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<p>The <a href="http://scala.sygneca.com/patterns/loan">Loan my Resource</a> pattern ensures that a resource is deterministically disposed of once it goes out of scope.</p>

<p>This pattern is built in to many Groovy helper methods. You should consider using it yourself if you need to work with resources in ways beyond what Groovy supports.</p>

<h3>Example</h3>

<p>Consider the following code which works with a file. First we might write some line to the file and then print its size:</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 f = new File('junk.txt')
f.withPrintWriter { pw ->
    pw.println(new Date())
    pw.println(this.class.name)
}
println f.size()
// => 42
</pre></td></tr></table>

<p>We could also read back the contents of the file a line at a time and print each line out:</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>
f.eachLine { line ->
    println line
}
// =>
// Mon Jun 18 22:38:17 EST 2007
// RunPattern
</pre></td></tr></table>

<p>Note that normal Java <code>Reader</code> and <code>PrintWriter</code> objects were used under the covers by Groovy but the code writer did not have to worry about explicitly creating or closing those resources. The built-in Groovy methods <em>loan</em> the respective reader or writer to the closure code and then tidy up after themselves. So, you are using this pattern without having to do any work.</p>

<p>Sometimes however, you wish to do things slightly differently to what you can get for free using Groovy's built-in mechanisms. You should consider utilising this pattern within your own resource-handling operations.</p>

<p>Consider how you might process the list of words on each line within the file. We could actually do this one too using Groovy's built-in functions, but bear with us and assume we have to do some resource handling ourselves. Here is how we might write the code without using this pattern:</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 reader = f.newReader()
reader.splitEachLine(' ') { wordList ->
    println wordList
}
reader.close()
// =>
// [ "Mon", "Jun", "18", "22:38:17", "EST", "2007" ]
// [ "RunPattern" ]
</pre></td></tr></table>

<p>Notice that we now have an explicit call to <code>close()</code> in our code. If we didn't code it just right (here we didn't surround the code in a <code>try ... finally</code> block, we run the risk of leaving the file handle open.</p>

<p>Let's now apply the loan pattern. First, we'll write a helper method:</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 withListOfWordsForEachLine(File f, Closure c) {
    def r = f.newReader()
    try {
        r.splitEachLine(' ', c)
    } finally {
        r?.close()
    }
}
</pre></td></tr></table>

<p>Now, we can re-write our code as follows:</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>
withListOfWordsForEachLine(f) { wordList ->
    println wordList    
}
// =>
// [ "Mon", "Jun", "18", "22:38:17", "EST", "2007" ]
// [ "RunPattern" ]
</pre></td></tr></table>

<p>This is much simpler and has removed the explicit <code>close()</code>. This is now catered for in one spot so we can apply the appropriate level of testing or reviewing in just one spot to be sure we have no problems.</p>