jsp自定义标签 线程安全
程序员文章站
2023-12-28 14:03:28
...
我们在编写自定义标签的时候设置属性如下
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：
<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
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
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
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);
_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) {
_005fjspx_005ftagPool_005fh3_005finput_0026_005fvalue_005furl_005ftype_005fstyleClass_005fstyle_005fpid_005fonclick_005fname_005fisValidated_005fid.reuse(_jspx_th_h3_005finput_005f0);
return true;
}
_005fjspx_005ftagPool_005fh3_005finput_0026_005fvalue_005furl_005ftype_005fstyleClass_005fstyle_005fpid_005fonclick_005fname_005fisValidated_005fid.reuse(_jspx_th_h3_005finput_005f0);
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：
/**
* 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 {
Tag 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);
}
}
/**
* 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：
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
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;
}