自定义EL函数的步骤

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第一步，首先定义一个public 的类，在该类中定义方法，这个方法必须是静态的，例如：

package com.tinysoft.cn.test;//自定义函数，定义一个类，这个类必须是public的//类中提供静态方法，实现功能public class MyFunction {    //必须是静态方法public static String toUpperCase(String s){//转化为大写return s.toUpperCase();}}

第二步，在WEB-INF目录下，新建一个tld结尾的xml文件，例如：

<?xml version="1.0" encoding="utf-8" ?><taglib xmlns="http://java.sun.com/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-jsptaglibrary_2_1.xsd" version="2.1">   <tlib-version>1.0</tlib-version>

   <!-- myfun和uri提供给jsp调用时引用，没有实际意思-->   <short-name>myfun</short-name>   <uri>http://www.itheima.com/jsp/myfunctions</uri>   <function>

  <!-- 注意函数名和上一步定义的静态方法要一致-->   <name>toUpperCase</name>

  <!-- 上面方法所在类的全路径名-->   <function-class>com.tinysoft.cn.test.MyFunction</function-class>

  <!-- 函数的签名，函数的参数和返回值为基本数据类型时，则不需要签名-->   <function-signature>java.lang.String toUpperCase(java.lang.String)</function-signature>   </function></taglib>

第三步，在jsp中调用这个定义的函数

 <body>   <%String s="abcdefghijklmnopqrstuvwxyz";     pageContext.setAttribute("s", "abddgfdgdgd");       %>    ${myfun:toUpperCase(s)}   </body>
//注意javaee也内置了很多EL函数
<?xml version="1.0" encoding="UTF-8" ?><taglib xmlns="http://java.sun.com/xml/ns/j2ee"  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"  xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-jsptaglibrary_2_0.xsd"  version="2.0">      <description>JSTL 1.1 functions library</description>  <display-name>JSTL functions</display-name>  <tlib-version>1.1</tlib-version>  <short-name>fn</short-name>  <uri>http://java.sun.com/jsp/jstl/functions</uri>  <function>    <description>      Tests if an input string contains the specified substring.    </description>    <name>contains</name>    <function-class>org.apache.taglibs.standard.functions.Functions</function-class>    <function-signature>boolean contains(java.lang.String, java.lang.String)</function-signature>    <example>      <c:if test="${fn:contains(name, searchString)}">    </example>  </function>  <function>    <description>      Tests if an input string contains the specified substring in a case insensitive way.    </description>    <name>containsIgnoreCase</name>    <function-class>org.apache.taglibs.standard.functions.Functions</function-class>    <function-signature>boolean containsIgnoreCase(java.lang.String, java.lang.String)</function-signature>    <example>      <c:if test="${fn:containsIgnoreCase(name, searchString)}">    </example>  </function>  <function>    <description>      Tests if an input string ends with the specified suffix.    </description>    <name>endsWith</name>    <function-class>org.apache.taglibs.standard.functions.Functions</function-class>    <function-signature>boolean endsWith(java.lang.String, java.lang.String)</function-signature>    <example>      <c:if test="${fn:endsWith(filename, ".txt")}">    </example>  </function>  <function>    <description>      Escapes characters that could be interpreted as XML markup.    </description>    <name>escapeXml</name>    <function-class>org.apache.taglibs.standard.functions.Functions</function-class>    <function-signature>java.lang.String escapeXml(java.lang.String)</function-signature>    <example>      ${fn:escapeXml(param:info)}    </example>  </function>  <function>    <description>      Returns the index withing a string of the first occurrence of a specified substring.    </description>    <name>indexOf</name>    <function-class>org.apache.taglibs.standard.functions.Functions</function-class>    <function-signature>int indexOf(java.lang.String, java.lang.String)</function-signature>    <example>      ${fn:indexOf(name, "-")}    </example>  </function>  <function>    <description>      Joins all elements of an array into a string.    </description>    <name>join</name>    <function-class>org.apache.taglibs.standard.functions.Functions</function-class>    <function-signature>java.lang.String join(java.lang.String[], java.lang.String)</function-signature>    <example>      ${fn:join(array, ";")}    </example>  </function>  <function>    <description>      Returns the number of items in a collection, or the number of characters in a string.    </description>    <name>length</name>    <function-class>org.apache.taglibs.standard.functions.Functions</function-class>    <function-signature>int length(java.lang.Object)</function-signature>    <example>      You have ${fn:length(shoppingCart.products)} in your shopping cart.    </example>  </function>  <function>    <description>      Returns a string resulting from replacing in an input string all occurrences      of a "before" string into an "after" substring.    </description>    <name>replace</name>    <function-class>org.apache.taglibs.standard.functions.Functions</function-class>    <function-signature>java.lang.String replace(java.lang.String, java.lang.String, java.lang.String)</function-signature>    <example>      ${fn:replace(text, "-", "•")}    </example>  </function>  <function>    <description>      Splits a string into an array of substrings.    </description>    <name>split</name>    <function-class>org.apache.taglibs.standard.functions.Functions</function-class>    <function-signature>java.lang.String[] split(java.lang.String, java.lang.String)</function-signature>    <example>      ${fn:split(customerNames, ";")}    </example>  </function>  <function>    <description>      Tests if an input string starts with the specified prefix.    </description>    <name>startsWith</name>    <function-class>org.apache.taglibs.standard.functions.Functions</function-class>    <function-signature>boolean startsWith(java.lang.String, java.lang.String)</function-signature>    <example>      <c:if test="${fn:startsWith(product.id, "100-")}">    </example>  </function>  <function>    <description>      Returns a subset of a string.    </description>    <name>substring</name>    <function-class>org.apache.taglibs.standard.functions.Functions</function-class>    <function-signature>java.lang.String substring(java.lang.String, int, int)</function-signature>    <example>      P.O. Box: ${fn:substring(zip, 6, -1)}    </example>  </function>  <function>    <description>      Returns a subset of a string following a specific substring.    </description>    <name>substringAfter</name>    <function-class>org.apache.taglibs.standard.functions.Functions</function-class>    <function-signature>java.lang.String substringAfter(java.lang.String, java.lang.String)</function-signature>    <example>      P.O. Box: ${fn:substringAfter(zip, "-")}    </example>  </function>  <function>    <description>      Returns a subset of a string before a specific substring.    </description>    <name>substringBefore</name>    <function-class>org.apache.taglibs.standard.functions.Functions</function-class>    <function-signature>java.lang.String substringBefore(java.lang.String, java.lang.String)</function-signature>    <example>      Zip (without P.O. Box): ${fn:substringBefore(zip, "-")}    </example>  </function>  <function>    <description>      Converts all of the characters of a string to lower case.    </description>    <name>toLowerCase</name>    <function-class>org.apache.taglibs.standard.functions.Functions</function-class>    <function-signature>java.lang.String toLowerCase(java.lang.String)</function-signature>    <example>      Product name: ${fn.toLowerCase(product.name)}    </example>  </function>  <function>    <description>      Converts all of the characters of a string to upper case.    </description>    <name>toUpperCase</name>    <function-class>org.apache.taglibs.standard.functions.Functions</function-class>    <function-signature>java.lang.String toUpperCase(java.lang.String)</function-signature>    <example>      Product name: ${fn.UpperCase(product.name)}    </example>  </function>  <function>    <description>      Removes white spaces from both ends of a string.    </description>    <name>trim</name>    <function-class>org.apache.taglibs.standard.functions.Functions</function-class>    <function-signature>java.lang.String trim(java.lang.String)</function-signature>    <example>      Name: ${fn.trim(name)}    </example>    </function></taglib>

引用这些EL函数，必须导入standard.jar这个包,这个包中封装了所有Javaee内置函数，如：


配置文件fn.tld，jsp中引用EL内置函数来自于Functions这个类里面的所有静态方法：如EL内置函数有：
package org.apache.taglibs.standard.functions;import java.lang.reflect.Array;import java.util.Collection;import java.util.Enumeration;import java.util.Iterator;import java.util.Map;import java.util.StringTokenizer;import javax.servlet.jsp.JspTagException;import org.apache.taglibs.standard.resources.Resources;import org.apache.taglibs.standard.tag.common.core.Util;public class Functions{  public static String toUpperCase(String input)  {    return input.toUpperCase();  }    public static String toLowerCase(String input)  {    return input.toLowerCase();  }    public static int indexOf(String input, String substring)  {    if (input == null) {      input = "";    }    if (substring == null) {      substring = "";    }    return input.indexOf(substring);  }    public static boolean contains(String input, String substring)  {    return indexOf(input, substring) != -1;  }    public static boolean containsIgnoreCase(String input, String substring)  {    if (input == null) {      input = "";    }    if (substring == null) {      substring = "";    }    String inputUC = input.toUpperCase();    String substringUC = substring.toUpperCase();    return indexOf(inputUC, substringUC) != -1;  }    public static boolean startsWith(String input, String substring)  {    if (input == null) {      input = "";    }    if (substring == null) {      substring = "";    }    return input.startsWith(substring);  }    public static boolean endsWith(String input, String substring)  {    if (input == null) {      input = "";    }    if (substring == null) {      substring = "";    }    int index = input.indexOf(substring);    if (index == -1) {      return false;    }    if ((index == 0) && (substring.length() == 0)) {      return true;    }    return index == input.length() - substring.length();  }    public static String substring(String input, int beginIndex, int endIndex)  {    if (input == null) {      input = "";    }    if (beginIndex >= input.length()) {      return "";    }    if (beginIndex < 0) {      beginIndex = 0;    }    if ((endIndex < 0) || (endIndex > input.length())) {      endIndex = input.length();    }    if (endIndex < beginIndex) {      return "";    }    return input.substring(beginIndex, endIndex);  }    public static String substringAfter(String input, String substring)  {    if (input == null) {      input = "";    }    if (input.length() == 0) {      return "";    }    if (substring == null) {      substring = "";    }    if (substring.length() == 0) {      return input;    }    int index = input.indexOf(substring);    if (index == -1) {      return "";    }    return input.substring(index + substring.length());  }    public static String substringBefore(String input, String substring)  {    if (input == null) {      input = "";    }    if (input.length() == 0) {      return "";    }    if (substring == null) {      substring = "";    }    if (substring.length() == 0) {      return "";    }    int index = input.indexOf(substring);    if (index == -1) {      return "";    }    return input.substring(0, index);  }    public static String escapeXml(String input)  {    if (input == null) {      return "";    }    return Util.escapeXml(input);  }    public static String trim(String input)  {    if (input == null) {      return "";    }    return input.trim();  }    public static String replace(String input, String substringBefore, String substringAfter)  {    if (input == null) {      input = "";    }    if (input.length() == 0) {      return "";    }    if (substringBefore == null) {      substringBefore = "";    }    if (substringBefore.length() == 0) {      return input;    }    StringBuffer buf = new StringBuffer(input.length());    int startIndex = 0;    int index;    while ((index = input.indexOf(substringBefore, startIndex)) != -1)    {      buf.append(input.substring(startIndex, index)).append(substringAfter);      startIndex = index + substringBefore.length();    }    return input.substring(startIndex);  }    public static String[] split(String input, String delimiters)  {    if (input == null) {      input = "";    }    if (input.length() == 0)    {      String[] array = new String[1];      array[0] = "";      return array;    }    if (delimiters == null) {      delimiters = "";    }    StringTokenizer tok = new StringTokenizer(input, delimiters);    int count = tok.countTokens();    String[] array = new String[count];    int i = 0;    while (tok.hasMoreTokens()) {      array[(i++)] = tok.nextToken();    }    return array;  }    public static int length(Object obj)    throws JspTagException  {    if (obj == null) {      return 0;    }    if ((obj instanceof String)) {      return ((String)obj).length();    }    if ((obj instanceof Collection)) {      return ((Collection)obj).size();    }    if ((obj instanceof Map)) {      return ((Map)obj).size();    }    int count = 0;    if ((obj instanceof Iterator))    {      Iterator iter = (Iterator)obj;      count = 0;      while (iter.hasNext())      {        count++;        iter.next();      }      return count;    }    if ((obj instanceof Enumeration))    {      Enumeration enum_ = (Enumeration)obj;      count = 0;      while (enum_.hasMoreElements())      {        count++;        enum_.nextElement();      }      return count;    }    try    {      return Array.getLength(obj);    }    catch (IllegalArgumentException ex)    {      throw new JspTagException(Resources.getMessage("FOREACH_BAD_ITEMS"));    }  }    public static String join(String[] array, String separator)  {    if (array == null) {      return "";    }    if (separator == null) {      separator = "";    }    StringBuffer buf = new StringBuffer();    for (int i = 0; i < array.length; i++)    {      buf.append(array[i]);      if (i < array.length - 1) {        buf.append(separator);      }    }    return buf.toString();  }}

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