springMVC--HandlerAdapter

HandlerAdapter的接口中定义了三个方法：

（1）boolean supports(Object handler); 判断是否支持传入的Handler

（2）ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)  用来使用Handler处理请求

（3）long getLastModified(HttpServletRequest request, Object handler); 用来获取资料的Last-Modified值。

接口源码如下：

public interface HandlerAdapter {  
  
    boolean supports(Object handler);  
  
    ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception;  
  
    long getLastModified(HttpServletRequest request, Object handler);  
  
}  

HandlerAdapter的执行操作，其执行过程在DispatcherServlet的doDispatch中，执行流程如下：

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {  
          
        ........  
  
        try {  
              
            try {  
                  
                //获取合适的HandlerAdapter实现类  
                HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());  
                  
            ........  
                  
                if (isGet || "HEAD".equals(method)) {  
                    long lastModified = ha.getLastModified(request, mappedHandler.getHandler());  
                      
                }  
            ........  
                //执行真正的请求操作  
                mv = ha.handle(processedRequest, response, mappedHandler.getHandler());  
  
        ........  
    }  

getHandlerAdapter的操作就是选择合适的HandlerAdapter来执行，设计模式中的适配器模式，handlerAdapters中的内容就是所有的HandlerAdapter的实现类。

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 +  
                "]: The DispatcherServlet configuration needs to include a HandlerAdapter that supports this handler");  
    }  

这样就完成了Handler的真正调用过程，最终调用过程是返回一个ModelAndView对象。