如何获取spring handlermapping

在基于Spring MVC的Web应用程序中，我们可以为DispatcherServlet提供多个Handler- Mapping供其使用。DispatcherServlet在选用HandlerMapping的过程中，将根据我们所指定的一系列HandlerMapping的优先级进行排序，然后优先使用优先级在前的HandlerMapping。如果当前的HandlerMapping能够返回可用的Handler，DispatcherServlet则使用当前返回的Handler进行Web请求的处理，而不再继续询问其他的HandlerMapping。否则，DispatcherServlet将继续按照各个HandlerMapping的优先级进行询问，直到获取一个可用的Handler为止。
HandlerMapping的优先级规定遵循Spring框架内一贯的Ordered接口所规定的语义。Spring MVC中可用的HandlerMapping实现全都实现了Ordered接口。假设我们优先使用SimpleUrl- HandlerMapping进行Handler的映射管理，其次使用BeanNameUrlHandlerMapping，那么就可以在DispatcherServlet特定的WebApplicationContext中增加如代码清单24-3所示的配置内容。
代码清单24-3　HandlerMapping指定优先级配置代码示例




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class="org.springframework.Web.servlet.
handler.BeanNameUrlHandlerMapping">

如果不为HandlerMapping明确指定order，那么默认值为Integer.MAX_VALUE，对应最低优先级。所以，拥有order值为1的SimpleUrlHandlerMapping较之BeanNameUrlHandlerMapping优先被调用。

SimpleUrlHandlerMapping中的注册实现代码：

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protected void registerHandlers(Map urlMap) throws BeansException {
if (urlMap.isEmpty()) {
logger.warn("Neither 'urlMap' nor 'mappings' set on SimpleUrlHandlerMapping");
}
else {
//urlMap信息是根据配置文件注入进来的
for (Map.Entry entry : urlMap.entrySet()) {
String url = entry.getKey();
Object handler = entry.getValue();
// Prepend with slash if not already present.
if (!url.startsWith("/")) {
url = "/" + url;
}
// Remove whitespace from handler bean name.
if (handler instanceof String) {
handler = ((String) handler).trim();
}
registerHandler(url, handler);
}
}
}

AbstractDetectingUrlHandlerMapping中的注册实现代码：

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protected void detectHandlers() throws BeansException {
if (logger.isDebugEnabled()) {
logger.debug("Looking for URL mappings in application context: " + getApplicationContext());
}
String[] beanNames = (this.detectHandlersInAncestorContexts ?
BeanFactoryUtils.beanNamesForTypeIncludingAncestors(getApplicationContext(), Object.class) :
getApplicationContext().getBeanNamesForType(Object.class));

// Take any bean name that we can determine URLs for.
for (String beanName : beanNames) {
String[] urls = determineUrlsForHandler(beanName);
if (!ObjectUtils.isEmpty(urls)) {
// URL paths found: Let's consider it a handler.
registerHandler(urls, beanName);
}
else {
if (logger.isDebugEnabled()) {
logger.debug("Rejected bean name '" + beanName + "': no URL paths identified");
}
}
}
}

单个的映射关系注册是在registerHandler方法中执行的：

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protected void registerHandler(String urlPath, Object handler) throws BeansException, IllegalStateException {
Assert.notNull(urlPath, "URL path must not be null");
Assert.notNull(handler, "Handler object must not be null");
Object resolvedHandler = handler;

// Eagerly resolve handler if referencing singleton via name.
//如果给定的handeler是字符串，则认为是bean name，直接到IoC容器中取得bean instance
if (!this.lazyInitHandlers && handler instanceof String) {
String handlerName = (String) handler;
if (getApplicationContext().isSingleton(handlerName)) {
resolvedHandler = getApplicationContext().getBean(handlerName);
}
}

Object mappedHandler = this.handlerMap.get(urlPath);
//同一path不能对应多个处理对象
if (mappedHandler != null) {
if (mappedHandler != resolvedHandler) {
throw new IllegalStateException(
"Cannot map handler [" + handler + "] to URL path [" + urlPath +
"]: There is already handler [" + resolvedHandler + "] mapped.");
}
}
else {
if (urlPath.equals("/")) {
if (logger.isInfoEnabled()) {
logger.info("Root mapping to handler [" + resolvedHandler + "]");
}
setRootHandler(resolvedHandler);
}
else if (urlPath.equals("/*")) {
if (logger.isInfoEnabled()) {
logger.info("Default mapping to handler [" + resolvedHandler + "]");
}
setDefaultHandler(resolvedHandler);
}
else {
//将path和handler放在handlerMap中，保存了它们之间的映射关系
this.handlerMap.put(urlPath, resolvedHandler);
if (logger.isInfoEnabled()) {
logger.info("Mapped URL path [" + urlPath + "] onto handler [" + resolvedHandler + "]");
}
}
}
}

通过以上代码，在HandlerMapping对象被创建的时候，即完成了url到handler之间的映射关系的注册。handlerMap已经被赋值，可以被使用了。
在完成url到handler映射关系的注册后，就可以使用getHandler方法，根据http请求获得handler对象了。下面，首先看一下getHandler方法的调用时序：

下面就主要关注一下上图中涉及到的方法的代码实现。

[java] view plain copy
public final HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
Object handler = getHandlerInternal(request);
//如果没有找到匹配的handler，则使用默认handler
if (handler == null) {
handler = getDefaultHandler();
}
if (handler == null) {
return null;
}
// Bean name or resolved handler?
//如果给定的handler是字符串类型，则认为是bean name
if (handler instanceof String) {
String handlerName = (String) handler;
handler = getApplicationContext().getBean(handlerName);
}
//如果Handler是HandlerExecutionChain类型，则只是添加拦截器，否则会创建一个新的HandlerExecutionChain
return getHandlerExecutionChain(handler, request);
}

将url信息与handler进行匹配查找的操作是在lookupHandler方法中执行的，下面就来看一下lookupHandler方法的实现：

[java] view plain copy
//这个方法可能的返回值是HandlerExecutionChain对象或者是null
//在HandlerExecutionChain对象中的handler，是根据handlerMap中取出来的bean name获得到的bean instance
protected Object lookupHandler(String urlPath, HttpServletRequest request) throws Exception {
// Direct match?
//直接匹配
Object handler = this.handlerMap.get(urlPath);
if (handler != null) {
// Bean name or resolved handler?
if (handler instanceof String) {
String handlerName = (String) handler;
//从IoC容器中取出handler
handler = getApplicationContext().getBean(handlerName);
}
validateHandler(handler, request);
//创建一个HandlerExecutionChain对象并返回
return buildPathExposingHandler(handler, urlPath, urlPath, null);
}
// Pattern match?
//根据一定的模式匹配规则
List matchingPatterns = new ArrayList();
for (String registeredPattern : this.handlerMap.keySet()) {
if (getPathMatcher().match(registeredPattern, urlPath)) {
matchingPatterns.add(registeredPattern);
}
}
String bestPatternMatch = null;
if (!matchingPatterns.isEmpty()) {
Collections.sort(matchingPatterns, getPathMatcher().getPatternComparator(urlPath));
if (logger.isDebugEnabled()) {
logger.debug("Matching patterns for request [" + urlPath + "] are " + matchingPatterns);
}
bestPatternMatch = matchingPatterns.get(0);
}
if (bestPatternMatch != null) {
//处理最佳匹配
handler = this.handlerMap.get(bestPatternMatch);
// Bean name or resolved handler?
if (handler instanceof String) {
String handlerName = (String) handler;
handler = getApplicationContext().getBean(handlerName);
}
validateHandler(handler, request);
String pathWithinMapping = getPathMatcher().extractPathWithinPattern(bestPatternMatch, urlPath);
Map uriTemplateVariables =
getPathMatcher().extractUriTemplateVariables(bestPatternMatch, urlPath);
//返回一个HandlerExecutionChain对象
return buildPathExposingHandler(handler, bestPatternMatch, pathWithinMapping, uriTemplateVariables);
}
// No handler found...
return null;
}