JSP/Servlet||EL表达式及其访问的用法

古城微笑少年丶 2021-10-30 01:24 506阅读 0赞

## 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：

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0pvY2tMaXU_size_16_color_FFFFFF_t_70][]

### 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

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0pvY2tMaXU_size_16_color_FFFFFF_t_70 1][]

### 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：

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0pvY2tMaXU_size_16_color_FFFFFF_t_70 2][]

### 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>

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0pvY2tMaXU_size_16_color_FFFFFF_t_70 3][]

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0pvY2tMaXU_size_16_color_FFFFFF_t_70 4][]

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[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0pvY2tMaXU_size_16_color_FFFFFF_t_70 3]: /images/20211029/4ffa9812701a4bfda63e279e8c48f4f5.png
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