EL从四大域设置属性值、获取数据

一、四大域

page:只对当前页面有效

request:只对一次请求内有效

session：只对一次会话内有效

application:只对当前应用有效

注意：从四大域中取值时，取值顺序从小到大page<request<session<application，直到找到为止。

二、为四大域设置属性

<%
  pageContext.setAttribute("username1","张三");
  request.setAttribute("username2","李四");
  session.setAttribute("username3","王五");
  application.setAttribute("username4","赵六");
%>

1.使用JSP表达式获取域的属性值

<h3>使用JSP表达式获取域中的属性值</h3>
从page域中获取：<%= pageContext.getAttribute("username1")%> <br>
从request域中获取：<%= request.getAttribute("username2")%> <br>
从session域中获取：<%= session.getAttribute("username3")%> <br>
从application域中获取：<%= application.getAttribute("username4")%>

2.使用EL表达式获取域的属性值

<h3>使用EL表达式获取域中的属性值</h3>
从page域中获取：${username1}<br>
从request域中获取：${username2}<br>
从session域中获取：${username3}<br>
从application域中获取：${username4}

 3.使用内置对象来获取域的属性值，获取指定域的属性值

<h3>使用内置对象获取域中的属性值</h3>
从page域中获取：${pageScope.username1}<br>
从request域中获取：${requestScope.username2}<br>
从session域中获取：${sessionScope.username3}<br>
从application域中获取：${applicationScope.username4}

EL中域相关的四个内置对象：

        pageScope:从page域中查找指定的key 

        requestScope:从request域中查找指定的key 

        sessionScope:从session域中查找指定的key 

        applicationScope:从application域中查找指定的key 

三、例子

        使用servlet处理请求，查询数据库的数据，并封装在List集合并存储在request域中，JSP使用    c标签循环获取request域的数据；

        使用c标签需要导入jstl的jar包

1.controller层

package servlet;

import lombok.SneakyThrows;
import model.User;
import service.UserService;

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.sql.SQLException;
import java.util.List;

@WebServlet("/getAll")
public class UserServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        try {
            List<User> userList = UserService.getAll();
            for (User user : userList) {
                System.out.println(user);
            }
            req.setAttribute("userList", userList);
            req.getRequestDispatcher("/index.jsp").forward(req,resp);
        } catch (SQLException e) {
            throw new RuntimeException(e);
        }

    }
}

2.service层

package service;

import dao.UserDao;
import model.User;

import java.sql.SQLException;
import java.util.List;

public class UserService {

    public static List<User> getAll() throws SQLException {
        return UserDao.getALL();
    }
}

3.dao层

package dao;

import model.User;
import org.apache.commons.dbutils.QueryRunner;
import org.apache.commons.dbutils.handlers.BeanHandler;
import org.apache.commons.dbutils.handlers.BeanListHandler;
import utils.C3p0Utils;

import java.sql.SQLException;
import java.util.List;

public class UserDao {

    public static List<User> getALL() throws SQLException {
        QueryRunner qr= new QueryRunner(C3p0Utils.getDataSource());
        String sql = "select * from user";
        return qr.query(sql,new BeanListHandler<>(User.class));
    }
}

4.model层

package model;

public class User {
    private int id;
    //用户名
    private String username;
    //电话
    private String phone;
    //密码
    private String password;
    //身份
    private int role;

    public int getId() {
        return id;
    }

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

    public String getUsername() {
        return username;
    }

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

    public String getPhone() {
        return phone;
    }

    public void setPhone(String phone) {
        this.phone = phone;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }

    public int getRole() {
        return role;
    }

    public void setRole(int role) {
        this.role = role;
    }

    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", username='" + username + '\'' +
                ", phone='" + phone + '\'' +
                ", password='" + password + '\'' +
                ", role=" + role +
                '}';
    }
}

5.index.jsp

<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
<%@ taglib prefix="fmt" uri="http://java.sun.com/jsp/jstl/fmt"%>

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>EL表达式的用法</title>
</head>
<body>
<c:forEach items="${requestScope.userList}" var="user">
    <p>${user.username}</p>
    <p>${user.password}</p>
    <c:if test="${user.role == 0}">
        <p>系统管理员</p>
    </c:if>
    <c:if test="${user.role == 1}">
        <p>员工</p>
    </c:if>
    <c:if test="${user.role == 2}">
        <p>司机</p>
    </c:if>
</c:forEach>

</body>
</html>

实现效果：