StartInConstructor.qhelp

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


<overview>
<p>Starting a thread within a constructor may cause unexpected results. If the class is extended, 
the thread may start before the subclass constructor has completed its initialization, 
which may not be intended.</p>

</overview>
<recommendation>

<p>Avoid starting threads in constructors.
Typically, the constructor of a class only <i>constructs</i> the thread object, 
and a separate <code>start</code> method should be provided to <i>start</i> the thread object
created by the constructor.
</p>

</recommendation>
<example>

<p>In the following example, because the <code>Test</code> constructor implicitly calls the
<code>Super</code> constructor, the thread created in the <code>Super</code> constructor may start 
before <code>this.name</code> has been initialized. Therefore, the program may
output "hello " followed by a null string. 
</p>

<sample src="StartInConstructor.java" />

<p>In the following modified example, the thread created in the <code>Super</code> constructor is not started
within the constructor; <code>main</code> starts the thread after <code>this.name</code> has been 
initialized. This results in the program outputting "hello my friend".</p>

<sample src="StartInConstructorGood.java" />
 
</example>
<references>


<li>
  IBM developerWorks:
  <a href="https://web.archive.org/web/20200417101823/http://www.ibm.com/developerworks/java/library/j-jtp0618/index.html#4">Don't start threads from within constructors</a>.
</li>


</references>
</qhelp>