JSP隐含变量和Spring中Model在EL表达式中的读取顺序

原文链接：http://www.cnblogs.com/rocky-AGE-24/p/5816193.html

结论:

1.jsp中用EL表达式（${attribute_name}）的读取顺序：model > session > application > requset

2.慎重使用session变量,这个范围太大了,如果因为一个模块把变量存在session中实现起来更容易,就这样做了,那么这就很容易为后面埋下bug.

测试代码：

controller类：

package com.my.innerParam.test;

import javax.servlet.ServletConfig;
import javax.servlet.ServletContext;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
import javax.servlet.jsp.JspWriter;
import javax.servlet.jsp.PageContext;

import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.web.bind.annotation.RequestMapping;

/**
 * 测试几个内置变量中在EL读取的顺序
 * @author rocky
 *
 */
@Controller
public class InnerParam
{
    @RequestMapping("/innerParamTest")
    public String innerParamTest(HttpServletRequest request,HttpServletResponse response,Model model)
    {
        HttpSession session = request.getSession();
        session.setAttribute("session", "session223");
        
        ServletContext application = request.getServletContext();
       // application.setInitParameter("appicationParam", "appicationParam");
        //Initialization parameters can not be set after the context has been initialized
        application.setAttribute("appicationAttr", "applicationAttr");
        request.setAttribute("request", "request");
       
        
        model.addAttribute("model", "model123");
        return "/testParam/testParam1";
    }
    
    //当变量名字相同时判断EL表达式用哪一个
    @RequestMapping("/innerParamTest2")
    public String innerParamTest2(HttpServletRequest request,HttpServletResponse response,Model model)
    {
        HttpSession session = request.getSession();
        session.setAttribute("test", "session223");
        
        ServletContext application = request.getServletContext();
       // application.setInitParameter("appicationParam", "appicationParam");
        //Initialization parameters can not be set after the context has been initialized
        application.setAttribute("test", "applicationAttr");
  
        request.setAttribute("test", "request");
        model.addAttribute("test", "model123");
        return "/testParam/testParam2";
    }
    
    //当model是null的时候
    @RequestMapping("/innerParamTest3")
    public String innerParamTest3(HttpServletRequest request,HttpServletResponse response,Model model)
    {
        HttpSession session = request.getSession();
        session.setAttribute("test", "session223");
        
        ServletContext application = request.getServletContext();
       // application.setInitParameter("appicationParam", "appicationParam");
        //Initialization parameters can not be set after the context has been initialized
        application.setAttribute("test", "applicationAttr");
  
        request.setAttribute("test", "request");
        model.addAttribute("test", null);
        return "/testParam/testParam3";
    }
}

jsp页面：

1、显示

session : ${session}
appicationAttr : '${appicationAttr}'
appicationParam : '${appicationParam}'
request : '${request}'
model : '${model}'

运行结果
session : session223

appicationAttr : 'applicationAttr'

appicationParam : ''

request : 'request'

model : 'model123'

2、 变量名字相同时

session : ${test}
appicationAttr : '${test}'
appicationParam : '${test}'
request : '${test}'
model : '${test}'

运行结果
session : model123

appicationAttr : 'model123'

appicationParam : 'model123'

request : 'model123'

model : 'model123'