转自：http://klcwt.iteye.com/blog/749652
我们在编写自定义标签的时候设置属性如下
Java代码
- public class InputTag extends TagSupport {
- private static final long serialVersionUID = 1L;
- private String onclick;
- private String style;
- private String styleClass;
- private String value;
- private String id;
在页面上如果同时使用两个标签：
Html代码
- <h3:input type="button" onclick="myFun()" name="name" id="id"
- style="style" styleClass="styleClass" value="中国人" url="url"
- pid="pid" isValidated="true">
- 中国人
- </h3:input>
- <h3:input type="button" onclick="myFun()" name="name" id="id"
- style="style" styleClass="styleClass" value="美国人" url="url"
- pid="pid" isValidated="true">
- </h3:input>
从后台发现打印的InpuTag都是同一个对象!
发现这个后，我十分担心线程安全问题！比如这些getType();setType(); ！
于是就看了下jsp生成的Servlet源代码
Java代码
- out.write("\t<body>\r\n");
- out.write("\t\t");
- //调用InputTag
- if (_jspx_meth_h3_005finput_005f0(_jspx_page_context))
- return;
- out.write("\r\n");
- out.write("\t\t\r\n");
- out.write("\t\t");
- //调用InputTag
- if (_jspx_meth_h3_005finput_005f1(_jspx_page_context))
- return;
- out.write("\r\n");
- out.write("\t</body>\r\n");
再接着看_jspx_meth_h3_005finput_005f0方法
Java代码
- private boolean _jspx_meth_h3_005finput_005f0(PageContext _jspx_page_context)
- throws Throwable {
- PageContext pageContext = _jspx_page_context;
- JspWriter out = _jspx_page_context.getOut();
- // h3:input
- <span style="color: #ff0000;"> tag.InputTag <span style="color: #0000ff;">_jspx_th_h3_005finput_005f0</span> = (tag.InputTag) _005fjspx_005ftagPool_005fh3_005finput_0026_005fvalue_005furl_005ftype_005fstyleClass_005fstyle_005fpid_005fonclick_005fname_005fisValidated_005fid.get(tag.InputTag.class);</span>
- _jspx_th_h3_005finput_005f0.setPageContext(_jspx_page_context);
- _jspx_th_h3_005finput_005f0.setParent(null);
- // /button2.jsp(12,2) name = type type = null reqTime = true required = false fragment = false deferredValue = false expectedTypeName = null deferredMethod = false methodSignature = null
- _jspx_th_h3_005finput_005f0.setType("button");
- // /button2.jsp(12,2) name = onclick type = java.lang.String reqTime = false required = true fragment = false deferredValue = false expectedTypeName = null deferredMethod = false methodSignature = null _jspx_th_h3_005finput_005f0.setPid("pid");
- // /button2.jsp(12,2) name = isValidated type = null reqTime = true required = false fragment = false deferredValue = false expectedTypeName = null deferredMethod = false methodSignature = null
- _jspx_th_h3_005finput_005f0.setIsValidated("true");
- int _jspx_eval_h3_005finput_005f0 = _jspx_th_h3_005finput_005f0.doStartTag();
- if (_jspx_eval_h3_005finput_005f0 != javax.servlet.jsp.tagext.Tag.SKIP_BODY) {
- do {
- out.write("\r\n");
- out.write("\t\t中国人\r\n");
- out.write("\t\t");
- int evalDoAfterBody = _jspx_th_h3_005finput_005f0.doAfterBody();
- if (evalDoAfterBody != javax.servlet.jsp.tagext.BodyTag.EVAL_BODY_AGAIN)
- break;
- } while (true);
- }
- if (_jspx_th_h3_005finput_005f0.doEndTag() == javax.servlet.jsp.tagext.Tag.SKIP_PAGE) {
- <span style="color: #ff0000;">_005fjspx_005ftagPool_005fh3_005finput_0026_005fvalue_005furl_005ftype_005fstyleClass_005fstyle_005fpid_005fonclick_005fname_005fisValidated_005fid.reuse(_jspx_th_h3_005finput_005f0);</span>
- return true;
- }
- <span style="color: #ff0000;"> _005fjspx_005ftagPool_005fh3_005finput_0026_005fvalue_005furl_005ftype_005fstyleClass_005fstyle_005fpid_005fonclick_005fname_005fisValidated_005fid.reuse(_jspx_th_h3_005finput_005f0);</span>
- return false;
- }
最关键就是这句了,看他如何获得自定义标签对象： tag.InputTag _jspx_th_h3_005finput_005f0 = (tag.InputTag) _005fjspx_005ftagPool_005fh3_005finput_0026_005fvalue_005furl_005ftype_005fstyleClass_005fstyle_005fpid_005fonclick_005fname_005fisValidated_005fid.get(tag.InputTag.class);
解释下：
_jspx_th_h3_005finput_005f0 是InputTag 的实例 也就是<h3:input.
而
005fjspx_005ftagPool_005fh3_005finput_0026_005fvalue_005furl_005ftype_005fstyleClass_005fstyle_005fpid_005fonclick_005fname_005fisValidated_005
是TagHandlerPool的实例
自定义标签是通过这个TagHandlerPool.get 来获取的！
举一反三，有借就有还TagHandlerPool.reuse用来回收这个对象！
TagHandlerPool.get
TagHandlerPool.reuse
方法如下：
Java代码
- /**
- * Gets the next available tag handler from this tag handler pool,
- * instantiating one if this tag handler pool is empty.
- *
- * @param handlerClass Tag handler class
- *
- * @return Reused or newly instantiated tag handler
- *
- * @throws JspException if a tag handler cannot be instantiated
- */
- public Tag get(Class handlerClass) throws JspException {
- g handler = null;
- synchronized( this ) {
- if (current >= 0) {
- handler = handlers[current--];
- return handler;
- }
- }
- // Out of sync block - there is no need for other threads to
- // wait for us to construct a tag for this thread.
- try {
- Tag instance = (Tag) handlerClass.newInstance();
- AnnotationHelper.postConstruct(annotationProcessor, instance);
- return instance;
- } catch (Exception e) {
- throw new JspException(e.getMessage(), e);
- }
- }
Java代码
- /**
- * Adds the given tag handler to this tag handler pool, unless this tag
- * handler pool has already reached its capacity, in which case the tag
- * handler's release() method is called.
- *
- * @param handler Tag handler to add to this tag handler pool
- */
- public void reuse(Tag handler) {
- synchronized( this ) {
- if (current < (handlers.length - 1)) {
- handlers[++current] = handler;
- return;
- }
- }
- // There is no need for other threads to wait for us to release
- handler.release();
- if (annotationProcessor != null) {
- try {
- AnnotationHelper.preDestroy(annotationProcessor, handler);
- } catch (Exception e) {
- log.warn("Error processing preDestroy on tag instance of "
- + handler.getClass().getName(), e);
- }
- }
- }
现在就明白了所有的tag对象都是有一个对象池来维护的，一是方便了重用，而是做到了线程同步。
总结：自定义标签是线程安全的，同时也是可重用的！
同时又有另一个疑问
synchronized( this ) {
if (current >= 0) {
handler = handlers[current--];
return handler;
}
}
感觉这种方法可能只能在一个页面上共享，另一个页面上的tag估计是不能共享的！
后来看了下生成的servelt代码
如下：
Java代码
- public void _jspInit() {
- _tagpool = org.apache.jasper.runtime.TagHandlerPool.getTagHandlerPool(getServletConfig());
- _el_expressionfactory = _jspxFactory.getJspApplicationContext(getServletConfig().getServletContext()).getExpressionFactory();
- _jsp_annotationprocessor = (org.apache.AnnotationProcessor) getServletConfig().getServletContext().getAttribute(org.apache.AnnotationProcessor.class.getName());
- }
可以看到_tagpool 是根据ServletConifg来生成的
TagHandlerPool.getTagHandlerPool代码如下
Java代码
- public static TagHandlerPool getTagHandlerPool( ServletConfig config) {
- TagHandlerPool result=null;
- String tpClassName=getOption( config, OPTION_TAGPOOL, null);
- if( tpClassName != null ) {
- try {
- Class c=Class.forName( tpClassName );
- result=(TagHandlerPool)c.newInstance();
- } catch (Exception e) {
- e.printStackTrace();
- result=null;
- }
- }
- if( result==null ) result=new TagHandlerPool();
- result.init(config);
- return result;
- }