【JavaWeb】EL表达式

EL表达式

EL表达式语言，用于简化JSP的输出；

EL表达式的基本语法：${表达式}；

示例：<h1>学生姓名：${student.name}</h1>

作用域对象

忽略书写作用域对象时，el则按作用域从小到大依次尝试获取（不建议忽略）

pageScope：从当前页面取值
requestScope：从当前请求中获取属性值
sessionScope：从当前会话中获取属性值
applicationScope：从当前应用获取全局属性值

EL表达式输出

语法：＄{[作用域.]属性名[.子属性]}
EL表达式支持将运算结果进行输出
EL支持绝大多数对象输出，本质是执行toString()方法

EL输出参数值

EL表达式内置param对象来简化参数的输出
语法：${param.参数名}

实例

Course.java

package el;

public class Course {
	private int id;
	private String name;
	private String category;

	public int getId() {
		return id;
	}

	public void setId(int id) {
		this.id = id;
	}

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	public String getCategory() {
		return category;
	}

	public void setCategory(String category) {
		this.category = category;
	}

	public Course() {
	}

	public Course(int id, String name, String category) {
		super();
		this.id = id;
		this.name = name;
		this.category = category;
	}

	@Override
	public String toString() {
		return "Course [id=" + id + ", name=" + name + ", category=" + category + "]";
	}

}

El.java

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

	/**
	 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		Course course = new Course(1, "小明", "计算机");
		request.setAttribute("course", course);
		request.getRequestDispatcher("/el.jsp").forward(request, response);
	}

}

el.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>
	<b style="color: red;">${ requestScope.course }</b>
</body>
</html>

浏览器输入：http://localhost:8080/EL/el

页面显示：Course [id=1, name=小明, category=计算机]

el2.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>
	<b style="color: red;">${ param.level }</b>
</body>
</html>

浏览器输入：http://localhost:8080/EL/el2.jsp?level=primary

页面显示：primary