1、問題：在request裡的 People 對象，有個屬性叫 men ，men 是一個Collection ，有許多個man 。現在，把 collection裡的man的名字都顯示出來。
顯然，這是一個嵌套Tag的問題。有三個Tag互相作用：最外層的Tag找到People對象，中間的Tag取得Collection，子Tag負責打印。
例如:
＜dIEgo:withObject value="${people}"＞
＜dIEgo:withCollection property="men"＞
＜dIEgo:elementout property="name"/＞
＜/dIEgo:withCollection＞
＜/dIEgo:withObject＞
思路如下:
1) 編寫WithObjectTag,負責從El表達式中取得對象
2) 編寫WithCollectionTag,負責從對象中取得 Collection ,遍歷 Collection ,每遍歷一次 Collection ,執行一次body
3) 編寫ElementoutTag ,把 Collection 中每個men對象的 name 打印出來
2. 完整程序如下：
在上例的dIEgoyun.vo包內，編寫 People 類
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;
}
}
編寫 withObject ，這是從request裡取得People對象的最外層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;
}
}
編寫WithCollectionTag，該Tag負責取得Collection，並遍歷執行子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對象,並且得到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 {
//設置第一個元素,然後執行子Tag
iterator = list.iterator();
if (iterator.hasNext())
element = iterator.next();
return EVAL_BODY_INCLUDE;
}
public int doAfterBody() {
if (iterator.hasNext()) {
//如果還存在子元素,設置子元素,並且再次執行子Tag
//循環由此而來
//否則不再執行子Tag
element = iterator.next();
return EVAL_BODY_AGAIN;
}
else
return EVAL_PAGE;
}
}
編寫 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
{
//判斷上層tag中是否存在該屬性名稱,如果存在,取得屬性值,否則報錯
propertyValue = PropertyUtils.getProperty(parent.getElement(), property);
}
catch (Exception e)
{
throw new JSPException(e);
}
}
public int doEndTag()throws JSPException
{
try
{
//簡單的把值打印到JSP頁面
pageContext.getOut().print(propertyValue);
}
catch (IOException e)
{
throw new JSPException(e);
}
return EVAL_PAGE;
}
}
編寫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＞
編寫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＞
運行,則可以看到:
Test loop tag:
dIEgo
Zidane
Rui