技术文章第一时间送达！
来源：tuhooo cnblogs.com/tuhooo/p/6491903.html
一、ServletContext对象获取Demo
Servlet容器在启动时会加载Web应用，并为每个Web应用创建唯一的ServletContext对象。
可以把ServletContext看作一个Web应用的服务器端组件的共享内存。在ServletContext中可以存放共享数据，有4个读取或者设置共享数据的方法：
CounterServlet.java
代码语言：javascript复制package com.servletContext.demo;
import java.io.IOException;
import javax.servlet.ServletConfig;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class CounterServlet extends HttpServlet {
@Override
public void init(ServletConfig config) throws ServletException {
super.init(config);
}
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
doPost(req, resp);
}
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
// 获得ServletContext的引用
ServletContext context = getServletContext();
// 从ServletContext读取count属性
Integer count = (Integer) context.getAttribute("count");
// 如果没有读到count属性，那么创建count属性，并设置初始值为0
if(count == null) {
System.out.println("context中还没有count属性呢");
count = new Integer(0);
context.setAttribute("count", count);
}
count = count 1;
// count增加之后还要写回去，引用为什么还要重新存回去
context.setAttribute("count", count);
System.out.println("您是第" count "个访问的！");
}
@Override
public void destroy() {
super.destroy();
}
}从上述代码中可见通过getServletContext()方法可以直接获得ServletContext的引用。
二、Spring和ServletContext的关系
缘何这两货会扯上关系呢？
在使用Spring的时候想必对如下代码肯定熟悉：
代码语言：javascript复制// 获取Spring容器
ApplicationContext ctx = new ClassPathXmlApplicationContext("bean.xml");
// 从Spring容器中根据id获得对象的引用
User user = (User) ctx.getBean("user");
// 调用对象的方法
user.add();这样做是最低级的，也就是通过加载配置文件来获得Spring容器，再来获取对象的应用，在Web项目中，每次都通过加载配置文件显得效率低下，而且繁琐，这里介绍一种另外的方法。想在Web项目启动的时候就把Spring容器也给启动了，不用每次都手动去启动。
这里就用到了上面介绍的ServletContext了，每次Web项目启动的时候都会创建ServletContext对象，而该对象又有一个ServletContextListener的接口，监视ServletContext的创建，这样就可以调用这个接口的回调方法来启动Spring容器了。（但是这里我有个疑问，随着项目启动的不止有ServletContext啊，过滤器好像也随着项目启动，为啥不在过滤器的init()方法里面启动Spring容器呢？）
先来看看这个接口是啥定义：
代码语言：javascript复制package javax.servlet;
import java.util.EventListener;
/**
* Implementations of this interface receive notifications about changes to the
* servlet context of the web application they are part of. To receive
* notification events, the implementation class must be configured in the
* deployment descriptor for the web application.
*/
public interface ServletContextListener extends EventListener {
/**
** Notification that the web application initialization process is starting.
* All ServletContextListeners are notified of context initialization before
* any filter or servlet in the web application is initialized.
*/
public void contextInitialized(ServletContextEvent sce);
/**
** Notification that the servlet context is about to be shut down. All
* servlets and filters have been destroy()ed before any
* ServletContextListeners are notified of context destruction.
*/
public void contextDestroyed(ServletContextEvent sce);
}第一段注释描述的是：这个接口的实现接受和Web应用关联的servlet context的变更的通知。为了接受通知事件，这个类的实现必须在web应用的部署描述符配置。
第二段注释的描述是：通知是在Web应用初始化的时候开始的。所有的ServletContextListeners都会在web应用中任何的filter和servlet初始话之前接收到context初始化的时候通知。
第三段注释的描述是：servlet context将要被关闭的时候的通知。所有的filter和servlet会在任何ServletContextListeners收到context销毁的通知之前就被销毁了。
另外再来看看ServeletContextEvent.java
代码语言：javascript复制package javax.servlet;
/**
* This is the event class for notifications about changes to the servlet
* context of a web application.
*
* @see ServletContextListener
* @since v 2.3
*/
public class ServletContextEvent extends java.util.EventObject {
private static final long serialVersionUID = 1L;
/**
* Construct a ServletContextEvent from the given context.
*
* @param source
* - the ServletContext that is sending the event.
*/
public ServletContextEvent(ServletContext source) {
super(source);
}
/**
* Return the ServletContext that changed.
*
* @return the ServletContext that sent the event.
*/
public ServletContext getServletContext() {
return (ServletContext) super.getSource();
}
}public ServletContextEvent(ServletContext source);这个方法是从一个给定的ServletContext构建一个ServletContextEvent。而public ServletContext getServletContext();则是返回已经改变的ServletContext，暂时不知道有啥用，是不是给监听器塞ServletContext用的啊？
想自己也写一个ServletContextListener呢！
代码语言：javascript复制package com.servletContext.demo;
import javax.servlet.ServletContext;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
public class MyServletContextListener implements ServletContextListener {
@Override
public void contextInitialized(ServletContextEvent sce) {
// 从web.xml中拿出添加的参数
ServletContext ctx = sce.getServletContext();
String initParam = ctx.getInitParameter("myContextListener");
System.out.println("我配置的初始化参数为：" initParam);
// 利用初始化参数找到配置文件机型初始化
System.out.println("context初始化了咯");
System.out.println("这里假装初始化Spring容器.....");
}
@Override
public void contextDestroyed(ServletContextEvent sce) {
// 在销毁之前获得ServletContext
ServletContext ctx = sce.getServletContext();
// 正好刚刚存了一个值进去了，销毁之前拿出来瞅瞅
Integer count = (Integer) ctx.getAttribute("count");
System.out.println("在销毁之前，count的值为：" count);
}
}这他喵的居然真的可以！
web.xml为：
代码语言：javascript复制<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" id="WebApp_ID" version="2.5">
<display-name>ServletContext</display-name>
<welcome-file-list>
<welcome-file>index.html</welcome-file>
<welcome-file>index.htm</welcome-file>
<welcome-file>index.jsp</welcome-file>
<welcome-file>default.html</welcome-file>
<welcome-file>default.htm</welcome-file>
<welcome-file>default.jsp</welcome-file>
</welcome-file-list>
<!-- 假装为Spring监听器提供启动参数，其实是给ServletContext提供的 -->
<context-param>
<param-name>myContextListener</param-name>
<!-- 这里如果bean.xml在包cn.ssh下，那么就应该写为：cn/ssh/bean.xml -->
<param-value>这是我设置的值</param-value>
</context-param>
<!-- 配置Spring的监听器 -->
<listener>
<listener-class>com.servletContext.demo.MyServletContextListener</listener-class>
</listener>
<servlet>
<servlet-name>count</servlet-name>
<servlet-class>com.servletContext.demo.CounterServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>count</servlet-name>
<url-pattern>/counter</url-pattern>
</servlet-mapping>
</web-app>测试结果为：
看来真的是可以了，这里关闭服务器的时候Console中的内容也被清除了，暂时没有看到ServletContext销毁时的消息。
Spring提供的是ContextLoaderListener，这个监听器实现了ServletContextListener接口，可以作为Listener使用，它会在创建的时候自动查找WEB-INF/下的applicationContext.xml文件，因此，如果只有一个配置文件，并且文件名为applicationContext.xml,则只需要在web.xml中加入对Listener的配置就可以。
如果有多个配置文件需要加载，则要考虑使用<context-param.../>元素来确定配置文件的文件名。ContextLoaderListener加载的时候，会查找名为contextConfigLocation的初始化参数。因此<context-param.../>时应该指定参数名为contextConfigLocation。
代码语言：javascript复制<!-- 为Spring监听器提供启动参数 -->
<context-param>
<param-name>contextConfigLocation</param-name>
<!-- 这里如果bean.xml在包cn.ssh下，那么就应该写为：cn/ssh/bean.xml -->
<param-value>classpath:bean.xml</param-value>
</context-param>
<!-- 配置Spring的监听器 -->
<listener>
<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>如果没有使用contextConfigLocation指定配置文件，则Spring自动查找applicationContext.xml配置文件；如果有contextConfigLocation，则利用该参数确定配置文件。如果无法找到适合的配置文件，Spring将无法初始化。
Spring根据指定的文件创建WebApplicationContext对象，并将其保存在Web应用的ServletContext中。大部分情况下，应用中的Bean无需感受到ApplicationContext的存在，只要用ApplicationContext中的IoC即可。
这个监听器所在的jar包为：
如果需要利用ApplicationContext的实例，可以通过如下代码获取：
代码语言：javascript复制package com.ssh.domain;
import javax.servlet.ServletContext;
import javax.servlet.http.HttpServletRequest;
import org.apache.struts2.ServletActionContext;
import org.springframework.web.context.WebApplicationContext;
import org.springframework.web.context.support.WebApplicationContextUtils;
import com.opensymphony.xwork2.ActionSupport;
import com.ssh.test.TestAdd;
public class TestAction extends ActionSupport {
@Override
public String execute() throws Exception {
HttpServletRequest request = ServletActionContext.getRequest();
ServletContext servletContext = request.getServletContext();
// 这里不是通过依赖注入，而是直接从容器中拿
WebApplicationContext ctx = WebApplicationContextUtils.getWebApplicationContext(servletContext);
// 也可以是下面这样的
WebApplicationContext ctx1 = (WebApplicationContext)
servletContext.getAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE);
if(ctx == ctx1) {
System.out.println("两次获得对象是一样的");
}
TestAdd testAdd = (TestAdd) ctx.getBean("testAdd");
testAdd.add();
return NONE;
}
}TestAdd.java
代码语言：javascript复制package com.ssh.test;
public class TestAdd {
public void add( ) {
System.out.println("通过WebContext获得的而打印....");
}
}测试结果为：
http://localhost:8080/spring_struts2/testAction
打开源码，就蛋疼了，有封装了一下：
代码语言：javascript复制package org.springframework.web.context;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
/**
* Bootstrap listener to start up and shut down Spring's root {@link WebApplicationContext}.
* Simply delegates to {@link ContextLoader} as well as to {@link ContextCleanupListener}.
*
* <p>This listener should be registered after {@link org.springframework.web.util.Log4jConfigListener}
* in {@code web.xml}, if the latter is used.
*
* <p>As of Spring 3.1, {@code ContextLoaderListener} supports injecting the root web
* application context via the {@link #ContextLoaderListener(WebApplicationContext)}
* constructor, allowing for programmatic configuration in Servlet 3.0 environments.
* See {@link org.springframework.web.WebApplicationInitializer} for usage examples.
*
* @author Juergen Hoeller
* @author Chris Beams
* @since 17.02.2003
* @see #setContextInitializers
* @see org.springframework.web.WebApplicationInitializer
* @see org.springframework.web.util.Log4jConfigListener
*/
public class ContextLoaderListener extends ContextLoader implements ServletContextListener {
/**
* Create a new {@code ContextLoaderListener} that will create a web application
* context based on the "contextClass" and "contextConfigLocation" servlet
* context-params. See {@link ContextLoader} superclass documentation for details on
* default values for each.
* <p>This constructor is typically used when declaring {@code ContextLoaderListener}
* as a {@code <listener>} within {@code web.xml}, where a no-arg constructor is
* required.
* <p>The created application context will be registered into the ServletContext under
* the attribute name {@link WebApplicationContext#ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE}
* and the Spring application context will be closed when the {@link #contextDestroyed}
* lifecycle method is invoked on this listener.
* @see ContextLoader
* @see #ContextLoaderListener(WebApplicationContext)
* @see #contextInitialized(ServletContextEvent)
* @see #contextDestroyed(ServletContextEvent)
*/
public ContextLoaderListener() {
}
/**
* Create a new {@code ContextLoaderListener} with the given application context. This
* constructor is useful in Servlet 3.0 environments where instance-based
* registration of listeners is possible through the {@link javax.servlet.ServletContext#addListener}
* API.
* <p>The context may or may not yet be {@linkplain
* org.springframework.context.ConfigurableApplicationContext#refresh() refreshed}. If it
* (a) is an implementation of {@link ConfigurableWebApplicationContext} and
* (b) has <strong>not</strong> already been refreshed (the recommended approach),
* then the following will occur:
* <ul>
* <li>If the given context has not already been assigned an {@linkplain
* org.springframework.context.ConfigurableApplicationContext#setId id}, one will be assigned to it</li>
* <li>{@code ServletContext} and {@code ServletConfig} objects will be delegated to
* the application context</li>
* <li>{@link #customizeContext} will be called</li>
* <li>Any {@link org.springframework.context.ApplicationContextInitializer ApplicationContextInitializer}s
* specified through the "contextInitializerClasses" init-param will be applied.</li>
* <li>{@link org.springframework.context.ConfigurableApplicationContext#refresh refresh()} will be called</li>
* </ul>
* If the context has already been refreshed or does not implement
* {@code ConfigurableWebApplicationContext}, none of the above will occur under the
* assumption that the user has performed these actions (or not) per his or her
* specific needs.
* <p>See {@link org.springframework.web.WebApplicationInitializer} for usage examples.
* <p>In any case, the given application context will be registered into the
* ServletContext under the attribute name {@link
* WebApplicationContext#ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE} and the Spring
* application context will be closed when the {@link #contextDestroyed} lifecycle
* method is invoked on this listener.
* @param context the application context to manage
* @see #contextInitialized(ServletContextEvent)
* @see #contextDestroyed(ServletContextEvent)
*/
public ContextLoaderListener(WebApplicationContext context) {
super(context);
}
/**
* Initialize the root web application context.
*/
@Override
public void contextInitialized(ServletContextEvent event) {
initWebApplicationContext(event.getServletContext());
}
/**
* Close the root web application context.
*/
@Override
public void contextDestroyed(ServletContextEvent event) {
closeWebApplicationContext(event.getServletContext());
ContextCleanupListener.cleanupAttributes(event.getServletContext());
}
}源码果然是个好东西，平时敲代码那会注意到这么多细节。这个类不复杂，两个构造方法，外加一个初始化的时候创建Spring容器和服务关闭的时候对容器的清理，封装了之后还要看其他的类，哎。
首先第一段注释是对这个类的描述：
这个启动监听器是用开启和关闭Spring的root的，这里他用了root而不是容器。简单的代理给了ContextLoader和ContextCleanupListener这两个类来处理。如果这个org.springframework.web.util.Log4jConfigListener被用到了，那么ContextLoaderListener应该在它之后注册。
在Spring3.1中，ContextLoaderListener支持通过ContextLoaderListener(WebApplicationContext)这个构造方法向应用上下文中注入root（也就是Spring的容器），这样可以以编程的方式来配置Servlet 3.0 的环境。
第二段注释是，新建一个ContextLoaderListener的类将会基于Servlet的"contextClass"和"contextCofigLocation"这两个参数来创建web应用的上下文。
翻译的好累啊，反正意思差不多就是这样5555....
来看这段代码：
代码语言：javascript复制/**
* Initialize the root web application context.
*/
@Override
public void contextInitialized(ServletContextEvent event) {
initWebApplicationContext(event.getServletContext());
}这个initWebApplicationContext方法是ContextLoad.java这个类里面的方法。
代码语言：javascript复制/**
* Initialize Spring's web application context for the given servlet context,
* using the application context provided at construction time, or creating a new one
* according to the "{@link #CONTEXT_CLASS_PARAM contextClass}" and
* "{@link #CONFIG_LOCATION_PARAM contextConfigLocation}" context-params.
* @param servletContext current servlet context
* @return the new WebApplicationContext
* @see #ContextLoader(WebApplicationContext)
* @see #CONTEXT_CLASS_PARAM
* @see #CONFIG_LOCATION_PARAM
*/
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;
}
}ContextLoad.initWebApplicationContext是为给定的servlet context来初始化web应用的上下文的。
业务逻辑解读：
首先从ServletContext中看看有没有Spring创建的这个容器；然后为ContextLoader存一份实例变量，使得在ServletContext关闭之后仍可以访问；
代码语言：javascript复制this.context = createWebApplicationContext(servletContext);这句就是创建一个WebApplicationContext相当于我们自己加载配置文件的那个类。
代码语言：javascript复制configureAndRefreshWebApplicationContext(cwac, servletContext);这句话也很明显，就是配置并且刷新WebAppCtx的。
代码语言：javascript复制servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);这句将创建的Spring的context作为属性放到servletContext中。
代码语言：javascript复制return this.context;然后就返回了Spring的容器了.....是不是简洁（装逼装不下去了），调用链好长。
暂时只能分析到这里！
