ch10.表达式语言

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：

${  ? : }