Servlet之EL

時間 2021-01-02

標籤 html java express cookie session app jsp this spa code 欄目 Tomcat 简体版

原文原文鏈接

EL表達式

使用傳統方式獲取request對象裏的屬性值

ElServlet.javahtml

public class ElServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        this.doGet(request,response);
    }
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        request.setCharacterEncoding("utf-8");
        response.setContentType("text/html;charset=utf-8");
        //從請求中獲取數據
        String name = request.getParameter("name");
        String pwd = request.getParameter("pwd");
        System.out.println("name="+name+",pwd="+pwd);
        //設置屬性的值爲字符串類型
        request.setAttribute("city","beijing");
        //Object類型
        User user = new User(1,"lisi",new Address("hubei","wuhan","hongshan"));
        request.setAttribute("user",user);
        //List類型
        ArrayList<User> list = new ArrayList<>();
        list.add(user);
        request.setAttribute("list",list);
        //HashMap類型
        HashMap<String, String> hm = new HashMap<>();
        hm.put("china","beijing");
        hm.put("hubei","wuhan");
        request.setAttribute("map",hm);
        HashMap<String, User> hm1 = new HashMap<>();
        hm1.put("user",user);
        request.setAttribute("suhm",hm1);
        //經過請求轉發方式跳到某jsp頁面
        request.getRequestDispatcher("el.jsp").forward(request,response);
    }
}

el.jspjava

<%@ page import="com.syf.entity.User" %>
<%@ page import="java.util.ArrayList" %>
<%@ page import="java.util.HashMap" %>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
<%--使用傳統方式獲取做用域中的值--%>
name:<%=request.getParameter("name")%>
pwd:<%=request.getParameter("pwd")%>
<br>
city:<%=request.getAttribute("city")%>
town:<%=((User)request.getAttribute("user")).getAddress().getTown()%>
<br>
<%=((User)((ArrayList)request.getAttribute("list")).get(0)).getAddress().getTown()%>
<br>
<%=((HashMap)request.getAttribute("map")).get("china")%>
<br>
<%=((HashMap)request.getAttribute("suhm")).get("user")%>
</body>
</html>

從上面代碼能夠看到，使用傳統方式得到request對象中的屬性值有如下缺點：express

必需要導入包
須要進行類型的強制轉換
層次結構比較複雜

EL表達式能夠解決上述問題
一、概念cookie

Expression Language 一種很是簡單的表達式，語法簡單易懂，便於使用。讓jsp寫起來更加方便，簡化jsp中獲取做用域或者請求數據的寫法。
語法結構：${expression}
使用.和[]兩種運算符獲取數據

二、用法
使用EL表達式進行上述輸出session

<html>
<head>
    <title>Title</title>
</head>
<body>
<%--使用EL表達式獲取做用域中的值--%>
name:${param.name}
pwd:${param.pwd}<br>
city:${city}<br>
town:${user.address.town}<br>
${list[0].address.town}<br>
${map.china}<br>
${suhm.user}
</body>
</html>

三、EL表達式做用域的順序
JSP的四大做用域，範圍由小到大爲：pageContext-->request-->session-->application
若是四個做用域中都有相同的key，獲取值的順序是怎樣的？app

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
<!--在四大做用域中設置屬性值-->
<%
 pageContext.setAttribute("key","this is pageContext");
 request.setAttribute("key","this is request");
 session.setAttribute("key","this is session");
 application.setAttribute("key","this is application");
%>
<%--獲取做用域中的值--%>
key：${key}
</body>
</html>

上述代碼獲取到的是pageContext，把pageContext這行註釋掉，再執行程序，獲取到的是request，同理往下依次是session，application。
這說明不一樣做用域中有相同key，獲取key的值的順序是由小做用域到大做用域。
如何獲取指定的key的值？jsp

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
<!--在四大做用域中設置屬性值-->
<%
 pageContext.setAttribute("key","this is pageContext");
 request.setAttribute("key","this is request");
 session.setAttribute("key","this is session");
 application.setAttribute("key","this is application");
%>
<%--獲取指定做用域中key的值--%>
pageContext：${pageScope.key}<br>
request：${requestScope.key}<br>
session：${sessionScope.key}<br>
application：${applicationScope.key}
</body>
</html>

四、EL表達式進行運算this

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
${1+1}<br><%--2--%>
${2-1}<br><%--1--%>
${2*3}<br><%--6--%>
${6/2}<br><%--3.0--%>
${7%3}<br><%--1--%>
${1>2?"好":"很差"}<br><%--很差--%>
${1+"2"}<br><%--3--%>
<%--${1+"abc"}<br> 報錯--%>
<%--${"abc"+"cdb"} 報錯--%>
<%--邏輯運算--%>
${true&&false}<br><%--false--%>
${true||false}<br><%--true--%>
</body>
</html>

EL表達式能夠進行算術運算與關係運算，關係運算返回true或false。須要注意的是，EL表達式中的+，表示加法，不是字符串鏈接符。
五、EL表達式其餘用法spa

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
<%--獲取請求頭數據--%>
${header}<br>
${header["host"]}<br>
${headerValues["accept-language"][0]}<br>
<hr>
<%--獲取cookie數據--%>
${cookie}<br>
${cookie.JSESSIONID}<br>
${cookie.JSESSIONID.name}<br>
${cookie.JSESSIONID.value}<br>
</body>
</html>