servlet中service doGet doPost 的联系和区别

在servlet中默认情况下，无论你是get还是post 提交过来 都会经过service（）方法来处理，然后转向到doGet

或是doPost方法，可以看HttpServlet 类的service方法：
Java代码

   1. protected void service(HttpServletRequest req, HttpServletResponse resp)   
   2.        throws ServletException, IOException   
   3.     {   
   4.         String method = req.getMethod();   
   5.        if(method.equals("GET"))   
   6.         {   
   7.            long lastModified = getLastModified(req);   
   8.            if(lastModified == -1L)   
   9.             {   
  10.                 doGet(req, resp);   
  11.             } else  
  12.             {   
  13.                long ifModifiedSince = req.getDateHeader("If-Modified-Since");   
  14.                if(ifModifiedSince < (lastModified / 1000L) * 1000L)   
  15.                 {   
  16.                     maybeSetLastModified(resp, lastModified);   
  17.                     doGet(req, resp);   
  18.                 } else  
  19.                 {   
  20.                     resp.setStatus(304);   
  21.                 }   
  22.             }   
  23.         } else  
  24.        if(method.equals("HEAD"))   
  25.         {   
  26.            long lastModified = getLastModified(req);   
  27.             maybeSetLastModified(resp, lastModified);   
  28.             doHead(req, resp);   
  29.         } else  
  30.        if(method.equals("POST"))   
  31.             doPost(req, resp);   
  32.        else  
  33.        if(method.equals("PUT"))   
  34.             doPut(req, resp);   
  35.        else  
  36.        if(method.equals("DELETE"))   
  37.             doDelete(req, resp);   
  38.        else  
  39.        if(method.equals("OPTIONS"))   
  40.             doOptions(req, resp);   
  41.        else  
  42.        if(method.equals("TRACE"))   
  43.         {   
  44.             doTrace(req, resp);   
  45.         } else  
  46.         {   
  47.             String errMsg = lStrings.getString("http.method_not_implemented");   
  48.             Object errArgs[] = new Object[1];   
  49.             errArgs[0] = method;   
  50.             errMsg = MessageFormat.format(errMsg, errArgs);   
  51.             resp.sendError(501, errMsg);   
  52.         }   
  53.     }  

protected void service(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException
    {
        String method = req.getMethod();
        if(method.equals("GET"))
        {
            long lastModified = getLastModified(req);
            if(lastModified == -1L)
            {
                doGet(req, resp);
            } else
            {
                long ifModifiedSince = req.getDateHeader("If-Modified-Since");
                if(ifModifiedSince < (lastModified / 1000L) * 1000L)
                {
                    maybeSetLastModified(resp, lastModified);
                    doGet(req, resp);
                } else
                {
                    resp.setStatus(304);
                }
            }
        } else
        if(method.equals("HEAD"))
        {
            long lastModified = getLastModified(req);
            maybeSetLastModified(resp, lastModified);
            doHead(req, resp);
        } else
        if(method.equals("POST"))
            doPost(req, resp);
        else
        if(method.equals("PUT"))
            doPut(req, resp);
        else
        if(method.equals("DELETE"))
            doDelete(req, resp);
        else
        if(method.equals("OPTIONS"))
            doOptions(req, resp);
        else
        if(method.equals("TRACE"))
        {
            doTrace(req, resp);
        } else
        {
            String errMsg = lStrings.getString("http.method_not_implemented");
            Object errArgs[] = new Object[1];
            errArgs[0] = method;
            errMsg = MessageFormat.format(errMsg, errArgs);
            resp.sendError(501, errMsg);
        }
    }

从上面可以看出 这里的service是用来转向的，但是如果你在自己的servlet类中覆盖了service方法，比如说你的service是这样的：

Java代码

   1. public void service(ServletRequest req, ServletResponse res)   
   2.                    throws ServletException, IOException {   
   3.          res.getOutputStream().print(   
   4.          "image is <img src='images/downcoin.gif'></img><br>");   
   5.      }  

public void service(ServletRequest req, ServletResponse res)
                   throws ServletException, IOException {
        res.getOutputStream().print(
         "image is <img src='images/downcoin.gif'></img><br>");
    }

那么这时service就不是用来转向的，而是用来处理业务的，现在不论你的客户端是用pos还是get来请求此servlet

都会执行service方法也只能执行servlet方法，不会去执行doPost或是doGet方法。

比如说：你的客户端代码是：
Java代码

   1. <%@page contentType="text/html; charset=utf-8"%>   
   2. <html>   
   3. <head><title>选择</title></head>   
   4. <body>   
   5. 请选择你喜欢的水果：<br>   
   6. <form action = "Test" method = "post">   
   7. <input type="checkbox" name="fruit" value ="apple" >苹果<br>   
   8. <input type="checkbox" name="fruit" value ="orange">桔子<br>   
   9. <input type="checkbox" name="fruit" value ="mango">芒果<br>   
  10. <input type="submit" value="提交">   
  11. </form>   
  12. </body>   
  13. </html>   
  14.   
  15. 服务端servlet是：Test类   
  16.   
  17. import java.io.IOException;   
  18.   
  19. import javax.servlet.ServletException;   
  20. import javax.servlet.ServletOutputStream;   
  21. import javax.servlet.ServletRequest;   
  22. import javax.servlet.ServletResponse;   
  23. import javax.servlet.http.HttpServlet;   
  24. import javax.servlet.http.HttpServletRequest;   
  25. import javax.servlet.http.HttpServletResponse;   
  26.   
  27. /**
  28. * 演示service方法
  29. */  
  30. public class Test extends HttpServlet {   
  31.   
  32. public void service(ServletRequest req, ServletResponse res)   
  33.    throws ServletException, IOException {   
  34.             res.getOutputStream().print("This is the service");   
  35.   
  36. }   
  37.   
  38. protected void doGet(HttpServletRequest request,   
  39.     HttpServletResponse response) throws ServletException, IOException {   
  40.    doPost(request,response);   
  41.   
  42. }   
  43. protected void doPost(HttpServletRequest request,   
  44.     HttpServletResponse response) throws ServletException, IOException {   
  45.    ServletOutputStream out=response.getOutputStream();   
  46.    String[] args=(String[])request.getParameterValues("fruit");   
  47.   for(int i=0;i<args.length;i++){   
  48.     out.print(args[i]+"<br>");   
  49.    }   
  50.      
  51. }   
  52. }