Jsp/Servlet复习笔记-----第8章 JSP表达式语言

8.1 表达式语言简介
JSP用于在网页上显示动态内容，通常需要在JSP页面中嵌入Java脚本以完成复杂功能。但是大量的Java脚本使得JSP页面难以维护。一种类似JavaScript语言—JSP表达式语言可用于在网页上生成动态内容，并代替JSP脚本元素。JSP表达式语言是从JSP2.0规范开始支持的技术。
8.2 禁用EL 表达式计算
8.3 变量
8.4 隐式对象
8.5 文字常量
8.6 运算符
8.7 保留字
8.8 表达式使用案例
   8.9 本章小结 

EL的主要功能：
精确的访问存储对象
Bean属性的简略记法
空值取代错误消息

之前代码：

<%
	pageContext.setAttribute("name","darkness");
%>
<%
	if(pageContext.getAttribute("name") != null)
	{
%>
		<%=pageContext.getAttribute("name")%>
<%
	}
%>

<!--可以发现，这样，代码较多-->
<%
	pageContext.setAttribute("name","darkness");
%>

${name}
使用表达式时不需要去判断属性是否为空

<!--如果在不同的属性范围中设置了同名的属性，则之后的同名属性无法找到-->
<!--按照四中标志位来寻找-->
<%
	pageContext.setAttribute("name","darkness");
	request.setAttribute("name","darkness");
%>
${name}

EL的主要功能在于MVC设计模式之中
后台：

package org.sky.darkness.servlet.el ;

import java.io.* ;
import javax.servlet.* ;
import javax.servlet.http.* ;

public class ELServletDemo01 extends HttpServlet
{
	public void doGet(HttpServletRequest req,HttpServletResponse resp) throws IOException,ServletException
	{
		req.setAttribute("name","www.sky.org") ;
		// 服务器端跳转
		req.getRequestDispatcher("eldemo03.jsp").forward(req,resp) ;
	}
	public void doPost(HttpServletRequest req,HttpServletResponse resp) throws IOException,ServletException
	{
		this.doGet(req,resp) ;
	}
};
/*
  <servlet>
	<servlet-name>el01</servlet-name>
	<servlet-class> org.sky.darkness.servlet.el.ELServletDemo01</servlet-class>
  </servlet>
  <servlet-mapping>
	<servlet-name>el01</servlet-name>
	<url-pattern>/eldemo/el_1</url-pattern>
  </servlet-mapping>
*/
后台：${name}

*访问Bean的属性 ：
1、点号记法
${beanName.beanProperty}

2、数组记法

${beanName[“beanProperty”]}

package org.sky.darkness.servlet.el ;

import java.io.* ;
import javax.servlet.* ;
import javax.servlet.http.* ;

public class ELServletDemo02 extends HttpServlet
{
	public void doGet(HttpServletRequest req,HttpServletResponse resp) throws IOException,ServletException
	{
		Simple s = new Simple() ;
		s.setName("darkness") ;
		req.setAttribute("sim",s) ;
		// 服务器端跳转
		req.getRequestDispatcher("eldemo04.jsp").forward(req,resp) ;
	}
	public void doPost(HttpServletRequest req,HttpServletResponse resp) throws IOException,ServletException
	{
		this.doGet(req,resp) ;
	}
};
/*
  <servlet>
	<servlet-name>el02</servlet-name>
	<servlet-class> org.sky.darkness.servlet.el.ELServletDemo02</servlet-class>
  </servlet>
  <servlet-mapping>
	<servlet-name>el02</servlet-name>
	<url-pattern>/eldemo/el_2</url-pattern>
  </servlet-mapping>
*/

	
package org.sky.darkness.servlet.el ;

public class Simple
{
	private String name ;
	public void setName(String name)
	{
		this.name = name ;
	}
	public String getName()
	{
		return this.name ;
	}
};

如果保存的是一个对象，则在 JSP页面中要写以下代码：

<%@ page import=”org.sky.darkness.servlet.el.*”%>
<%
If(request.getAttribute(“sim”) != null){
    Simple  simp = (Simple)request.getAttribute(“sim”);
%>
        <%=simp.getName()%>
<%
    }
%>
<h1>${sim.name}</h1>
嵌套调用： ${ emp.name.firstName }

package org.sky.darkness.servlet.el ;

public class NameBean
{
	private String firstName ;
	private String lastName ;

	public void setFirstName(String firstName)
	{
		this.firstName = firstName ;
	}
	public void setLastName(String lastName)
	{
		this.lastName = lastName ;
	}
	public String getFirstName()
	{
		return this.firstName ;
	}
	public String getLastName()
	{
		return this.lastName ;
	}
};

package org.sky.darkness.servlet.el ;

public class EmployeeBean
{
	private NameBean name ;
	private CompanyBean company ;

	public void setName(NameBean name)
	{
		this.name = name ;
	}
	public void setCompany(CompanyBean company)
	{
		this.company = company ;
	}
	public NameBean getName()
	{
		return this.name ;
	}
	public CompanyBean getCompany()
	{
		return this.company ;
	}
};

package org.sky.darkness.servlet.el ;

public class CompanyBean
{
	private String companyName ;
	private String business ;
	
	public void setCompanyName(String companyName)
	{
		this.companyName = companyName ;
	}
	public void setBusiness(String business)
	{
		this.business = business ;
	}
	public String getCompanyName()
	{
		return this.companyName ;
	}
	public String getBusiness()
	{
		return this.business ;
	}
};

package org.sky.darkness.servlet.el ;

import java.io.* ;
import javax.servlet.* ;
import javax.servlet.http.* ;

public class ELServletDemo03 extends HttpServlet
{
	public void doGet(HttpServletRequest req,HttpServletResponse resp) throws IOException,ServletException
	{
		NameBean nb = new NameBean() ;
		nb.setFirstName("darkness") ;
		nb.setLastName("depravedAngel") ;
		
		CompanyBean cb = new CompanyBean() ;
		cb.setCompanyName("www.sky.org") ;
		cb.setBusiness("JAVA EE、JAVA SE、JAVA ME") ;

		EmployeeBean eb = new EmployeeBean() ;
		eb.setName(nb) ;
		eb.setCompany(cb) ;

		// 保存属性
		req.setAttribute("emp",eb) ;

		// 服务器端跳转
		req.getRequestDispatcher("eldemo05.jsp").forward(req,resp) ;
	}
	public void doPost(HttpServletRequest req,HttpServletResponse resp) throws IOException,ServletException
	{
		this.doGet(req,resp) ;
	}
};
/*
  <servlet>
	<servlet-name>el03</servlet-name>
	<servlet-class> org.sky.darkness.servlet.el.ELServletDemo03</servlet-class>
  </servlet>
  <servlet-mapping>
	<servlet-name>el03</servlet-name>
	<url-pattern>/eldemo/el_3</url-pattern>
  </servlet-mapping>
*/

<h1>FirstName:${emp.name.firstName}</h1>
<h1>LastName:${emp.name.lastName}</h1>
<h1>CompanyName:${emp.company.companyName}</h1>
<h1>Business:${emp.company.business}</h1>

访问集合操作：

package org.sky.darkness.servlet.el ;

import java.io.* ;
import java.util.* ;
import javax.servlet.* ;
import javax.servlet.http.* ;

public class ELServletDemo04 extends HttpServlet
{
	public void doGet(HttpServletRequest req,HttpServletResponse resp) throws IOException,ServletException
	{
		List l = new ArrayList() ;
		Simple s = null ;
		s = new Simple() ;
		s.setName("darkness") ;
		l.add(s) ;

		s = new Simple() ;
		s.setName("wind") ;
		l.add(s) ;

		s = new Simple() ;
		s.setName("cloud") ;
		l.add(s) ;

		req.setAttribute("all",l) ;
		// 服务器端跳转
		req.getRequestDispatcher("eldemo06.jsp").forward(req,resp) ;
	}
	public void doPost(HttpServletRequest req,HttpServletResponse resp) throws IOException,ServletException
	{
		this.doGet(req,resp) ;
	}
};
/*
  <servlet>
	<servlet-name>el02</servlet-name>
	<servlet-class> org.sky.darkness.servlet.el.ELServletDemo02</servlet-class>
  </servlet>
  <servlet-mapping>
	<servlet-name>el02</servlet-name>
	<url-pattern>/eldemo/el_2</url-pattern>
  </servlet-mapping>
*/

<%@page import="java.util.*"%>
<%--
	要使用Iterator进行输出
--%>
<%
	// List要使用Iterator输出
	List l = (List)request.getAttribute("all") ;
	Iterator iter = l.iterator() ;
	while(iter.hasNext())
	{
		// 将取出的对象放在page范围之中
		pageContext.setAttribute("sim",iter.next()) ;
%>
            //调用隐式对象
		<h1>${pageScope.sim.name}</h1>
<%
	}
%>