DateFormatThreadUnsafe.qhelp

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


<overview>
<p>Static fields of type <code>java.text.DateFormat</code> or its descendants
should be avoided because the class <code>DateFormat</code> is not thread-safe.
</p>

</overview>
<recommendation>

<p>Use instance fields instead and synchronize access where necessary.</p>

</recommendation>
<example>

<p>In the following example, <code>DateFormattingThread</code> declares a static field <code>dateF</code>
of type <code>DateFormat</code>. When instances of <code>DateFormattingThread</code> are created 
and run by <code>DateFormatThreadUnsafe</code>, erroneous results are output because <code>dateF</code>
is shared by all instances of <code>DateFormattingThread</code>.</p>

<sample src="DateFormatThreadUnsafe.java" />

<p>In the following modification of the above example, <code>DateFormattingThread</code> declares an
<i>instance</i> field <code>dateF</code> of type <code>DateFormat</code>. When instances of 
<code>DateFormattingThread</code> are created and run by <code>DateFormatThreadUnsafeFix</code>, 
correct results are output because there is a separate instance of <code>dateF</code> for each 
instance of <code>DateFormattingThread</code>.</p>

<sample src="DateFormatThreadUnsafeGood.java" />

</example>
<references>


<li>
  Java API Specification:
  <a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/text/DateFormat.html#synchronization">java.text.DateFormat synchronization</a>.
</li>

</references>
</qhelp>