Spring容器启动流程
简单分析解读一下Web容器启动时Spring容器的初始化过程
1 启动web容器
web容器启动时读取web.xml，如下
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://xmlns.jcp.org/xml/ns/javaee" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd" id="WebApp_ID" version="3.1">
<context-param>
<param-name>contextConfigLocationparam-name>
<param-value>classpath:applicationContext.xmlparam-value>
context-param>
<listener>
<listener-class>org.springframework.web.context.ContextLoaderListenerlistener-class>
listener>
<servlet>
<servlet-name>springDispatcherServletservlet-name>
<servlet-class>org.springframework.web.servlet.DispatcherServletservlet-class>
<load-on-startup>1load-on-startup>
servlet>
web-app>
web容器的初始化过程为：
- 读取web.xml文件
- web容器创建servlet上下文对象（servletContext）
- 读取context-param节点并转化为键值对的形式传给servletContext
- 读取linstener节点的监听器，其中接节点中的监听器类必须实现ServletContextListener接口
- 执行监听器中从ServletContextListener接口实现的contextInitialized()方法
ServletContextListener接口能监听整个servletContext的生命周期，也就是web容器的生命周期，当web容器启动或关闭时都会触发ServletContextEvent事件，然后由ServletContextListener来处理。其中容器启动调用contextInitialized()方法，关闭调用contextDestroyed()方法
2 initWebApplicationContext()
进入listener节点中的ContextLoaderListener类中
public class ContextLoaderListener extends ContextLoader implements ServletContextListener {
/**
* 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());
}
}
ContextLoaderListener继承自ContextLoader类并且实现了ServletContextListeneri接口
在contextInitialized()中调用了initWebApplicationContext()方法，并且由event对象传入了servletContext对象
整个Spring容器的初始化也就是在这个方法中完成
进入initWebApplicationContext()
public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {
//判断是否已经存在容器对象，如果已经存在就不再创建
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!");
}
//打印日志并开始计算启动时间
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 {
//创建WebApplicationContext
if (this.context == null) {
this.context = createWebApplicationContext(servletContext);
}
// 判断context有没有父context，取决于web.xml配置文件中locatorFactorySelector参数，如果有父context，则加载它
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);
}
// refresh容器，最重要的一步，这一步会创建beans
configureAndRefreshWebApplicationContext(cwac, servletContext);
}
}
//把当前Spring的容器对象放到全局的Servlet容器（ServletContext）中
servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);
//将spring容器context赋值给currentContext变量，保存下来
ClassLoader ccl = Thread.currentThread().getContextClassLoader();
if (ccl == ContextLoader.class.getClassLoader()) {
currentContext = this.context;
}
else if (ccl != null) {
currentContextPerThread.put(ccl, this.context);
}
//一些日志信息
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;
}
}
在方法中最重要的有三步
1 创建web环境下的Spring容器实现类WebApplicationContext，通过createWebApplicationContext()
2 加载配置文件，初始化Spring容器，通过configureAndRefreshWebApplicationContext()方法
3 把Spring容器对象放到Servlet上下文对象中，通过servletContext.setAttribute()
3 createWebApplicationContext() 创建Spring容器
protected WebApplicationContext createWebApplicationContext(ServletContext sc) {
//获取WebApplicationContext实现类的class对象，通过determineContextClass()
Class<?> contextClass = determineContextClass(sc);
//获取的WebApplicationContext实现类必须继承自ConfigurableWebApplicationContext
if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
throw new ApplicationContextException("Custom context class [" + contextClass.getName() +
"] is not of type [" + ConfigurableWebApplicationContext.class.getName() + "]");
}
//根据calss对象创建实例对象
return (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
}
创建Spring是通过WebApplicationContext的实现类的class对象创建的实例对象
接下来进入determineContextClass()看一下怎么获取的class对象
protected Class<?> determineContextClass(ServletContext servletContext) {
//获取web.xml中name为contextClass的context-param节点的的值
String contextClassName = servletContext.getInitParameter(CONTEXT_CLASS_PARAM);
//如果在web.xml中配置了contextClass节点则直接根据类名来创建容器
if (contextClassName != null) {
try {
return ClassUtils.forName(contextClassName, ClassUtils.getDefaultClassLoader());
}
catch (ClassNotFoundException ex) {
throw new ApplicationContextException(
"Failed to load custom context class [" + contextClassName + "]", ex);
}
}
//如果没有配置contextClass，则创建Spring在web环境下的默认容器
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);
}
}
}
defaultStrategies的声明和上面这些方法一样都在ContexLoader类中，看下面
private static final String DEFAULT_STRATEGIES_PATH = "ContextLoader.properties";
private static final Properties defaultStrategies;
static {
try {
ClassPathResource resource = new ClassPathResource(DEFAULT_STRATEGIES_PATH, ContextLoader.class);
defaultStrategies = PropertiesLoaderUtils.loadProperties(resource);
}
catch (IOException ex) {
throw new IllegalStateException("Could not load 'ContextLoader.properties': " + ex.getMessage());
}
}
在ContextLoader中直接声明了一个静态代码块对defaultStrategies进行了初始化
根据DEFAULT_STRATEGIES_PATH这个静态常量作为路径读取配置的properties文件然后加载这个文件
路径直接时文件名所以文件就在ContextLoader这个类的同路径下，直接找到ContextLoader.properties打开
# Default WebApplicationContext implementation class for ContextLoader.
# Used as fallback when no explicit context implementation has been specified as context-param.
# Not meant to be customized by application developers.
org.springframework.web.context.WebApplicationContext=org.springframework.web.context.support.XmlWebApplicationContext
只有一句话，指明了Spring的容器初始化类为XmlWebApplicationContext，也就是说，Spring在Web环境下的初始化类就是XmlWebApplicationContext
4 configureAndRefreshWebApplicationContext() 主要初始化方法
创建Spring容器之后就会加载Spring的配置文件并根据配置文件初始化Beans，整个过程就是在configureAndRefreshWebApplicationContext()中
进入configureAndRefreshWebApplicationContext()方法
protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac, ServletContext sc) {
//设置容器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()));
}
}
//把Servlet容器设置到Spring容器中
wac.setServletContext(sc);
//获取Web.xml中name为contextConfigLocation的context-param节点的值
//public static final String CONFIG_LOCATION_PARAM = "contextConfigLocation"
String configLocationParam = sc.getInitParameter(CONFIG_LOCATION_PARAM);
//如果配置了contextConfigLocation则直接作为路径加载配置文件
if (configLocationParam != null) {
wac.setConfigLocation(configLocationParam);
}
//初始化Servlet的propertySources
ConfigurableEnvironment env = wac.getEnvironment();
if (env instanceof ConfigurableWebEnvironment) {
((ConfigurableWebEnvironment) env).initPropertySources(sc, null);
}
//个性化配置，一般用默认即可
customizeContext(sc, wac);
//主要工作就是在这里完成
wac.refresh();
}
configureAndRefreshWebApplicationContext()会先读取web.xml中声明的contextConfigLocation的值，通过它来找到Spring的配置文件，然后在最后的refresh()中读取配置文件，并初始化和创建Beans，接下来进入refresh()
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// Prepare this context for refreshing.
prepareRefresh();
// Tell the subclass to refresh the internal bean factory.
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
// Prepare the bean factory for use in this context.
prepareBeanFactory(beanFactory);
try {
// Allows post-processing of the bean factory in context subclasses.
postProcessBeanFactory(beanFactory);
// Invoke factory processors registered as beans in the context.
invokeBeanFactoryPostProcessors(beanFactory);
// Register bean processors that intercept bean creation.
registerBeanPostProcessors(beanFactory);
// Initialize message source for this context.
initMessageSource();
// Initialize event multicaster for this context.
initApplicationEventMulticaster();
// Initialize other special beans in specific context subclasses.
onRefresh();
// Check for listener beans and register them.
registerListeners();
// Instantiate all remaining (non-lazy-init) singletons.
finishBeanFactoryInitialization(beanFactory);
// Last step: publish corresponding event.
finishRefresh();
}
catch (BeansException ex) {
if (logger.isWarnEnabled()) {
logger.warn("Exception encountered during context initialization - " +
"cancelling refresh attempt: " + ex);
}
// Destroy already created singletons to avoid dangling resources.
destroyBeans();
// Reset 'active' flag.
cancelRefresh(ex);
// Propagate exception to caller.
throw ex;
}
finally {
resetCommonCaches();
}
}
}
refresh()的方法有很多也很复杂，在下一篇文章里再进行详细的分析