ch08.html

java2高级编程

Thread</EM>
 onto the stack. </P>
<PRE CLASS="CODE"><A NAME="pgfId-1087478"></A>public Pool (int max, Class workerClass)   throws Exception {     </PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087479"></A>  _max = max;</PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087480"></A>  _waiting = new Stack();</PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087481"></A>  _workerClass = workerClass;</PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087482"></A>  Worker worker;     </PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087483"></A>  WorkerThread w;     </PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087484"></A>  for ( int i = 0; i &lt; _max; i++ ) {   </PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087485"></A>     worker = (Worker)_workerClass.newInstance();</PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087486"></A>     w = new WorkerThread (Worker#+i, worker);</PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087487"></A>     w.start();</PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087488"></A>     _waiting.push(w); </PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087489"></A>   }</PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087490"></A> }</PRE>
<P CLASS="Body">
<A NAME="pgfId-1087491"></A>Besides the <EM CLASS="CODE">
run</EM>
 method, the <EM CLASS="CODE">
WorkerThread</EM>
 class has a <EM CLASS="CODE">
wake</EM>
 method. When work comes in, the <EM CLASS="CODE">
wake</EM>
 method is called, which assigns the data and notifies the sleeping <EM CLASS="CODE">
WorkerThread</EM>
 (the one initialized by the <EM CLASS="CODE">
Pool</EM>
) to resume running. The <EM CLASS="CODE">
wake</EM>
 method's call to <EM CLASS="CODE">
notify</EM>
 causes the blocked <EM CLASS="CODE">
WorkerThread</EM>
 to fall out of its <EM CLASS="CODE">
wait</EM>
 state, and the <EM CLASS="CODE">
run</EM>
 method of the <EM CLASS="CODE">
HttpServerWorker</EM>
 class (page 285) is executed. Once the work is done, the <EM CLASS="CODE">
WorkerThread</EM>
 is either put back onto the <EM CLASS="CODE">
Stack</EM>
 (assuming the thread pool is not full) or terminates. </P>
<PRE CLASS="CODE"><A NAME="pgfId-1087492"></A>synchronized void wake (Object data) {</PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087493"></A>  _data = data;</PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087494"></A>  notify();</PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087495"></A>}</PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087496"></A>synchronized public void run(){</PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087497"></A>  boolean stop = false;</PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087498"></A>  while (!stop){</PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087499"></A>    if ( _data == null ){</PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087500"></A>    try {</PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087501"></A>      wait(); </PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087502"></A>    } catch (InterruptedException e){</PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087503"></A>      e.printStackTrace(); continue;</PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087504"></A>    }</PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087505"></A>    if( _data != null ){</PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087506"></A>      _worker.run(_data); </PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087507"></A>    }
    _data = null; </PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087508"></A>    stop = !(_waiting.push(this));</PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087509"></A>  }
}</PRE>
<P CLASS="Body">
<A NAME="pgfId-1087511"></A>At its highest level, incoming work is handled by the <EM CLASS="CODE">
performWork</EM>
<A NAME="marker-1087510"></A> method in the <EM CLASS="CODE">
Pool</EM>
<A NAME="marker-1087512"></A> class (shown below). As work comes in, an existing <EM CLASS="CODE">
WorkerThread</EM>
 is popped off of the <EM CLASS="CODE">
Stack</EM>
 (or a new one is created if the <EM CLASS="CODE">
Pool</EM>
 is empty). The sleeping <EM CLASS="CODE">
WorkerThread</EM>
 is then activated by a call to its <EM CLASS="CODE">
wake</EM>
 method. </P>
<PRE CLASS="CODE"><A NAME="pgfId-1087513"></A>public void performWork (Object data)   throws InstantiationException{</PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087514"></A>  WorkerThread w = null;</PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087515"></A>  synchronized (_waiting){</PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087516"></A>    if( _waiting.empty() ){</PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087517"></A>      try {</PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087518"></A>        w = new WorkerThread (additional worker,
                                (Worker)_workerClass.newInstance());</PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087519"></A>        w.start(); </PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087520"></A>      } catch (Exception e){</PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087521"></A>        throw new InstantiationException(
                 Problem creating instance of Worker.class: + e.getMessage());</PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087522"></A>      } else{</PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087523"></A>        w = (WorkerThread)_waiting.pop(); </PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087524"></A>     }</PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087525"></A>   } </PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087526"></A>   w.wake (data);</PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087527"></A> }</PRE>
<P CLASS="Body">
<A NAME="pgfId-1087528"></A>The <EM CLASS="CODE">
HttpServer</EM>
 class (page 286) constructor creates a new <EM CLASS="CODE">
Pool</EM>
 instance to service <EM CLASS="CODE">
HttpServerWorker</EM>
 instances. <EM CLASS="CODE">
HttpServerWorker</EM>
<A NAME="marker-1087529"></A><A NAME="marker-1087530"></A> instances are created and stored as part of the <EM CLASS="CODE">
WorkerThread</EM>
 data. When a <EM CLASS="CODE">
WorkerThread</EM>
 is activated by a call to its <EM CLASS="CODE">
wake</EM>
 method, the <EM CLASS="CODE">
HttpServerWorker</EM>
 instance is invoked by way of its <EM CLASS="CODE">
run</EM>
 method. </P>
<PRE CLASS="CODE"><A NAME="pgfId-1087531"></A>  try{</PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087532"></A>    _pool = new Pool (poolSize, HttpServerWorker.class);</PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087533"></A>  } catch (Exception e){</PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087534"></A>    e.printStackTrace(); </PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087535"></A>    throw new InternalError (e.getMessage()); </PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087536"></A>  }</PRE>
<P CLASS="Body">
<A NAME="pgfId-1087537"></A>This next code is in the <EM CLASS="CODE">
run</EM>
 method of the <EM CLASS="CODE">
HttpServer</EM>
 class (page 286). Every time a request comes in, the data is initialized and the <EM CLASS="CODE">
Thread</EM>
 starts work.    </P>
<UL>
<P CLASS="NOTE">
<A NAME="pgfId-1087538"></A>NOTE If creating a new <EM CLASS="CODE">
Hashtable</EM>
 for each <EM CLASS="CODE">
WorkerThread</EM>
 presents too much overhead, just modify the code so it does not use the <EM CLASS="CODE">
Worker</EM>
 abstraction.     </P>
</UL>
<PRE CLASS="CODE"><A NAME="pgfId-1087539"></A>try {</PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087540"></A>   Socket s = _serverSocket.accept();</PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087541"></A>  Hashtable data = new Hashtable();</PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087542"></A>   data.put (Socket, s);</PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087543"></A>   data.put (HttpServer, this);</PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087544"></A>   _pool.performWork (data); </PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087545"></A>} catch (Exception e){ </PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087546"></A>  e.printStackTrace(); </PRE>
<PRE CLASS="CODE"><A NAME="pgfId-1087547"></A>}</PRE>
<P CLASS="Body">
<A NAME="pgfId-1087548"></A>Thread pooling is an effective performance-tuning technique that puts the expensive thread startup process at the startup of an application. This way, the negative impact on performance occurs once at program startup where it is least likely to be noticed. </P>
</DIV>
<DIV>
<H4 CLASS="A">
<A NAME="pgfId-1087550"></A><A NAME="78167"></A>Connection Pooling</H4>
<P CLASS="Body">
<A NAME="pgfId-1087553"></A><A NAME="marker-1087551"></A><A NAME="marker-1087552"></A>If you have used SQL or another similar tool to connect to a database and act on the data, you probably know that getting the connection and logging in is the part that takes the most time. An application can easily spend several seconds every time it needs to establish a connection. </P>
<P CLASS="Body">
<A NAME="pgfId-1087554"></A>In releases prior to JDBC 2.0 every database session required a new connection and login even if the previous connection and login used the same table and user account. If you are using a JDBC release prior to 2.0 and want to improve performance, you can <A NAME="marker-1087555"></A><A NAME="marker-1087556"></A>cache JDBC connections instead of creating a new connection and login.</P>
<P CLASS="Body">
<A NAME="pgfId-1087557"></A>Cached connections are kept in a run-time object pool and can be used and reused as needed by the application. One way to implement the object pool is to make a simple hashtable of connection objects. However, a more flexible way to do it is to write a wrapper JDBC Driver that is an intermediary between the client application and database. </P>
<P CLASS="Body">
<A NAME="pgfId-1087558"></A>The wrapper approach works particularly well in an enterprise bean that uses bean-managed persistence for two reasons: (1) Only one <EM CLASS="CODE">
Driver</EM>
 class is loaded per bean and (2) specific connection details are handled outside the bean. This section explains how to write a wrapper <EM CLASS="CODE">
JDBC Driver</EM>
 class. </P>
<DIV>
<H5 CLASS="B">
<A NAME="pgfId-1087559"></A>Wrapper Classes</H5>
<P CLASS="Body">
<A NAME="pgfId-1087562"></A><A NAME="marker-1087560"></A><A NAME="marker-1087561"></A>The wrapper<EM CLASS="CODE">
 JDBC Driver</EM>
 created for this example consists of the following three classes: </P>
<UL>
<LI CLASS="BL">
<A NAME="pgfId-1087563"></A><EM CLASS="CODE">
JDCConnectionDriver</EM>
</LI>
<LI CLASS="BL">
<A NAME="pgfId-1087564"></A><EM CLASS="CODE">
JDCConnectionPool</EM>
 </LI>
<LI CLASS="BL">
<A NAME="pgfId-1087566"></A><EM CLASS="CODE">
JDCConnectio</EM>
<EM CLASS="A">
</EM>
<A NAME="drive"></A><EM CLASS="CODE">
n</EM>
</LI>
</UL>
<P CLASS="Body">
<A NAME="pgfId-1087568"></A><EM CLASS="Bold">