Java web----EL函数库

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1 What is EL函数库

EL函数库是由第三方对EL的扩展，我们现在学习的EL函数库是由JSTL添加的。JSTL明天再学！

EL函数库就是定义一些有返回值的静态方法。然后通过EL语言来调用它们！当然，不只是JSTL可以定义EL函数库，我们也可以自定义EL函数库。

　　EL函数库中包含了很多对字符串的操作方法，以及对集合对象的操作。例如：${fn:length(“abc”)}会输出3，即字符串的长度。

2 导入函数库

因为是第三方的东西，所以需要导入。导入需要使用taglib指令！

<%@ taglib prefix="fn"uri="http://java.sun.com/jsp/jstl/functions" %>

3 EL函数库介绍

String toUpperCase(String input)：
String toLowerCase(String input)：
int indexOf(String input, String substring)：
boolean contains(String input, String substring)：
boolean containsIgnoreCase(String input, String substring)：
boolean startsWith(String input, String substring)：
boolean endsWith(String input, String substring)：
String substring(String input, int beginIndex, int endIndex)：
String substringAfter(String input, String substring)：hello-world, “-“
substringBefore(String input, String substring)：hello-world, “-“
String escapeXml(String input)：把字符串的“>”、“<”。。。转义了！
String trim(String input)：
String replace(String input, String substringBefore, StringsubstringAfter)：
String[] split(String input, String delimiters)：
int length(Object obj)：可以获取字符串、数组、各种集合的长度！
String join(String array[], String separator)：

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%><%@page import="com.cug.bean01.Person" %><%@taglib prefix="fn" uri="http://java.sun.com/jsp/jstl/functions" %><%String path = request.getContextPath();String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";%><!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><html>  <head>    <base href="<%=basePath%>">        <title>My JSP 'person.jsp' starting page</title>    <meta http-equiv="pragma" content="no-cache"><meta http-equiv="cache-control" content="no-cache"><meta http-equiv="expires" content="0">    <meta http-equiv="keywords" content="keyword1,keyword2,keyword3"><meta http-equiv="description" content="This is my page"><!--<link rel="stylesheet" type="text/css" href="styles.css">-->  </head>    <body>   <%   String[] strs = {"a","b","c"};   List list = new ArrayList();   list.add("a");   pageContext.setAttribute("arr", strs);   pageContext.setAttribute("list", list);   %>   ${fn:length(arr) }<br/><!-- 3 -->   ${fn:length(list) }<br/><!-- 1 -->   ${fn:toLowerCase("Hello") }<br/><!-- hello -->   ${fn:toUpperCase("Hello") }<br/><!-- HELLO -->   ${fn:contains("abc","a") }<br/><!-- true -->   ${fn:containsIgnoreCase("abc","Ab") }<br/><!-- true -->   ${fn:contains(arr,"a") }<br/><!-- true -->   ${fn:containsIgnoreCase(list,"A") }<br/><!-- true -->   ${fn:endsWith("Hello.java",".java") }<br/><!-- true -->   ${fn:startsWith("Hello.java","Hell") }<br/><!-- true -->   ${fn:indexOf("hello-world","-") }<br/><!-- 5 -->   ${fn:join(arr,";") }<br/><!-- a;b;c -->   ${fn:replace("Hello-world","-","+") }<br/><!-- Hello+world -->   ${fn:substring("0123456789",6,9) }<br/><!-- 678 -->   ${fn:substring("0123456789",5,-1) }<br/><!-- 56789 -->   ${fn:substringAfter("hello-world","-") }<br/><!-- 56789 -->   ${fn:substringBefore("hello-world","-") }<br/><!-- 56789 -->   ${fn:trim(   "hello"   ) }<br><!-- hello -->   ${fn:escapeXml("<html></html>") }<br/><!-- <html></html> -->  </body></html>

4 自定义EL函数库

写一个类，写一个有返回值的静态方法；
编写saylove.tld文件，可以参数fn.tld文件来写，把saylove.tld文件放到/WEB-INF目录下；
在页面中添加taglib指令，导入自定义标签库。

saylove.tld

<?xml version="1.0" encoding="UTF-8"?><taglib xmlns="http://java.sun.com/xml/ns/j2ee"  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"  xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-jsptaglibrary_2_0.xsd"  version="2.0">      <description>JSTL 1.1 functions library of mine</description>  <display-name>JSTL functions of mine</display-name> <tlib-version>1.1</tlib-version> <short-name>love</short-name>  <uri>http://cug.com/jsp/jstl/myfunctions</uri>  <function>        <name>sayLove</name>    <function-class>com.cug.bean01.MyFunction</function-class>    <function-signature>java.lang.String sayLove()</function-signature>  </function> </taglib>

MyFunction.java

package com.cug.bean01;public class MyFunction {public static String sayLove(){return "I Love you!!!";}}

index.jsp

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%><%@page import="com.cug.bean01.Person" %><%@taglib prefix="love" uri="http://cug.com/jsp/jstl/myfunctions" %><%String path = request.getContextPath();String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";%><!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><html>  <head>    <base href="<%=basePath%>">        <title>My JSP 'person.jsp' starting page</title>    <meta http-equiv="pragma" content="no-cache"><meta http-equiv="cache-control" content="no-cache"><meta http-equiv="expires" content="0">    <meta http-equiv="keywords" content="keyword1,keyword2,keyword3"><meta http-equiv="description" content="This is my page"><!--<link rel="stylesheet" type="text/css" href="styles.css">-->  </head>    <body>${love:sayLove() }  </body></html>

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