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<h2>Where it all starts</h2>

<p>Say you have a Groovy code like</p>
<table class="wysiwyg-macro" data-macro-name="code" style="background-image: url(/plugins/servlet/confluence/placeholder/macro-heading?definition=e2NvZGV9&amp;locale=en_GB&amp;version=2); background-repeat: no-repeat;" data-macro-body-type="PLAIN_TEXT"><tr><td class="wysiwyg-macro-body"><pre>println 'hi'
</pre></td></tr></table>
<p>As explained in <a class="confluence-link" href="/display/GROOVY/Groovy+Backstage" data-linked-resource-id="19168" data-linked-resource-type="page" data-linked-resource-default-alias="Groovy Backstage" data-base-url="http://docs.codehaus.org">Groovy Backstage</a>, there is bytecode generated to achieve the desired behaviour of printing to stdout.</p>

<p>The easiest way of looking at the generated bytecode is to groovyc your Groovy source to a class file and process it with a Java Decompiler (e.g. JAD). See also: <a class="confluence-link" href="/display/GROOVY/From+source+code+to+bytecode" data-linked-resource-id="22517" data-linked-resource-type="page" data-linked-resource-default-alias="From source code to bytecode" data-base-url="http://docs.codehaus.org">From source code to bytecode</a></p>

<p>The resulting code looks as follows (only the relevant snippet):</p>
<table class="wysiwyg-macro" data-macro-name="code" style="background-image: url(/plugins/servlet/confluence/placeholder/macro-heading?definition=e2NvZGV9&amp;locale=en_GB&amp;version=2); background-repeat: no-repeat;" data-macro-body-type="PLAIN_TEXT"><tr><td class="wysiwyg-macro-body"><pre>...
Object aobj[] = { "hi" };
ScriptBytecodeAdapter.invokeMethod(this, "println", ((Object) (aobj)));
...
</pre></td></tr></table>
<h2>Invocation scheme</h2>

<p>There is a delegation scheme like</p>
<ul class="alternate">
	<li><code>ScriptBytecodeAdapter.invokeMethod(...)</code> (static method)
	<ul class="alternate">
		<li><code>InvokerHelper.invokeMethod(...)</code> (static method)
		<ul class="alternate">
			<li><code>Invoker.invokeMethod(...)</code> (instance method called on InvokerHelper's single instance)</li>
		</ul>
		</li>
	</ul>
	</li>
</ul>

<h2>MetaClass and MetaClassRegistry</h2>

<p>Invoker calls <code>invokeMethod(...)</code> on the <strong>MetaClass</strong> of our class (with exceptions, see below). It finds this MetaClass by looking it up in the <strong>MetaClassRegistry</strong>. The Invoker holds a single instance of this registry.</p>
<table class="wysiwyg-macro" data-macro-name="note" style="background-image: url(/plugins/servlet/confluence/placeholder/macro-heading?definition=e25vdGV9&amp;locale=en_GB&amp;version=2); background-repeat: no-repeat;" data-macro-body-type="RICH_TEXT"><tr><td class="wysiwyg-macro-body"><p> When working with the MetaClassRegistry, <code>InvokerHelper.getInstance().getMetaRegistry()</code> is the only instance, you should ever use.</p></td></tr></table>
<p>Exceptions (when <code>MetaClass.invokeMethod(...)</code> is not used):</p>
<ul class="alternate">
	<li>for Closures, <code>Closure.invoke(...)</code> is used</li>
	<li>for GroovyObjects <em>obj</em> of type <code>GroovyInterceptable</code>, <code>obj.invokeMethod(methodName,asArray(arguments))</code> is called</li>
	<li>for any other <code>GroovyObject</code> <em>obj</em> when method invokation through its MetaClass fails, <code>obj.invokeMethod(methodName,asArray(arguments))</code> is called</li>
</ul>

<p><code>MetaClass.invokeMethod(...)</code> finally cares for the invokation, either by reflection or by dynamic bytecode generation. Dynamic bytecode generation is supposed to be faster. For a class <code>MyClass</code> it generates <code>gjdk.groovy.lang.MyClass_GroovyReflector</code> with an <code>invoke</code> method.</p>

<p><em>Does MyClass_GroovyReflector contain methods according to MyClass.groovy that can be called directly</em> <img class="emoticon emoticon-question" data-emoticon-name="question" border="0" src="/s/en_GB/3278/15/_/images/icons/emoticons/help_16.png" alt="(question)" title="(question)" /></p>

<p>The cool thing about MetaClass is that you can dynamically add or remove methods to it. One can even replace the whole MetaClass in the MetaClassRegistry. See ProxyMetaClass for an example.</p>

<p>back to <a class="confluence-link" href="/display/GROOVY/Groovy+Backstage" data-linked-resource-id="19168" data-linked-resource-type="page" data-linked-resource-default-alias="Groovy Backstage" data-base-url="http://docs.codehaus.org">Groovy Backstage</a></p>