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<p><table class="wysiwyg-macro" data-macro-name="excerpt" data-macro-parameters="atlassian-macro-output-type=BLOCK|hidden=true" style="background-image: url(/plugins/servlet/confluence/placeholder/macro-heading?definition=e2V4Y2VycHQ6aGlkZGVuPXRydWV8YXRsYXNzaWFuLW1hY3JvLW91dHB1dC10eXBlPUJMT0NLfQ&amp;locale=en_GB&amp;version=2); background-repeat: no-repeat;" data-macro-body-type="RICH_TEXT"><tr><td class="wysiwyg-macro-body"><p>Adding methods to an instance</p></td></tr></table></p>

<h1>Adding Methods to an Instance</h1>

<p>Normally when you add a <code>MetaMethod</code>, it is added for <em>all instances</em> of that class.  However, you can dynamically add methods to individual instances by giving that instance its own <code>MetaClass</code>.</p>

<h2>For Groovy version 1.6 and higher - any Groovy or Java object</h2>

<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>
def str = "hello"
str.metaClass.test = { "test" }
assert "test" == str.test()
</pre></td></tr></table>

<h2>For Groovy prior to version 1.6 - <code>GroovyObjects</code> only!</h2>

<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>
def test = "test"
def gstr = "hello $test"     // this is a GString, which implements GroovyObject

def emc = new ExpandoMetaClass( gstr.class, false )
emc.test = { println "test" }
emc.initialize()

gstr.metaClass = emc
gstr.test()                  // prints "test"
</pre></td></tr></table>
<p>Note that you <strong>cannot</strong> do this:</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>
gstr.metaClass = new ExpandoMetaClass( gstr.class )
gstr.metaClass.test = { println "test" }
</pre></td></tr></table>
<p>because you <em>must</em> call <code>emc.initialize()</code> before making any method calls on the instance.  But you can't add MetaMethods after calling <code>initialize()</code>! This is bit of a catch 22 because the ExpandoMetaClass is intercepting methods to itself. The solution is (as shown in the first example) to simply add the MetaMethods before assigning the new MetaClass to your instance.</p>

<p>The other option is to set the set <code>emc.allowChangesAfterInit = true</code>.  This will allow you to add additional methods on the MetaClass after it is in use.</p>
<table class="wysiwyg-macro" data-macro-name="note" data-macro-parameters="title=Note" style="background-image: url(/plugins/servlet/confluence/placeholder/macro-heading?definition=e25vdGU6dGl0bGU9Tm90ZX0&amp;locale=en_GB&amp;version=2); background-repeat: no-repeat;" data-macro-body-type="RICH_TEXT"><tr><td class="wysiwyg-macro-body">
<p>Be sure to use the proper constructor, <code>new ExpandoMetaClass(MyClass,</code><code><strong>false</strong></code><code>)</code>.  The <code>false</code> parameter keeps the MetaClass from being inserted into the Registry. Otherwise your new MetaClass will be used for all instances of <code>MyClass</code>, not just the instance it is assigned to.</p></td></tr></table>
<table class="wysiwyg-macro" data-macro-name="note" data-macro-parameters="title=Compatibility" style="background-image: url(/plugins/servlet/confluence/placeholder/macro-heading?definition=e25vdGU6dGl0bGU9Q29tcGF0aWJpbGl0eX0&amp;locale=en_GB&amp;version=2); background-repeat: no-repeat;" data-macro-body-type="RICH_TEXT"><tr><td class="wysiwyg-macro-body">
<p>Only works in Groovy 1.1-beta-3 and above. Use the Proxy class in older versions to achieve a per-instance behaviour change.</p></td></tr></table>

<h2>If your Instance is not a GroovyObject</h2>

<p>If your instance is a plain Java type, it will not implement <code>GroovyObject</code>, and consequently, will not have a <code>metaClass</code> property.  In this case you must wrap your instance in a <a href="http://groovy.codehaus.org/api/groovy/util/Proxy.html">groovy.util.Proxy</a>:</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>
ExpandoMetaClass emc = new ExpandoMetaClass( Object, false )
emc.boo = { "Surprise!" }
emc.initialize()

def obj = new groovy.util.Proxy().wrap( new Object() )
obj.setMetaClass( emc )
assert obj.boo() == "Surprise!"
</pre></td></tr></table>
<p>Note that this example is calling the <code>setMetaClass(..)</code> method rather than using the property notation in the previous example.  This is because <code>Proxy</code> intercepts <em>method calls</em> only, not property access.</p>