SpringMVC API缓存 LastModified的实现总结

      Servlet的生态中，提供了一种友好机制用来提高系统的响应速度，当客户端查询的数据在没有变化的情况下直接返回，以减少服务器压力和网络传输的压力。Spring MVC也很好的继承了该机制，下面我们就学习下怎么使用LastModified。

1.handler是实现了LastModified的Servlet

public class MyServlet extends HttpServlet{
	
	/**
	 * 
	 */
	private static final long serialVersionUID = -2471386444207404619L;
	private static Long count= -1L;

	@Override
	public void doGet(HttpServletRequest req, HttpServletResponse resp) throws IOException
	{
		System.out.println("req....");
		count= System.currentTimeMillis();
		resp.getWriter().write("hello" +count);
	}
	@Override
	public long getLastModified(HttpServletRequest req) {
        return count;
    }
}

Spring-mvc.xml配置如下：

因为handler是Servlet，所以此处使用了BeanNameUrlHandlerMapping和SimpleServletHandlerAdapter。

然后在对该请求的时候，就能在request headers中发现If-Modified-Since头信息，response中发现Last-Modified。

2.handler继承AbstractController并实现LastModified接口

public class InfoController extends AbstractController implements LastModified{

	private static Long count= -1L;
	public String test() {
		return "hello";
	}

	@Override
	protected ModelAndView handleRequestInternal(HttpServletRequest request,
			HttpServletResponse response)
			throws Exception {
		count= System.currentTimeMillis();
		response.getWriter().write("hello" +count);
		return null;
	}

	@Override
	public long getLastModified(HttpServletRequest request) {
		// TODO Auto-generated method stub
		return count;
	}
	
}

 

因为handler是Controller，所以Adapter使用了SimpleControllerHandlerAdapter。

请求的时候在request headers中发现If-Modified-Since头信息，response中发现Last-Modified。

3.handler是使用注解的@RestController,url的匹配是@requestMapping

@RestController
@RequestMapping("/info")
public class JJController 
{
	private static Long count = 0L;

	@GetMapping("/jj")
	public String test(WebRequest request) {
		if(request.checkNotModified(count)) {
			return null;
		}
		System.out.println("******************************8");
		count = System.currentTimeMillis();
		return "hello";
	}

}

为了找到对应的处理器和正确处理请求，handlerMapping和handlerAdapter的配置如上。

 但是并没有实现LastModified接口，原因，请看源码RequestMappingHandlerAdapter(由于3.2版本之后，AnnotationMethodHandlerAdapter已经不被推荐使用，替代为RequestMappingHandlerAdapter)中的描述：

我们再看看 WebRequest中关于checkNotModified方法的说明：

/**
	 * Check whether the requested resource has been modified given the
	 * supplied last-modified timestamp (as determined by the application).
	 * 
This will also transparently set the "Last-Modified" response header
	 * and HTTP status when applicable.
	 * 
Typical usage:
	 * 
	 * public String myHandleMethod(WebRequest webRequest, Model model) {
	 *   long lastModified = // application-specific calculation
	 *   if (request.checkNotModified(lastModified)) {
	 *     // shortcut exit - no further processing necessary
	 *     return null;
	 *   }
	 *   // further request processing, actually building content
	 *   model.addAttribute(...);
	 *   return "myViewName";
	 * }
	 * 
This method works with conditional GET/HEAD requests, but
	 * also with conditional POST/PUT/DELETE requests.
	 * 
Note: you can use either
	 * this {@code #checkNotModified(long)} method; or
	 * {@link #checkNotModified(String)}. If you want enforce both
	 * a strong entity tag and a Last-Modified value,
	 * as recommended by the HTTP specification,
	 * then you should use {@link #checkNotModified(String, long)}.
	 * 
If the "If-Modified-Since" header is set but cannot be parsed
	 * to a date value, this method will ignore the header and proceed
	 * with setting the last-modified timestamp on the response.
	 * @param lastModifiedTimestamp the last-modified timestamp in
	 * milliseconds that the application determined for the underlying
	 * resource
	 * @return whether the request qualifies as not modified,
	 * allowing to abort request processing and relying on the response
	 * telling the client that the content has not been modified
	 */
	boolean checkNotModified(long lastModifiedTimestamp);

大意就是注解方式中，一个controller会有多个方法，我们只能给每个@requestMapping的方法进行一次判断，只要传入WebRequest参数，并调用其checkNotModified(lastModified)方法，为true则return null，表示内容没有被更改过，不需要重新查询。这个方法只能是在Get或者Head请求中适用。其中的例子代码如下：

 public String myHandleMethod(WebRequest webRequest, Model model) {
      long lastModified = // application-specific calculation
       if (request.checkNotModified(lastModified)) {
        // shortcut exit - no further processing necessary
         return null;
        }
       // further request processing, actually building content
       model.addAttribute(...);
       return "myViewName";
     }