本节一起学习DispatcherServlet的启动和初始化
前面已经分析了Spring MVC的工作，下面就DispatcherServlet的启动和初始化进行详细的分析
作为Servlet，DispatcherServlet的启动和Servlet的启动过程是相关联的，在Servlet的初始化过程中，Servlet的init会被调用，进行初始化。下面看一下DispatcherServlet的基类HttpServletBean中的初始化过程。
@Override
public final void init() throws ServletException {
if (logger.isDebugEnabled()) {
logger.debug("Initializing servlet '" + getServletName() + "'");
}
// 获取Servlet的初始化参数，对Bean属性进行配置
PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
if (!pvs.isEmpty()) {
try {
BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);
ResourceLoader resourceLoader = new ServletContextResourceLoader(getServletContext());
bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, getEnvironment()));
initBeanWrapper(bw);
bw.setPropertyValues(pvs, true);
}
catch (BeansException ex) {
if (logger.isErrorEnabled()) {
logger.error("Failed to set bean properties on servlet '" + getServletName() + "'", ex);
}
throw ex;
}
}
// 调用子类的initServletBean进行具体的初始化
initServletBean();
if (logger.isDebugEnabled()) {
logger.debug("Servlet '" + getServletName() + "' configured successfully");
}
}
在初始化开始时，要读取配置在ServletContext中的Bean属性参数，这些属性参数设置在web.xml的Web容器初始化参数中。使用编程式的方式设置这些Bean属性。接着会执行DispatcherServlet持有的IOC容器的初始化过程，在这个初始化过程中，一个新的上下文被建立起来，这个DispatcherServlet持有的上下文被设置为根上下文的子上下文。根上下文和Web应用相对应的一个上下文，而DispatcherServlet持有的上下文时和Servlet相对性的一个上下文。DispatcherServlet持有的上下文被建立起来后，也需要和其他IOC容器一样完成初始化，这个初始化也是通过refresh方法完成的。最后，DispatcherServlet给自己持有上下文命名，并把它设置到Web容器的上下文中，这个名词和web.xml设置的DispatcherServlet的Servlet的名称有关，从而保证这个上下文在web环境上下文体系中的唯一性。
@Override
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();
//初始化上下文
try {
this.webApplicationContext = initWebApplicationContext();
initFrameworkServlet();
}
catch (ServletException | 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");
}
}
protected WebApplicationContext initWebApplicationContext() {
WebApplicationContext rootContext =
WebApplicationContextUtils.getWebApplicationContext(getServletContext());//得到根上下文
WebApplicationContext wac = null;
if (this.webApplicationContext != null) {
// A context instance was injected at construction time -> use it
wac = this.webApplicationContext;
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
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);
}
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
wac = findWebApplicationContext();
}
if (wac == null) {
// No context instance is defined for this servlet -> create a local one
wac = createWebApplicationContext(rootContext);
}
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);
}
if (this.publishContext) {
// 把当前建立的上下文存到ServletContext中
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;
}
在这里MVC的上下文建立起来了，具体获取根上下文的过程在WebApplicationContextUtils中实现。
@Nullable
public static WebApplicationContext getWebApplicationContext(ServletContext sc) {
return getWebApplicationContext(sc, WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE);
}
@Nullable
public static WebApplicationContext getWebApplicationContext(ServletContext sc, String attrName) {
Assert.notNull(sc, "ServletContext must not be null");
Object attr = sc.getAttribute(attrName);
if (attr == null) {
return null;
}
if (attr instanceof RuntimeException) {
throw (RuntimeException) attr;
}
if (attr instanceof Error) {
throw (Error) attr;
}
if (attr instanceof Exception) {
throw new IllegalStateException((Exception) attr);
}
if (!(attr instanceof WebApplicationContext)) {
throw new IllegalStateException("Context attribute is not of type WebApplicationContext: " + attr);
}
return (WebApplicationContext) attr;
}
这个根上下文是ContextLoader设置到ServletContext中的，使用的属性是ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE，同时对这个IOC容器的Bean配置文件，ContextLoader也进行了设置，默认的位置在/WEB-INF/applicationContext.xml中。由于这个根上下文是DispatcherServlet建立的上下文的双亲上下文，所以跟上下文中管理的Bean也是可以被DispatcherServlet的上下文使用的。通过getBean向IOC容器获取Bean时，容器会先到它的双亲Ioc容器中获得getBean。
protected WebApplicationContext createWebApplicationContext(@Nullable WebApplicationContext parent) {
return createWebApplicationContext((ApplicationContext) parent);
}
protected WebApplicationContext createWebApplicationContext(@Nullable ApplicationContext parent) {
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 + "]");
}
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");
}
ConfigurableWebApplicationContext wac =
(ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
//使用反射实例化上下文，默认的XmlWebApplicationContext
wac.setEnvironment(getEnvironment());
wac.setParent(parent);//设置双亲IOC
String configLocation = getContextConfigLocation();
if (configLocation != null) {
wac.setConfigLocation(configLocation);
}//初始化
configureAndRefreshWebApplicationContext(wac);
return wac;
}这里创建上下文的过程与建立根上下文的过程很类似，建立DispatcherServlet的上下文需要把根上下文参数传递给它。然后使用反射技术实例化上下文对象，并为它设置参数。最后通过refresh完成IOC容器的最终初始化。
在Spring MVC DispatcherServlet的初始化过程中，以对HandlerMapping的初始化调用作为触发点，了解Spring MVC模块初始化的方法调用关系。这个调用关系最初是由HttpServletBean的init方法触发的，这个HttpServletBean是HttpServlet的子类，接着会在HttpServletBean的子类FrameworkServlet对IOC容器完成初始化，在初始化中，会调用DispatcherServlet的initStrategies方法，这个initStrategies方法，启动整个Spring MVC框架的初始化。
protected void initStrategies(ApplicationContext context) {
initMultipartResolver(context);
initLocaleResolver(context);
initThemeResolver(context);
initHandlerMappings(context);
initHandlerAdapters(context);
initHandlerExceptionResolvers(context);
initRequestToViewNameTranslator(context);
initViewResolvers(context);
initFlashMapManager(context);
}
initStrategies完成了对各种MVC框架的实现元素，比如支持国际化的LocaleResolver、支持request映射的HandlerMappings以及视图生成的ViewResolvers等的初始化。
以HandlerMappings为例子，说明这个initHandlerMappings的过程，这里的Mapping关系的作用是，为Http请求找到相应的Controller控制器，从而利用这些控制器Controller完成设计好的数据处理工作。HandlerMappings完成对MVC中Controller的定义和配置，只不过在Web特定的应用环境中，这些控制器是与具体的HTTP请求相对应的。
private void initHandlerMappings(ApplicationContext context) {
this.handlerMappings = null;
//detectAllHandlerMappings默认为true,这里导入所有的HandlerMappings Bean，这些Bean可以在当前的DispatcherServlet的IOC容器中，也可能在双亲上下文中
if (this.detectAllHandlerMappings) {
// Find all HandlerMappings in the ApplicationContext, including ancestor contexts.
Map<String, HandlerMapping> matchingBeans =
BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerMapping.class, true, false);
if (!matchingBeans.isEmpty()) {
this.handlerMappings = new ArrayList<>(matchingBeans.values());
// We keep HandlerMappings in sorted order.
AnnotationAwareOrderComparator.sort(this.handlerMappings);
}
}
else {//也可以根据名称从当前的IOC容器中通过getBean获得handlerMapping
try {
HandlerMapping hm = context.getBean(HANDLER_MAPPING_BEAN_NAME, HandlerMapping.class);
this.handlerMappings = Collections.singletonList(hm);
}
catch (NoSuchBeanDefinitionException ex) {
// Ignore, we'll add a default HandlerMapping later.
}
}
// 如果没有找到handlerMapping，那么需要Servlet设定默认的handlerMappings
if (this.handlerMappings == null) {
this.handlerMappings = getDefaultStrategies(context, HandlerMapping.class);
if (logger.isDebugEnabled()) {
logger.debug("No HandlerMappings found in servlet '" + getServletName() + "': using default");
}
}
}