JSTL标签库与EL表达式的使用

使用JSTL标签库前需添加相应的jstl.jar包，并在JSP页面中添加taglib指令标签

JSTL核心标签的使用

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

< c:if test="表达式">

判断表达式的值，如果表达式的值为 true ，则执行其主体内容，否则不显示。

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%--导入核心标签库--%>
<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
<c:if test="">
    test默认为false<br>
</c:if>
<c:if test="true">
    test=true时显示内容<br>
</c:if>
<c:if test="false">
    test=false时不显示内容<br>
</c:if>
<c:if test="${1>2}">
    用EL表达式做判断，1>2为false，不显示内容<br>
</c:if>
<c:if test="${1<2}">
    用EL表达式做判断，1<2为true,显示内容<br>
</c:if>
</body>
</html>

< c:choose >, < c:when >, < c:otherwise >

<c:choose>对应java中的switch语句
<c:when>对应Java中的case语句，有多个when满足条件时，只返回第一个
<c:otherwise>对应Java中的default语句

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%--导入核心标签库--%>
<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
<c:choose>
<%--    不满足条件--%>
    <c:when test="${1==2}">
        when 1
    </c:when>
<%--    满足条件--%>
    <c:when test="${2==2}">
        when 2
    </c:when>
<%--    满足条件--%>
    <c:when test="${3==3}">
        when 3
    </c:when>
    <c:otherwise>
        otherwise
    </c:otherwise>
</c:choose>
</body>
</html>

< c:forEach >

迭代器，用于迭代集合。

varStatus 属性

current: 当前这次迭代的集合中的项
index: 当前这次迭代从 0 开始的迭代索引
count: 当前这次迭代从 1 开始的迭代计数
first: 用来表明当前这轮迭代是否为第一次迭代的标志
last: 用来表明当前这轮迭代是否为最后一次迭代的标志

<%@ page import="java.util.ArrayList" %>
<%@ page import="java.util.List" %>
<%--language指定JSP中可以嵌套的语言--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%--导入核心标签库--%>
<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
<%--    创建list集合--%>
    List<Character> list=new ArrayList<>();
    list.add('w');
    list.add('e');
    list.add('l');
    list.add('c');
    list.add('o');
    list.add('m');
    list.add('e');
<%--    放进request域中--%>
    request.setAttribute("list",list);
%>
<c:forEach items="${requestScope.list}" var="list" varStatus="li" begin="0">
    ${list}----${li.index}----${li.current}----${li.count}----${li.first}----${li.last}<br>
</c:forEach>
</body>
</html>

使用ForEach迭代List

需求：1）创建Users对象，含有userid，username属性。

public class Users {
    private Integer userid;
    private String username;

    public Users(Integer userid, String username) {
        this.userid = userid;
        this.username = username;
    }

    public Users() {
    }

    public Integer getUserid() {
        return userid;
    }

    public void setUserid(Integer userid) {
        this.userid = userid;
    }

    public String getUsername() {
        return username;
    }

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

}

2）创建一个Servlet，在Servlet中创建多个Users对象并放到List集合中

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.ArrayList;
import java.util.List;
@WebServlet("/findusers")
public class FindUserServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
       this.doPost(req,resp);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //创建测试数据
        Users users1=new Users(1,"张三");
        Users users2=new Users(2,"李四");
        Users users3=new Users(3,"王五");

        //将数据放进list中
        List<Users> list=new ArrayList<>();
        list.add(users1);
        list.add(users2);
        list.add(users3);
        //将list添加到request对象中
        req.setAttribute("list",list);
        //通过请求转发做跳转
        req.getRequestDispatcher("showUsers.jsp").forward(req,resp);

    }
}

　　　3）在showUsers.jsp的页面中显示所有的Users对象的信息。

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
    <table border="1" align="center">
        <tr width="100px">
            <th>用户ID</th>
            <th>用户名</th>
        </tr>
        <c:forEach items="${requestScope.list}" var="list" varStatus="li">
            <tr>
                <td>${list.userid}</td>
                <td>${list.username}</td>
            </tr>
        </c:forEach>
    </table>
</body>
</html>

使用ForEach迭代Map

需求： 1）创建Users对象，含有userid，username属性。

public class Users {
    private Integer userid;
    private String username;

    public Users(Integer userid, String username) {
        this.userid = userid;
        this.username = username;
    }

    public Users() {
    }

    public Integer getUserid() {
        return userid;
    }

    public void setUserid(Integer userid) {
        this.userid = userid;
    }

    public String getUsername() {
        return username;
    }

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

　　　 2）创建一个Servlet，在Servlet中创建多个Users对象并放到Map集合中

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.HashMap;
import java.util.Map;
@WebServlet("/finduser2")
public class FindUserServlet2 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        this.doPost(req,resp);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //创建测试数据
        Users users1=new Users(6,"张三");
        Users users2=new Users(7,"李四");
        Users users3=new Users(8,"王五");

        //实例化Map,hashmap不能保证添加顺序
        Map<String,Users> map=new HashMap<>();
        //将数据放进map中
        map.put("user1",users1);
        map.put("user2",users2);
        map.put("user3",users3);
        //将map添加到request对象中
        req.setAttribute("map",map);
        //通过请求转发做跳转
        req.getRequestDispatcher("showUsers2.jsp").forward(req,resp);
    }
}

　　　 3）在showUsers2.jsp的页面中显示所有的Users对象的信息。

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
<table border="1" align="center">
    <tr>
        <th>Map的key</th>
        <th>用户ID</th>
        <th>用户名</th>
    </tr>
    <c:forEach items="${requestScope.map}" var="map" varStatus="li">
        <tr>
            <td>${map.key}</td>
            <td>${map.value.userid}</td>
            <td>${map.value.username}</td>
        </tr>
    </c:forEach>
</table>
</body>
</html>