mvc中Action前HttpPost的作用

[HttpPost]  
public string post_test(string str)  
...{  
    return "post的字符串是："+str;  
} 

		

function post_test()  
...{  
   $.post("/test/post_test", ...{ str: "John" }, function (data) 
   ...{  
      $('#lbl_show').text(data);  
   });  
}  

		

$.ajax(...{
            type: "POST",
            url: "/test/post_test",
            data: "str='" + John+ "'",
           success: function(msg) ...{
               $('#lbl_show').text(data);  
            }
      });