SpringMVC源码解析之DispatcherServlet：生命周期init()和destroy()
SpringMVC源码解析之Servlet
SpringMVC源码解析之GenericServlet和HttpServlet
一、DispatcherServlet类
DispatcherServlet是SpringMVC的基础Servlet，基于Java-bean提供的应用上下文（application context）的集成。
Dispatcher主要提供了以下功能：
（1）通过Servlet提供了对WebApplicationContext实例的管理。Servlet的配置由Servlet命名空间内的beans确定。
（2）在处理请求时提供了事件机制，无论是否成功都会发布事件。
二、DispatcherServlet初始化
在上篇博客中已经提到，HttpServlet将初始化的逻辑委托给了无参init方法处理，下面我们从无参的init方法开始了解DispatcherServlet的初始化过程。
1. HttpServletBean#init()
public final void init() throws ServletException {
if (logger.isDebugEnabled()) {
logger.debug("Initializing servlet '" + getServletName() + "'");
}
//配置的参数封装到pvs
// Set bean properties from init parameters.
PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
if (!pvs.isEmpty()) {
try {
//Spring中bean的包装工具类，用于直接修改bean对象的属性值
BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);
ResourceLoader resourceLoader = new ServletContextResourceLoader(getServletContext());
bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, getEnvironment()));
//空的模板方法，留给子类重写
initBeanWrapper(bw);
//根据配置的参数值修改Servlet属性
bw.setPropertyValues(pvs, true);
}
catch (BeansException ex) {
if (logger.isErrorEnabled()) {
logger.error("Failed to set bean properties on servlet '" + getServletName() + "'", ex);
}
throw ex;
}
}
// Let subclasses do whatever initialization they like.
//空的模板方法，留给子类重写
initServletBean();
if (logger.isDebugEnabled()) {
logger.debug("Servlet '" + getServletName() + "' configured successfully");
}
}
2. FrameworkServlet#initServletBean()
FrameServlet对initServletBean方法进行了重写。
protected final void initServletBean() throws ServletException {
getServletContext().log("Initializing Spring FrameworkServlet '" + getServletName() + "'");
if (this.logger.isInfoEnabled()) {
this.logger.info("FrameworkServlet '" + getServletName() + "': initialization started");
}
long startTime = System.currentTimeMillis();
try {
//初始化成员变量webApplicationContext
this.webApplicationContext = initWebApplicationContext();
//默认空的模板方法
initFrameworkServlet();
}
catch (ServletException ex) {
this.logger.error("Context initialization failed", ex);
throw ex;
}
catch (RuntimeException ex) {
this.logger.error("Context initialization failed", ex);
throw ex;
}
if (this.logger.isInfoEnabled()) {
long elapsedTime = System.currentTimeMillis() - startTime;
this.logger.info("FrameworkServlet '" + getServletName() + "': initialization completed in " +
elapsedTime + " ms");
}
}
可以看到，除了日志记录初始化时间外，主要功能是初始化webApplicationContext。
3. FrameworkServlet#initWebApplicationContext()
protected WebApplicationContext initWebApplicationContext() {
//获取根容器rootContext。Spring默认会将根容器设置为ServletContext的属性，
//默认key值为：String ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE = WebApplicationContext.class.getName() + ".ROOT";
WebApplicationContext rootContext =
WebApplicationContextUtils.getWebApplicationContext(getServletContext());
WebApplicationContext wac = null;
//webApplicationContext已经在构造方法中被注入
if (this.webApplicationContext != null) {
// A context instance was injected at construction time -> use it
wac = this.webApplicationContext;
//wac instanceof ConfigurableWebApplicationContext类型需要保证active
if (wac instanceof ConfigurableWebApplicationContext) {
ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) wac;
if (!cwac.isActive()) {
// The context has not yet been refreshed -> provide services such as
// setting the parent context, setting the application context id, etc
//如果父容器未设置，以根容器为父容器
if (cwac.getParent() == null) {
// The context instance was injected without an explicit parent -> set
// the root application context (if any; may be null) as the parent
cwac.setParent(rootContext);
}
//配置并刷新容器
configureAndRefreshWebApplicationContext(cwac);
}
}
}
if (wac == null) {
// No context instance was injected at construction time -> see if one
// has been registered in the servlet context. If one exists, it is assumed
// that the parent context (if any) has already been set and that the
// user has performed any initialization such as setting the context id
//webApplicationContext已经在构造方法中没有被注入，从ServletContext中查找
wac = findWebApplicationContext();
}
if (wac == null) {
// No context instance is defined for this servlet -> create a local one
//ServletContext中也没有找到，进行创建
wac = createWebApplicationContext(rootContext);
}
//刷新事件没有出发时，进行刷新。刷新方法也是默认空的模板方法。确保onRefresh至少被调用过一次。
if (!this.refreshEventReceived) {
// Either the context is not a ConfigurableApplicationContext with refresh
// support or the context injected at construction time had already been
// refreshed -> trigger initial onRefresh manually here.
onRefresh(wac);
}
//作为属性注册到servletContext（注册之后与findWebApplicationContext对应）
//默认为true
if (this.publishContext) {
// Publish the context as a servlet context attribute.
//前缀（SERVLET_CONTEXT_PREFIX = FrameworkServlet.class.getName() + ".CONTEXT."） + servletName
String attrName = getServletContextAttributeName();
getServletContext().setAttribute(attrName, wac);
if (this.logger.isDebugEnabled()) {
this.logger.debug("Published WebApplicationContext of servlet '" + getServletName() +
"' as ServletContext attribute with name [" + attrName + "]");
}
}
return wac;
}
(1)获取根容器rootContext
(2)获取或生成容器webApplicationContext并设置, 其保证至少执行了一次onRefresh方法
(2.1)webApplicationContext已经在构造方法中传入
(2.2)webApplicationContext已经注入到servletContext，根据getServletContextAttributeName进行查找
(2.3)以根容器作为父容器通过createWebApplicationContext进行生成
(3)如果需要，将webApplicationContext注册到servletContext。
这一步可以和（2.2）对应。
4.FrameworkServlet#createWebApplicationContext(WebApplicationContext)
protected WebApplicationContext createWebApplicationContext(@Nullable ApplicationContext parent) {
//WebApplicationContext的class类型，默认为XmlWebApplicationContext
Class contextClass = getContextClass();
if (this.logger.isDebugEnabled()) {
this.logger.debug("Servlet with name '" + getServletName() +
"' will try to create custom WebApplicationContext context of class '" +
contextClass.getName() + "'" + ", using parent context [" + parent + "]");
}
//检验，contextClass应该是ConfigurableWebApplicationContext或其子类
if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
throw new ApplicationContextException(
"Fatal initialization error in servlet with name '" + getServletName() +
"': custom WebApplicationContext class [" + contextClass.getName() +
"] is not of type ConfigurableWebApplicationContext");
}
//初始化
ConfigurableWebApplicationContext wac =
(ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
//属性配置
wac.setEnvironment(getEnvironment());
wac.setParent(parent);
String configLocation = getContextConfigLocation();
if (configLocation != null) {
wac.setConfigLocation(configLocation);
}
//配置并刷新
configureAndRefreshWebApplicationContext(wac);
return wac;
}
根据父容器创新新的容器。
首先获取容器的class类型，默认为XmlWebApplicationContext。
接着进行校验，class类型必须属于ConfigurableWebApplicationContext。
然后初始化并设置属性。
最后进行配置和刷新。
5.FrameworkServlet#configureAndRefreshApplicationContext(ConfigurableApplicationContext)
protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac) {
//设置id属性
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
if (this.contextId != null) {
wac.setId(this.contextId);
}
else {
// Generate default id...
wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX +
ObjectUtils.getDisplayString(getServletContext().getContextPath()) + '/' + getServletName());
}
}
wac.setServletContext(getServletContext());
wac.setServletConfig(getServletConfig());
wac.setNamespace(getNamespace());
//添加监听器SourceFilteringListener
wac.addApplicationListener(new SourceFilteringListener(wac, new ContextRefreshListener()));
// 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(getServletContext(), getServletConfig());
}
//默认空的模板方法
postProcessWebApplicationContext(wac);
applyInitializers(wac);
wac.refresh();
}
可以看到此处给ApplicationContext添加了监听器SourceFilteringListener，，而SourceFilteringListenerh使用了委派模式，实际使用的是ContextRefreshListener的监听逻辑。
ContextRefreshListener是FrameworkServlet的内部类，监听刷新事件ContextRefreshedEvent。
private class ContextRefreshListener implements ApplicationListener {
@Override
public void onApplicationEvent(ContextRefreshedEvent event) {
FrameworkServlet.this.onApplicationEvent(event);
}
}
//修改refreshEventReceived标志并调用onRefresh方法，
//FrameworkServlet#initWebApplication()方法中根据refreshEventReceived标志判断是否调用onRefresh对应
public void onApplicationEvent(ContextRefreshedEvent event) {
this.refreshEventReceived = true;
onRefresh(event.getApplicationContext());
}
回到FrameworkServlet#configureAndRefreshApplicationContext(ConfigurableApplicationContext)可以看到最后其进行了一次刷新，以触发onRefresh方法。
再回到FrameworkServlet#initWebApplicationContext()方法中的
if (!this.refreshEventReceived) {
// Either the context is not a ConfigurableApplicationContext with refresh
// support or the context injected at construction time had already been
// refreshed -> trigger initial onRefresh manually here.
onRefresh(wac);
}
可以看到，如果没有执行过onRefresh方法，则触发执行，确保至少执行了一次。
6. FrameworkServlet#applyInitializers(ConfigurableApplicationContext)
protected void applyInitializers(ConfigurableApplicationContext wac) {
//获取配置文件中的globalInitializerClasses参数
String globalClassNames = getServletContext().getInitParameter(ContextLoader.GLOBAL_INITIALIZER_CLASSES_PARAM);
if (globalClassNames != null) {
for (String className : StringUtils.tokenizeToStringArray(globalClassNames, INIT_PARAM_DELIMITERS)) {
this.contextInitializers.add(loadInitializer(className, wac));
}
}
if (this.contextInitializerClasses != null) {
for (String className : StringUtils.tokenizeToStringArray(this.contextInitializerClasses, INIT_PARAM_DELIMITERS)) {
this.contextInitializers.add(loadInitializer(className, wac));
}
}
AnnotationAwareOrderComparator.sort(this.contextInitializers);
for (ApplicationContextInitializer initializer : this.contextInitializers) {
initializer.initialize(wac);
}
}
根据contextInitializerClasses属性和配置文件中的globalInitializerClasses参数进行初始化得到contextInitializers，并调用其初始化initialize方法.。
public interface ApplicationContextInitializer {
/**
* Initialize the given application context.
* @param applicationContext the application to configure
*/
void initialize(C applicationContext);
}
ApplicationContextInitializer是ConfigurableApplicationContext初始化的回调接口，用来做一些初始化动作。
7. DispatcherServlet#onRefresh(ApplicationContext)
DispatcherServlet对onRefresh进行了重写，用于在ApplicationContext刷新后进行策略组件的初始化。
DispatcherServlet#onRefresh(ApplicationContext)
protected void onRefresh(ApplicationContext context) {
initStrategies(context);
}
//初始化9个策略组件
protected void initStrategies(ApplicationContext context) {
initMultipartResolver(context);
initLocaleResolver(context);
initThemeResolver(context);
initHandlerMappings(context);
initHandlerAdapters(context);
initHandlerExceptionResolvers(context);
initRequestToViewNameTranslator(context);
initViewResolvers(context);
initFlashMapManager(context);
}
private void initMultipartResolver(ApplicationContext context) {
try {
this.multipartResolver = context.getBean(MULTIPART_RESOLVER_BEAN_NAME, MultipartResolver.class);
if (logger.isDebugEnabled()) {
logger.debug("Using MultipartResolver [" + this.multipartResolver + "]");
}
}
catch (NoSuchBeanDefinitionException ex) {
// Default is no multipart resolver.
this.multipartResolver = null;
if (logger.isDebugEnabled()) {
logger.debug("Unable to locate MultipartResolver with name '" + MULTIPART_RESOLVER_BEAN_NAME +
"': no multipart request handling provided");
}
}
}
以DispatcherServlet#initMultipartResolver为例，用于确定成员变量multipartResolver的值。
其它8个方法也类似。
这一点和前面对DispatcherServlet的介绍相吻合。
除了MultipartResolver外，其它的8个组件都指定了默认值，默认值的类型可以查看DispatcherServlet.properties。
# Default implementation classes for DispatcherServlet's strategy interfaces.
# Used as fallback when no matching beans are found in the DispatcherServlet context.
# Not meant to be customized by application developers.
org.springframework.web.servlet.LocaleResolver=org.springframework.web.servlet.i18n.AcceptHeaderLocaleResolver
org.springframework.web.servlet.ThemeResolver=org.springframework.web.servlet.theme.FixedThemeResolver
org.springframework.web.servlet.HandlerMapping=org.springframework.web.servlet.handler.BeanNameUrlHandlerMapping,\
org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerMapping
org.springframework.web.servlet.HandlerAdapter=org.springframework.web.servlet.mvc.HttpRequestHandlerAdapter,\
org.springframework.web.servlet.mvc.SimpleControllerHandlerAdapter,\
org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter
org.springframework.web.servlet.HandlerExceptionResolver=org.springframework.web.servlet.mvc.method.annotation.ExceptionHandlerExceptionResolver,\
org.springframework.web.servlet.mvc.annotation.ResponseStatusExceptionResolver,\
org.springframework.web.servlet.mvc.support.DefaultHandlerExceptionResolver
org.springframework.web.servlet.RequestToViewNameTranslator=org.springframework.web.servlet.view.DefaultRequestToViewNameTranslator
org.springframework.web.servlet.ViewResolver=org.springframework.web.servlet.view.InternalResourceViewResolver
org.springframework.web.servlet.FlashMapManager=org.springframework.web.servlet.support.SessionFlashMapManager
通过Servlet提供了对WebApplicationContext实例的管理。Servlet的配置由Servlet命名空间内的beans确定。
三、DispatcherServlet销毁
1. FrameworkServlet#destroy()
public void destroy() {
getServletContext().log("Destroying Spring FrameworkServlet '" + getServletName() + "'");
// Only call close() on WebApplicationContext if locally managed...
if (this.webApplicationContext instanceof ConfigurableApplicationContext && !this.webApplicationContextInjected) {
((ConfigurableApplicationContext) this.webApplicationContext).close();
}
}
销毁逻辑很简单，只需要打印日志和关闭webApplicationContext。