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specify the JAR file that contains all of <CODE>MyApplet's</CODE> related 
files. The executable specified by the <CODE>CODE</CODE> tag is sometimes
called the <CODE>code base</CODE>.

<P>
For security reasons the JAR files listed by the <CODE>archive</CODE> 
parameter must be in the same directory or a sub-directory as the 
applets <CODE>codebase</CODE>. If no <CODE>codebase</CODE> parameter 
is supplied the directory from where the applet was loaded is used as 
the <CODE>codebase</CODE>.

<P>
The following example specifies <CODE>jarfile</CODE> as the JAR file that 
contains the related files for the <CODE>MyApplet.class</CODE> executable. 

<P>
&lt;APPLET CODE=&quot;MyApplet.class&quot; ARCHIVE=&quot;jarfile&quot;
	WIDTH=&quot;100&quot; HEIGHT=&quot;200&quot;&gt;
&lt;/APPLET&gt;

<P>
If the applet download uses multiple JAR files as shown in the next HTML 
segment, the <CODE>ClassLoader</CODE> loads each JAR file when the applet 
starts. So, if your applet uses some resource files infrequently, the JAR 
file containing those infrequently used files is downloaded, regardless of 
whether the resources are actually used during that session or not.

<P>
&lt;APPLET CODE=&quot;MyApplet.class&quot; ARCHIVE=&quot;jarfile1, jarfile2&quot;
        WIDTH=&quot;100&quot; HEIGHT=&quot;200&quot;&gt;
&lt;/APPLET&gt;

<P>
To improve performance when an applet has infrequently used files, put the
frequently used files into the JAR file and the infrequently used files into 
the applet class directory. Infrequently used files are then located and
downloaded by the browser only when needed. 

<A NAME="pool"></A>
<H3>Thread Pooling</H3>

The Java Developer Connection<FONT SIZE="-2"><SUP>SM</SUP></FONT> 
(JDC) applet servers and the Java Web Server<FONT SIZE="-2"><SUP>TM</SUP></FONT>
make extensive use of  thread pooling to improve performance. Thread pooling 
is creating a ready supply of sleeping threads at the beginning of execution. 
Because the thread startup process is expensive in terms of system resources, 
thread pooling makes the startup process a little slower, but improves runtime 
performance because sleeping (or suspended) threads are awakened only when 
they are needed to perform new tasks. 

<P>
This code sample taken from the <A HREF="./Code/Pool.java">Pool.java</A>
class shows one way to implement thread pooling.  In the pool's constructor 
(shown below), the <CODE>WorkerThreads</CODE> are initialized and started. 
The call to the <CODE>start</CODE> method executes the <CODE>run</CODE> method 
of the <CODE>WorkerThread</CODE>, and the call to <CODE>wait</CODE> in the 
<CODE>run</CODE> method suspends the <CODE>Thread</CODE> while the 
<CODE>Thread</CODE> waits for work to arrive. The last line of the constructor 
pushes the sleeping <CODE>Thread</CODE> onto the stack.

</FONT>

<PRE><FONT SIZE="-1">
public Pool (int max, Class workerClass) 
                         throws Exception {

    _max = max;
    _waiting = new Stack();
    _workerClass = workerClass;
    Worker worker;
    WorkerThread w;
    for ( int i = 0; i &lt; _max; i++ ) {
        worker = (Worker)_workerClass.newInstance();
        w = new WorkerThread (&quot;Worker#&quot;+i, worker);
        w.start();
        _waiting.push (w);
    }
}
</FONT>
</PRE>

<FONT FACE="Verdana, Arial, Helvetica, sans-serif">

<P>
Besides the <CODE>run</CODE> method, the <CODE>WorkerThread</CODE> class has 
a <CODE>wake</CODE> method.  When work comes in, the <CODE>wake</CODE> method 
is called, which assigns the data and notifies the sleeping 
<CODE>WorkerThread</CODE> (the one initialized by the <CODE>Pool</CODE>) 
to resume running.  The <CODE>wake</CODE> method's call to <CODE>notify</CODE>
causes the blocked <CODE>WorkerThread</CODE> to fall out of its wait state, 
and the <CODE>run</CODE> method of the 
<A HREF="./Code/HttpServerWorker.java">HttpServerWorker</A> class is executed.
Once the work is done, the <CODE>WorkerThread</CODE> is either put back onto 
the <CODE>Stack</CODE> (assuming the Thread <CODE>Pool</CODE> is not full) or 
terminates.

</FONT>

<PRE><FONT SIZE="-1">  
synchronized void wake (Object data) {
  	_data = data; 
	notify();
  }

  synchronized public void run(){
    boolean stop = false; 
    while (!stop){ 
	if ( _data == null ){ 
	  try{
	     wait();
	  }catch (InterruptedException e){ 
		e.printStackTrace();
		continue;
	  }
	}

	if ( _data != null ){
	  _worker.run(_data);
	}

	_data = null;
	stop = !(_push (this));
    }
  }
</FONT>
</PRE>

<FONT FACE="Verdana, Arial, Helvetica, sans-serif">

At its highest level, incoming work is handled by the <CODE>performWork</CODE> 
method in the <CODE>Pool</CODE> class (shown below). As work comes in, an 
existing <CODE>WorkerThread</CODE> is popped off of the <CODE>Stack</CODE> 
(or a new one is created if the <CODE>Pool</CODE> is empty). The sleeping 
<CODE>WorkerThread</CODE> is then activated by a call to its <CODE>wake</CODE> 
method.

</FONT>

<PRE><FONT SIZE="-1">
public void performWork (Object data) 
               throws InstantiationException{
  WorkerThread w = null;
  synchronized (_waiting){
     if ( _waiting.empty() ){
	try{
	  w = new WorkerThread (&quot;additional worker&quot;,
	  (Worker)_workerClass.newInstance());
	  w.start();
	}catch (Exception e){
	  throw new InstantiationException (
	              &quot;Problem creating 
	              instance of Worker.class: &quot; 
	              + e.getMessage());
	}
      }else{
	w = (WorkerThread)_waiting.pop();
      }
  }
  w.wake (data);
}
</FONT>
</PRE>

<FONT FACE="Verdana, Arial, Helvetica, sans-serif">

The <A HREF="./Code/HttpServer.java">HttpServer.java</A> class constructor
creates a new <CODE>Pool</CODE> instance to service 
<A HREF="./Code/HttpServerWorker.java">HttpServerWorker</A> instances.
<CODE>HttpServerWorker</CODE> instances are created and stored as part of the 
<CODE>WorkerThread</CODE> data. When a <CODE>WorkerThread</CODE> is activated 
by a call to its <CODE>wake</CODE> method, the <CODE>HttpServerWorker</CODE> 
instance is invoked by way of its <CODE>run</CODE> method.
 
</FONT>

<PRE><FONT SIZE="-1">  
try{
    _pool = new Pool (poolSize, 
                  HttpServerWorker.class);
  }catch (Exception e){
    e.printStackTrace();
    throw new InternalError (e.getMessage());
  }
</FONT>
</PRE>

<FONT FACE="Verdana, Arial, Helvetica, sans-serif">

<P>
This next code is in the <CODE>run</CODE> method of the
<A HREF="./Code/HttpServer.java">HttpServer.java</A> class. Every time a 
request comes in, the data is initialized and the <CODE>Thread</CODE> starts 
work.  

  <BLOCKQUOTE>
  <HR>
  <STRONG>Note:</STRONG> If creating a new <CODE>Hashtable</CODE> for 
  each <CODE>WorkerThread</CODE> presents too much overhead, just modify 
  the code so it does not use the <CODE>Worker</CODE> abstraction. 
  
  <HR>
  </BLOCKQUOTE>

</FONT>

<PRE><FONT SIZE="-1">  
try{
	Socket s = _serverSocket.accept();
	Hashtable data = new Hashtable();
	data.put (&quot;Socket&quot;, s); 
	data.put (&quot;HttpServer&quot;, this);
	_pool.performWork (data); 
  }catch (Exception e){ 
	e.printStackTrace();
  }
</FONT>
</PRE>

<FONT FACE="Verdana, Arial, Helvetica, sans-serif">

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.

<HR>
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