新書Java併發程式設計系統與模型已上線，歡迎拜讀。
在使用Spring MVC的時候，標準的配置是如下這樣的：
1.ContextLoaderListener配置：
<!-- Spring讀取Spring的配置檔案 -->
<context-param>
<!-- 名稱 -->
<param-name>contextConfigLocation</param-name>
<!-- 檔案的位置 -->
<param-value>classpath:application-context.xml</param-value>
</context-param>
<context-param>
<param-name>webAppRootKey</param-name>
<param-value>meipian</param-value>
</context-param>
<!-- Spring 的監聽器配置 -->
<listener>
<!-- 在Spring-web包下 context -->
<listener-class>org.springframework.web.context.ContextLoaderListener
</listener-class>
</listener>複製程式碼2.DispatcherServlet的配置：
<servlet>
<servlet-name>meipian</servlet-name>
<!-- 配置SpringMVC核心控制器 -->
<servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
<init-param>
<param-name>contextConfigLocation</param-name>
<param-value>classpath:springmvc.xml</param-value>
</init-param>
</servlet>
<servlet-mapping>
<servlet-name>meipian</servlet-name>
<url-pattern>/*</url-pattern>
</servlet-mapping>複製程式碼ContextLoaderListener監聽器的作用就是啟動Web容器時，自動裝配ApplicationContext的配置資訊。因為它實現了ServletContextListener這個介面，在web.xml配置這個監聽器，啟動容器時，就會預設執行它實現的方法。
Spring MVC的使用的容器是WebApplicationContext。那麼他和ContextLoaderListener所初始化的ApplicationContext有什麼關係呢？
可以從DispatcherServlet的初始化過程說起：DispatcherServlet攔截了所有的請求，所以訪問任何一個介面都會初始化DispatcherServlet物件。
初始化DispatcherServlet只是做了一個很簡單的事：
public DispatcherServlet() {
super();
setDispatchOptionsRequest(true);
}複製程式碼其父類FrameworkServlet初始化什麼也沒有:複製程式碼 public FrameworkServlet() {
}複製程式碼然後Tomcat會呼叫DispatcherServlet的init方法，在 Servlet 的生命期中，僅執行一次 init() 方法。它是在伺服器裝入 Servlet 時執行的。 可以配置伺服器，以在啟動伺服器或客戶機首次訪問 Servlet 時裝入 Servlet。 無論有多少客戶機訪問 Servlet，都不會重複執行 init() 。別問我為什麼會呼叫init方法，這是servlet的規範。DispatcherServlet的init方法是在父類FrameworkServlet的父類HttpServlet中實現的:
@Override
public final void init() throws ServletException {
if (logger.isDebugEnabled()) {
logger.debug("Initializing servlet '" + getServletName() + "'");
}
// Set bean properties from init parameters.
try {
PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
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) {
logger.error("Failed to set bean properties on servlet '" + getServletName() + "'", ex);
throw ex;
}
// Let subclasses do whatever initialization they like.
initServletBean();
if (logger.isDebugEnabled()) {
logger.debug("Servlet '" + getServletName() + "' configured successfully");
}
}複製程式碼會呼叫DispatcherServlet的initServletBean()方法就是初始化WebApplicationContext的方法。這個方法在其子類FrameServlet方法中實現：
@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 {
this.webApplicationContext = initWebApplicationContext(); //初始化WebApplicationContext物件
initFrameworkServlet();
}
catch (ServletException ex) {
this.logger.error("Context initialization failed", ex);
throw ex;
}
catch (RuntimeException ex) {
this.logger.error("Context initialization failed", ex);
throw ex;
}
if (this.logger.isInfoEnabled()) {
long elapsedTime = System.currentTimeMillis() - startTime;
this.logger.info("FrameworkServlet '" + getServletName() + "': initialization completed in " +
elapsedTime + " ms");
}
}複製程式碼initWebApplicationContext方法如下所示：
protected WebApplicationContext initWebApplicationContext() {
WebApplicationContext rootContext =
WebApplicationContextUtils.getWebApplicationContext(getServletContext());
WebApplicationContext wac = null;
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(cwac);
}
}
}
if (wac == null) {
// 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) {
// No context instance is defined for this servlet -> create a local one
wac = createWebApplicationContext(rootContext);
}
if (!this.refreshEventReceived) {
// 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) {
// 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;
}複製程式碼會預設走到createWebApplicationContext方法:複製程式碼protected WebApplicationContext createWebApplicationContext(ApplicationContext parent) {
Class<?> contextClass = getContextClass();
if (this.logger.isDebugEnabled()) {
this.logger.debug("Servlet with name '" + getServletName() +
"' will try to create custom WebApplicationContext context of class '" +
contextClass.getName() + "'" + ", using parent context [" + parent + "]");
}
if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
throw new ApplicationContextException(
"Fatal initialization error in servlet with name '" + getServletName() +
"': custom WebApplicationContext class [" + contextClass.getName() +
"] is not of type ConfigurableWebApplicationContext");
}
ConfigurableWebApplicationContext wac =
(ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
wac.setEnvironment(getEnvironment());
wac.setParent(parent);
wac.setConfigLocation(getContextConfigLocation());
configureAndRefreshWebApplicationContext(wac);
return wac;
}複製程式碼在沒有 contexloaderListener的情況下,parent是空的。
在有 contexloaderListener的情況下，發現parent不是空的。
而且在Spring MVC的配置中，如果你將Service的配置與mvc的配置寫在一起，有沒有contexloaderListener無所謂的。
這就說明了contexloaderListener作用是什麼了。