前言
<servlet>
<servlet-name>spring-mvc</servlet-name>
<servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
<init-param>
<param-name>contextConfigLocation</param-name>
<param-value>classpath:config/spring-mvc.xml</param-value>
</init-param>
<load-on-startup>1</load-on-startup>
</servlet>
<servlet-mapping>
<servlet-name>spring-mvc</servlet-name>
<url-pattern>/</url-pattern>
</servlet-mapping>
根据web.xml的配置，我们可以知道springMVC的主体控制器部分就是DispatcherServlet，同时也不难看出DispatcherServlet的本质其实就是一个Servlet。那我们这次来剖析一下DispatcherServlet的初始化流程。
DispatcherServlet类图
DispatcherServlet类图
其中HttpServletBean、FrameworkServlet和DispatcherServlet都是springMVC框架中的东西。
HttpServletBean类的init方法
写java web时，我们知道如果需要初始化，那我们就需要重载HttpServlet的init方法。那么我们首先来寻找init方法的位置，可以发现它在HttpServletBean类中。
public final void init() throws ServletException {
if (logger.isDebugEnabled()) {
logger.debug("Initializing servlet '" + getServletName() + "'");
}
// Set bean properties from init parameters.
try {
PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
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) {
logger.error("Failed to set bean properties on servlet '" + getServletName() + "'", ex);
throw ex;
}
// Let subclasses do whatever they like.
initServletBean();
if (logger.isDebugEnabled()) {
logger.debug("Servlet '" + getServletName() + "' configured successfully");
}
}
该方法主要做了如下两件事：
- 将初始化参数设置为该servlet的属性
- 调用子类的initServletBean方法
附：以我的理解来解释一下初始化参数设置为属性这一步骤，
<servlet>
<servlet-name>spring-mvc</servlet-name>
<servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
<init-param>
<param-name>contextConfigLocation</param-name>
<param-value>classpath:config/spring-mvc.xml</param-value>
</init-param>
<load-on-startup>1</load-on-startup>
</servlet>
<servlet-mapping>
<servlet-name>spring-mvc</servlet-name>
<url-pattern>/</url-pattern>
</servlet-mapping>
在配置文件中我们可以发现DispatcherServlet的初始化参数为contextConfigLocation=classpath:config/spring-mvc.xml，我们可以在FrameworkServlet的属性列表中找到contextConfigLocation这个属性。那么这块地方做的工作就是将初始化参数中contextConfigLocation的值配置给DispatcherServlet的contextConfigLocation属性。
FrameworkServlet类的initServletBean方法
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 {
this.webApplicationContext = initWebApplicationContext();
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");
}
}
该函数中主要调用了两个方法：initWebApplicationContext()和initFrameworkServlet()。
initFrameworkServlet()是用来给子类做初始化扩展的，事实上FrameworkServlet中是一个空实现，DispatchServlet也并没有重载该方法。
FrameworkServlet类的initWebApplicationContext方法
初始化WebApplicationContext，并且将根WebApplicationContext设置为其父context。
protected WebApplicationContext initWebApplicationContext() {
WebApplicationContext rootContext =
WebApplicationContextUtils.getWebApplicationContext(getServletContext());
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) {
// Publish the context as a servlet context attribute.
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;
}
DispatcherServlet类的onRefresh方法
我们可以发现，无论怎样，initWebApplicationContext方法最终都会调用到onRefresh方法。onRefresh在DispatcherServlet中被重载了，那我们直接看onRefresh部分的源码。
protected void onRefresh(ApplicationContext context) {
initStrategies(context);
}
protected void initStrategies(ApplicationContext context) {
initMultipartResolver(context);
initLocaleResolver(context);
initThemeResolver(context);
initHandlerMappings(context);
initHandlerAdapters(context);
initHandlerExceptionResolvers(context);
initRequestToViewNameTranslator(context);
initViewResolvers(context);
initFlashMapManager(context);
}
我们可以看到onRefresh调用了initStrategies方法，而initStrategies则对MultipartResolver, LocaleResolver, ThemeResolve, HandlerMappings, HandlerAdapters, HandlerExceptionResolvers, RequestToViewNameTranslator, ViewResolvers和FlashMapManager做了初始化。
不难知道，我们之前阅读的HandlerMapping, HandlerAdapter及ViewResolver等都是在这一步进行初始化的操作。
initHandlerMappings
我们取其中的HandlerMapping的初始化来继续分析初始化流程,
private void initHandlerMappings(ApplicationContext context) {
this.handlerMappings = null;
// 如果detectAllHandlerMappings属性设置为真，则从用户配置文件中查找所有的HandlerMapping，并排序
if (this.detectAllHandlerMappings) {
Map<String, HandlerMapping> matchingBeans =
BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerMapping.class, true, false);
if (!matchingBeans.isEmpty()) {
this.handlerMappings = new ArrayList<HandlerMapping>(matchingBeans.values());
OrderComparator.sort(this.handlerMappings);
}
}
// 否则只使用名称为"handlerMapping"的HandlerMapping
else {
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，则使用默认配置
if (this.handlerMappings == null) {
this.handlerMappings = getDefaultStrategies(context, HandlerMapping.class);
if (logger.isDebugEnabled()) {
logger.debug("No HandlerMappings found in servlet '" + getServletName() + "': using default");
}
}
}
从源码中可以知道，HandlerMapping的初始化流程主要分为两种，
- 使用用户自己的配置
- 使用默认配置
对于自己的配置没有什么可说的，那我们接下来来看默认配置，
protected <T> List<T> getDefaultStrategies(ApplicationContext context, Class<T> strategyInterface) {
String key = strategyInterface.getName();
String value = defaultStrategies.getProperty(key);
if (value != null) {
String[] classNames = StringUtils.commaDelimitedListToStringArray(value);
List<T> strategies = new ArrayList<T>(classNames.length);
for (String className : classNames) {
try {
Class<?> clazz = ClassUtils.forName(className, DispatcherServlet.class.getClassLoader());
Object strategy = createDefaultStrategy(context, clazz);
strategies.add((T) strategy);
}
catch (ClassNotFoundException ex) {
throw new BeanInitializationException(
"Could not find DispatcherServlet's default strategy class [" + className +
"] for interface [" + key + "]", ex);
}
catch (LinkageError err) {
throw new BeanInitializationException(
"Error loading DispatcherServlet's default strategy class [" + className +
"] for interface [" + key + "]: problem with class file or dependent class", err);
}
}
return strategies;
}
else {
return new LinkedList<T>();
}
}
我们可以看到里面有一个关键的属性defaultStrategies。我们来看看这个defaultStrategies到底是怎么一回事，
private static final String DEFAULT_STRATEGIES_PATH = "DispatcherServlet.properties";
private static final Properties defaultStrategies;
static {
// Load default strategy implementations from properties file.
// This is currently strictly internal and not meant to be customized
// by application developers.
try {
ClassPathResource resource = new ClassPathResource(DEFAULT_STRATEGIES_PATH, DispatcherServlet.class);
defaultStrategies = PropertiesLoaderUtils.loadProperties(resource);
}
catch (IOException ex) {
throw new IllegalStateException("Could not load 'DispatcherServlet.properties': " + ex.getMessage());
}
}
可以发现defaultStrategies里的属性来自于DispatcherServlet.properties这个文件，那么我们继续找到该文件，
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.annotation.DefaultAnnotationHandlerMapping
org.springframework.web.servlet.HandlerAdapter=org.springframework.web.servlet.mvc.HttpRequestHandlerAdapter,\
org.springframework.web.servlet.mvc.SimpleControllerHandlerAdapter,\
org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter
org.springframework.web.servlet.HandlerExceptionResolver=org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerExceptionResolver,\
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
到这里我们的疑惑应该就已经全部解清了。也知道之前阅读的HandlerMapping、HandlerAdater等默认值为什么是那些了。当然了，如果我们只需要其中的部分HandlerMapping等，也并不需要修改该文件，从getDefaultStrategies函数中我们可以知道，只需要我们在配置文件中有定义相应的属性，那么就不会使用默认的。
