Web学习之自定义标签

1、编写一个实现Tag接口的Java类(标签处理器类)

 package me.gacl.web.tag;

 import java.io.IOException;

 import javax.servlet.http.HttpServletRequest;
 import javax.servlet.jsp.JspException;
 import javax.servlet.jsp.JspWriter;
 import javax.servlet.jsp.PageContext;
 import javax.servlet.jsp.tagext.Tag;

 public class ViewIPTag implements Tag {

     //接收传递进来的PageContext对象
     private PageContext pageContext;

     @Override
     public int doEndTag() throws JspException {
         System.out.println("调用doEndTag()方法");
         return 0;
     }

     @Override
     public int doStartTag() throws JspException {
         System.out.println("调用doStartTag()方法");
         HttpServletRequest request =(HttpServletRequest) pageContext.getRequest();
         JspWriter out = pageContext.getOut();
         String ip = request.getRemoteAddr();
         try {
             //这里输出的时候会抛出IOException异常
             out.write(ip);
         } catch (IOException e) {
             //捕获IOException异常后继续抛出
             throw new RuntimeException(e);
         }
         return 0;
     }

     @Override
     public Tag getParent() {
         return null;
     }

     @Override
     public void release() {
         System.out.println("调用release()方法");
     }

     @Override
     public void setPageContext(PageContext pageContext) {
         System.out.println("setPageContext(PageContext pageContext)");
         this.pageContext = pageContext;
     }

     @Override
     public void setParent(Tag arg0) {

     }

 }

2、在WEB-INF/目录下新建tld文件，在tld文件中对标签处理器类进行描述

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　　gacl.tld文件的代码如下：

 <?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用来添加对taglib(标签库)的描述 -->
     <description>孤傲苍狼开发的自定义标签库</description>
     <!--taglib(标签库)的版本号 -->
     <tlib-version>1.0</tlib-version>
     <short-name>GaclTagLibrary</short-name>
     <!--
         为自定义标签库设置一个uri，uri以/开头，/后面的内容随便写，如这里的/gacl ，
         在Jsp页面中引用标签库时，需要通过uri找到标签库
         在Jsp页面中就要这样引入标签库：<%@taglib uri="/gacl" prefix="gacl"%>
     -->
     <uri>/gacl</uri>

     <!--一个taglib(标签库)中包含多个自定义标签，每一个自定义标签使用一个tag标记来描述  -->
     <!-- 一个tag标记对应一个自定义标签 -->
      <tag>
         <description>这个标签的作用是用来输出客户端的IP地址</description>
         <!--
             为标签处理器类配一个标签名，在Jsp页面中使用标签时是通过标签名来找到要调用的标签处理器类的
             通过viewIP就能找到对应的me.gacl.web.tag.ViewIPTag类
          -->
         <name>viewIP</name>
         <!-- 标签对应的处理器类-->
         <tag-class>me.gacl.web.tag.ViewIPTag</tag-class>
         <body-content>empty</body-content>
     </tag>

 </taglib>

2.2、在Jsp页面中使用自定义标签

　　1、使用"<%@taglib uri="标签库的uri"  prefix="标签的使用前缀"%>"指令引入要使用的标签库。

例如：在jspTag_Test1.jsp中引用gacl标签库

 <%@ page language="java" pageEncoding="UTF-8"%>
 <!-- 使用taglib指令引用gacl标签库，标签库的前缀(prefix)可以随便设置，如这里设置成 prefix="xdp" -->
 <%@taglib uri="/gacl"  prefix="xdp"%>
 <!DOCTYPE HTML>
 <html>
   <head>
     <title>输出客户端的IP</title>
   </head>

   <body>
     你的IP地址是(使用java代码获取输出)：
     <%
         //在jsp页面中使用java代码获取客户端IP地址
         String ip = request.getRemoteAddr();
         out.write(ip);
     %>
     <hr/>
      你的IP地址是(使用自定义标签获取输出)：
      <%--使用自定义标签viewIP --%>
      <xdp:viewIP/>
   </body>
 </html>

　标签的运行效果如下：

　　

　　从运行效果种可以看到，使用自定义标签就可以将jsp页面上的java代码移除掉，如需要在jsp页面上输出客户端的IP地址时，使用 <xdp:viewIP/>标签就可以代替jsp页面上的这些代码：

 <%
 2         //在jsp页面中使用java代码获取客户端IP地址
 3         String ip = request.getRemoteAddr();
 4         out.write(ip);
 5 %>

JSP引擎遇到自定义标签时，首先创建标签处理器类的实例对象，然后按照JSP规范定义的通信规则依次调用它的方法。
    1、public void setPageContext(PageContext pc)， JSP引擎实例化标签处理器后，将调用setPageContext方法将JSP页面的pageContext对象传递给标签处理器，标签处理器以后可以通过这个pageContext对象与JSP页面进行通信。
    2、public void setParent(Tag t)，setPageContext方法执行完后，WEB容器接着调用的setParent方法将当前标签的父标签传递给当前标签处理器，如果当前标签没有父标签，则传递给setParent方法的参数值为null。
    3、public int doStartTag()，调用了setPageContext方法和setParent方法之后，WEB容器执行到自定义标签的开始标记时，就会调用标签处理器的doStartTag方法。
    4、public int doEndTag()，WEB容器执行完自定义标签的标签体后，就会接着去执行自定义标签的结束标记，此时，WEB容器会去调用标签处理器的doEndTag方法。
    5、public void release()，通常WEB容器执行完自定义标签后，标签处理器会驻留在内存中，为其它请求服务器，直至停止web应用时，web容器才会调用release方法。

　　我们在tomcat服务器的"work\Catalina\localhost\JavaWeb_JspTag_study_20140816\org\apache\jsp"目录下可以找到将jspTag_Test1.jsp翻译成Servlet后的java源代码，如下图所示：

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" alt="" />

　　打开jspTag_005fTest1_jsp.java文件，可以看到setPageContext(PageContext pc)、setParent(Tag t)、doStartTag()、doEndTag()、release()这5个方法的调用顺序和过程。

 package org.apache.jsp;

 import javax.servlet.*;
 import javax.servlet.http.*;
 import javax.servlet.jsp.*;

 public final class jspTag_005fTest1_jsp extends org.apache.jasper.runtime.HttpJspBase
     implements org.apache.jasper.runtime.JspSourceDependent {

   private static final JspFactory _jspxFactory = JspFactory.getDefaultFactory();

   private static java.util.List _jspx_dependants;

   static {
     _jspx_dependants = new java.util.ArrayList(1);
     _jspx_dependants.add("/WEB-INF/gacl.tld");
   }

   private org.apache.jasper.runtime.TagHandlerPool _005fjspx_005ftagPool_005fxdp_005fviewIP_005fnobody;

   private javax.el.ExpressionFactory _el_expressionfactory;
   private org.apache.AnnotationProcessor _jsp_annotationprocessor;

   public Object getDependants() {
     return _jspx_dependants;
   }

   public void _jspInit() {
     _005fjspx_005ftagPool_005fxdp_005fviewIP_005fnobody = org.apache.jasper.runtime.TagHandlerPool.getTagHandlerPool(getServletConfig());
     _el_expressionfactory = _jspxFactory.getJspApplicationContext(getServletConfig().getServletContext()).getExpressionFactory();
     _jsp_annotationprocessor = (org.apache.AnnotationProcessor) getServletConfig().getServletContext().getAttribute(org.apache.AnnotationProcessor.class.getName());
   }

   public void _jspDestroy() {
     _005fjspx_005ftagPool_005fxdp_005fviewIP_005fnobody.release();
   }

   public void _jspService(HttpServletRequest request, HttpServletResponse response)
         throws java.io.IOException, ServletException {

     PageContext pageContext = null;
     HttpSession session = null;
     ServletContext application = null;
     ServletConfig config = null;
     JspWriter out = null;
     Object page = this;
     JspWriter _jspx_out = null;
     PageContext _jspx_page_context = null;

     try {
       response.setContentType("text/html;charset=UTF-8");
       pageContext = _jspxFactory.getPageContext(this, request, response,
                   null, true, 8192, true);
       _jspx_page_context = pageContext;
       application = pageContext.getServletContext();
       config = pageContext.getServletConfig();
       session = pageContext.getSession();
       out = pageContext.getOut();
       _jspx_out = out;

       out.write("\r\n");
       out.write("<!-- 引用gacl标签库，标签库的前缀(prefix)可以随便设置，如这里设置成 prefix=\"gacl\" -->\r\n");
       out.write("\r\n");
       out.write("<!DOCTYPE HTML>\r\n");
       out.write("<html>\r\n");
       out.write("  <head>\r\n");
       out.write("    <title>输出客户端的IP</title>\r\n");
       out.write("  </head>\r\n");
       out.write("  \r\n");
       out.write("  <body>\r\n");
       out.write("    你的IP地址是(使用java代码获取输出)：\r\n");
       out.write("    ");

         //在jsp页面中使用java代码获取客户端IP地址
         String ip = request.getRemoteAddr();
         out.write(ip);

       out.write("\r\n");
       out.write("    <hr/>\r\n");
       out.write("     你的IP地址是(使用自定义标签获取输出)：");
       if (_jspx_meth_xdp_005fviewIP_005f0(_jspx_page_context))
         return;
       out.write("\r\n");
       out.write("  </body>\r\n");
       out.write("</html>\r\n");
     } catch (Throwable t) {
       if (!(t instanceof SkipPageException)){
         out = _jspx_out;
         if (out != null && out.getBufferSize() != 0)
           try { out.clearBuffer(); } catch (java.io.IOException e) {}
         if (_jspx_page_context != null) _jspx_page_context.handlePageException(t);
       }
     } finally {
       _jspxFactory.releasePageContext(_jspx_page_context);
     }
   }

   private boolean _jspx_meth_xdp_005fviewIP_005f0(PageContext _jspx_page_context)
           throws Throwable {
     PageContext pageContext = _jspx_page_context;
     JspWriter out = _jspx_page_context.getOut();
     //  xdp:viewIP
     me.gacl.web.tag.ViewIPTag _jspx_th_xdp_005fviewIP_005f0 = (me.gacl.web.tag.ViewIPTag) _005fjspx_005ftagPool_005fxdp_005fviewIP_005fnobody.get(me.gacl.web.tag.ViewIPTag.class);
     _jspx_th_xdp_005fviewIP_005f0.setPageContext(_jspx_page_context);
     _jspx_th_xdp_005fviewIP_005f0.setParent(null);
     int _jspx_eval_xdp_005fviewIP_005f0 = _jspx_th_xdp_005fviewIP_005f0.doStartTag();
     if (_jspx_th_xdp_005fviewIP_005f0.doEndTag() == javax.servlet.jsp.tagext.Tag.SKIP_PAGE) {
       _005fjspx_005ftagPool_005fxdp_005fviewIP_005fnobody.reuse(_jspx_th_xdp_005fviewIP_005f0);
       return true;
     }
     _005fjspx_005ftagPool_005fxdp_005fviewIP_005fnobody.reuse(_jspx_th_xdp_005fviewIP_005f0);
     return false;
   }
 }

　  下面重点分析一下上述代码中标红色的那个 private boolean _jspx_meth_xdp_005fviewIP_005f0(PageContext _jspx_page_context)方法中的代码

　　①、这里是实例化一个viewIP标签处理器类me.gacl.web.tag.ViewIPTag的对象

1  //  xdp:viewIP
2     me.gacl.web.tag.ViewIPTag _jspx_th_xdp_005fviewIP_005f0 = (me.gacl.web.tag.ViewIPTag) _005fjspx_005ftagPool_005fxdp_005fviewIP_005fnobody.get(me.gacl.web.tag.ViewIPTag.class);

　　②、实例化标签处理器后，调用setPageContext方法将JSP页面的pageContext对象传递给标签处理器

1 _jspx_th_xdp_005fviewIP_005f0.setPageContext(_jspx_page_context);

　　③、setPageContext方法执行完后，接着调用的setParent方法将当前标签的父标签传递给当前标签处理器，如果当前标签没有父标签，则传递给setParent方法的参数值为null

1 _jspx_th_xdp_005fviewIP_005f0.setParent(null);

　　④、调用了setPageContext方法和setParent方法之后，WEB容器执行到自定义标签的开始标记时，就会调用标签处理器的doStartTag方法

1 int _jspx_eval_xdp_005fviewIP_005f0 = _jspx_th_xdp_005fviewIP_005f0.doStartTag();

　　⑤、WEB容器执行完自定义标签的标签体后，就会接着去执行自定义标签的结束标记，此时，WEB容器会去调用标签处理器的doEndTag方法

1 if (_jspx_th_xdp_005fviewIP_005f0.doEndTag() == javax.servlet.jsp.tagext.Tag.SKIP_PAGE)

　　这就是自定义标签的执行流程。