JSP学习指南，JSTL表达式，EL表达式，JavaBean，常见问题收集

青旅半醒 2023-07-02 03:15 41阅读 0赞

**目录**

JSTL表达式

1.通用基本标签（set,out,remove)

2.条件标签

3.循环foreach的使用：

的EL表达式

1.基本用法

2.常用表达式

3.EL表达式操作集合

4.EL表达式的隐式对象

JSP的JavaBean标签用法

用法1：

用法2（快速将表单数据封装为实体对象，并提交给Servlet做业务处理）

常见问题收集

--------------------

## JSTL表达式 ##

JSTL（Java Server Page Standard Tag Library) JSP标准标签库，用来替代JSP页面脚本，它与EL（Expression Language）配合可以有效地减少JSP页面中的Java代码。

### 1.通用基本标签（set,out,remove) ###

<%--var：变量名,value:变量值--%>
    <c:set var="count" value="99" />
    <c:out value="${count}"/><hr>
    <%--如果变量未被定义值，可声明默认值--%>
    <c:out value="${happy}" default="我是默认值"/><hr>
    <%--移除值--%>
    <c:remove var="count"/>
    <c:out value="${count}"/><hr>

输出结果：

![20200124170944987.png][]

###  2.条件标签 ###

<c:if test="${8>4}">
        <h3>你好 2020</h3>
    </c:if>
    
    <c:set var="score" value="70"/>
    <c:choose>
        <c:when test="${score>=90 }">优秀</c:when>
        <c:when test="${score>=80 }">良好</c:when>
        <c:when test="${score>=70 }">中等</c:when>
        <c:when test="${score>=60 }">及格</c:when>
        <c:otherwise>不及格</c:otherwise>
    </c:choose>

其中要注意两点

*  无<c:else>标签
 *  choose,when组合即Java中Swith语句的效果

### 3.循环foreach的使用： ###

**一.简单循环：**

<c:forEach var="i" begin="0" end="10" step="2">
        ${i}<br>
    </c:forEach><hr>

![20200124172826298.png][]

**二.集合的遍历：**

新建TestServlet.java:

package com.qianqian.practice.servlet;
    
    import com.qianqian.practice.entity.User;
    
    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(name = "TestServlet", value = "/test")
    public class TestServlet extends HttpServlet {
        protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
            doGet(request, response);
        }
    
        protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
            List<User> userList=new ArrayList<>();
            userList.add(new User("4567654","李华","7545","man"));
            userList.add(new User("5452545","张梁","8775","woman"));
            userList.add(new User("6456455","康康","2421","woman"));
            userList.add(new User("1312313","小明","5457","man"));
            request.setAttribute("userList",userList);
            request.getRequestDispatcher("/index.jsp").forward(request, response);
        }
    }

index.jsp:

<body>
    
    <table border="1" width="30%" bordercolor="black" cellspacing="0" align="center">
        <tr>
            <th>ID</th>
            <th>NAME</th>
            <th>PASSWORD</th>
            <th>GENDER</th>
        </tr>
    
        <c:forEach items="${userList}" var="user">
            <tr>
                <td>${user.userId}</td>
                <td>${user.userName}</td>
                <td>${user.userPassword}</td>
                <td>${user.userGender}</td>
            </tr>
        </c:forEach>
    </table>
    </body>

结果：

![20200124181209635.png][]

## 的EL表达式 ##

**JSP的设计目的就是为了解决Servlet在视图层的复杂应用，因此要尽可能的少出现java代码，EL（Expression Language)表达式的出现很好的解决了这个问题:**

> **格式：$\{表达式\}**

### 1.基本用法 ###

新建一个实体类User

package com.qianqian.practice.entity;
    
    /**
     * 用户实体
     */
    public class User {
        private String userId;
        private String userName;
        private String userPassword;
        private String userGender;
    
        public User(String userId, String userName, String userPassword, String userGender) {
            this.userId = userId;
            this.userName = userName;
            this.userPassword = userPassword;
            this.userGender = userGender;
        }    
    }

然后再TestServlet.java新建对象并转发

package com.qianqian.practice.servlet;
    
    import com.qianqian.practice.entity.User;
    
    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 = "TestServlet", value = "/test")
    public class TestServlet extends HttpServlet {
        protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
            doGet(request, response);
        }
    
        protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
            User user = new User("4646794654", "小可爱", "4455455", "女");
            request.setAttribute("user1", user);
            request.getRequestDispatcher("/index.jsp").forward(request, response);
        }
    }

接下来就要在index.jsp中来接受此对象了，如下：

<%@ page import="com.qianqian.practice.entity.User" %><%--
      Created by IntelliJ IDEA.
      User: Swing
      Date: 2020/1/20
      Time: 9:46
      To change this template use File | Settings | File Templates.
    --%>
    <%@ page contentType="text/html;charset=UTF-8" language="java" pageEncoding="utf-8" %>
    <html>
    <head>
        <title>Expression Language</title>
    </head>
    <body>
    <%--如果未声明作用域，则更具作用域从小到大查找--%>
    userId:${user1.userId}<br>
    userName:${user1.userName}<br>
    userPassword:${user1.userPassword}<br>
    <%--如果声明了作用域，则在此作用域中查找--%>
    userGender:${requestScope.user1.userGender}<br>
    </body>
    </html>

### 2.常用表达式 ###

基本表达式与Java类似，如下举例：

算术运算
    	${10+20 }<br/>
    	${10-20 }<br/>
    	${10*20 }<br/>
    	${3%5}<br/>
    	关系运算
    	${10==9 }<br/>
    	${"abc"=="abc" }<br/>
    	${s1==s2 } equals<br/>
    	${s1 eq s2 }<br/>
    	${10>20}<br/>
    	${10 gt 20}<br/>
    	${5 < 10}<br/>
    	${5 lt 10}<br/>
    	${5 != 10}<br/>
    	${5 ne 10}<br/>
    	
    
    	三目运算
    	${score>80?"优秀":"一般" }<br/>  
    	${flag==0?"没有激活":flag==1?"激活":"删除" }<br/>  
    	${sex==0?"男":"女" }<br/>  
    	
         逻辑运算
    	${true &&  false}<br/> 
    	${true || false}<br/> 
    	${!true}<br/> 
    	${true and  false}<br/> 
    	${true or false}<br/> 
    	${not true}<br/>

此处要特别注意empty关键字

<%
        String s1 = "";
        String s2 = null;
        String s3 = "122222";
        List list1 = new ArrayList();
    
        pageContext.setAttribute("s1", s1);
        pageContext.setAttribute("s2", s2);
        pageContext.setAttribute("s3", s3);
        pageContext.setAttribute("list1", list1);
    %>
    
    ${empty s1}<br>
    ${empty s2}<br>
    ${empty  s3}<br>
    ${empty  list1}<br>

![20200124123811259.png][]

### 3.EL表达式操作集合 ###

<%
        List<String> stringList=new ArrayList<>();
        stringList.add("a");
        stringList.add("b");
        stringList.add("c");
        application.setAttribute("list",stringList);
    
        Map<String,String> stringStringMap=new HashMap<>();
        stringStringMap.put("A","abstract");
        stringStringMap.put("B","bool");
        stringStringMap.put("C","character");
        application.setAttribute("map",stringStringMap);
    %>
    List:<br>
    ${list}<br>
    ${list[0]}<br>
    ${list.get(1)}<br>
    
    <br>Map:<br>
    ${map}<br>
    ${map.keySet()}<br>
    ${map.values()}<br>
    ${map.A}<br>
    ${map["B"]}<br>

### 4.EL表达式的隐式对象 ###

大部分对应的是JSP的内置对象，由于不提倡在JSP中过多的使用Java代码，所有将内置对象映射到EL表达式的隐式对象

> **pageContext：**页面上下文，可以获取jsp中其他八个内置对象
> 
> **pageScope、requestScope、sessionScope、applicationScope**表示四个域对象中集合
> 
> **param** 表示request.getParameter("username"); $\{param.username\}
> 
> **paramvalues** 表示request.getPrameterValues("hobby"); $\{paramValues.hobby\}
> 
> **header** 表示 request.getHeader("accept"); $\{header.accept\};
> 
> **headerValues** 表示 request.getHeaderValues("accept-encoding"); $\{headerValues"accept-encoding"\}
> 
> **cookie** 表示 request.getCookies(); $\{cookie.JSESSIONID\}
> 
> **initParam** 表示 ServletContext(application)初始化参数

## JSP的JavaBean标签用法 ##

> jsp:useBean 创建一个对象 id class
> 
> jsp:setProperty给指定的对象属性赋值 name property value
> 
> jsp:getProperty取出指定的对象属值 name property

首先新建一个实体类，用来说明此动作标签的用法：

package com.qianqian.practice.entity;
    
    /**
     * 用户实体
     */
    public class User {
        private String userId;
        private String userName;
        private String userPassword;
        private String userGender;
    
        @Override
        public String toString() {
            return "User{" +
                    "userId='" + userId + '\'' +
                    ", userName='" + userName + '\'' +
                    ", userPassword='" + userPassword + '\'' +
                    ", userGender='" + userGender + '\'' +
                    '}';
        }
    }

###  用法1： ###

进行简单的新建类对象，在index.jsp中使用如下代码

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
    <html>
    <head>
        <title>结果</title>
    </head>
    <body>
    <%--实例化一个对象，id为对象标识，class为对象类地址--%>
    <jsp:useBean id="user" class="com.qianqian.practice.entity.User"/>
    <%--设置对象成员变量--%>
    <jsp:setProperty name="user" property="userId" value="748387483"/>
    <jsp:setProperty name="user" property="userName" value="阿狗"/>
    <jsp:setProperty name="user" property="userPassword" value="12345"/>
    <jsp:setProperty name="user" property="userGender" value="男"/>
    <%--获取成员变量--%>
    <jsp:getProperty name="user" property="userId"/><br>
    <jsp:getProperty name="user" property="userName"/><br>
    <jsp:getProperty name="user" property="userPassword"/><br>
    <jsp:getProperty name="user" property="userGender"/><br>
    </body>
    </html>

运行服务器，访问index.jsp，结果如下:

![20200123172630108.png][]

很明显，以上用法，也可以使用new关键字示例化对象，再使用set/get方法即可替代

### 用法2（快速将表单数据封装为实体对象，并提交给Servlet做业务处理） ###

新建login.html:

<!DOCTYPE html>
    <html lang="en">
    <head>
        <meta charset="UTF-8">
        <title>登录</title>
    </head>
    <body>
    <form method="post" action="index.jsp">
        用户id<input type="text" name="userId"/><br>
        用户名<input type="text" name="userName"/><br>
        用户密码<input type="password" name="userPassword"/><br>
        性别
        <input type="radio" name="userGender" value="女" checked/>女
        <input type="radio" name="userGender" value="男"/>男<br>
        <input type="submit" value="提交">
    </form>
    </body>
    </html>

将index.jsp中的内容改成如下：

<%@ page contentType="text/html;charset=UTF-8" language="java" pageEncoding="utf-8" %>
    <html>
    <head>
        <title>结果</title>
    </head>
    <body>
    <%--实例化一个对象，id为对象标识，class为对象类地址--%>
    <jsp:useBean id="user" class="com.qianqian.practice.entity.User"/>
    <%--可从表单中自动获取对应实体的所有属性，完成实体类对象的构建--%>
    <jsp:setProperty name="user" property="*"/>
    用户名id:<jsp:getProperty name="user" property="userId"/><br>
    用户名:<jsp:getProperty name="user" property="userName"/><br>
    用户密码:<jsp:getProperty name="user" property="userPassword"/><br>
    性别:<jsp:getProperty name="user" property="userGender"/><br>
    </body>
    </html>

此处可能会出现中文乱码，简单的一个过滤器即可解决

EncodeFilter.java:

package com.qianqian.practice.filter;
    
    import javax.servlet.*;
    import javax.servlet.annotation.WebFilter;
    import java.io.IOException;
    
    @WebFilter(filterName = "EncodeFilter",value = "/*")
    public class EncodeFilter implements Filter {
        public void destroy() {
        }
    
        public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws ServletException, IOException {
            req.setCharacterEncoding("utf-8");
            resp.setCharacterEncoding("utf-8");
            chain.doFilter(req, resp);
        }
    
        public void init(FilterConfig config) throws ServletException {
    
        }
    
    }

运行tomcat服务器，输入信息，点击提交，结果如下

![20200123205259318.png][]

![20200123205312230.png][]

由此可以体现出JavaBean在处理表单对象的便捷优势,我们可以将表单信息封装成对象，转发到servlet处理：

index.jsp:

<%@ page contentType="text/html;charset=UTF-8" language="java" pageEncoding="utf-8" %>
    <html>
    <head>
        <title>结果</title>
    </head>
    <body>
    <%--实例化一个对象，id为对象标识，class为对象类地址--%>
    <jsp:useBean id="user" class="com.qianqian.practice.entity.User"/>
    <%--可从表单中自动获取对应实体的所有属性，完成实体类对象的构建--%>
    <jsp:setProperty name="user" property="*"/>
    <%
        request.setAttribute("userInstance", user);
        request.getRequestDispatcher("/test").forward(request, response);
    %>
    </body>
    </html>

TestServlet.java:

点击提交，控制台输出如下：

![2020012321063464.png][]

说明对象成功传到Servlet中。

## 常见问题收集 ##

在使用JSTL的 c 标签时，出现下面小问题

![2020011021501246.png][]

解决方法：

将taglib指令改成如下：

![2020011021512988.png][]

[20200124170944987.png]: https://img-blog.csdnimg.cn/20200124170944987.png
[20200124172826298.png]: https://img-blog.csdnimg.cn/20200124172826298.png
[20200124181209635.png]: https://img-blog.csdnimg.cn/20200124181209635.png
[20200124123811259.png]: https://img-blog.csdnimg.cn/20200124123811259.png
[20200123172630108.png]: https://img-blog.csdnimg.cn/20200123172630108.png
[20200123205259318.png]: https://img-blog.csdnimg.cn/20200123205259318.png
[20200123205312230.png]: https://img-blog.csdnimg.cn/20200123205312230.png
[2020012321063464.png]: https://img-blog.csdnimg.cn/2020012321063464.png
[2020011021501246.png]: https://img-blog.csdnimg.cn/2020011021501246.png
[2020011021512988.png]: https://img-blog.csdnimg.cn/2020011021512988.png