30.JSTL与EL表达式

1.初识EL表达式

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1.1 简单认识EL

//Stduent.java
package com.imooc.el;

public class Student {
    public String name;
    public String mobile;
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public String getMobile() {
        return mobile;
    }
    public void setMobile(String mobile) {
        this.mobile = mobile;
    }
    
    
}
//StduentServlet.java
package com.imooc.el;

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

/**
 * Servlet implementation class StduentServlet
 */
@WebServlet("/StduentServlet")
public class StduentServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;
       
    /**
     * @see HttpServlet#HttpServlet()
     */
    public StduentServlet() {
        super();
        // TODO Auto-generated constructor stub
    }

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        Student stu = new Student();
        stu.setName("hachiman");
        stu.setMobile(null);
        String grade = "A";
        request.setAttribute("student", stu);
        request.setAttribute("grade", grade);
        request.getRequestDispatcher("/el_info.jsp").forward(request, response);
        
    }

}

用普通的jsp表达式

//info.jsp
<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8" import="com.imooc.el.Student"%>
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Insert title here</title>
</head>
<body>
    <% 
        Student stu = (Student)request.getAttribute("student");
        String grade = (String)request.getAttribute("grade");
        out.println("<h1>姓名："+stu.getName()+"</h1>");
        out.println("<h2>手机："+stu.getMobile()+"</h2>");
        out.println("<h2>教师评级："+grade+"</h2>");
    %>
</body>
</html>

注意需要导入类import="com.imooc.el.Student"
用el表达式，则无需导入类，而且输出也简化很多

//el_info.jsp
<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Insert title here</title>
</head>
<body>
    <h1>姓名:${requestScope.student.name}</h1>
    <h2>手机:${requestScope.student.mobile }</h2>
    <h2>评级:${requestScope.grade}</h2>
</body>
</html>

1.2EL作用域对象

image.png
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        Student stu = new Student();
        stu.setName("hachiman");
        stu.setMobile(null);
        String grade = "A";
        HttpSession session = request.getSession();
        session.setAttribute("student", stu);
        session.setAttribute("grade", grade);
        /*在不同的作用域设置同名属性*/
        request.setAttribute("grade", "A");//request作用域
        session.setAttribute("grade", "B");//session作用域
        request.getServletContext().setAttribute("grade", "C");//context全局作用域
        
        //request.setAttribute("student", stu);
        //request.setAttribute("grade", grade);
        request.getRequestDispatcher("/el_info.jsp").forward(request, response);
        
    }
//jsp
<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Insert title here</title>
</head>
<body>
    <h1>姓名:${sessionScope.student.name}</h1>
    <h2>手机:${sessionScope.student.mobile }</h2>
    <h2>评级:${sessionScope.grade}</h2>
    
    <h1>姓名:${student.name}</h1>
    <h2>手机:${student.mobile }</h2>
    <h2>评级:${grade}</h2>
</body>
</html>

1.3 EL表达式输出

image.png

image.png

zh

注意：
1.对于直接输出类，本质上是执行类的toString方法，因此重写toString方法可以修改输出内容
2.对于EL输出时，EL人性化将输出为null的对象，输出为空，在页面上就看不到null了

1.4EL输出参数

image.png

http://localhost:8080/ServletProj/StduentServlet?teacher=andy
可以用<h2>讲师:${param.teacher}</h2>直接获取

2.JSTL标签库

2.1简介

JSTL标签库

*JSTL(JSP Standard Tag Library),JSP标准标签库
*JSTL用于简化JSP开发，提高代码的可读性和可维护性

JSTL的标签库种类
JSTL按功能划分为五类标签库
核心标签库 -core
格式化输出标签库 -fmt
SQL操作标签库 -sql
XML操作标签库 -xml
函数标签库 -functions

引用JSTL核心库
核心标签库（Core）是JSTL最重要的标签库 提供了JSTL的基础功能
<%@taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core"%>
JSTL核心标签库在taglib-standard-impl.jar由META-INF/c.tld定义

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2.2判断标签

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注意：单分支<c:if>是没有else的，多分支就不要写这个

//ServletJstl.java
package com.imooc.jstl;

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

/**
 * Servlet implementation class ServletJstl
 */
@WebServlet("/jstl")
public class ServletJstl extends HttpServlet {
    private static final long serialVersionUID = 1L;
       
    /**
     * @see HttpServlet#HttpServlet()
     */
    public ServletJstl() {
        super();
        // TODO Auto-generated constructor stub
    }

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        request.setAttribute("score",58);
        request.setAttribute("grade", "B");
        request.getRequestDispatcher("/core.jsp").forward(request, response);
    }

}

//core.jsp
<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>    

<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Insert title here</title>
</head>
<body>
    <!-- 单分支 -->
    <h1>${requestScope.score}</h1>
    <c:if test = "${ score >= 60}">
        <h1 style = "color:green">恭喜，通过测试</h1>
    </c:if>
    <c:if test = "${requestScope.score < 60}">
        <h1 style="color:red">抱歉，请再接再厉</h1>
    </c:if>
    
    <!-- 多分支 -->
    <h1>${grade}</h1>
    <c:choose>
        <c:when test = "${grade == 'A' }">
            <h2>你很优秀</h2>
        </c:when>
        <c:when test = "${grade == 'B' }">
            <h2>不错哦</h2>
        </c:when>
        <c:when test = "${grade == 'C' }">
            <h2>水平一般，需要提高</h2>
        </c:when>
        <c:when test = "${grade == 'D'}">
            <h2>需要努力啦，不要气馁</h2>
        </c:when>
        <c:otherwise>
            <h2>一切随缘吧</h2>
        </c:otherwise>
    </c:choose>
    
</body>
</html>

2.3 遍历标签

image.png
//Company.java
package com.imooc.jstl;

public class Company {
    private String cname;
    private String url;
    public Company(String cname,String url) {
        this.cname = cname;
        this.url = url;
    }
    public String getCname() {
        return cname;
    }
    public void setCname(String cname) {
        this.cname = cname;
    }
    public String getUrl() {
        return url;
    }
    public void setUrl(String url) {
        this.url = url;
    }
    
}


//ServletJstl.java
@WebServlet("/jstl")
public class ServletJstl extends HttpServlet {
    private static final long serialVersionUID = 1L;
       
    /**
     * @see HttpServlet#HttpServlet()
     */
    public ServletJstl() {
        super();
        // TODO Auto-generated constructor stub
    }

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        List<Company> list = new ArrayList();
        list.add(new Company("腾讯","www.tencent.com"));
        list.add(new Company("百度","www.baidu.com"));
        list.add(new Company("慕课网","www.imooc.com"));
        request.setAttribute("companies", list);
        request.getRequestDispatcher("/core.jsp").forward(request, response);
    }

}

//core.jsp
<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>    

<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Insert title here</title>
</head>
<body>

   <!-- 遍历标签 --> 
    <c:forEach items="${requestScope.companies}" var="c" varStatus="idx">
        <h2>${idx.index+1}.${c.cname}-${c.url }</h2>
    </c:forEach>
    
</body>
</html>

2.4 fmt标签

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//fmt.jsp
<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<%@ taglib uri="http://java.sun.com/jsp/jstl/fmt" prefix="fmt"%>
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Insert title here</title>
</head>
<body>
    <%
        request.setAttribute("amt", 19876543.567);
        request.setAttribute("now",new java.util.Date());
        request.setAttribute("html","<a href='index.html'>index</a>");
        request.setAttribute("nothing", null);
    %>
    <h1>${requestScope.now}</h1>
    
    <!-- formatDate pattern 
        yyyy - 四位年
        MM - 两位月
        dd - 两位日
        HH - 24小时制
        hh - 12小时制
        mm - 分钟
        ss - 秒
        SSS - 毫秒
    -->
    <h1>
        <fmt:formatDate value="${requestScope.now }" pattern="yyyy年MM月dd日HH时mm分ss秒SSS毫秒"/>
    </h1>
    
    <h1>${requestScope.amt}</h1>
    <h1>
        <!-- 保留小数点后两位 -->
        <fmt:formatNumber value="${requestScope.amt}" pattern="0.00"></fmt:formatNumber>
    </h1>
    <h1>
        <!-- 增加分割逗号 -->
        ￥<fmt:formatNumber value="${requestScope.amt}" pattern="0,000.00"></fmt:formatNumber>元
    </h1>
    <!-- c:out 与 `uri="http://java.sun.com/jsp/jstl/core" prefix="c" `标签库有关 -->
    <h1>
        <!-- 修改null输出为空的情况，该为自定义 -->
        null默认值：<c:out value="${nothing}" default="无"></c:out>
    </h1>
    <h1>
        <!-- 输出带有特殊字符，选择是否需要进行转译，保证原样输出 -->
        <c:out value="${html}" escapeXml="true"></c:out>
    </h1>
</body>
</html>
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