EL表达式详解

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一、EL表达式简介

EL 全名为Expression Language。EL主要作用：

1、获取数据

EL表达式主要用于替换JSP页面中的脚本表达式，以从各种类型的web域中检索java对象、获取数据。(某个web域中的对象，访问javabean的属性、访问list集合、访问map集合、访问数组)

2、执行运算

利用EL表达式可以在JSP页面中执行一些基本的关系运算、逻辑运算和算术运算，以在JSP页面中完成一些简单的逻辑运算。${user==null}

3、获取web开发常用对象

EL 表达式定义了一些隐式对象，利用这些隐式对象，web开发人员可以很轻松获得对web常用对象的引用，从而获得这些对象中的数据。

4、调用Java方法

EL表达式允许用户开发自定义EL函数，以在JSP页面中通过EL表达式调用Java类的方法。

二、EL表达式的功能

1、获取数据

代码语言：javascript复制

 1 <%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
 5 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
 6 <html>
 7   <head>
 8     <title>el表达式获取数据</title>
 9   </head>
10   
11   <body>
12   
13       <br/>----------------获取域中存的数据-------------------------<br/>
14     <% 
15         String data = "abcd"; 16  request.setAttribute("data",data); 17     %>
18     ${data }  <%--pageContext.findAttribute("data") ""--%>
19     
20     
21     <br/>----------------获取域中存的javabean的数据-------------------------<br/>
22     <% 
23  Person p = new Person(); 24  p.setName("aaaa"); 25         
26  request.setAttribute("person",p); 27     %>
28  ${person.name } 29     
30     
31      <br/>----------------获取域中存的javabean中的javabean的数据-------------------------<br/>
32     <% 
33  Person p1 = new Person(); 34  Address a = new Address(); 35  a.setCity("上海"); 36  p1.setAddress(a); 37  request.setAttribute("p1",p1); 38     %>
39  ${p1.address.city } 40    
41     
42 <br/>----------------获取域中存的list集合中的数据-------------------------<br/> 43 44 <% 45  List list = new ArrayList(); 46  list.add(new Person("aaa")); 47  list.add(new Person("bbb")); 48  request.setAttribute("list",list); 49 %> 50 ${list['1'].name } 51     
52     
53 <br/>----------------获取域中map集合中的数据-------------------------<br/> 54 <% 55  Map map = new HashMap(); 56  map.put("aa",new Person("aaaaa")); 57  map.put("bb",new Person("bbbbb")); 58  map.put("cc",new Person("ccccc")); 59  map.put("dd",new Person("ddddd")); 60  map.put("111",new Person("eeeee")); 61  request.setAttribute("map111",map); 62 %> 63 ${map111.cc.name } 64     
65    
66     
67 <br/>----------------其它几个常用的el表达式-------------------------<br/> 68 ${pageContext.request.contextPath } 69 <a href="${pageContext.request.contextPath }/index.jsp">点点</a>
70     
71     
72   </body>
73 </html>

EL表达式语句在执行时，会调用pageContext.findAttribute方法，用标识符为关键字，分别从page、request、session、application四个域中查找相应的对象，找到则返回相应对象，找不到则返回”” （注意，不是null，而是空字符串）。

2、执行运算

语法：${运算表达式}，EL表达式支持如下运算符：

empty运算符：检查对象是否为null或“空”

二元表达式：${user!=null?user.name : “”}

[ ] 和 . 号运算符

代码语言：javascript复制

 1 <%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
 2 
 3 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
 4 <html>
 5   <head>
 6     <title>运算符示例</title>
 7   </head>
 8   
 9   <body>
10     
11     <% 
12  request.setAttribute("username","aaa"); 13  request.setAttribute("password","124"); 14     %>
15     
16  ${username=='aaa' && password=='123' } 17     
18     <br/>----------------empty运算符-------------------------<br/>
19     
20 <% 21 //request.setAttribute("list",null); 22  request.setAttribute("list",new ArrayList()); 23 %> 24 ${empty(list) } 25     
26 <br/>----------------二元运算符-------------------------<br/> 27 ${user!=null ? user.username : '' } 28  
29     
30     <br/>----------------二元运算符(数据回显)-------------------------<br/>
31     
32 <% 33  request.setAttribute("gender","male"); 34 %> 35 36 <input type="radio" name="gender" value="male" ${gender=='male'?'checked':'' }>男 37 <input type="radio" name="gender" vlaue="female" ${gender=='female'?'checked':'' }>女
49   </body>
50 </html>

3、获取web开发常用对象

EL表达式语言中定义了11个隐含对象，使用这些隐含对象可以很方便地获取web开发中的一些常见对象，并读取这些对象的数据。

语法：${隐式对象名称} ：获得对象的引用。

代码语言：javascript复制

 1 <%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
 2 
 3 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
 4 <html>
 5   <head>
 6     <title>隐式对象</title>
 7   </head>
 8   
 9   <body>
10     
11  ${pageContext} 12     
15     <br/>----------------使用el隐式对象访问指定的域-------------------------<br/>
16     <% 
17  pageContext.setAttribute("aa","123"); 18     %>
19  ${aa } 20  ${pageScope.aa } <!--在域里面参数都是保存在一个map集合中--> 21     
22  ${sessionScope.user!=null } 23     
24     
25     
26     <br/>----------------获取保存了所有请求参数的Map对象-------------------------<br/>
27 ${param.name } 28     
29 ${paramValues.name[0] }<!--参数同名--> 30 ${paramValues.name[1] } 31     
32     
33     
34     <br/>----------------获取请求头-------------------------<br/>
35  ${header['Accept-Language'] }

代码语言：javascript复制

36     
37     <br/>---------------获取cookie-------------------------<br/>
38     ${cookie.JSESSIONID.name }<br/>
39  ${cookie.JSESSIONID.value } 40     
41     
42  ${initParam.xx } 43   </body>
44 </html>

重要应用：

可以从指定的域中直接取数据，提高查找效率

param隐含对象用于数据回显，之前的做法是把表单提交的数据封装到javabean,在服务端进行校验，如果校验失败就把bean存到request域中，再从页面中从request取出来回显

三、使用EL调用java方法

EL表达式语法允许开发人员开发自定义函数，以调用Java类的方法。

示例：${prefix：method(params)}

在EL表达式中调用的只能是Java类的静态方法。

这个Java类的静态方法需要在TLD文件中描述，才可以被EL表达式调用。

EL自定义函数用于扩展EL表达式的功能，可以让EL表达式完成普通Java程序代码所能完成的功能

1、EL Function开发步骤

一般来说， EL自定义函数开发与应用包括以下三个步骤：

编写一个Java类的静态方法

编写标签库描述符（tld）文件，在tld文件中描述自定义函数。

在JSP页面中导入和使用自定义函数

实例：开发对html标签进行转义的EL函数

java类的静态方法：

代码语言：javascript复制

 1 public class MyEL {  2     //进行html标签转义
 3     public static String filter(String message) {  4 
 5         if (message == null)  6             return (null);  7 
 8         char content[] = new char[message.length()];  9         message.getChars(0, message.length(), content, 0); 10         StringBuffer result = new StringBuffer(content.length   50); 11         for (int i = 0; i < content.length; i  ) { 12             switch (content[i]) { 13             case '<': 14                 result.append("&lt;"); 15                 break; 16             case '>': 17                 result.append("&gt;"); 18                 break; 19             case '&': 20                 result.append("&amp;"); 21                 break; 22             case '"': 23                 result.append("&quot;"); 24                 break; 25             default: 26  result.append(content[i]); 27  } 28  } 29         return (result.toString()); 30 
31  } 32     
33     //进行两个字符串连接
34     public static String add(String s1,String s2){ 35         return s1   s2; 36  } 37 }

标签库描述文件：

代码语言：javascript复制

 1 <?xml version="1.0" encoding="UTF-8" ?>
 2 <!--
 3  Licensed to the Apache Software Foundation (ASF) under one or more  4  contributor license agreements. See the NOTICE file distributed with  5  this work for additional information regarding copyright ownership.  6  The ASF licenses this file to You under the Apache License, Version 2.0  7  (the "License"); you may not use this file except in compliance with  8  the License. You may obtain a copy of the License at  9 
10  http://www.apache.org/licenses/LICENSE-2.0 11 
12  Unless required by applicable law or agreed to in writing, software 13  distributed under the License is distributed on an "AS IS" BASIS, 14  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 15  See the License for the specific language governing permissions and 16  limitations under the License. 17 -->
18 
19 <taglib xmlns="http://java.sun.com/xml/ns/j2ee"
20  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
21  xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-jsptaglibrary_2_0.xsd"
22  version="2.0">
23     <description>A tag library exercising SimpleTag handlers.</description>
24     <tlib-version>1.0</tlib-version>
25     <short-name>SimpleTagLibrary</short-name>
26     <uri>/my</uri>
27     
28 <function> 29 <name>filter</name> 30 <function-class>cn.itcast.demo.MyEL</function-class> 31 <function-signature>java.lang.String filter( java.lang.String )</function-signature> 32 </function> 33 34 <function> 35 <name>add</name> 36 <function-class>cn.itcast.demo.MyEL</function-class> 37 <function-signature>java.lang.String add( java.lang.String,java.lang.String )</function-signature> 38 </function>
39 </taglib>

使用EL函数：

代码语言：javascript复制

 1 <%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
 2 <%@taglib uri="/my" prefix="my" %>
 3 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
 4 <html>
 5   <head>
 6     <title>My JSP '4.jsp' starting page</title>
 7   </head>
 8   
 9   <body>
10   ${my:filter("<a href=''>点点</a>") } 11   </body>
12 </html>

2、开发EL函数注意事项

编写完标签库描述文件后，需要将它放置到<web应用>WEB-INF目录中或WEB-INF目录下的除了classes和lib目录之外的任意子目录中。
TLD文件中的<uri> 元素用指定该TLD文件的URI，在JSP文件中需要通过这个URI来引入该标签库描述文件。
<function>元素用于描述一个EL自定义函数，其中： <name>子元素用于指定EL自定义函数的名称。 <function-class>子元素用于指定完整的Java类名， <function-signature>子元素用于指定Java类中的静态方法的签名，方法签名必须指明方法的返回值类型及各个参数的类型，各个参数之间用逗号分隔。
EL表达式保留关键字

所谓保留字的意思是指变量在命名时，应该避开上述的名字，以免程序编译时发生错误。

四、sun公司的el函数库

1、el函数库所有标签描述

代码语言：javascript复制

 1 <?xml version="1.0" encoding="UTF-8" ?>
 2 
 3 <taglib xmlns="http://java.sun.com/xml/ns/j2ee"
 4  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 5  xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-jsptaglibrary_2_0.xsd"
 6  version="2.0">
 7     
 8   <description>JSTL 1.1 functions library</description>
 9   <display-name>JSTL functions</display-name>
 10   <tlib-version>1.1</tlib-version>
 11   <short-name>fn</short-name>
 12   <uri>http://java.sun.com/jsp/jstl/functions</uri>
 13 
 14   <function>
 15     <description>
 16  Tests if an input string contains the specified substring.  17     </description>
 18     <name>contains</name>
 19     <function-class>org.apache.taglibs.standard.functions.Functions</function-class>
 20     <function-signature>boolean contains(java.lang.String, java.lang.String)</function-signature>
 21     <example>
 22       &lt;c:if test="${fn:contains(name, searchString)}">  23     </example>
 24   </function>
 25 
 26   <function>
 27     <description>
 28  Tests if an input string contains the specified substring in a case insensitive way.  29     </description>
 30     <name>containsIgnoreCase</name>
 31     <function-class>org.apache.taglibs.standard.functions.Functions</function-class>
 32     <function-signature>boolean containsIgnoreCase(java.lang.String, java.lang.String)</function-signature>
 33     <example>
 34       &lt;c:if test="${fn:containsIgnoreCase(name, searchString)}">  35     </example>
 36   </function>
 37 
 38   <function>
 39     <description>
 40  Tests if an input string ends with the specified suffix.  41     </description>
 42     <name>endsWith</name>
 43     <function-class>org.apache.taglibs.standard.functions.Functions</function-class>
 44     <function-signature>boolean endsWith(java.lang.String, java.lang.String)</function-signature>
 45     <example>
 46       &lt;c:if test="${fn:endsWith(filename, ".txt")}">  47     </example>
 48   </function>
 49 
 50   <function>
 51     <description>
 52  Escapes characters that could be interpreted as XML markup.  53     </description>
 54     <name>escapeXml</name>
 55     <function-class>org.apache.taglibs.standard.functions.Functions</function-class>
 56     <function-signature>java.lang.String escapeXml(java.lang.String)</function-signature>
 57     <example>
 58  ${fn:escapeXml(param:info)}  59     </example>
 60   </function>
 61 
 62   <function>
 63     <description>
 64  Returns the index withing a string of the first occurrence of a specified substring.  65     </description>
 66     <name>indexOf</name>
 67     <function-class>org.apache.taglibs.standard.functions.Functions</function-class>
 68     <function-signature>int indexOf(java.lang.String, java.lang.String)</function-signature>
 69     <example>
 70  ${fn:indexOf(name, "-")}  71     </example>
 72   </function>
 73 
 74   <function>
 75     <description>
 76  Joins all elements of an array into a string.  77     </description>
 78     <name>join</name>
 79     <function-class>org.apache.taglibs.standard.functions.Functions</function-class>
 80     <function-signature>java.lang.String join(java.lang.String[], java.lang.String)</function-signature>
 81     <example>
 82  ${fn:join(array, ";")}  83     </example>
 84   </function>
 85 
 86   <function>
 87     <description>
 88  Returns the number of items in a collection, or the number of characters in a string.  89     </description>
 90     <name>length</name>
 91     <function-class>org.apache.taglibs.standard.functions.Functions</function-class>
 92     <function-signature>int length(java.lang.Object)</function-signature>
 93     <example>
 94  You have ${fn:length(shoppingCart.products)} in your shopping cart.  95     </example>
 96   </function>
 97 
 98   <function>
 99     <description>
100  Returns a string resulting from replacing in an input string all occurrences 101  of a "before" string into an "after" substring. 102     </description>
103     <name>replace</name>
104     <function-class>org.apache.taglibs.standard.functions.Functions</function-class>
105     <function-signature>java.lang.String replace(java.lang.String, java.lang.String, java.lang.String)</function-signature>
106     <example>
107       ${fn:replace(text, "-", "&#149;")} 108     </example>
109   </function>
110 
111   <function>
112     <description>
113  Splits a string into an array of substrings. 114     </description>
115     <name>split</name>
116     <function-class>org.apache.taglibs.standard.functions.Functions</function-class>
117     <function-signature>java.lang.String[] split(java.lang.String, java.lang.String)</function-signature>
118     <example>
119  ${fn:split(customerNames, ";")} 120     </example>
121   </function>
122 
123   <function>
124     <description>
125  Tests if an input string starts with the specified prefix. 126     </description>
127     <name>startsWith</name>
128     <function-class>org.apache.taglibs.standard.functions.Functions</function-class>
129     <function-signature>boolean startsWith(java.lang.String, java.lang.String)</function-signature>
130     <example>
131       &lt;c:if test="${fn:startsWith(product.id, "100-")}"> 132     </example>
133   </function>
134 
135   <function>
136     <description>
137  Returns a subset of a string. 138     </description>
139     <name>substring</name>
140     <function-class>org.apache.taglibs.standard.functions.Functions</function-class>
141     <function-signature>java.lang.String substring(java.lang.String, int, int)</function-signature>
142     <example>
143  P.O. Box: ${fn:substring(zip, 6, -1)} 144     </example>
145   </function>
146 
147   <function>
148     <description>
149  Returns a subset of a string following a specific substring. 150     </description>
151     <name>substringAfter</name>
152     <function-class>org.apache.taglibs.standard.functions.Functions</function-class>
153     <function-signature>java.lang.String substringAfter(java.lang.String, java.lang.String)</function-signature>
154     <example>
155  P.O. Box: ${fn:substringAfter(zip, "-")} 156     </example>
157   </function>
158 
159   <function>
160     <description>
161  Returns a subset of a string before a specific substring. 162     </description>
163     <name>substringBefore</name>
164     <function-class>org.apache.taglibs.standard.functions.Functions</function-class>
165     <function-signature>java.lang.String substringBefore(java.lang.String, java.lang.String)</function-signature>
166     <example>
167  Zip (without P.O. Box): ${fn:substringBefore(zip, "-")} 168     </example>
169   </function>
170 
171   <function>
172     <description>
173  Converts all of the characters of a string to lower case. 174     </description>
175     <name>toLowerCase</name>
176     <function-class>org.apache.taglibs.standard.functions.Functions</function-class>
177     <function-signature>java.lang.String toLowerCase(java.lang.String)</function-signature>
178     <example>
179  Product name: ${fn.toLowerCase(product.name)} 180     </example>
181   </function>
182 
183   <function>
184     <description>
185  Converts all of the characters of a string to upper case. 186     </description>
187     <name>toUpperCase</name>
188     <function-class>org.apache.taglibs.standard.functions.Functions</function-class>
189     <function-signature>java.lang.String toUpperCase(java.lang.String)</function-signature>
190     <example>
191  Product name: ${fn.UpperCase(product.name)} 192     </example>
193   </function>
194 
195   <function>
196     <description>
197  Removes white spaces from both ends of a string. 198     </description>
199     <name>trim</name>
200     <function-class>org.apache.taglibs.standard.functions.Functions</function-class>
201     <function-signature>java.lang.String trim(java.lang.String)</function-signature>
202     <example>
203  Name: ${fn.trim(name)} 204     </example>  
205   </function>
206 
207 </taglib>

2、el函数库使用

代码语言：javascript复制

 1 <%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
 2 <%@taglib uri="http://java.sun.com/jsp/jstl/functions" prefix="fn" %> 3 <%@taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
 4 
 5 
 6 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
 7 <html>
 8   <head>
 9     <title>My JSP '5.jsp' starting page</title>
10   </head>
11   
12   <body>
13     
14  ${fn:toLowerCase("AAAA") } 15     
16     <% 
17  request.setAttribute("arr",new String[5]); 18     %>
19     
20  ${fn:length(arr) } 21     
22     
23     <% 
24  List list = new ArrayList(); 25  list.add("aa"); 26  list.add("bb"); 27  request.setAttribute("list",list); 28     %>
29 <c:forEach var="i" begin="0" end="${fn:length(list)}"> 30 ${list[i] } 31 </c:forEach>
32     
33     
34  ${fn:join(fn:split("www,itcast,cn",","),".") } 35     
36  ${fn:contains("aaaabbbcc","ab") } 37     
38    
39     ${fn:escapeXml("<a href=''>点点</a>")} 40     
41     
42  <br/>---------------取出用户填写的爱好回显------------------------<br/> 43 <% 44  request.setAttribute("likes",new String[]{"football","sing"}); 45 %> 46 47 <input type="checkbox" name="likes" value="sing" ${fn:contains(fn:join(likes,","),"sing")?'checked':'' }>唱歌 48 <input type="checkbox" name="likes" value="dance" ${fn:contains(fn:join(likes,","),"dance")?'checked':'' }>跳舞 49 <input type="checkbox" name="likes" value="basketball" ${fn:contains(fn:join(likes,","),"basketball")?'checked':'' }>蓝球 50 <input type="checkbox" name="likes" value="football" ${fn:contains(fn:join(likes,","),"football")?'checked':'' }>足球 51   </body>
52 </html>

注意：el表达式不支持两个字符串的连接，如：

代码语言：javascript复制

 1   <body>
 2     
 3     <% 
 4  User user = new User();  5  user.setUsername("aaaa");  6         //session.setAttribute("user",user);  7     %>
 8     
 9  ${user!=null?"欢迎您" user.name:""} 10     
11   </body>

要想进行两个字符串连接可以开发el函数实现，如：

代码语言：javascript复制

1     //进行两个字符串连接
2      public static String add(String s1,String s2){ 3          return s1   s2; 4      }

在tld文件描述后再使用

代码语言：javascript复制

 1   <body>
 2     
 3     <% 
 4  User user = new User();  5  user.setUsername("aaaa");  6         //session.setAttribute("user",user);  7     %>
 8     
 9 ${user!=null?my:add("欢迎您：",user.username):''} 10     
11   </body>

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