jsp tag 实例

有关JSP中自定义TAG的一个实例  

2007-07-21 10:52:08|  分类： java和j2ee学习 |字号 订阅

 

　一、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 。如今，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互相做用：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=$＞
＜diego:withCollection property=men＞
＜diego:elementout property=name/＞
＜/diego:withCollection＞
＜/diego:withObject＞

　　思路以下:

　　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 中每一个men对象的 name 打印出来

　　2. 完整程序以下：

　　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,而且获得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,而后执行子Tag
iterator = list.iterator();
if (iterator.hasNext())
element = iterator.next();
return EVAL_BODY_INCLUDE;
}

　public int doAfterBody() {
if (iterator.hasNext()) {
//若是还存在子元素,设置子元素,而且再次执行子Tag
//循环由此而来
//不然再也不执行子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,若是存在,取得属性值,不然报错
propertyValue = PropertyUtils.getProperty(parent.getElement(), property);
}
catch (Exception e)
{
throw new JspException(e);
}
}
public int doEndTag()throws JspException

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=$＞
＜diego:withCollection property=men＞
＜diego:elementout property=name/＞
＜br＞
＜/diego:withCollection＞
＜/diego:withObject＞
＜/body＞
＜/html＞

　　运行,则能够看到:　

Test loop tag: diego Zidane Rui