jsp tag实例

　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=$＞

＜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.PropertyUt