j-customtags-3-7.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="3.简单标签" src="imagemaster/titlebar3.jpg" border="0" height="25" width="562"></td>
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<td width="90%"><font size="4" face="Verdana, Arial, Helvetica"><b>第 5 步：实现 doStartTag() 方法</b></font></td><td width="200" align="right"><font size="1" face="Verdana, Arial, Helvetica"><nobr>   第 7 页（共9 页）</nobr></font></td>
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<p>
          

  标签开始时调用 <code>doStartTag()</code> 方法 —— 从开发人员的角度看，这是当引擎遇到 

 <code>&lt;map:mapDefine&gt;</code> 时发生的。如果 <code>doStartTag()</code> 返回 <code>SKIP_BODY</code>，那么将不处理标签正文。 

 如果它返回一个 <code>EVAL_BODY_INCLUDE</code>，那么将处理正文。
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<p>
						
<code>MapDefine</code> 类的 <code>doStartTag()</code> 方法完成以下工作：
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<ul>
<li>根据 <code>type</code> 属性确定要创建的 map 的属性。</li>
<li>根据 <code>scope</code> 属性确定新的 map 对象放在什么范围内。</li>
<li>根据 <code>id</code> 属性确定新 map 对象要放入的范围的名字。</li>
</ul>
<p>
 让我们更详细地分析这个过程。<code>MapDefine</code> 类检查 <code>type</code> 属性是设置为 <code>FASTTREE</code>、<code>HASH</code>、<code>TREE</code> 还是 <code>FASTHASH</code>。然后创建相应的 map，如下所示：

 

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

     /* String constants for the different types of maps we support */

     public static final String FASTHASH = "FASTHASH";

     public static final String FASTTREE = "FASTTREE";

     public static final String HASH = "HASH";

     public static final String TREE = "TREE";

     

     /** The map we are going to create */

     private Map map = null;

     

     /** The member variable that holds the type attribute */

     private String type = FASTTREE;

     ...

 

     public int doStartTag() throws JspException {

 

         /** Based on the type attribute, determines which type of Map to create */

         if (type.equalsIgnoreCase(FASTTREE)) {

             map = new FastTreeMap();

         } else if (type.equalsIgnoreCase(HASH)) {

             map = new HashMap();

         } else if (type.equalsIgnoreCase(TREE)) {

             map = new TreeMap();

         } else if (type.equalsIgnoreCase(FASTHASH)) {

             map = new FastHashMap();

         }

         

 </code>
</pre>
<p>
  

 

 

 然后，用 <code>id</code> 和 <code>scope</code> 属性将 hashmap 以一个给定的名字设置到一个给定范围中：

 

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

 

     private String id;

     private String scope;

 

     public int doStartTag() throws JspException {

 

         ...        

         if (scope == null){

             pageContext.setAttribute(id, map);

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

             pageContext.setAttribute(id, map);

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

             pageContext.getRequest().setAttribute(id, map);

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

             pageContext.getSession().setAttribute(id, map);

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

             pageContext.getServletContext().setAttribute(id, map);

 

         }

         

         return EVAL_BODY_INCLUDE;

     }

 

 </code>
</pre>
<p>
  

 

 如果范围属性是 <code>null</code>，那么 map 将放入页范围。否则，参数将放入通过 <code>scope</code> 属性传递的范围名所对应的范围中。 

 </p>
<p>

 到目前为止，我们已经有一个非常简单的标签，它有三个属性：<code>id</code>、<code>scope</code> 和 <code>type</code>。

我们将用给定的名字将 map 放到一个范围中。但是，我们还有一件事没做，就是声明 scriptlet 变量。

 为了做到这一点，需要向 TLD 再添加一项，这就是我们在下一小节中所要做的事。 

 

 


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