在项目的web.xml中配置
<listener>
<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>
此配置为spring容器加载入口，因为其javax.servlet.ServletContextListener接口。
下面代码为ServletContextListener的源码：
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 );
}
其中contextInitialized方法为web应用加载入口，实现此方法即可在其中加载自定义初始化web应用。
回过头看ContextLoaderListener中的初始化工作：
/**
* Initialize the root web application context.
*/
public void contextInitialized(ServletContextEvent event) {
this.contextLoader = createContextLoader();
if (this.contextLoader == null) {
this.contextLoader = this;
}
this.contextLoader.initWebApplicationContext(event.getServletContext());
}
其具体初始化工作在其父类org.springframework.web.context.ContextLoader中的initWebApplicationContext方法中进行：
/**
* 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;
}
}
下面我们来分析一下上面的方法中的具体工作：
1）方法determineContextClass其返回一个WebApplicationContext的实现类，要么是默认的XmlWebApplicationContext，要么是一个指定的自定义的类。
先看方法源码：
/**
* Return the WebApplicationContext implementation class to use, either the
* default XmlWebApplicationContext or a custom context class if specified.
* @param servletContext current servlet context
* @return the WebApplicationContext implementation class to use
* @see #CONTEXT_CLASS_PARAM
* @see org.springframework.web.context.support.XmlWebApplicationContext
*/
protected Class<?> determineContextClass(ServletContext servletContext) {
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);
}
}
}
先查看是否有用户自定义的context类，其配置方式是在web.xml中配置初始化属性其name为contextClass，原因是：
/**
* Config param for the root WebApplicationContext implementation class to use: {@value}
* @see #determineContextClass(ServletContext)
* @see #createWebApplicationContext(ServletContext, ApplicationContext)
*/
public static final String CONTEXT_CLASS_PARAM = "contextClass";
在web.xml中的配置例如：
<context-param>
<param-name>contextClass</param-name>
<param-value>xxxxxxxxxx</param-value>
</context-param>
如果没有自定义配置的话，实现加载其默认WebApplicationContext实现类org.springframework.web.context.support.XmlWebApplicationContext。
2）方法createWebApplicationContext将determineContextClass方法返回的Class类实例化。并将其实例类向上转型为ConfigurableWebApplicationContext类型。
默认的返回应该是ConfigurableWebApplicationContext的子类org.springframework.web.context.support.XmlWebApplicationContext实例。
下面是方法的源码：
/**
* Instantiate the root WebApplicationContext for this loader, either the
* default context class or a custom context class if specified.
* <p>This implementation expects custom contexts to implement the
* {@link ConfigurableWebApplicationContext} interface.
* Can be overridden in subclasses.
* <p>In addition, {@link #customizeContext} gets called prior to refreshing the
* context, allowing subclasses to perform custom modifications to the context.
* @param sc current servlet context
* @return the root WebApplicationContext
* @see ConfigurableWebApplicationContext
*/
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);
}
3）方法configureAndRefreshWebApplicationContext配置web应用
源码为：
protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac, ServletContext sc) {
if (ObjectUtils.identityToString(wac).equals(wac.getId())) {
// The application context id is still set to its original default value
// -> assign a more useful id based on available information
String idParam = sc.getInitParameter(CONTEXT_ID_PARAM);
if (idParam != null) {
wac.setId(idParam);
}
else {
// Generate default id...
if (sc.getMajorVersion() == 2 && sc.getMinorVersion() < 5) {
// Servlet <= 2.4: resort to name specified in web.xml, if any.
wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX +
ObjectUtils.getDisplayString(sc.getServletContextName()));
}
else {
wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX +
ObjectUtils.getDisplayString(sc.getContextPath()));
}
}
}
wac.setServletContext(sc);
String initParameter = sc.getInitParameter(CONFIG_LOCATION_PARAM);
if (initParameter != null) {
wac.setConfigLocation(initParameter);
}
customizeContext(sc, wac);
wac.refresh();
}
其中String initParameter = sc.getInitParameter(CONFIG_LOCATION_PARAM);获取在web.xml中配置的contextConfigLocation参数来确定spring配置文件的位置。
其中的ConfigurableWebApplicationContext类型参数实际上是其子类org.springframework.web.context.support.XmlWebApplicationContext的类型参数，
其中方法customizeContext是定制化conext的方法，如果没有在web.xml中配置contextInitializerClasses初始化参数则此方法不做任何工作。
其中wac.refresh();方法其是在org.springframework.web.context.support.XmlWebApplicationContext的父类org.springframework.context.support.AbstractApplicationContext中实现。它才开始真正的spring配置工作。