Taglib原理和实现之循环的Tag JSP 关键词:Taglib原理,实现之循环,Tag...

Taglib原理和实现之循环的Tag
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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:
＜diegwithObject value="${people}"＞
＜diegwithCollection property="men"＞
＜diegelementout property="name"/＞
＜/diegwithCollection＞
＜/diegwithObject＞
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＞
＜diegwithObject value="${people}"＞
＜diegwithCollection property="men"＞
＜diegelementout property="name"/＞
＜br＞
＜/diegwithCollection＞
＜/diegwithObject＞
＜/body＞
＜/html＞
运行,则可以看到:
Test loop tag:
diego
Zidane
Rui