Taglib原理和实现之循环的Tag

来源： 互联网 作者： 若水   2008-05-05/15:09

软件频道 编程学院 Jsp 精华文摘 正文

• 循环
• 实现
• 原理
• public
• import
• Collect

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; #p#分页标题#e# 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