本文探討Spring MVC中DispatcherServlet是如何初始化的，DispatcherServlet初始化指的是init()生命周期方法被執行，而不是DispatcherServlet被實例化的過程。
DispatcherServlet類
DispatcherServlet的類層次如下圖所示
DispatcherServlet.png
不管DispatcherServlet被如何包裝，它本質上是一個servlet，servlet的生命周期是init -> service -> destroy，因此本文從init()方法入手分析DispatcherServlet的初始化過程。（如果你對servlet不熟悉，我建議你看一下這篇入門指南：Java Servlet完全教程）
init()方法
DispatcherServlet的init()方法在父類HttpServletBean中定義，其代碼如下所示：
@Override
public final void init() throws ServletException {
if (logger.isDebugEnabled()) {
logger.debug("Initializing servlet '" + getServletName() + "'");
}
// Set bean properties from init parameters.
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;
}
}
// Let subclasses do whatever initialization they like.
initServletBean();
if (logger.isDebugEnabled()) {
logger.debug("Servlet '" + getServletName() + "' configured successfully");
}
}
- ServletConfigPropertyValues用于解析web.xml定義中<servlet>元素的子元素<init-param>中的參數值。
若<init-param>元素如下，則ServletConfigPropertyValues就會擁有這些參數<servlet> <servlet-name>dispatcher</servlet-name> <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class> <init-param> <param-name>contextConfigLocation</param-name> <param-value>/WEB-INF/spring/dispatcherServlet.xml</param-value> </init-param> <load-on-startup>-1</load-on-startup> </servlet>
ServletConfigPropertyValues.png
- BeanWrapper把DispatcherServlet當做一個Bean去處理，這也是HttpServletBean類名的含義。bw.setPropertyValues(pvs, true) 將上一步解析的servlet初始化參數值綁定到DispatcherServlet對應的字段上；
- init()方法是一個模板方法，initBeanWrapper和initServletBean兩個方法由子類去實現。
initServletBean()方法
DispatcherServlet的initServletBean()方法在父類FrameworkServlet中定義，它調用initWebApplicationContext方法初始化DispatcherServlet自己的應用上下文：
@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();
try {
this.webApplicationContext = initWebApplicationContext();
initFrameworkServlet();
}
// 省略一些代碼
}
protected void initFrameworkServlet() throws ServletException {
}
初始化servlet應用上下文
initWebApplicationContext方法負責初始化DispatcherServlet自己的應用上下文，其代碼如下所示：
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;
}
該方法的概要流程如下：
- 獲得ContextLoaderListener創建的根應用上下文；
- 為DispatcherServlet創建自己的應用上下文；
- 刷新DispatcherServlet自己的應用上下文。
獲得根應用上下文
利用WebApplicationContextUtils類的getWebApplicationContext靜態方法取得根應用上下文，相關代碼如下：
public static WebApplicationContext getWebApplicationContext(ServletContext sc) {
return getWebApplicationContext(sc, WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE);
}
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;
}
前面文章指出根應用上下文已經通過ContextLoaderListener被容器初始化，其類型默認是XmlWebApplicationContext類，啟動過程中會將WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE（即org.springframework.web.context.WebApplicationContext.ROOT）和根應用上下文通過ServletContext.setAttribute設置到應用的ServletContext。
rootContext.png
創建DispatcherServlet的應用上下文
- 若this.webApplicationContext不為null，則說明DispatcherServlet在實例化期間已經被注入了應用上下文。這種情況發生在Spring Boot應用啟動時，由于父類FrameworkServlet實現了ApplicationContextAware接口，所以setApplicationContext回調函數被調用時將字段webApplicationContext設置為根應用上下文，注意這并不是在init()初始化方法中完成的，而是在實例化DispatcherServlet的過程中完成的。
@Override
public void setApplicationContext(ApplicationContext applicationContext) {
if (this.webApplicationContext == null && applicationContext instanceof WebApplicationContext) {
this.webApplicationContext = (WebApplicationContext) applicationContext;
this.webApplicationContextInjected = true;
}
}
- 若this.webApplicationContext為null，則說明DispatcherServlet在實例化期間沒有被注入應用上下文。首先通過findWebApplicationContext方法嘗試尋找先前創建的應用上下文。
protected WebApplicationContext findWebApplicationContext() {
String attrName = getContextAttribute();
if (attrName == null) {
return null;
}
WebApplicationContext wac =
WebApplicationContextUtils.getWebApplicationContext(getServletContext(), attrName);
if (wac == null) {
throw new IllegalStateException("No WebApplicationContext found: initializer not registered?");
}
return wac;
}
public String getContextAttribute() {
return this.contextAttribute;
}
該方法從ServletContext的屬性中找到該servlet初始化屬性(<init-param>元素)contextAttribute的值對應的應用上下文，若沒有找到則報錯。
- 找到的WebApplicationContext是其他servlet初始化時設置到ServletContext屬性中的，具體是由initWebApplicationContext方法最后幾行做的，其代碼如下。
String attrName = getServletContextAttributeName(); getServletContext().setAttribute(attrName, wac);
可見設置的鍵值是FrameworkServlet.class.getName() + ".CONTEXT."加上servlet的<servlet-name>值。public String getServletContextAttributeName() { return SERVLET_CONTEXT_PREFIX + getServletName(); } public static final String SERVLET_CONTEXT_PREFIX = FrameworkServlet.class.getName() + ".CONTEXT."; - 以下面的web.xml片段為例，MyServlet會被容器在啟動的時候初始化，而dispatcher則是延遲初始化，它們均是DispatcherServlet類型。MyServlet初始化時會在ServletContext中設置以org.springframework.web.servlet.FrameworkServlet.CONTEXT.MyServlet為鍵，XmlWebApplicationContext對象為值的屬性，當dispatcher初始化時，其contextAttribute值恰是由MyServlet初始化應用上下文時設置的鍵，因此dispatcher初始化時應用上下文就是MyServlet初始化的應用上下文（雖然從源碼分析如此但筆者并未在Spring MVC的文檔里找到與contextAttribute有關的資料）。
<servlet> <servlet-name>MyServlet</servlet-name> <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class> <init-param> <param-name>contextConfigLocation</param-name> <param-value>/WEB-INF/spring/dispatcherServlet.xml</param-value> </init-param> <load-on-startup>0</load-on-startup> </servlet> <servlet> <servlet-name>dispatcher</servlet-name> <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class> <init-param> <param-name>contextAttribute</param-name> <param-value>org.springframework.web.servlet.FrameworkServlet.CONTEXT.MyServlet</param-value> </init-param> <load-on-startup>-1</load-on-startup> </servlet>
- 若第2步沒有找到之前初始化的應用上下文那么就需要通過createWebApplicationContext方法為DispatcherServlet創建一個以根應用上下文為父的應用上下文。這個應用上下文的類型是由父類FrameworkServlet的contextClass字段指定的，可以在web.xml中配置，默認是XmlWebApplicationContext類型，可以參見DispatcherServlet配置文檔。
protected WebApplicationContext createWebApplicationContext(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);
wac.setEnvironment(getEnvironment());
wac.setParent(parent);
wac.setConfigLocation(getContextConfigLocation());
configureAndRefreshWebApplicationContext(wac);
return wac;
}
具體的創建過程如下：
- 實例化contextClass指定類型的應用上下文，同時將根應用上下文設置為它的父上下文，并將<servlet>的contextConfigLocation初始化參數值設置到對應屬性；
- 從代碼中拋出異常的條件看contextClass屬性指定的類必須實現ConfigurableWebApplicationContext接口，而不是文檔說明的WebApplicationContext接口，對此問題筆者咨詢了Spring作者，見SPR-17414；
- configureAndRefreshWebApplicationContext方法先進一步為DispatcherServlet自己的應用上下文設置了屬性，然后調用了各ApplicationContextInitializer實現類的回調函數，最后做了刷新操作實例化各單例bean，其代碼如下所示：
applyInitializers方法執行各ApplicationContextInitializer的initialize回調函數，這里的ApplicationContextInitializer分為兩種：protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac) { 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()); 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()); } postProcessWebApplicationContext(wac); applyInitializers(wac); wac.refresh(); } protected void postProcessWebApplicationContext(ConfigurableWebApplicationContext wac) { } protected void applyInitializers(ConfigurableApplicationContext wac) { 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<ConfigurableApplicationContext> initializer : this.contextInitializers) { initializer.initialize(wac); } }- 全局ApplicationContextInitializer類由部署描述符中名為globalInitializerClasses的<context-param>初始化參數指定；
- DispatcherServlet自己的ApplicationContextInitializer類由部署描述符中<servlet>元素內名為contextInitializerClasses的<init-param>初始化參數指定；
- 這兩個參數的值都是由各個類名組成的以逗號、分號或空白符分隔的字符串。
-
最后的疑問是部署描述符web.xml中的contextClass等參數是如何被綁定到FrameworkServlet類或DispatcherServlet類的對應字段的呢？這是由上文提到的init()方法中ServletConfigPropertyValues和BeanWrapper完成的。
contextClass.png
刷新DispatcherServlet的應用上下文
onRefresh方法刷新DispatcherServlet自己的應用上下文，DispatcherServlet類重寫了父類FrameworkServlet的onRefresh方法，該方法調用initStrategies()方法實例化MultipartResolver、LocaleResolver、HandlerMapping、HandlerAdapter和ViewResolver等組件。
@Override
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);
}
以實例化HandlerMapping的initHandlerMappings方法為例，其代碼如下：
private void initHandlerMappings(ApplicationContext context) {
this.handlerMappings = null;
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<HandlerMapping>(matchingBeans.values());
// We keep HandlerMappings in sorted order.
AnnotationAwareOrderComparator.sort(this.handlerMappings);
}
}
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.
}
}
// Ensure we have at least one HandlerMapping, by registering
// a default HandlerMapping if no other mappings are found.
if (this.handlerMappings == null) {
this.handlerMappings = getDefaultStrategies(context, HandlerMapping.class);
if (logger.isDebugEnabled()) {
logger.debug("No HandlerMappings found in servlet '" + getServletName() + "': using default");
}
}
}
- handlerMappings是DispatcherServlet的一個List<HandlerMapping>；
- detectAllHandlerMappings是DispatcherServlet的一個布爾值屬性，表示是否要發現所有的HandlerMapping，若為true則從DispatchServlet自己的應用上下文和根應用上下文獲得所有已實例化的HandlerMapping單例，否則只獲取名為handlerMapping的HandlerMapping單例；
- 若不存在已實例化的HandlerMapping，那么用默認策略實例化HandlerMapping。
用getDefaultStrategies方法獲取默認策略：
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<>(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(
"Unresolvable class definition for DispatcherServlet's default strategy class [" +
className + "] for interface [" + key + "]", err);
}
}
return strategies;
}
else {
return new LinkedList<>();
}
}
- defaultStrategies是Properties類型的靜態變量，保存了策略名到默認工廠實現類的映射關系，它被DispatcherServlet的靜態代碼塊所填充。默認的策略定義在spring-webmvc包下的DispatcherServlet.properties文件中，該文件內容如下：
# 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 - 以HandlerMapping為例，獲取HandlerMapping默認實現的調用是getDefaultStrategies(context, HandlerMapping.class)，接著會查找以HandlerMapping.class.getName()為鍵的值，從文件中可以看到是org.springframework.web.servlet.handler.BeanNameUrlHandlerMapping和org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerMapping兩個工廠實現類；
- 其他組件的策略同理，在此不再贅述。
總結
至此，本文完成了對DispatcherServlet的init方法的分析，它已準備好提供服務了，對請求處理的分析請看后續文章。
