SpringMVC中的HandlerAdapter
還記得在DispatcherServlet中SpringMVC處理請求的邏輯嗎，網上有一個非常棒的圖，闡明瞭請求分發處理的整個過程。
映射處理器HandlerMapping根據Request返回一個HandlerExecutionChain實例，HandlerExecutionChain並不是真正的處理器Handler，而是包含了Handler以及攔截器HandlerInterceptor的封裝後的調用鏈對象。系統中可能存在多個HandlerMapping，不同的HandlerMapping對應着不同的策略。DispatcherServlet根據先後順序，哪個HandlerMapping最先找到處理器，則使用當前的處理器並且立即停止繼續查找。
DispatcherServlet獲取到處理器Handler後，它並不知道如何使用這個處理器。因爲處理器可能多種多樣，沒有遵循一種標準，它只是一個Object。所以這裏需要多Handler進行適配，使得所有的處理器共同擁有一個標準的接口處理請求。Spring使用的適配器的名字就是HandlerAdapter，看它的定義：
public interface HandlerAdapter {
boolean supports(Object handler);
ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception;
long getLastModified(HttpServletRequest request, Object handler);
} protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {
for (HandlerAdapter ha : this.handlerAdapters) {
if (logger.isTraceEnabled()) {
logger.trace("Testing handler adapter [" + ha + "]");
}
if (ha.supports(handler)) {
return ha;
}
}
throw new ServletException("No adapter for handler [" + handler +
"]: Does your handler implement a supported interface like Controller?");
}HandlerAdapter的初始化
和HandlerMapping一樣，HandlerAdapter的初始化也在DispatcherServlet的initStrategies方法中，具體初始化方法如下：
private void initHandlerAdapters(ApplicationContext context) {
this.handlerAdapters = null;
if (this.detectAllHandlerAdapters) {
// Find all HandlerAdapters in the ApplicationContext, including ancestor contexts.
Map<String, HandlerAdapter> matchingBeans =
BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerAdapter.class, true, false);
if (!matchingBeans.isEmpty()) {
this.handlerAdapters = new ArrayList<HandlerAdapter>(matchingBeans.values());
// We keep HandlerAdapters in sorted order.
OrderComparator.sort(this.handlerAdapters);
}
}
else {
try {
HandlerAdapter ha = context.getBean(HANDLER_ADAPTER_BEAN_NAME, HandlerAdapter.class);
this.handlerAdapters = Collections.singletonList(ha);
}
catch (NoSuchBeanDefinitionException ex) {
// Ignore, we'll add a default HandlerAdapter later.
}
}
// Ensure we have at least some HandlerAdapters, by registering
// default HandlerAdapters if no other adapters are found.
if (this.handlerAdapters == null) {
this.handlerAdapters = getDefaultStrategies(context, HandlerAdapter.class);
if (logger.isDebugEnabled()) {
logger.debug("No HandlerAdapters found in servlet '" + getServletName() + "': using default");
}
}
}若容器中沒有找到HandlerAdapter，則spring會加載使用默認的HandlerAdapter。其實不僅HandlerAdapter是這樣，DispatcherServlet的一些其它組件在某些情況也會使用默認的配置。它們通過一個共同的方法獲取：
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>();
}
}這個defaultStrategies加載自配置文件，加載過程如下
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());
}
}找到對應位置的配置文件，並且找到關於HandlerAdapter的配置
org.springframework.web.servlet.HandlerAdapter=org.springframework.web.servlet.mvc.HttpRequestHandlerAdapter,\
org.springframework.web.servlet.mvc.SimpleControllerHandlerAdapter,\
org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter如上所示，容器中HandlerAdapter的時候就會使用配置好的上面的三種HandlerAdapter。如果在spring的配置文件中加入 <mvc:annotation-driven />，則它的實現類AnnotationDrivenBeanDefinitionParser會自動向容器註冊RequestMappingHandlerAdapter和RequestMappingHandlerMapping。
