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<h3>Introduction</h3>

<p><strong>backport175</strong> has been designed to work in highly dynamic environment (such as load-time and runtime weaving for AOP). In environments like this the bytecode on disk is not the &quot;current&quot; one being executed. Since the bytecode has been transformed during the execution of the application. What this means is that annotations can have been added or removed. </p>

<p>We therefore need two things:</p>

<ol>
	<li>a way for the vendor/user to define a bytecode provider since the current one is not available on disk</li>
	<li>an API for refreshing/updating the annotation reader, so we know that the annotations we have reflects the annotation in the latest bytecode</li>
</ol>

<p>All these operation should be fully thread-safe, if not, then we have a bug. </p>

<h3>Defining the bytecode provider</h3>

<p>This is done by implementing the <code>org.codehaus.backport175.reader.bytecode.spi.BytecodeProvider</code> interface. This interface is a strategy with one single method you need to implement:</p>

<table class="wysiwyg-macro" data-macro-name="code" data-macro-default-parameter="java" style="background-image: url(/plugins/servlet/confluence/placeholder/macro-heading?definition=e2NvZGU6amF2YX0&amp;locale=en_GB&amp;version=2); background-repeat: no-repeat;" data-macro-body-type="PLAIN_TEXT"><tr><td class="wysiwyg-macro-body"><pre>
public interface BytecodeProvider {
    /**
     * Returns the bytecode for a specific class.
     *
     * @param className the fully qualified name of the class
     * @param loader the class loader that has loaded the class
     * @return the bytecode
     */
    byte[] getBytecode(String className, ClassLoader loader);
}
</pre></td></tr></table>

<h3>Registering the bytecode provider</h3>

<p>The bytecode provider implementation class is registered in the <code>org.codehaus.backport175.reader.bytecode.AnnotationReader</code> class by invoking:</p>

<p>If no bytecode provider is registered then a default one that uses <code>getResourceAsStream()</code>, e.g. reads in the bytecode from disk, is used.</p>

<p>It is also possible to set a bytecode provider on a per class basis, which can be usefull in some circumstances.<br />
Refer to the API documentation for more details.</p>

<h3>Refreshing the annotation reader</h3>

<p>When a change to the bytecode has been made it is needed to refresh the annotation reader, so that it can pull the new bytecode from the <code>BytecodeProvider</code>, parse it and update the reader with the new information about the annotations.</p>

<p>This is done by invoking one of the <code>refresh(..)</code> methods in the <code>org.codehaus.backport175.reader.bytecode.AnnotationReader</code> class:</p>

// refreshes all classes (that has been accessed through the Annotations interface so far) 
AnnotationReader.refreshAll();
</pre></td></tr></table>