ch10.表达式语言

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表达式语言（EL）可以方便的访问标志位（JSP中有四种标志位：page(pageContext), request, session, application）中的属性，可以避免许多scriptlet代码。

在JSP中，只有三种代码可以出现：

接收属性
判断语句
迭代输出

但是开发中这点很难做到，因为JSP的输出要接收VO对象，所以，要通过表达式来避免导入vo包

表达式语言

四种标识为就是四种属性范围，可以直接通过表达式语言访问：

语法格式：

${属性名称}

表达式语言的主要功能就是进行内容的显示，通过不同内置对象的设置，表达式语言可以输出不同的内容：

内置对象的定义：

本图来自《Java Web 开发实战经典 --基础篇》视频

如果在不同的属性范围中保存了同名属性，则EL的查找范围就是 page --> request ---> session--->application,这是就可以通过内置对象区分四种属性

代码如下：ELdemo.jsp

<%@ page contentType = "text/html" pageEncoding="GBK"%><html>  <head>        <title>My JSP 'ELdemo.jsp' starting page</title>  </head>    <body>  <%  pageContext.setAttribute("info", "page属性");  request.setAttribute("info", "request属性") ;  session.setAttribute("info", "session属性");  application.setAttribute("info", "application属性");  %>  <h3>${pageScope.info}</h3>  <h3>${requestScope.info}</h3>  <h3>${sessionScope.info }</h3>  <h3>${applicationScope.info }</h3>  </body></html>

显示结果：

接收请求参数：

这个功能与request.getParameter()类似，语法如下：

${param.参数名称}

<h3><%=request.getParameter("ref") %></h3><h3>${param.ref }</h3>

也可以通过${paramValues.参数名称}

接收一组参数

集合操作

集合有 Collection(List, Set), Map, Iterator

所有的集合都要通过Iterator方法进行输出，而且List和Set的本质区别在于List接口对Collection接口进行扩充，而Set接口并没有扩充

Map每次保存一对内容，而且都是用Map.Entry接口对象

在MVC中应用：

在：org.thystar.eldemo.vo包中写入Dept.java

package org.thystar.eldemo.vo;public class Dept {private int deptno;private String dname;private String loc;public int getDeptno() {return deptno;}public void setDeptno(int deptno) {this.deptno = deptno;}public String getDname() {return dname;}public void setDname(String dname) {this.dname = dname;}public String getLoc() {return loc;}public void setLoc(String loc) {this.loc = loc;}}

在：org.thystar.eldemo.servlet包中写入ELServlet.java

package org.thystar.eldemo.servlet;import java.io.IOException;import java.util.ArrayList;import java.util.List;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;import org.thystar.eldemo.vo.Dept;public class ELServlet extends HttpServlet {/** *  */private static final long serialVersionUID = 1L;/** * The doGet method of the servlet. <br> * * This method is called when a form has its tag value method equals to get. *  * @param request the request send by the client to the server * @param response the response send by the server to the client * @throws ServletException if an error occurred * @throws IOException if an error occurred */public void doGet(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {List<Dept> all = new ArrayList<Dept>();Dept dept = null;dept = new Dept();dept.setDeptno(1);dept.setDname("xxxx");dept.setLoc("yyyy");all.add(dept);dept = new Dept();dept.setDeptno(2);dept.setDname("xxxxxx");dept.setLoc("yyyyyyyy");all.add(dept);request.setAttribute("alldept", all);request.getRequestDispatcher("ELdemo.jsp").forward(request, response);}/** * The doPost method of the servlet. <br> * * This method is called when a form has its tag value method equals to post. *  * @param request the request send by the client to the server * @param response the response send by the server to the client * @throws ServletException if an error occurred * @throws IOException if an error occurred */public void doPost(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {this.doGet(request, response);}}

在：ELdemo.jsp中写入

<%@ page contentType = "text/html" pageEncoding="GBK" import = "java.util.*"%><html>  <head>        <title>My JSP 'ELdemo.jsp' starting page</title>  </head>    <body>  <%  List all = (List)request.getAttribute("alldept");  if(all != null){   %>   <table border = "1" width = "90%">   <tr>   <td>部门编号</td>   <td>部门名称</td>   <td>部门位置</td>   </tr>   <%   Iterator iter = all.iterator(); while(iter.hasNext()){ pageContext.setAttribute("dept", iter.next());    %>    <tr>    <td>${dept.deptno }</td>    <td>${dept.dname }</td>    <td>${dept.loc }</td>    </tr>    <%    }    %>     </table>    <%     }    %>  </body></html>

结果如下：

运算符：

简单的运算符可以用表达式完成，复杂的应该交给Servlet完成

主要用到的是三目运算符：

${ ? : }

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