JSP/Servlet||EL表达式及其访问的用法

最新推荐文章于 2021-10-31 17:22:29 发布

Jock.Liu

最新推荐文章于 2021-10-31 17:22:29 发布

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分类专栏： JSP/Servlet 文章标签： EL表达式及其访问的用法

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本文详细介绍了JSP中的EL（Expression Language）表达式，包括EL语法、如何访问PageContext,Request,Session和Application中的对象、访问对象属性、集合以及参数、头信息、cookie和初始化参数的方法。通过示例代码展示了EL表达式的使用，帮助读者掌握EL的强大功能。" 80587762,7731652,Matlab实现马科维茨投资组合理论笔记（四）,"['投资理论', '风险管理', '金融工程', 'Matlab编程', '金融模型']

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EL表达式

EL是JSP 2.0增加的技术规范，其全称是表达式语言(Expression Language)。EL语言是一种在页面中访问数据的语言，简洁，易于维护。

1.EL语法

在JSP中访问模型对象是通过EL表达式的语法来表达。所有EL表达式的格式都是以“${}”表示。

例如，${ userinfo}代表获取变量userinfo的值。

2.EL表达式访问，PageContext,Request,Session，Application中的对象

页面中有四个范围（scope）,分别是pageScope页面范围，requestScope请求范围，sessionScope会话范围，applicationScope服务器范围

示例：先创建一个Servlet，在上述四个范围中存入数据，转发到index.jsp页面

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

@WebServlet(name = "DataServlet",urlPatterns = "/data")
public class DataServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //存数据
        request.setAttribute ("a","个人练习生");
        request.setAttribute ("b","蔡徐坤");
        request.setAttribute ("c","鸡你太美");


        request.getSession ().setAttribute ("a","个人练习生");
        request.getSession ().setAttribute ("b","蔡徐坤");
        request.getSession ().setAttribute ("c","鸡你太美");

        request.getServletContext ().setAttribute ("a","个人练习生");
        request.getServletContext ().setAttribute ("b","蔡徐坤");
        request.getServletContext ().setAttribute ("c","鸡你太美");

        //转发
        request.getRequestDispatcher ("/index.jsp").forward (request,response);

    }
}

在index.jsp页面中使用范围取值的EL表达式：

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>$Title$</title>
</head>
<body>

<h3>获取request中的常量</h3>
<h4>
    a:${requestScope.a}<br/>
    b:${requestScope.b}<br/>
    c:${requestScope.c}<br/>
</h4>

<h3>获取Session中的常量</h3>
<h4>
    a:${sessionScope.a}<br/>
    b:${sessionScope.b}<br/>
    c:${sessionScope.c}<br/>
</h4>

<h3>获取servletContext中的常量</h3>
<h4>
    a:${applicationScope.a}<br/>
    b:${applicationScope.b}<br/>
    c:${applicationScope.c}<br/>
</h4>

<h3>EL表达式访问范围的默认顺序 page=>session=>application</h3>
<h4>
    a:${a}<br/>
    b:${b}<br/>
    c:${c}<br/>
</h4>

</body>
</html>

输出结果：

3.EL表达式访问对象

首先如果存在某Scope范围的数据是对象，我们要访问对象中的属性时，使用.引用法，如${student.name}

示例：

创建Student对象，在Servlet中存入request

创建Student对象及其内的Adress对象

//Student
package edu.xalead.entity;
public class Student {
    public Student(int id, String name, int age,Address adr) {
        this.id = id;
        this.name = name;
        this.age = age;
        this.address = adr;
    }
    public Student() {

    }

    private int id;
	private String name;
    private int age;
    
    public Address getAddress() {
        return address;
    }

    public void setAddress(Address address) {
        this.address = address;
    }

    private Address address;
    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

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

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }
}
//Address
package edu.xalead.entity;

public class Address {
    private String homeAdr;

    public Address(String homeAdr, String busiAdr) {
        this.homeAdr = homeAdr;
        this.busiAdr = busiAdr;
    }

    private String busiAdr;

    public String getHomeAdr() {
        return homeAdr;
    }

    public void setHomeAdr(String homeAdr) {
        this.homeAdr = homeAdr;
    }

    public String getBusiAdr() {
        return busiAdr;
    }

    public void setBusiAdr(String busiAdr) {
        this.busiAdr = busiAdr;
    }
}

创建ObjectDataServlet :

import edu.xalead.star.Address;
import edu.xalead.star.Student;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

@WebServlet(name = "ObjectDataServlet",urlPatterns = "/obj")
public class ObjectDataServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {


        Student stu = new Student (10088,"蔡徐坤",23,new Address ("韩国","湖南卫视"));
        //把对象传入request
        request.setAttribute ("stu",stu);
        //转发到index.jsp
        request.getRequestDispatcher ("/index1.jsp").forward (request,response);

    }
}

创建转发到的页面index1.jsp

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
  <head>
    <title>$Title$</title>
    <meta charset="utf-8"/>
  </head>
  <body>

  <h3>EL表达式获取对象中的数据</h3>
  <h4>
    Student对象的<br/>
    id属性 : ${s.id} <br/>
    name属性 : ${s.name} <br/>
    age属性 : ${s.age} <br/>
    Address属性：<br/>
       homeAdr: ${s.address.homeAdr}<br/>
       busiAdr: ${s.address.busiAdr}<br/>
  </h4>
  </body>
</html>

输出结果

4.EL表达式访问集合

示例:

创建保存List,Set，Map集合的ListDataServlet

import edu.xalead.entity.Address;
import edu.xalead.entity.Student;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.util.*;

@WebServlet(name = "ListDataServlet",urlPatterns = "/list")
public class ListDataServlet extends HttpServlet {
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //创建List集合
        List<Student> students = new ArrayList<>();
        students.add(new Student(2222,"张三",20,new Address("西安","北京")));
        students.add(new Student(2232,"李四",22,new Address("西安","北京")));
        students.add(new Student(3322,"王麻子",21,new Address("西安","北京")));
        //存入request
        request.setAttribute("list",students);

        // 创建Set集合
        Set<Student> ss = new HashSet<>();

        ss.add(new Student(2222,"张三",20,new Address("西安","北京")));
        ss.add(new Student(2232,"李四",22,new Address("西安","北京")));
        ss.add(new Student(3322,"王麻子",21,new Address("西安","北京")));
        // 存入request
        request.setAttribute("ss",ss.toArray());


        //访问map集合的数据
        Map<String,Student> map = new HashMap<>();

        map.put("2222",new Student(2222,"张三",20,new Address("西安","北京")));
        map.put("2232",new Student(2232,"李四",22,new Address("西安","北京")));
        map.put("3322",new Student(3322,"王麻子",21,new Address("西安","北京")));
        //存入request
        request.setAttribute("map",map);

        // 转发给index2.jsp
        request.getRequestDispatcher("/index2.jsp").forward(request,response);
    }
}

注意，set集合EL表达式没法直接访问其中的元素，所以我们先ss.toArray()转换成了数组

创建index2.jsp页面

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
   <h3>访问List集合的数据</h3>
   <h4>
       访问第一个元素: ${list[0].id} : ${list[0].name} : ${list[0].age} <br/>
       访问第三个元素: ${list[2].id} : ${list[2].name} : ${list[2].age}<br/>
   </h4>
   <h3>访问Set集合的数据</h3>
   <h4>
       访问第一个元素: ${ss[0].id} : ${ss[0].name} : ${ss[0].age} <br/>
       访问第三个元素: ${ss[2].id} : ${ss[2].name} : ${ss[2].age}<br/>
   </h4>
   <h3>访问Map集合的数据</h3>
   <h4>
       访问键为2232元素: ${map["2232"].id} : ${map["2232"].name} : ${map["2232"].age} <br/>
       访问第三个元素: ${map["3322"].id} : ${map["3322"].name} : ${map["3322"].age}<br/>
   </h4>
</body>
</html>

输出结果：

5.EL访问参数（查询字符串，表单的属性）,头，cookie，InitParam

EL表达式中通过params访问参数，通过header访问头属性，通过cookie访问cookie

示例：

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
<h3>EL表达式访问查询字符串参数</h3>
<h4>
name:${param["name"]}<br/>
sex:${param.sex}<br/>
</h4>
<h3>EL表达式访问表单</h3>
<h4>
    name:${param["account"]}<br/>
    sex:${param.password}<br/>
</h4>
<h3>EL表达式访问头</h3>
<h4>
    Host:${header["Host"]}<br/>
    cookie:${header.cookie}<br/>
</h4>
<h3>EL表达式访问Cookie</h3>
<h4>
    JSeesionId:${cookie["JSESSIONID"].name}:${cookie["JSESSIONID"].value}<br/>
    Password:${cookie.password.name}:${cookie["password"].value}<br/>
</h4>

</body>
</html>

访问contextParam

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
         version="4.0">
    <context-param>
        <param-name>driverClass</param-name>
        <param-value>com.mysql.jdbc.Driver</param-value>
    </context-param>
    <context-param>
        <param-name>url</param-name>
        <param-value>jdbc:mysql://192.168.93.88:3306/mydb</param-value>
    </context-param>
</web-app>

在页面中添加

<h3>EL表达式访问Context-Param</h3>
<h4>
    driverClass:${initParam["driverClass"]}<br/>
    url:${initParam.url}<br/>
</h4>