Web容器启动过程,主要讲解Servlet和Spring容器结合的内容。
流程图如下:
Web容器启动的Root Context是有ContextLoaderListener,一般使用spring,都会在web.xml中配置这个监听器。
<listener>
<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>
下面就从这里入手,看看它是如何启动spring容器。
public class ContextLoaderListener extends ContextLoader implements ServletContextListener { 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());
} }
这个类继承了ContextLoader,同时实现了ServletContextListener,从注释上可以看出,web应用会调用contextInitalized方法进行初始化。
先看一下ServletContextListener.
public interface ServletContextListener extends EventListener {
void contextInitialized(ServletContextEvent var1); void contextDestroyed(ServletContextEvent var1);
}
从名字可以看出,一个是初始化方法,另一个是销毁是调用的方法。
回到上面而initWebApplicationContext方法,通过createWebApplicationContext方法获取WebApplicationContext,调用determineContextClass方法如下:
/**
* 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);
}
}
}
从注释上可以看出返回的是XmlWebApplicationContext。
回到initWebApplicationContext方法中,有一段
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);
}
}
会调用configureAndRefreshWebApplication方法,感觉是配置和刷新,进入方法。
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...
wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX +
ObjectUtils.getDisplayString(sc.getContextPath()));
}
} wac.setServletContext(sc);
//获取配置的参数信息
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);
} customizeContext(sc, wac);
//刷新上下文-->AbsreactApplicationContext.refresh()
wac.refresh();
}
至此,通过wac.refresh()启动Spring容器。