[DRP]-EL表达式

E L（Expression Language）目的：为了使JSP写起来更加简单。表达式语言的灵感来自于 ECMAScript 和 XPath 表达式语言，它提供了在 JSP 中简化表达式的方法，让Jsp的代码更加简化。

index 页面：目的，连接servelet类加载数据，然后转发页面。

 <a href="servlet/JstlElServlet">测试EL表达式</a><br>

JstlElServlet 类：

package com.bjpowernode.jstl;

import java.io.IOException;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * 测试el表达式
 * @author Administrator
 *
 */
public class JstlElServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        //普通字符串
        request.setAttribute("hello", "Hello World");
        
        //结构
         
        Group group = new Group();
        group.setName("动力节点603班");
        
        User user = new User();
        user.setUsername("张三");
        user.setAge(23);
        user.setGroup(group);
        request.setAttribute("user", user);
        
        //map
        Map map = new HashMap();
        map.put("k1", "v1");
        map.put("k2", "v2");
        request.setAttribute("map", map);
        
        //字符串数组
        String[] strArray = new String[]{"a", "b", "c"};
        request.setAttribute("str_array", strArray);
        
        //对象数组
        User[] users = new User[10];
        for (int i=0; i<users.length; i++) {
            users[i] = new User();
            users[i].setUsername("张三_" + i);
        }
        request.setAttribute("users", users);
        
        //list
        List userList = new ArrayList();
        for (int i=0; i<10; i++) {
            User usr = new User();
            usr.setUsername("李四_" + i);
            userList.add(usr);
        }
        request.setAttribute("userList", userList);
    
        //empty
        request.setAttribute("v2", "");
        request.setAttribute("v3", new ArrayList());
        request.setAttribute("v4", "12345");
        request.setAttribute("v5", null);
        
        request.getRequestDispatcher("/jstl_el.jsp").forward(request, response);
    }

}

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Group类：

package com.bjpowernode.jstl;

public class Group {
    
    private String name;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
    

}

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user类：

package com.bjpowernode.jstl;

public class User {
 
    private String username;
    private int age;
    
    private Group group;

    public Group getGroup() {
        return group;
    }

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public void setGroup(Group group) {
        this.group = group;
    }
    
}

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jstl_el.jsp页面：

<%@ page language="java" contentType="text/html; charset=GB18030"
    pageEncoding="GB18030"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=GB18030">
<title>Insert title here</title>
</head>
<body>
    <h1>测试EL表达式</h1>
    <hr>
    <li>普通字符串</li><br>
    hello(jsp脚本):<%=request.getAttribute("hello") %><br>
    hello(el表达式，语法：$和{}):${hello }<br>
    hello(el表达式，el的内置对象pageScope,requestScope,sessionScope,applicationScope)<br>
    如果不指定范围，它的搜索顺序为pageScope~applicationScope):${requestScope.hello }<br>
    hello(el表达式，指定范围从session取得):${sessionScope.hello }<br>
    <p>
    <li>结构,采用.进行导航，或称为存取器</li><br>
    姓名：${user.username }<br>
    年龄：${user.age }<br>
    所属组：${user.group.name }<br>
    <p>
    <li>map</li><br>
    map.k1:${map.k1 }<br>
    map.k2:${map.k2 }<br>
    <p>
    <li>字符串数组，采用[]下标</li><br>
    strArray[1]:${str_array[1] }<br>
    <p>
    <li>对象数组，采用[]下标</li><br>
    users[5].username:${users[5].username }<br>
    <p>
    <li>list，采用[]下标</li><br>
    userList[6].username:${userList[6].username }<br>
    <p>
    <li>el表达式对运算符的支持</li><br>
    1+1=${1+1 }<br>
    10/5=${10/5 }<br>
    10 div 5=${10 div 5 }<br>
    10 % 3=${10 % 3 }<br>
    10 mod 3 = ${ 10 mod 3 }<br>
    
    <!-- 
        ==/eq
        !=/ne
        </lt
        >/gt
        &&/and
        ||/or
        !/not
        //div
        %/mod
     --> 
    <p>
    <li>测试empty</li><br>
    v1:${empty v1}<br>
    v2:${empty v2}<br>
    v3:${empty v3}<br>
    v4:${empty v4}<br>
    v5:${empty v5}<br>
        
</body>
</html>

View Code

感觉和session 、cookie 的作用差不多，但是EL表达式只是后台先前台页面传值。数据不能共享！

让自己越来越优秀！