FutileSynchOnField.qhelp

<!DOCTYPE qhelp PUBLIC
  "-//Semmle//qhelp//EN"
  "qhelp.dtd">
<qhelp>


<overview>
<p>
A block of code that synchronizes on a field and updates that field while the
lock is held is unlikely to provide the desired thread safety. Such a synchronized 
block does not prevent multiple unsynchronized assignments to that field because it 
obtains a lock on the object stored <i>in</i> the field rather than the field itself.
</p>

</overview>
<recommendation>

<p>Instead of synchronizing on the field itself, consider synchronizing on a separate lock object 
when you want to avoid simultaneous updates to the field.
You can do this by declaring a synchronized method and using it for any field updates.
</p>

</recommendation>
<example>

<p>In the following example, in class A, synchronization takes place on the field that is updated in the body of the <code>setField</code> method.</p>

<sample src="FutileSynchOnField.java" />

<p>In class B, the recommended approach is shown, where synchronization takes place on a separate lock 
object.</p>

<sample src="FutileSynchOnFieldGood.java" />

</example>
<references>


<li>
  Java Language Specification:
  <a href="https://docs.oracle.com/javase/specs/jls/se11/html/jls-14.html#jls-14.19">The synchronized Statement</a>,
  <a href="https://docs.oracle.com/javase/specs/jls/se11/html/jls-8.html#jls-8.4.3.6">synchronized Methods</a>.
</li>
<li>
  The Java Tutorials:
  <a href="https://docs.oracle.com/javase/tutorial/essential/concurrency/newlocks.html">Lock Objects</a>.
</li>


</references>
</qhelp>