j-customtags-6-3.html

这是用java编写自定义标签的一些源码和笔记

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<a name="navskip"></a><td height="25" colspan="4"><img alt="6.用自定义标签控制流程" src="imagemaster/titlebar6.jpg" border="0" height="25" width="562"></td>
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<td width="90%"><font size="4" face="Verdana, Arial, Helvetica"><b>实现 doStartTag() 方法</b></font></td><td width="200" align="right"><font size="1" face="Verdana, Arial, Helvetica"><nobr>   第 3 页（共4 页）</nobr></font></td>
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<code>doStartTag()</code> 方法从范围中抓取集合，然后从集合中抓取迭代器。

 如果迭代器中没有任何项（<code>iter.hasNext()</code>），那么 <code>doStartTag()</code> 方法就返回 <code>SKIP_BODY</code>，从而实现了逻辑 <code>if</code>。在这种情况下，这个 <code>if</code> 等同于&ldquo;如果集合为空，则跳过正文判断&rdquo;。 

 然后迭代器从集合中抓取第一个 map。

 

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<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
  

 public class MapPrintMapsTag extends BodyTagSupport {

 

 private String name;

 private Iterator iter;

 private Map map;

 private String scope;

 

   public int doStartTag() throws JspException {

     Collection collection = null; 

     

     /* Grab the collection out of scope using the scope attribute. */

     if (scope == null){

         collection = (Collection) pageContext.findAttribute(name);

     }else if("page".equalsIgnoreCase(scope)){

         collection = (Collection) pageContext.getAttribute(name);

     }else if("request".equalsIgnoreCase(scope)){

         collection = (Collection) pageContext.getRequest().getAttribute(name);

     }else if("session".equalsIgnoreCase(scope)){

         collection = (Collection) pageContext.getSession().getAttribute(name);

     }else if("application".equalsIgnoreCase(scope)){

         collection = (Collection) pageContext.getServletContext().getAttribute(name);

     }

 

     /* Get the iterator from the collection. */

     iter = collection.iterator();

     

     /* If the collection is empty skip the body evaluation. */

     if (iter.hasNext()==false) return SKIP_BODY;

     

     /* Grab the first map out of the collection. */

     map = (Map)iter.next();

     return EVAL_BODY_BUFFERED;

   }

 

 </code>
</pre>
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