简单介绍Spring的ContextLoaderListener
在开发Spring的Web项目中，通常我们都会在web.xml中配置一个Spring的核心监听器，就是把Spring的IOC容器纳入Servlet容器中，配置如下：
<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>
查看ContextLoaderListener源码如下：
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());
}
}
ContextLoaderListener官方源码定义是Spring的root(也就是WebApplicationContext)启动和关闭的引导监听器，当调用ContextLoaderListener的无参构造器时，就会创建一个新的ContextLoaderListener,也将会创建一个web应用上下文基于web.xml中的contextLoader和contextConfigLocation的servlet context-param参数，创建的应用上下文会被注册到ServletContext中通过WebApplicationContext里的ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE这个属性。
ContextLoaderListener继承自ContextLoader并且实现了ServletContextListener接口，实现ServletContextListener作用不言而喻，就是要让Spring和Servlet容器关联起来，让Spring容器伴随着Servlet容器的启动而启动，其实这个类中最主要的一个方法就是contextInitialized()，作用就是Spring上下文初始化，里面的initWebApplicationContext()是从父类ContextLoader中继承过来的，查看源码：
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;
}
}
关于多次出现的ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE这个参数，我个人理解的就是它是Servlet容器识别Spring的一个标识，如果为null，说明Spring还有没注册进Servlet容器中，
紧接着就是创建WebApplicationContext，configureAndRefreshWebApplicationContext()这个方法最主要就是配置applicationContext.xml这个文件，
servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);这句代码就是把Spring容器注册进ServletContext中。
关于WebApplicationContext这个接口：
public interface WebApplicationContext extends ApplicationContext {
String ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE = WebApplicationContext.class.getName() + ".ROOT";
String SCOPE_REQUEST = "request";
String SCOPE_SESSION = "session";
String SCOPE_GLOBAL_SESSION = "globalSession";
String SCOPE_APPLICATION = "application";
String SERVLET_CONTEXT_BEAN_NAME = "servletContext";
String CONTEXT_PARAMETERS_BEAN_NAME = "contextParameters";
String CONTEXT_ATTRIBUTES_BEAN_NAME = "contextAttributes";
ServletContext getServletContext();
}
WebApplicationContext继承自ApplicationContext，这个接口添加了一个getServletContext()方法，并且定义了一些root context在启动进程中必须绑定的application属性，和一般的应用上下文一样，web application contexts也是分层的，每一个应用都有一个root context,并且应用里的每个Servlet都有自己的子context,除了标准的应用上下文声明周期，WebApplicationContext实现需要探测ServletContextAware beans和调用setServletContext()方法。
WebApplicationContext里面定义了一些常用的bean作用域属性，比如request,session,globalSession,application,servletContext.
其实可以在相应的类中打断点调试，这样更能加深印象，有一点不明白的是
else if (ccl != null) {
currentContextPerThread.put(ccl, this.context);
}
这段代码中ccl明明不为空，可是断点调试却不会执行，不知道哪位能解释一下。
