傳統的SpringMVC項目中，需要在web.xml中配置Contextlistener。ContextLoaderListener是負責引導啟動和關閉Spring的Root上下文的監聽器。主要將處理委托給ContextLoader和ContextCleanupListener。
類的繼承關系。
ContextLoaderListener實現了ServletContextListener接口。該接口主要定義了兩個行為：監聽上下文創建（contextInitialized）和監聽上下文銷毀（contextDestroyed）。
ContextLoaderListener只是將方法的處理委托給ContextLoader。
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);
}
//初始化Root WebApplication
@Override
public void contextInitialized(ServletContextEvent event) {
//委托給ContextLoader
initWebApplicationContext(event.getServletContext());
}
//銷毀Root WebApplication
@Override
public void contextDestroyed(ServletContextEvent event) {
//委托給ContextLoader
closeWebApplicationContext(event.getServletContext());
//委托給ContextCleanupListener
ContextCleanupListener.cleanupAttributes(event.getServletContext());
}
}
ContextLoader的initWebApplicationContext方法負責創建root web application context。
//初始化root web application context
public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {
//校驗是否已經存在root application，
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.
//創建WebApplicationContext，並保存到變量中，等關閉時使用
if (this.context == null) {
this.context = createWebApplicationContext(servletContext);
}
//如果是ConfigurableWebApplicationContext，則配置上下文並刷新
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);
}
}
//在ServletContext中設置屬性，表明已經配置了root web application context
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;
}
}
該方法主要分為四步：
- 校驗是否已經存在root WebApplicationContext
- 創建root WebApplicationContext
- 配置上下文並刷新
- 綁定root WebApplicationContext到servlet context上
這四步具體分析如下：
1.校驗
校驗的過程比較簡單，主要是判斷Servlet Context是否已經綁定過WebApplicationContext
2.創建對象
createWebApplicationContext()方法負責創建WebApplicationContext對象，主要過程是確定WebApplicationContext類，並實例化：
protected WebApplicationContext createWebApplicationContext(ServletContext sc) {
//查找WebApplicationContext的類
Class<?> contextClass = determineContextClass(sc);
if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
throw new ApplicationContextException("Custom context class [" + contextClass.getName() +
"] is not of type [" + ConfigurableWebApplicationContext.class.getName() + "]");
}
//實例化對象
return
(ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
}
決定WebApplicationContext類的主要策略是先判斷ServletContext是否配置了contextClass屬性，如果是，則用該屬性指定的類作為WebApplicationContext類，否則則是否默認策略。默認策略是根絕classpath下的ContextLoader.properties中配置的類作為WebApplicatioNCOntext類：
protected Class<?> determineContextClass(ServletContext servletContext) {
//優先根據servletContext配置的contextClass屬性
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);
}
}
//在通過classpath下的`ContextLoader.properties`文件制定的類作為WebApplicationContext類
//默認是XmlWebApplicationContext
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);
}
}
}
配置與刷新
創建WebApplicationContext對象后，ContextLoader會判斷是否是ConfigurableWebApplicationContext。
ConfigurableWebApplicationContext拓展了WebApplicationContext的能力。其不僅擴展了自身方法，還繼承了ConfigurableApplicationContext接口。
配置與刷新的步驟也正是利用了這些接口提供的能力。
//配置並刷新上下文
protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac, ServletContext sc) {
//設置ID
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...
wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX +
ObjectUtils.getDisplayString(sc.getContextPath()));
}
}
//WebApplicationContext綁定Servlet Context
wac.setServletContext(sc);
String configLocationParam = sc.getInitParameter(CONFIG_LOCATION_PARAM);
if (configLocationParam != null) {
wac.setConfigLocation(configLocationParam);
}
// The wac environment's #initPropertySources will be called in any case when the context
// is refreshed; do it eagerly here to ensure servlet property sources are in place for
// use in any post-processing or initialization that occurs below prior to #refresh
ConfigurableEnvironment env = wac.getEnvironment();
if (env instanceof ConfigurableWebEnvironment) {
((ConfigurableWebEnvironment) env).initPropertySources(sc, null);
}
//自定義上下文初始過程，留給用戶的拓展機制
customizeContext(sc, wac);
//刷新上下文，加載bean
wac.refresh();
}
創建流程圖如下：
