Spring MVC 源码学习札记（四）HandlerMapping和HandlerAdapter（2）
上一节我们看到 当***-serlvet.xml中未声明HandlerMapping的实体bean时，默认构造
org.springframework.web.servlet.handler.BeanNameUrlHandlerMapping和org.springframework.web.servlet.mvc.annotation.DefaultAnnotationHandlerMapping
的实例。
所以本节就从这两个类入手研究，HandlerMapping中最重要的方法是 getHandler 用于获取HandlerExcutionChain的对象。
先来看BeanNameUrlHandlerMapping类，它的内部没有实现getHandler方法，而它继承自AbstractDetectingUrlHandlerMapping，所以自底向上我们去查看AbstractDetectingUrlHandlerMapping类的实现。
在这个类中，我看到了一个方法
/**
* Register all handlers found in the current ApplicationContext.
* <p>The actual URL determination for a handler is up to the concrete
* {@link #determineUrlsForHandler(String)} implementation. A bean for
* which no such URLs could be determined is simply not considered a handler.
* @throws org.springframework.beans.BeansException if the handler couldn't be registered
* @see #determineUrlsForHandler(String)
*/
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");
}
}
}
}whether to detect handler beans in ancestor ApplicationContexts.
private boolean detectHandlersInAncestorContexts = false;
该方法首先查询了上下文中所有的bean，因为查询的Type是Object.class，得到了所有的bean名称，之后解析bean名称，查看是否存是urlName，determineUrlsForHandler方法在BeanNameUrlHandlerMapping类中具体实现：
/**
* Checks name and aliases of the given bean for URLs, starting with "/".
*/
@Override
protected String[] determineUrlsForHandler(String beanName) {
List<String> urls = new ArrayList<String>();
if (beanName.startsWith("/")) {
urls.add(beanName);
}
String[] aliases = getApplicationContext().getAliases(beanName);
for (int i = 0; i < aliases.length; i++) {
if (aliases[i].startsWith("/")) {
urls.add(aliases[i]);
}
}
return StringUtils.toStringArray(urls);
}可以看出它查找了所有beanName以“/”开头的bean，将它们的name作为url返回。
接下来就对那些作为处理url请求的bean进行handler的注册，registerHandler方法在AbstractDetectingUrlHandlerMapping的父类AbstractUrlHandlerMapping类中：
/**
* Register the specified handler for the given URL paths.
* @param urlPaths the URLs that the bean should be mapped to
* @param beanName the name of the handler bean
* @throws BeansException if the handler couldn't be registered
* @throws IllegalStateException if there is a conflicting handler registered
*/
protected void registerHandler(String[] urlPaths, String beanName) throws BeansException, IllegalStateException {
Assert.notNull(urlPaths, "URL path array must not be null");
for (String urlPath : urlPaths) {
registerHandler(urlPath, beanName);
}
} /**
* Register the specified handler for the given URL path.
* @param urlPath the URL the bean should be mapped to
* @param handler the handler instance or handler bean name String
* (a bean name will automatically be resolved into the corresponding handler bean)
* @throws BeansException if the handler couldn't be registered
* @throws IllegalStateException if there is a conflicting handler registered
*/
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.
if (!this.lazyInitHandlers && handler instanceof String) {
String handlerName = (String) handler;
if (getApplicationContext().isSingleton(handlerName)) {
resolvedHandler = getApplicationContext().getBean(handlerName);
}
}
Object mappedHandler = this.handlerMap.get(urlPath);
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 {
this.handlerMap.put(urlPath, resolvedHandler);
if (logger.isInfoEnabled()) {
logger.info("Mapped URL path [" + urlPath + "] onto handler [" + resolvedHandler + "]");
}
}
}
}handlerMap就是一个K-V保存url对应的handler，初始时我们传入的Object handler参数是String对象beanNames，lazyInit表示是延迟初始化handler，如果设为true那么在注册handler的时候我们存入的handler还是String，如果是false，该方法中被用来取得bean实例，所以handlerMap中存放的V就是bean的对象，使用Object泛型是因为bean的不一致。
private final Map<String, Object> handlerMap = new LinkedHashMap<String, Object>();
注意上面的setDefaultHandler方法，它是AbstractUrlHandlerMapping类的父类AbstractHandlerMapping中的方法，在AbstractHandlerMapping中有一个Object对象属性：
private Object defaultHandler;
从字面意思可以理解为默认的处理器，因为它对应的映射是"/*"，即处理所有符合该表达式的url请求。
阅读完BeanNameUrlHandlerMapping和AbstractDetectingUrlHandlerMapping两个类后，我们发现它们中的方法主要进行了handler的set，而handler的get却没有在它们中体现，所以再向上走，我们查看最高的父类AbstractHandlerMapping类，它直接实现了HandlerMapping接口，所以剖析它的getHandler方法更容易让我们理解，handler获取的流程。
public final HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
Object handler = getHandlerInternal(request);
if (handler == null) {
handler = getDefaultHandler();
}
if (handler == null) {
return null;
}
// Bean name or resolved handler?
if (handler instanceof String) {
String handlerName = (String) handler;
handler = getApplicationContext().getBean(handlerName);
}
return getHandlerExecutionChain(handler, request);
}该方法首先调用了getHandlerInternal处理request，getHandlerInternal是其子类AbstractUrlHandlerMapping实现的方法，用来查询handlerMap查找request的url对应的handler。
@Override
protected Object getHandlerInternal(HttpServletRequest request) throws Exception {
String lookupPath = this.urlPathHelper.getLookupPathForRequest(request);
Object handler = lookupHandler(lookupPath, request);
if (handler == null) {
// We need to care for the default handler directly, since we need to
// expose the PATH_WITHIN_HANDLER_MAPPING_ATTRIBUTE for it as well.
Object rawHandler = null;
if ("/".equals(lookupPath)) {
rawHandler = getRootHandler();
}
if (rawHandler == null) {
rawHandler = getDefaultHandler();
}
if (rawHandler != null) {
// Bean name or resolved handler?
if (rawHandler instanceof String) {
String handlerName = (String) rawHandler;
rawHandler = getApplicationContext().getBean(handlerName);
}
validateHandler(rawHandler, request);
handler = buildPathExposingHandler(rawHandler, lookupPath, lookupPath, null);
}
}
if (handler != null && this.mappedInterceptors != null) {
Set<HandlerInterceptor> mappedInterceptors =
this.mappedInterceptors.getInterceptors(lookupPath, this.pathMatcher);
if (!mappedInterceptors.isEmpty()) {
HandlerExecutionChain chain;
if (handler instanceof HandlerExecutionChain) {
chain = (HandlerExecutionChain) handler;
} else {
chain = new HandlerExecutionChain(handler);
}
chain.addInterceptors(mappedInterceptors.toArray(new HandlerInterceptor[mappedInterceptors.size()]));
}
}
if (handler != null && logger.isDebugEnabled()) {
logger.debug("Mapping [" + lookupPath + "] to handler '" + handler + "'");
}
else if (handler == null && logger.isTraceEnabled()) {
logger.trace("No handler mapping found for [" + lookupPath + "]");
}
return handler;
}下面来分析getHandlerInternal方法内部，第一句使用类中UrlPathHelper对象urlPathHelper调用getLookupPathForRequest方法解析request，从而得到请求的urlPath。
第二句调用lookupHandler方法,用于查找url path对应的handler，从上面的regist我们可以很快理解，这里肯定是在handlerMap中进行查找，它是AbstractUrlHandlerMapping实现的方法：
/**
*Look up a handler instance for the given URL path.
*/
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);
}
}
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) {
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<String, String> uriTemplateVariables =
getPathMatcher().extractUriTemplateVariables(bestPatternMatch, urlPath);
return buildPathExposingHandler(handler, bestPatternMatch, pathWithinMapping, uriTemplateVariables);
}
// No handler found...
return null;
}果不其然，从上面的注释//直接匹配 我们可以看到该方法查找handlerMap中key为urlPath的handler value。
当查找到后，判断我们取出的handler是否是String对象，因为前面我们已经说明了lazyInit，所以这里不难理解。如果是String那么就构造bean，如果不是代码就继续向下走，请允许我先跳过buildPathExposingHandler方法，因为后面还会用到，在这里不是至关重要的，等下再说明它。
当直接匹配没有成功的时候，就进行//模式匹配，从handlerMap的keySet中查找到最符合urlPath模式的key，从而得到handler，如果没有找到，那就返回null。
回到getHandlerInternal方法中，当我们没有查找到urlPath对应的handler时，也就是返回handler==null，则进入第一个if条件中，首先要检查两种情况，因为我们在注册时将"/"set到了rootHandler中，而将"/*"set到了defaultHandler中，当从这两种情况中的一种得到handler后，也是判断是否为String，如果是则构造bean，然后调用buildPathExposingHandler方法。
细心的朋友会注意到getDefaultHandler方法在AbstractHandlerMapping的getHandler方法也被调用了一次，这是当getHandlerInternal返回null时调用的。
到这里我们就来介绍一下buildPathExposingHandler方法：
protected Object buildPathExposingHandler(Object rawHandler,
String bestMatchingPattern,
String pathWithinMapping,
Map<String, String> uriTemplateVariables) {
HandlerExecutionChain chain = new HandlerExecutionChain(rawHandler);
chain.addInterceptor(new PathExposingHandlerInterceptor(bestMatchingPattern, pathWithinMapping));
if (!CollectionUtils.isEmpty(uriTemplateVariables)) {
chain.addInterceptor(new UriTemplateVariablesHandlerInterceptor(uriTemplateVariables));
}
return chain;
}该方法中，使用handler作为参数构造了一个HandlerExecutionChain的对象，不熟悉HandlerExecutionChain的朋友可以打开该类的源码，可以看到它保存了两个最重要的对象，一个handler和一组interceptor
private final Object handler; private HandlerInterceptor[] interceptors; private List<HandlerInterceptor> interceptorList;
然后调用addInterceptor方法，为chain添加了一个HandlerInterceptor----PathExposingHandlerInterceptor
public void addInterceptor(HandlerInterceptor interceptor) {
initInterceptorList();
this.interceptorList.add(interceptor);
} private void initInterceptorList() {
if (this.interceptorList == null) {
this.interceptorList = new ArrayList<HandlerInterceptor>();
}
if (this.interceptors != null) {
this.interceptorList.addAll(Arrays.asList(this.interceptors));
this.interceptors = null;
}
}当uriTemplateVariables（这是模式匹配时传入buildPathExposingHandler的Map）不为空时，又添加一个HandlerInterceptor-----UriTemplateVariablesHandlerInterceptor
上述的两个interceptor都是AbstractUrlHandlerMapping类的内部类，这里我们先不讨论拦截器，等后面的文章会进行详细阅读。
后面直到getHandler方法完结，均是对handler和handlerinceptor的拼装，就先不介绍了。后面会着重阅读HandlerInceptor接口及其子类。