Copy Page

You are not logged in. Any changes you make will be marked as anonymous. You may want to Log In if you already have an account. You can also Sign Up for a new account.

<h2>What is a Closure?</h2>

<p>A Groovy Closure is like a &quot;code block&quot; or a method pointer. It is a piece of code that is defined and then executed at a later point. It has some special properties like implicit variables, support for currying and support for free variables (which we'll see later on). We'll ignore the nitty gritty details for now (see the formal definition if you want those) and look at some simple examples.</p>

<h4>Simple Example</h4>

<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 clos = { println "hello!" }

println "Executing the Closure:"
clos()                          //prints "hello!"
</pre></td></tr></table>
<p>Note that in the above example, &quot;hello!&quot; is printed when the Closure is <em>called</em>, not when it is defined.</p>

<h2>Parameters</h2>

<p>Closure parameters are listed before the <code>-&gt;</code> token, like so:</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>
def printSum = { a, b -> print a+b }
printSum( 5, 7 )                       //prints "12"
</pre></td></tr></table>
<p>The <code>-&gt;</code> token is optional and may be omitted if your Closure definition takes fewer than two parameters.</p>

<h4>Parameter notes</h4>

<p>A Closure without -&gt; , i.e. {} , is a Closure with one argument that is implicitly named as 'it'. (see below for details) In some cases, you need to construct a Closure with zero arguments, e.g. using <a class="confluence-link" href="/display/GROOVY/Strings+and+GString" data-linked-resource-id="2771" data-linked-resource-type="page" data-linked-resource-default-alias="Strings and GString" data-base-url="http://docs.codehaus.org">GString for templating</a>, defining <a class="confluence-link" href="/display/GROOVY/ExpandoMetaClass+-+Properties" data-linked-resource-id="79523" data-linked-resource-type="page" data-linked-resource-default-alias="ExpandoMetaClass - Properties" data-base-url="http://docs.codehaus.org">EMC Property</a> etc. You have to explicity define your Closure as { -&gt; } instead of just { }</p>

<p>You can also use varargs as parameters, refer to the <a class="confluence-link" href="/display/GROOVY/Closures+-+Formal+Definition" data-linked-resource-id="11403384" data-linked-resource-type="page" data-linked-resource-default-alias="Closures - Formal Definition" data-base-url="http://docs.codehaus.org">Formal Guide</a> for details. A JavaScript-style dynamic args could be simulated, refer to the <a class="confluence-link" href="/display/GROOVY/Closures+-+Informal+Guide" data-linked-resource-id="2729" data-linked-resource-type="page" data-linked-resource-default-alias="Closures - Informal Guide" data-base-url="http://docs.codehaus.org">Informal Guide</a>.</p>

<h2>Free variables</h2>

<p>Closures may refer to variables not listed in their parameter list. Such variables are referred to as &quot;free&quot; variables. They are &quot;bound&quot; to variables within the scope where they are defined:</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>
def myConst = 5
def incByConst = { num -> num + myConst }
println incByConst(10) // => 15
</pre></td></tr></table>
<p>Or another example:</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>
def localMethod() {
  def localVariable = new java.util.Date()
  return { println localVariable }
}

def clos = localMethod()

println "Executing the Closure:"
clos()                          //prints the date when "localVariable" was defined
</pre></td></tr></table>

<h2>Implicit variables</h2>

<p>Within a Groovy Closure, several variables are defined that have special meaning:</p>

<p>If you have a Closure that takes a single argument, you may omit the parameter definition of the Closure, like so:</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>
def clos = { print it }
clos( "hi there" )              //prints "hi there"
</pre></td></tr></table>

<h4>this, owner, and delegate</h4>

<p><strong>this</strong> : as in Java, <code>this</code> refers to the instance of the enclosing class where a Closure is defined<br />
<strong>owner</strong> : the enclosing object (<code>this</code> or a surrounding Closure)<br />
<strong>delegate</strong> : by default the same as <code>owner</code>, but changeable for example in a <a class="confluence-link" href="/display/GROOVY/Builders" data-linked-resource-id="16958" data-linked-resource-type="page" data-linked-resource-default-alias="Builders" data-base-url="http://docs.codehaus.org">builder</a> or <a class="confluence-link" href="/display/GROOVY/ExpandoMetaClass" data-linked-resource-id="79517" data-linked-resource-type="page" data-linked-resource-default-alias="ExpandoMetaClass" data-base-url="http://docs.codehaus.org">ExpandoMetaClass</a></p>

<p>Example:</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>
class Class1 {
  def closure = {
    println this.class.name
    println delegate.class.name
    def nestedClos = {
      println owner.class.name
    }
    nestedClos()
  }
}

def clos = new Class1().closure
clos.delegate = this
clos()
/*  prints:
 Class1
 Script1
 Class1$_closure1  */
</pre></td></tr></table>

<h2>Closures as Method Arguments</h2>

<p>When a method takes a Closure as the last parameter, you can define the Closure inline, like so:</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>
def list = ['a','b','c','d']
def newList = []

list.collect( newList ) {
  it.toUpperCase()
}
println newList           //  ["A", "B", "C", "D"]
</pre></td></tr></table>
<p>In the above example, the <code><a href="http://groovy.codehaus.org/groovy-jdk.html#meth223">collect </a></code> method accepts a <code>List</code> and a <code>Closure</code> argument. The same could be accomplished like so (although it is more verbose):</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>
def list = ['a','b','c','d']
def newList = []

def clos = { it.toUpperCase() }
list.collect( newList, clos )

assert newList == ["A", "B", "C", "D"]
</pre></td></tr></table>

<h2>More Information</h2>

<p>Groovy extends <code>java.lang.Object</code> and many of the <code>Collection</code> and <code>Map</code> classes with a number of methods that accept Closures as arguments. See <a class="confluence-link" href="/display/GROOVY/GDK+Extensions+to+Object" data-linked-resource-id="11403319" data-linked-resource-type="page" data-linked-resource-default-alias="GDK Extensions to Object" data-base-url="http://docs.codehaus.org">GDK Extensions to Object</a> for practical uses of Groovy's Closures.</p>