Spring 容器的启动过程
...大约 4 分钟
Spring 容器的启动过程
包含
- 含父子容器，含全注解驱动，含web.xml启动
- ContextLoader源码分析
- SpringBoot容器
0.前言
主要介绍Spring容器（父子容器）的启动过程。
当ContextLoaderListener和DispatcherServlet一起使用时, ContextLoaderListener 先创建一个根applicationContext，然后DispatcherSerlvet创建一个子applicationContext并且绑定到根applicationContext
在Spring的应用一启动会加载感兴趣的WebApplicationInitializer接口下的所有组件，这里只有上节我们自己继承AbstractAnnotationConfigDispatcherServletInitializer的类MyWebAppInitializer。
if (!waiClass.isInterface() && !Modifier.isAbstract(waiClass.getModifiers()) &&
WebApplicationInitializer.class.isAssignableFrom(waiClass)) {
try {
initializers.add((WebApplicationInitializer)
ReflectionUtils.accessibleConstructor(waiClass).newInstance());
}
catch (Throwable ex) {
throw new ServletException("Failed to instantiate WebApplicationInitializer class", ex);
}
}
...
// 排序后，循环调用onStartup方法，进行初始化
AnnotationAwareOrderComparator.sort(initializers);
for (WebApplicationInitializer initializer : initializers) {
initializer.onStartup(servletContext);
}
接下来看看onStart()方法的实现AbstractDispatcherServletInitializer#onStartup：
public void onStartup(ServletContext servletContext) throws ServletException {
super.onStartup(servletContext);
registerDispatcherServlet(servletContext);
}
// super
public void onStartup(ServletContext servletContext) throws ServletException {
registerContextLoaderListener(servletContext);
}
到此我们发现我们注解驱动，和我们的web.xml驱动可以说就一样了。分两步了：
- registerContextLoaderListener(servletContext)：注册ContextLoaderListener监听器，让它去初始化Spring父容器
- registerDispatcherServlet(servletContext);注册DispatcherServlet，让它去初始化Spring MVC的子容器
protected void registerContextLoaderListener(ServletContext servletContext) {
// 创建父容器
WebApplicationContext rootAppContext = createRootApplicationContext();
if (rootAppContext != null) {
// 创建listener 并且把已经创建好的容器放进去
ContextLoaderListener listener = new ContextLoaderListener(rootAppContext);
// //放入监听器需要的一些上下文，此处木有。一般都为null即可~~~。若有需要（自己定制），子类复写此方法即可
listener.setContextInitializers(getRootApplicationContextInitializers());
// 把监听器加入进来 这样该监听器就能监听ServletContext了，并且执行contextInitialized方法
servletContext.addListener(listener);
}
else {
logger.debug("No ContextLoaderListener registered, as " +
"createRootApplicationContext() did not return an application context");
}
}
监听器是是干啥的：未来会通过这个叫监听器的东西搞些事情
createRootApplicationContext：如下，创建了一个AnnotationConfigWebApplicationContext并且把配置文件注册进去了
@Override
@Nullable //Spring告诉我们，这个是允许返回null的，也就是说是允许返回null的，后面会专门针对这里如果返回null会是怎么样处理说明
protected WebApplicationContext createRootApplicationContext() {
Class<?>[] configClasses = getRootConfigClasses();
if (!ObjectUtils.isEmpty(configClasses)) {
AnnotationConfigWebApplicationContext context = new AnnotationConfigWebApplicationContext();
context.register(configClasses);
return context;
}
else {
return null;
}
}
继续往下走：执行registerDispatcherServlet
protected void registerDispatcherServlet(ServletContext servletContext) {
//Servlet名称 一般用系统默认的即可(dispatcher)，否则自己复写此方法也成
String servletName = getServletName();
Assert.hasLength(servletName, "getServletName() must not return null or empty");
//创建web的子容易。创建的代码和上面差不多，也是使用调用者提供的配置文件，创建AnnotationConfigWebApplicationContext. 备注：此处不可能为null
WebApplicationContext servletAppContext = createServletApplicationContext();
Assert.notNull(servletAppContext, "createServletApplicationContext() must not return null");
//创建DispatcherServlet，并且把子容器传进去了。其实就是new一个出来，最后加到容器里，就能够执行一些init初始化方法了~
FrameworkServlet dispatcherServlet = createDispatcherServlet(servletAppContext);
Assert.notNull(dispatcherServlet, "createDispatcherServlet(WebApplicationContext) must not return null");
//同样的 getServletApplicationContextInitializers()一般也为null即可
dispatcherServlet.setContextInitializers(getServletApplicationContextInitializers());
//注册servlet到web容器里面，这样就可以接收请求了
ServletRegistration.Dynamic registration = servletContext.addServlet(servletName, dispatcherServlet);
if (registration == null) {
throw new IllegalStateException("Failed to register servlet with name '" + servletName + "'. " +
"Check if there is another servlet registered under the same name.");
}
//1表示立马执行哦，没有第一次惩罚了
registration.setLoadOnStartup(1);
registration.addMapping(getServletMappings());//调用者必须实现
registration.setAsyncSupported(isAsyncSupported());//默认就是开启了支持异步的
//处理自定义的Filter进来，一般我们Filter不这么加进来，而是自己@WebFilter，或者借助Spring，
// 备注：这里添加进来的Filter都仅仅只拦截过滤上面注册的dispatchServlet
Filter[] filters = getServletFilters();
if (!ObjectUtils.isEmpty(filters)) {
for (Filter filter : filters) {
registerServletFilter(servletContext, filter);
}
}
//这个很清楚：调用者若相对dispatcherServlet有自己更个性化的参数设置，复写此方法即可
customizeRegistration(registration);
}
然后继续执行，就来到了ContextLoaderListener#contextInitialized执行此监听器的初始化方法（注意：到了此处，就和web.xml方式一模一样了）
但是不一样的是，注解驱动的此时候，我们的ContextLoaderListener对象已经持有WebApplicationContext的引用了（但是还没有放进ServletContext里面去，需要注意），所以会稍微有点不一样。
public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {
// 虽然注解驱动传进来的监听器对象持有WebApplicationContext的引用，但是并没有放进ServletContext容器
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.
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.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;
}
}