Taglib 原理和实现:第四章 循环的Tag

第四篇 支持循环的Tag

1。问题：在request里的 People 对象，有个属性叫 men ，men 是一个Collection ，有许多个man 。现在，把 collection里的man的名字都显示出来

 显然，这是一个嵌套Tag的问题。有三个Tag互相作用：最外层的Tag找到People对象，中间的Tag取得Collection，子Tag负责打印。
 例如:
 <diego:withObject value="${people}">
  <diego:withCollection property="men">
   <diego:elementout property="name"/>  
  </diego:withCollection>
 </diego:withObject>
 
 思路如下:
 1.编写WithObjectTag,负责从El表达式中取得对象
 2.编写WithCollectionTag,负责从对象中取得 Collection ,遍历 Collection ,每遍历一次 Collection ,执行一次body
 3.编写ElementoutTag ,把 Collection 中每个men对象的 name 打印出来
 
 
2. 完整程序如下：
 
在上例的diegoyun.vo包内，编写 People 类

package diegoyun.vo;
import java.util.Collection;
public class People
{
 private Collection men = null; 
 public Collection getMen()
 {
  return men;
 }
 public void setMen(Collection men)
 {
  this.men = men;
 }
}

编写 withObject ，这是从request里取得People对象的最外层Tag

package diegoyun;
import javax.servlet.jsp.JspException;
import javax.servlet.jsp.tagext.BodyTagSupport;
import org.apache.taglibs.standard.lang.support.ExpressionEvaluatorManager;
public class WithObjectTag extends BodyTagSupport
{
 private Object value = null;

 public Object getValue()
 {
  return value;
 }
 public void setValue(Object value)throws JspException
 {
  this.value = ExpressionEvaluatorManager.evaluate(
            "value", value.toString(), Object.class, this, pageContext);
 }
 public int doStartTag()
 {  
  return EVAL_BODY_INCLUDE;
 }
 public int doEndTag()throws JspException
 {  
  return EVAL_PAGE;
 }
}

 

编写WithCollectionTag，该Tag负责取得Collection，并遍历执行子Tag
package diegoyun;

import java.util.Collection;
import java.util.Iterator;

import javax.servlet.jsp.JspException;
import javax.servlet.jsp.tagext.BodyTagSupport;

import org.apache.commons.beanutils.PropertyUtils;

public class WithCollectionTag extends BodyTagSupport {
 private Object element = null;

 private Collection list = null;

 private Iterator iterator = null;

 public Object getElement() {
  return element;
 }

 public void setProperty(String property) throws JspException {
 //取得父Tag对象,并且得到Collection
  WithObjectTag parent = (WithObjectTag) getParent();
  if (parent == null)
   throw new JspException("parent tag is null");
  try {
   Object propertyValue = PropertyUtils.getProperty(parent.getValue(),
     property);
   this.list = (Collection) propertyValue;
   if (list == null)
    throw new JspException("Collection is null");
  } catch (Exception e) {
   throw new JspException(e);
  }
 }

 public int doStartTag() throws JspException {
 //设置第一个元素,然后执行子Tag
  iterator = list.iterator();
  if (iterator.hasNext())
   element = iterator.next();

  return EVAL_BODY_INCLUDE;
 }

 public int doAfterBody() {
  if (iterator.hasNext()) {
  //如果还存在子元素,设置子元素,并且再次执行子Tag
  //循环由此而来
  //否则不再执行子Tag
   element = iterator.next();
   return EVAL_BODY_AGAIN;
  }
  else
   return EVAL_PAGE;
 }
}

编写 ElementOutputTag

package diegoyun;
import java.io.IOException;

import javax.servlet.jsp.JspException;
import javax.servlet.jsp.tagext.TagSupport;

import org.apache.commons.beanutils.PropertyUtils;

public class ElementOutputTag extends TagSupport
{
 private Object propertyValue  = null;
 public void setProperty(String property)throws JspException
 {
  WithCollectionTag parent = (WithCollectionTag)getParent();
  if(parent == null)
   throw new JspException("parent tag is null");
  try
  {
  //判断上层tag中是否存在该属性名称,如果存在,取得属性值,否则报错
   propertyValue = PropertyUtils.getProperty(parent.getElement(), property);
  }
  catch (Exception e)
  {
   throw new JspException(e);
  }
 }
 public int doEndTag()throws JspException
 {
  try
  {
  //简单的把值打印到jsp页面
   pageContext.getOut().print(propertyValue);
  }
  catch (IOException e)
  {
   throw new JspException(e);
  }
  return EVAL_PAGE;
 }
}

编写tld
<!--WithObjectTag-->
 <tag>
  <name>withObject</name>
  <tag-class>diegoyun.WithObjectTag</tag-class>
  <body-content>JSP</body-content>
  <attribute>
   <name>value</name>
   <required>false</required>
   <rtexprvalue>true</rtexprvalue>
  </attribute>
 </tag>
 <!--WithCollectionTag-->
 <tag>
  <name>withCollection</name>
  <tag-class>diegoyun.WithCollectionTag</tag-class>
  <body-content>JSP</body-content>
  <attribute>
   <name>property</name>
   <required>false</required>
   <rtexprvalue>true</rtexprvalue>
  </attribute>
 </tag>
 <!--ElementOutputTag-->
 <tag>
  <name>elementout</name>
  <tag-class>diegoyun.ElementOutputTag</tag-class>
  <body-content>empty</body-content>
  <attribute>
   <name>property</name>
   <required>false</required>
   <rtexprvalue>true</rtexprvalue>
  </attribute>
 </tag>
 
编写jsp
<%@ page language="java" %>
<%@ page import="diegoyun.vo.*"%>
<%@ page import="java.util.*"%>
<%@ taglib uri="/WEB-INF/tlds/diego.tld" prefix="diego"%>

<html>
<body bgcolor="#FFFFFF">
<%
Collection c = new ArrayList();

Man man1 = new Man();
man1.setName("diego");
c.add(man1);

Man man2 = new Man();
man2.setName("Zidane");
c.add(man2);

Man man3 = new Man();
man3.setName("Rui");
c.add(man3);

People p =new People();
p.setMen(c);
request.setAttribute("people",p);
%>
Test loop tag:
<br>
<diego:withObject value="${people}">
 <diego:withCollection property="men">
  <diego:elementout property="name"/>
 <br>
 </diego:withCollection>
</diego:withObject>
</body>
</html>

运行,则可以看到:
Test loop tag:
diego
Zidane
Rui