Spring源码解析-Web容器启动过程

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。