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    <font face="arial, helvetica" size="-1" color="#336633"><b>Mastering Java Threads</b></font>
    <br>
    <font face="arial, helvetica" size="-1"><i>by Marc Adler and David Herst</i>
    <br>
    DDC Publishing, Inc.
    <br>
    <b>ISBN:</b>&nbsp;1562438425<b>&nbsp;&nbsp;&nbsp;Pub Date:</b>&nbsp;05/01/99</font>&nbsp;&nbsp;<A HREF="http://www.digitalguru.com/dgstore/product.asp?isbn=1562438425&ac%5Fid=28" TARGET="anotherwindows"><img src="/images/buyit.gif" width=64 height=23 hspace="5" align="middle" alt="Buy It" border="0"></a>
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<P><BR></P>
<H4 ALIGN="LEFT"><A NAME="Heading14"></A><FONT COLOR="#000077">The wait[] &amp; notify[] Methods</FONT></H4>
<P>The methods <B>wait()</B>, <B>notify()</B>, and <B>notifyAll()</B> are used to control synchronization within Java applications. Note that these methods are part of Java&#146;s Object class, not the Thread class.</P>
<P>There are three forms associated with the <B>wait()</B> method, as listed below:</P>
<DL>
<DD><B>1.</B>&nbsp;&nbsp;<SMALL>public final void wait() throws InterruptedException</SMALL>
<DD><B>2.</B>&nbsp;&nbsp;<SMALL>public final void wait(long timeout) throws InterruptedException</SMALL>
<DD><B>3.</B>&nbsp;&nbsp;<SMALL>public final void wait(long timeout, int nanos) throws InterruptedException</SMALL>
</DL>
<P>Note the following points regarding the <B>wait()</B> and <B>notify()</B> methods.</P>
<DL>
<DD><B>&#149;</B>&nbsp;&nbsp;The first form of <B>wait()</B> will wait forever; the other two forms will wait for a particular period of time.
<DD><B>&#149;</B>&nbsp;&nbsp;The <B>wait()</B> method will cause a thread to wait until it is notified or the specified timeout expires. It can only be called from within a synchronized method by the thread that holds the lock. The <B>wait()</B> method will also release the monitor.
<TABLE WIDTH="90%">
<TR>
<TD ALIGN="LEFT" VALIGN="TOP" WIDTH="5%"><IMG SRC="images/09-04i.jpg"></TD>
<TD ALIGN="LEFT" VALIGN="TOP" WIDTH="85%">A thread will awaken if the <B>notify()</B> or <B>notifyAll()</B> methods are called.</TD>
</TR>
</TABLE>
<DD><B>&#149;</B>&nbsp;&nbsp;The <B>notify()</B> method notifies a single waiting thread of a change in condition of another thread. The thread effecting the change notifies the waiting thread using <B>notify()</B>. Threads that want to wait for a condition to change before proceeding can call <B>wait()</B>.
<DD><B>&#149;</B>&nbsp;&nbsp;The <B>notifyAll()</B> method notifies <I>all</I> threads waiting for a condition to change. Threads that are waiting are generally waiting for another thread to change some condition. Thus, the thread effecting a change for which more than one thread is waiting notifies all of the waiting threads using the method <B>notifyAll()</B>. One of the waiting threads will grab the monitor and proceed. The other waiting threads will be forced to wait longer.
<DD><B>&#149;</B>&nbsp;&nbsp;The <B>notify()</B> and <B>notifyAll()</B> methods can only be called from within a synchronized method by the thread that holds the lock.
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<TD ALIGN="LEFT" VALIGN="TOP" WIDTH="5%"><IMG SRC="images/09-05i.jpg"></TD>
<TD ALIGN="LEFT" VALIGN="TOP" WIDTH="85%">Most often, you should use <B>notifyAll()</B> over the <B>notify()</B> method.</TD>
</TR>
</TABLE>
</DL>
<P><FONT SIZE="+1"><B>Producer &amp; Consumer Interaction</B></FONT></P>
<P>Table 9-1 implements a timeline so the interaction between the Producer and Consumer with the monitor can be seen.
</P>
<TABLE WIDTH="100%" BORDER><TR>
<CAPTION ALIGN="CENTER" VALIGN="BOTTOM"><B>Table 9-1: Interaction of the Producer and the Consumer with the monitor</B></CAPTION>
</TR>
<TR>
<TH ALIGN="CENTER" VALIGN="TOP" WIDTH="20%">Time</TH>
<TH ALIGN="CENTER" VALIGN="TOP" WIDTH="40%">Consumer</TH>
<TH ALIGN="CENTER" VALIGN="TOP" WIDTH="40%">Producer</TH>
</TR>
<TR>
<TD ALIGN="CENTER" VALIGN="TOP">T1</TD>
<TD ALIGN="LEFT" VALIGN="TOP"><B>Run()</B> calls <B>get()</B> and the Consumer acquires the monitor when it enters <B>Get()</B>.</TD>
<TD ALIGN="LEFT" VALIGN="TOP">Polling thread pulls some stock info off of the server.</TD>
</TR>
<TR>
<TD ALIGN="CENTER" VALIGN="TOP">T2</TD>
<TD ALIGN="LEFT" VALIGN="TOP">&nbsp;</TD>
<TD ALIGN="LEFT" VALIGN="TOP">Thread calls <B>put()</B>, but because the Consumer has the monitor, the Producer is forced to sleep at the entrance to <B>put()</B>.</TD>
</TR>
<TR>
<TD ALIGN="CENTER" VALIGN="TOP">T3</TD>
<TD ALIGN="LEFT" VALIGN="TOP">No stock available, so calls <B>wait()</B>. which releases the monitor</TD>
<TD ALIGN="LEFT" VALIGN="TOP">Locked out...</TD>
</TR>
<TR>
<TD ALIGN="CENTER" VALIGN="TOP">T4</TD>
<TD ALIGN="LEFT" VALIGN="TOP">waiting...</TD>
<TD ALIGN="LEFT" VALIGN="TOP">Wakes up and enters <B>put()</B>. The Producer now has the monitor.</TD>
</TR>
<TR>
<TD ALIGN="CENTER" VALIGN="TOP">T5</TD>
<TD ALIGN="LEFT" VALIGN="TOP">waiting...</TD>
<TD ALIGN="LEFT" VALIGN="TOP">Puts stock information into a Stock object.</TD>
</TR>
<TR>
<TD ALIGN="CENTER" VALIGN="TOP">T6</TD>
<TD ALIGN="LEFT" VALIGN="TOP">waiting...</TD>
<TD ALIGN="LEFT" VALIGN="TOP">Sets <SMALL>m_bStockPending</SMALL> to true</TD>
</TR>
<TR>
<TD ALIGN="CENTER" VALIGN="TOP">T7</TD>
<TD ALIGN="LEFT" VALIGN="TOP">waiting...</TD>
<TD ALIGN="LEFT" VALIGN="TOP">Calls <B>notifyAll()</B></TD>
</TR>
<TR>
<TD ALIGN="CENTER" VALIGN="TOP">T8</TD>
<TD ALIGN="LEFT" VALIGN="TOP">Woken and waiting for its time slice</TD>
<TD ALIGN="LEFT" VALIGN="TOP">Exits <B>put()</B> and releases the monitor</TD>
</TR>
<TR>
<TD ALIGN="CENTER" VALIGN="TOP">T9</TD>
<TD ALIGN="LEFT" VALIGN="TOP">Gets out of the while loop. Reacquires the monitor.</TD>
<TD ALIGN="LEFT" VALIGN="TOP">Polls for another stock. it is locked out of <B>put()</B> until the Consumer finishes.</TD>
</TR>
<TR>
<TD ALIGN="CENTER" VALIGN="TOP">T10</TD>
<TD ALIGN="LEFT" VALIGN="TOP">Fetches the stock object and inserts it into the database.</TD>
<TD ALIGN="LEFT" VALIGN="TOP">Gets another stock and calls <B>put()</B>.</TD>
</TR>
<TR>
<TD ALIGN="CENTER" VALIGN="TOP">T11</TD>
<TD ALIGN="LEFT" VALIGN="TOP">&nbsp;</TD>
<TD ALIGN="LEFT" VALIGN="TOP">Locked out of put until the monitor is released</TD>
</TR>
<TR>
<TD ALIGN="CENTER" VALIGN="TOP">T12</TD>
<TD ALIGN="LEFT" VALIGN="TOP">Calls <B>notifyAll()</B> and exits <B>get()</B>. The monitor is released.</TD>
<TD ALIGN="LEFT" VALIGN="TOP">&nbsp;</TD>
</TR>
<TR>
<TD ALIGN="CENTER" VALIGN="TOP">T13</TD>
<TD ALIGN="LEFT" VALIGN="TOP">&nbsp;</TD>
<TD ALIGN="LEFT" VALIGN="TOP">Enters <B>put()</B> and acquires the monitor.</TD>
</TR>
<TR>
<TD ALIGN="CENTER" VALIGN="TOP">T14</TD>
<TD ALIGN="LEFT" VALIGN="TOP"><B>Run()</B> calls <B>get()</B>, but the Consumer is locked out.</TD>
<TD ALIGN="LEFT" VALIGN="TOP">&nbsp;</TD>
</TR>
</TABLE>
<H3><A NAME="Heading15"></A><FONT COLOR="#000077">Another Example of Synchronization</FONT></H3>
<P>The following example is by Chuck McManis, as published in <I>Java World Magazine</I><SUP><SMALL><B>1</B></SMALL></SUP>. It is a simple simulation of a ping pong game involving two players, Bob and Alice. The shared resource is a ping pong table, which is represented by the Java object named <SMALL>PingPong</SMALL>.</P>

<BLOCKQUOTE>
<HR>
<SUP><SMALL><B>1</B></SMALL></SUP><FONT SIZE="-1">April 1996 issue.
</FONT>
<HR>
</BLOCKQUOTE>

<H4 ALIGN="LEFT"><A NAME="Heading16"></A><FONT COLOR="#000077">How the Program Starts</FONT></H4>
<P>The program starts off by creating a <SMALL>PingPong</SMALL> object, and then creates two players, each of which plays the game in a separate thread. After sleeping for 5 seconds, the game is terminated.</P>
<P>The <SMALL>Player</SMALL> object consists of a pointer to the <SMALL>PingPong</SMALL> table object and the name of the player&#146;s opponent. The Player&#146;s <B>run()</B> method loops continuously, calling <B>PingPong.hit()</B> until the PingPong&#146;s hit method returns <B>false</B>.</P>
<H4 ALIGN="LEFT"><A NAME="Heading17"></A><FONT COLOR="#000077">Guts of the Program</FONT></H4>
<P>The real guts of the program are in the <B>PingPong</B> method. You can consider the &#147;ball&#148; as being the critical shared resource; only one player can hit the ball at a time. The action of hitting the &#147;ball&#148; is represented by the following statement:</P>
<!-- CODE SNIP //-->
<PRE>
         System.out.println(&#147;PING! (&#147;&#43;x&#43;&#148;) &#148;);
</PRE>
<!-- END CODE SNIP //-->
<P>Only one player can reach this code at a time. More importantly, a thread cannot execute this statement twice in a row; this statement must be executed alternatively by thread one and thread two.
</P><P><BR></P>
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