ContextLoader启动后的操作
在文章Spring源码解析(I) 基于SSM看Spring的使用和Spring启动监听中，讲述了web容器启动后会触发的方法实现中生成Context的部分，回顾下核心方法：
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!");
}
servletContext.log("Initializing Spring root WebApplicationContext");
Log logger = LogFactory.getLog(ContextLoader.class);
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.
//如果非空 是针对Servlet 3.1+自定义Context的情况
if (this.context == null) {
//创建一个ApplicationContext
this.context = createWebApplicationContext(servletContext);
}
// 0. 对于默认的XmlWebApplicationContext 肯定会进入这一分支
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);
}
//在此处会加载具体的配置项，例如contextConfigLocation
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);
}
至此已经加载完成
if (logger.isInfoEnabled()) {
long elapsedTime = System.currentTimeMillis() - startTime;
logger.info("Root WebApplicationContext initialized in " + elapsedTime + " ms");
}
return this.context;
}
catch (RuntimeException | Error ex) {
logger.error("Context initialization failed", ex);
servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, ex);
throw ex;
}
}
我们已经分析到了0.处，他对我们生成的容器做了一个判断，对于web.xml监听初始化的Context，其生成的WebApplicationContext都是ConfigurableWebApplicationContext的子类，所以必然会进入if分支。
首先通过loadParentContext先加载了父容器，默认是null。然后调用了configureAndRefreshWebApplicationContext方法进行初始化和配置项的读取。
这回我们知道启动日志中的如下两行是哪来的了(是使用了Springboot的日志所以开始前输出略微不同)：
web.xml配置文件读取：
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 + //WebApplicationContext.class.getName() + ":";
ObjectUtils.getDisplayString(sc.getContextPath()));
}
}
wac.setServletContext(sc);
//读取web.xml的配置参数
// <init-param>
// <param-name>contextConfigLocation</param-name>
// <param-value>classpath:spring/spring-*.xml</param-value>
// </init-param>
String configLocationParam = sc.getInitParameter(CONFIG_LOCATION_PARAM); //contextConfigLocation
if (configLocationParam != null) {
wac.setConfigLocation(configLocationParam);
}
//初始化environment，主要是处理配置中的占位符属性资源，这样一些InitParam也能读取到了
// 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);
}
//refresh之前的回调，会实例化继承了ApplicationContextInitializer的类
customizeContext(sc, wac);
//refresh()是真正加载beans的地方，是Spring的核心方法
wac.refresh();
}
ApplicationContextInitializer——refresh前的回调
在如上调用refresh之前，先调用了customizeContext方法，此方法是refresh之前的回调方法，负责初始化一些继承了ApplicationContextInitializer类，实例化并调用其initialize方法，例如我们定义了一个类继承于ApplicationContextInitializer：
public class MyApplicationContextInitializer implements ApplicationContextInitializer {
@Override
public void initialize(ConfigurableApplicationContext configurableApplicationContext) {
System.out.println("----- initialize -----")
String beans[] = configurableApplicationContext.getBeanDefinitionNames();
for(String bean:beans){
System.out.println("[bean]: "+ bean);
}
}
}
然后在web.xml中将其注册（多个用逗号分割，从左向右加载）：
<context-param> <param-name>contextInitializerClasses</param-name> <param-value>com.xxx.MyApplicationContextInitializer</param-value> </context-param>
这样在程序启动时，我们便能在“Root WebApplicationContext initialized in xxx ms ”的日志之前看到这样的日志：
2020-08-09鱼鱼