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第四章 循环的Tag

2004-11-24 11:47 411 查看
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4

日期：2004-11-13 03:23:31
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

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