BusyWait.qhelp

<!DOCTYPE qhelp PUBLIC
  "-//Semmle//qhelp//EN"
  "qhelp.dtd">
<qhelp>
<overview>
<p>
Trying to control thread interaction by periodically calling <code>Thread.sleep</code> within a 
loop while waiting for a condition to be satisfied is less effective than waiting for a notification. 
This is because the waiting thread may either sleep for an unnecessarily long time or wake up too
frequently. This approach may also result in race conditions and, therefore, incorrect code.
</p>

<p>
Trying to control thread interaction by repeatedly checking a synchronized data structure without 
calling <code>Thread.sleep</code> or waiting for a notification may waste a lot of system resources 
and cause noticeable performance problems.
</p>

</overview>
<recommendation>

<p>See if communication between threads can be improved by using either of the following solutions:</p>

<ul>
    <li>The <code>java.util.concurrent</code> library, preferably</li>
    <li>The <code>Object.wait</code> and <code>Object.notifyAll</code> methods</li> 
</ul>

<p>
If following one of these recommendations is not feasible, ensure that race conditions cannot occur 
and precise timing is not required for program correctness.
</p>

</recommendation>
<example>

<p>In the following example, the <code>Receiver</code> thread sleeps for an unnecessarily long time
(up to five seconds) until it has received the message.</p>

<sample src="BusyWait.java" />

<p>In the following modification of the above example, the <code>Receiver</code> thread uses the 
recommended approach of waiting for a notification that the 
message has been sent. This means that the thread can respond immediately instead of sleeping.</p>

<sample src="BusyWaitGood.java" />

</example>
<references>


    <li>J. Bloch, <em>Effective Java (second edition)</em>, Item 72. Addison-Wesley, 2008.</li>
    <li>Java API Documentation:
    <a href="http://docs.oracle.com/javase/6/docs/api/java/lang/Object.html#wait%28%29">Object.wait()</a>,
    <a href="http://docs.oracle.com/javase/6/docs/api/java/lang/Object.html#notifyAll%28%29">Object.notifyAll()</a>,
    <a href="http://docs.oracle.com/javase/6/docs/api/java/util/concurrent/package-summary.html#package_description">java.util.concurrent</a>.</li>
    <li>The Java Tutorials: <a href="http://docs.oracle.com/javase/tutorial/essential/concurrency/guardmeth.html">Guarded Blocks</a>,
    <a href="http://docs.oracle.com/javase/tutorial/essential/concurrency/highlevel.html">High Level Concurrency Objects</a>.</li>


</references>
</qhelp>