attribute is not of type WebApplicationContext: " + attr);

         }

         return (WebApplicationContext) attr;

}

d) 我们根据上面的两句代码: Object attr = sc.getAttribute(attrName);  / return (WebApplicationContext) attr; 不难看出,将WebApplicationContext对象,从servletcontext对象中取出,强制转换并返回。

7、至此

a) 我们就取得了WebApplicationContext对象,并可以使用该对象来获取bean。而WebApplicationContext对象存放在servletcontext对象中,而servletcontext对象会一直存在于内存中,除非关闭servlet容器。这样就保证了只存在一份WebApplicationContext对象在内存中,同时也就保证了spring核心配置文件只会被加载一次。