GetClassGetResource.qhelp

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


<overview>
<p>Using the <code>Class.getResource</code> method is a common way of including
some non-code resources with an application.</p>

<p>There are problems when this is called using <code>x.getClass().getResource()</code>,
for some variable <code>x</code>. This is not a safe way to retrieve a resource. The method 
<code>getClass</code> returns the <em>run-time</em> class of <code>x</code> (that is, its actual, "most
derived" class, rather than its declared type), which causes two potential problems:</p>

<ul>
<li>If the run-time type of the receiving object is a subclass of the declared type and is in
a different package, the resource path may be interpreted differently. According to
its contract, <code>Class.getResource</code> qualifies non-absolute paths with the
current package name, thus potentially returning a different resource or failing to find
the requested resource.</li>
<li><code>Class.getResource</code> delegates finding the resource to the class loader that loaded the class.
At run time, there is no guarantee that all subclasses of
a particular type are loaded by the same class loader, resulting in resource lookup
failures that are difficult to diagnose.</li>
</ul>

</overview>
<recommendation>
<p>Rather than using the <code>getClass</code> method, which relies on dynamic dispatch
and run-time types, use <code>class</code> literals instead. For example, instead of calling 
<code>getClass().getResource()</code> on an object of type <code>Foo</code>,
call <code>Foo.class.getResource()</code>. Class literals always refer to the
declared type they are used on, removing the dependency on run-time types.</p>

</recommendation>
<example>

<p>In the following example, the calls to <code>getPostalCodes</code> return different results,
depending on which class the call is made on: the class <code>Address</code> is in the package
<code>framework</code> and the class <code>UKAddress</code> is in the package <code>client</code>.</p>

<sample src="GetClassGetResource.java" />

<p>In the following corrected example, the implementation of <code>getPostalCodes</code> is changed
so that it always calls <code>getResource</code> on the same class.</p>

<sample src="GetClassGetResourceGood.java" />

</example>
<references>

<li>Java Platform, Standard Edition 7, API Specification:
<a href="http://docs.oracle.com/javase/7/docs/api/java/lang/Class.html#getResource(java.lang.String)">class.getResource()</a>.</li>



</references>
</qhelp>