WaitWithTwoLocks.qhelp

<!DOCTYPE qhelp PUBLIC
  "-//Semmle//qhelp//EN"
  "qhelp.dtd">
<qhelp>
<overview>
<p>Calling <code>Object.wait</code> while two locks are held
may cause deadlock, because only one lock is released by <code>wait</code>.
</p>

</overview>
<recommendation>

<p>See if one of the locks should continue to be held while waiting for a condition on the other
lock. If not, release one of the locks before calling <code>Object.wait</code>.</p>

</recommendation>
<example>

<p>In the following example of the problem, <code>printText</code> locks both <code>idLock</code> and
<code>textLock</code> before it reads the value of <code>text</code>. It
then calls <code>textLock.wait</code>, which releases the lock on <code>textLock</code>.
However, <code>setText</code> needs to lock <code>idLock</code> but it cannot because
<code>idLock</code> is still locked by <code>printText</code>. Thus, deadlock is caused.</p>

<sample src="WaitWithTwoLocks.java" />

<p>In the following modification of the above example, <code>id</code> and <code>text</code> are
included in the class <code>Message</code>. The method <code>printText</code>
synchronizes on the field <code>message</code> before it reads the value of <code>message.text</code>. It then calls
<code>message.wait</code>, which releases the lock on <code>message</code>.
This enables <code>setText</code> to lock <code>message</code> so that it
can proceed.</p>

<sample src="WaitWithTwoLocksGood.java" />

</example>
<references>


    <li>Java API Specification: <a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Object.html#wait()">Object.wait()</a>.</li>



</references>
</qhelp>