之前分析过SpringMVC中的DispatcherServlet，分析了SpringMVC处理请求的过程。但忽略了一些DispatcherServlet协助请求处理的组件，例如SpringMVC中的HandlerMapping、HandlerAdapter、ViewResolvers等等。
HandlerMappings
HandlerMappings在DispathServlet中主要作用是为请求的urlpath匹配对应的Controller，建立一个映射关系，根据请求查找Handler、Interceptor。HandlerMappings将请求传递到HandlerExecutionChain上，HandlerExecutionChain包含了一个能够处理该请求的处理器，还可以包含拦截改请求的拦截器。
在没有处理器映射相关配置情况下，DispatcherServlet会为你创建一个BeanNameUrlHandlerMapping作为默认映射的配置。在DispatchServlet.properties文件中对于HandlerMapping的默认配置是：
org.springframework.web.servlet.HandlerMapping=org.springframework.web.servlet.handler.BeanNameUrlHandlerMapping,\
org.springframework.web.servlet.mvc.annotation.DefaultAnnotationHandlerMapping
HandlerMapping的配置策略一般分为配置式BeanNameUrlHandlerMapping和注解式DefaultAnnotationHandlerMapping。不过DefaultAnnotationHandlerMapping已经被放弃了，取代它的是RequestMappingHandlerMapping，不知道为啥SpringMVC这个默认配置尚未做修改。
AbstractHandlerMapping
AbstractHandlerMapping是HandlerMapping的抽象实现，是所有HandlerMapping实现类的父类。
AbstractHandlerMapping的作用是是为了初始化Interceptors。AbstractHandlerMapping重写了WebApplicationObjectSupport的initApplicationContext方法。
protected void initApplicationContext() throws BeansException {
extendInterceptors(this.interceptors);
detectMappedInterceptors(this.adaptedInterceptors);
initInterceptors();
}
extendInterceptors方法，Springmvc并没有做出具体实现，这里留下一个拓展，子类可以重写这个模板方法，为子类添加或者修改Interceptors。detectMappedInterceptors方法将SpringMVC容器中所有MappedInterceptor类的bean添加到adaptedInterceptors中。最后调用
initInterceptors初始化拦截器。遍历interceptors将WebRequestInterceptor和HandlerInterceptor类型的拦截器添加到adaptedInterceptors中。
HandlerMapping通过getHandler方法来获取请求的处理器Handler和拦截器Interceptor。在getHandlerExecutionChain方法中将遍历之前初始化的adaptedInterceptors，为当前的请求选择对应的MappedInterceptors和adaptedInterceptors。
AbstractUrlHandlerMapping
AbstractUrlHandlerMapping
AbstractUrlHandlerMapping继承于AbstractHandlerMapping，它是通过URL来匹配具体的Handler。AbstractUrlHandlerMapping维护一个handlerMap来存储Url和Handler的映射关系。
AbstractUrlHandlerMapping重写了AbstractHandlerMapping类中的getHandlerInternal方法。HandlerMapping通过getHandler方法，就会调用这里的getHandlerInternal方法来获取Handler。getHandlerInternal方法中关键调用lookupHandler方法去获取handler。
protected Object lookupHandler(String urlPath, HttpServletRequest request) throws Exception {
// Direct match?
Object handler = this.handlerMap.get(urlPath);
if (handler != null) {
// Bean name or resolved handler?
if (handler instanceof String) {
String handlerName = (String) handler;
handler = getApplicationContext().getBean(handlerName);
}
validateHandler(handler, request);
return buildPathExposingHandler(handler, urlPath, urlPath, null);
}
// Pattern match?
List<String> matchingPatterns = new ArrayList<String>();
for (String registeredPattern : this.handlerMap.keySet()) {
if (getPathMatcher().match(registeredPattern, urlPath)) {
matchingPatterns.add(registeredPattern);
}
else if (useTrailingSlashMatch()) {
if (!registeredPattern.endsWith("/") && getPathMatcher().match(registeredPattern + "/", urlPath)) {
matchingPatterns.add(registeredPattern +"/");
}
}
}
String bestMatch = null;
Comparator<String> patternComparator = getPathMatcher().getPatternComparator(urlPath);
if (!matchingPatterns.isEmpty()) {
Collections.sort(matchingPatterns, patternComparator);
if (logger.isDebugEnabled()) {
logger.debug("Matching patterns for request [" + urlPath + "] are " + matchingPatterns);
}
bestMatch = matchingPatterns.get(0);
}
if (bestMatch != null) {
handler = this.handlerMap.get(bestMatch);
if (handler == null) {
if (bestMatch.endsWith("/")) {
handler = this.handlerMap.get(bestMatch.substring(0, bestMatch.length() - 1));
}
if (handler == null) {
throw new IllegalStateException(
"Could not find handler for best pattern match [" + bestMatch + "]");
}
}
// Bean name or resolved handler?
if (handler instanceof String) {
String handlerName = (String) handler;
handler = getApplicationContext().getBean(handlerName);
}
validateHandler(handler, request);
String pathWithinMapping = getPathMatcher().extractPathWithinPattern(bestMatch, urlPath);
// There might be multiple 'best patterns', let's make sure we have the correct URI template variables
// for all of them
Map<String, String> uriTemplateVariables = new LinkedHashMap<String, String>();
for (String matchingPattern : matchingPatterns) {
if (patternComparator.compare(bestMatch, matchingPattern) == 0) {
Map<String, String> vars = getPathMatcher().extractUriTemplateVariables(matchingPattern, urlPath);
Map<String, String> decodedVars = getUrlPathHelper().decodePathVariables(request, vars);
uriTemplateVariables.putAll(decodedVars);
}
}
if (logger.isDebugEnabled()) {
logger.debug("URI Template variables for request [" + urlPath + "] are " + uriTemplateVariables);
}
return buildPathExposingHandler(handler, bestMatch, pathWithinMapping, uriTemplateVariables);
}
// No handler found...
return null;
}
- 首先调用
lookupHandler方法来获取handler。在lookupHandler方法中，先通过URL在handlerMap查找是否有合适的handler。 - 如果没有获取到
handler，遍历handlerMap利用正则匹配的方法，找到符合要求的handlers(有可能是多个)。 - 正则匹配是采用
Ant风格，将会通过排序筛选出一个匹配程度最高的Handler。 - 最后调用
buildPathExposingHandler方法构建一个handler,添加PathExposingHandlerInterceptor和UriTemplateVariablesHandlerInterceptor两个拦截器并返回。
上面介绍获取handler的过程中，会先从handlerMap查找。下面看一下handlerMap是如何初始化的。AbstractUrlHandlerMapping是通过registerHandler初始化handlerMap的。AbstractUrlHandlerMapping共有两个registerHandler方法。分别是注册多个url到一个handler和注册一个url到一个handler。首先判断handlerMap是否有此handler。如果存在的话，判断是否一致，不一致则抛出异常，如果不存在的话，如果url是/、/*，则，返回root handler、default handler，如果不是将添加到handlerMap中。
SimpleUrlHandlerMapping
SimpleUrlHandlerMapping继承于AbstractUrlHandlerMapping。SimpleUrlHandlerMapping重写了父类AbstractHandlerMapping中的初始化方法initApplicationContext。在initApplicationContext方法中调用registerHandlers方法。
protected void registerHandlers(Map<String, Object> urlMap) throws BeansException {
if (urlMap.isEmpty()) {
logger.warn("Neither 'urlMap' nor 'mappings' set on SimpleUrlHandlerMapping");
}
else {
for (Map.Entry<String, Object> 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);
}
}
}
判断是url是否以/开头，如果不是，默认补齐/,确保所有的url都是以/开头，然后依次调用父类的registerHandler方法注册到AbstractUrlHandlerMapping中的handlerMap。
在使用SimpleUrlHandlerMapping时，需要在注册的时候配置其urlmap否则会抛异常。
AbstractDetectingUrlHandlerMapping
AbstractDetectingUrlHandlerMapping类继承于AbstractUrlHandlerMapping类，重写了initApplicationContext方法，在initApplicationContext方法中调用了detectHandlers方法。
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");
}
}
}
}
获取所有容器的beanNames，遍历所有的beanName，调用determineUrlsForHandler方法解析url，这里的determineUrlsForHandler也是运用了模板方法设计模式，具体的实现在其子类中，如果解析到子类，将注册到父类的handlerMap中。
BeanNameUrlHandlerMapping
BeanNameUrlHandlerMapping类的类图大致如下：
BeanNameUrlHandlerMapping类继承于AbstractDetectingUrlHandlerMapping类。重写了父类中的determineUrlsForHandler方法。
protected String[] determineUrlsForHandler(String beanName) {
List<String> urls = new ArrayList<String>();
if (beanName.startsWith("/")) {
urls.add(beanName);
}
String[] aliases = getApplicationContext().getAliases(beanName);
for (String alias : aliases) {
if (alias.startsWith("/")) {
urls.add(alias);
}
}
return StringUtils.toStringArray(urls);
}
其通过beanName解析Url规则也很简单，判断beanName是否以/开头。
BeanNameUrlHandlerMapping是SpringMVC的默认映射配置。
AbstractHandlerMethodMapping
通常我们也习惯于用@Controller、@Re questMapping来定义Handler，AbstractHandlerMethodMapping可以将method作为Handler来使用。
AbstractHandlerMethodMapping实现了InitializingBean接口，实现了afterPropertiesSet方法。当容器启动的时候会调用initHandlerMethods注册委托handler中的方法。
public void afterPropertiesSet() {
initHandlerMethods();
}
protected void initHandlerMethods() {
if (logger.isDebugEnabled()) {
logger.debug("Looking for request mappings in application context: " + getApplicationContext());
}
String[] beanNames = (this.detectHandlerMethodsInAncestorContexts ?
BeanFactoryUtils.beanNamesForTypeIncludingAncestors(getApplicationContext(), Object.class) :
getApplicationContext().getBeanNamesForType(Object.class));
for (String beanName : beanNames) {
if (!beanName.startsWith(SCOPED_TARGET_NAME_PREFIX)) {
Class<?> beanType = null;
try {
beanType = getApplicationContext().getType(beanName);
}
catch (Throwable ex) {
// An unresolvable bean type, probably from a lazy bean - let's ignore it.
if (logger.isDebugEnabled()) {
logger.debug("Could not resolve target class for bean with name '" + beanName + "'", ex);
}
}
if (beanType != null && isHandler(beanType)) {
detectHandlerMethods(beanName);
}
}
}
handlerMethodsInitialized(getHandlerMethods());
}
在initHandlerMethods方法中，做了以下工作：
- 首先通过
BeanFactoryUtils扫描应用上下文，获取所有的bean。 - 遍历所有的
beanName，调用isHandler方法判断是目标bean是否包含@Controller或@RequestMapping注解。 - 对于带有
@Controller或@RequestMapping注解的类，调用detectHandlerMethods委托处理,获取所有的method，并调用registerHandlerMethod注册所有的方法。
在detectHandlerMethods方法负责将Handler保存到Map中。
protected void detectHandlerMethods(final Object handler) {
// 获取handler的类型
Class<?> handlerType = (handler instanceof String ?
getApplicationContext().getType((String) handler) : handler.getClass());
final Class<?> userType = ClassUtils.getUserClass(handlerType);
Map<Method, T> methods = MethodIntrospector.selectMethods(userType,
new MethodIntrospector.MetadataLookup<T>() {
@Override
public T inspect(Method method) {
try {
return getMappingForMethod(method, userType);
}
catch (Throwable ex) {
throw new IllegalStateException("Invalid mapping on handler class [" +
userType.getName() + "]: " + method, ex);
}
}
});
if (logger.isDebugEnabled()) {
logger.debug(methods.size() + " request handler methods found on " + userType + ": " + methods);
}
for (Map.Entry<Method, T> entry : methods.entrySet()) {
Method invocableMethod = AopUtils.selectInvocableMethod(entry.getKey(), userType);
T mapping = entry.getValue();
registerHandlerMethod(handler, invocableMethod, mapping);
}
}
selectMethods方法中重写了MetadataLookup中的inspect方法，inspect方法中调用了子类RequestMappingHandlerMapping实现了getMappingForMethod模板方法，用于构建RequestMappingInfo。
public static <T> Map<Method, T> selectMethods(Class<?> targetType, final MetadataLookup<T> metadataLookup) {
final Map<Method, T> methodMap = new LinkedHashMap<Method, T>();
Set<Class<?>> handlerTypes = new LinkedHashSet<Class<?>>();
Class<?> specificHandlerType = null;
if (!Proxy.isProxyClass(targetType)) {
handlerTypes.add(targetType);
specificHandlerType = targetType;
}
handlerTypes.addAll(Arrays.asList(targetType.getInterfaces()));
for (Class<?> currentHandlerType : handlerTypes) {
final Class<?> targetClass = (specificHandlerType != null ? specificHandlerType : currentHandlerType);
ReflectionUtils.doWithMethods(currentHandlerType, new ReflectionUtils.MethodCallback() {
@Override
public void doWith(Method method) {
Method specificMethod = ClassUtils.getMostSpecificMethod(method, targetClass);
T result = metadataLookup.inspect(specificMethod);
if (result != null) {
Method bridgedMethod = BridgeMethodResolver.findBridgedMethod(specificMethod);
if (bridgedMethod == specificMethod || metadataLookup.inspect(bridgedMethod) == null) {
methodMap.put(specificMethod, result);
}
}
}
}, ReflectionUtils.USER_DECLARED_METHODS);
}
return methodMap;
}
在selectMethods通过反射获取所有的方法，重写了doWith方法，将handler中的method和请求对应的RequestMappingInfo保存到methodMap中。
最终detectHandlerMethods将遍历这个methodMap，调用registerHandlerMethod注册HandlerMethod到MappingRegistry。
在AbstractHandlerMethodMapping类中，有个内部类MappingRegistry，用来存储mapping和 handler methods注册关系，并提供了并发访问方法。
AbstractHandlerMethodMapping通过getHandlerInternal来为一个请求选择对应的handler。
protected HandlerMethod getHandlerInternal(HttpServletRequest request) throws Exception {
// 根据request获取对应的urlpath
String lookupPath = getUrlPathHelper().getLookupPathForRequest(request);
if (logger.isDebugEnabled()) {
logger.debug("Looking up handler method for path " + lookupPath);
}
// 获取读锁
this.mappingRegistry.acquireReadLock();
try {
// 调用lookupHandlerMethod方法获取请求对应的HandlerMethod
HandlerMethod handlerMethod = lookupHandlerMethod(lookupPath, request);
if (logger.isDebugEnabled()) {
if (handlerMethod != null) {
logger.debug("Returning handler method [" + handlerMethod + "]");
}
else {
logger.debug("Did not find handler method for [" + lookupPath + "]");
}
}
return (handlerMethod != null ? handlerMethod.createWithResolvedBean() : null);
}
finally {
this.mappingRegistry.releaseReadLock();
}
}
lookupHandlerMethod的具体实现如下：
protected HandlerMethod lookupHandlerMethod(String lookupPath, HttpServletRequest request) throws Exception {
List<Match> matches = new ArrayList<Match>();
// 通过lookupPath获取所有匹配到的path
List<T> directPathMatches = this.mappingRegistry.getMappingsByUrl(lookupPath);
if (directPathMatches != null) {
// 将匹配条件添加到matches
addMatchingMappings(directPathMatches, matches, request);
}
if (matches.isEmpty()) {
// 如果没有匹配条件，将所有的匹配条件都加入matches
addMatchingMappings(this.mappingRegistry.getMappings().keySet(), matches, request);
}
if (!matches.isEmpty()) {
Comparator<Match> comparator = new MatchComparator(getMappingComparator(request));
Collections.sort(matches, comparator);
if (logger.isTraceEnabled()) {
logger.trace("Found " + matches.size() + " matching mapping(s) for [" +
lookupPath + "] : " + matches);
}
// 选取排序后的第一个作为最近排序条件
Match bestMatch = matches.get(0);
if (matches.size() > 1) {
if (CorsUtils.isPreFlightRequest(request)) {
return PREFLIGHT_AMBIGUOUS_MATCH;
}
Match secondBestMatch = matches.get(1);
// 前两个匹配条件排序一样抛出异常
if (comparator.compare(bestMatch, secondBestMatch) == 0) {
Method m1 = bestMatch.handlerMethod.getMethod();
Method m2 = secondBestMatch.handlerMethod.getMethod();
throw new IllegalStateException("Ambiguous handler methods mapped for HTTP path '" +
request.getRequestURL() + "': {" + m1 + ", " + m2 + "}");
}
}
// 将lookupPath设为请求request的PATH_WITHIN_HANDLER_MAPPING_ATTRIBUTE属性
handleMatch(bestMatch.mapping, lookupPath, request);
return bestMatch.handlerMethod;
}
else {
return handleNoMatch(this.mappingRegistry.getMappings().keySet(), lookupPath, request);
}
}
整个过程以Match作为载体，Match是个内部类，封装了匹配条件和handlerMethod两个属性，默认的实现是将lookupPath设置为请求的属性。
总结
本文从源码角度上分析了HandlerMapping的各种实现。主要功能是为请求找到合适的handler和interceptors，并组合成HandlerExecutionChain。查找handler的过程通过getHandlerInternal方法实现，每个子类都其不同的实现。
所有的HandlerMapping的实现都继承于AbstarctHandlerMapping，AbstarctHandlerMapping主要作用是完成拦截器的初始化工作。而通过AbstarctHandlerMapping又衍生出两个系列，AbstractUrlHandlerMapping和AbstractHandlerMethodMapping。
AbstractUrlHandlerMapping也有很多子类的实现，如SimpleUrlHandlerMapping、AbstractDetectingUrlHandlerMapping。总体来说，AbstractUrlHandlerMapping需要用到一个保存url和handler的对应关系的map，map的初始化工作由子类实现。不同的子类会有自己的策略，可以在配置文件中注册，也可以在spring容器中找。
AbstractHandlerMethodMapping系列则通常用于注解的方法，解析包含@Controller或者@RequestMapping注解的类，建立url和method的直接对应关系，这也是目前使用最多的一种方式。
