83. Let Go of That Instance!

Sometimes your variable (e.g. a class instance) can end up holding a large amount of data. For example, you might have placed the text of the complete works of Shakespeare into a property. The JVM performs a process called Garbage Collection so as to clean up data that you are no longer using. By assigning a variable to null we flag that the data previously held by the variable is no longer needed:

myInstance = null

For small Groovy scripts this may never be an issue but, for long running applications, data hanging around and not being used can start to soak up a serious amount of resources, especially memory. Once you’ve finished with a variable it’s worth assigning its value to null to let the JVM know you don’t need it anymore.

You don’t always need to explicitly set variables to null - this would make your code far less readable. The JVM knows that once a variable is no longer in scope, it’s no longer accessible and, thus, is no longer needed. This means that if you have a variable declared inside a method, its value is no longer needed once the method has completed. The same goes for variables declared within scopes such as loops. This can get a bit tricky when regarding closures and object references so it’s not all plain sailing.

There is an important caveat: if multiple variables refer to the same data then the JVM can only release resources once all references have “unlatched”. Let’s examine this in the code below:

class SampleText {
    def text
}

def shakespeare = new SampleText(text: 'It was the best of times....')
def marlow = shakespeare
shakespeare = null
println marlow.text

I’ve defined a variable (shakespeare) to hold a new instance of the SampleText class and then said that another variable (marlow) points to that instance of SampleText. My call to marlow.text will still work despite my setting shakespeare to null. In this case we say that “marlow still holds a reference to the SampleText instance”. This means that the JVM can’t release the resources held by the instance until all references are set to null. I need to set marlow to null to completely release the resources.

There’s a lot more to garbage collection than I want to cover here but if you spend a few moments searching “Java Garbage Collection” you’ll be able to delve deep into the topic.