SleepWithLock.qhelp

<!DOCTYPE qhelp PUBLIC
  "-//Semmle//qhelp//EN"
  "qhelp.dtd">
<qhelp>
<overview>
<p>Calling <code>Thread.sleep</code> with a lock held may lead to very poor performance
or even deadlock. This is because <code>Thread.sleep</code> does not cause a thread to release its locks.</p>

</overview>
<recommendation>

<p>
<code>Thread.sleep</code> should be called only outside of a <code>synchronized</code> block.
However, a better way for threads to yield execution time to other threads may be to use either of 
the following solutions:</p>

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

</recommendation>
<example>

<p>In the following example of the problem, two threads, <code>StorageThread</code> and <code>OtherThread</code>,
are started. Both threads output a message to show that they have started but then 
<code>StorageThread</code> locks <code>counter</code> and goes to sleep. The lock prevents 
<code>OtherThread</code> from locking <code>counter</code>, so it has to wait until 
<code>StorageThread</code> has woken up and unlocked <code>counter</code> before it can continue.</p>

<sample src="SleepWithLock.java" />

<p>To avoid this problem, <code>StorageThread</code> should call <code>Thread.sleep</code> outside 
the <code>synchronized</code> block instead, so that <code>counter</code> is unlocked.</p>

</example>
<references>


    <li>Java API Specification: <a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Thread.html#sleep(long)">Thread.sleep()</a>,
    <a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Object.html#wait()">Object.wait()</a>,
    <a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Object.html#notifyAll()">Object.notifyAll()</a>,
    <a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/concurrent/package-summary.html">java.util.concurrent</a>.</li>


</references>
</qhelp>