Are lambdas garbage collected?

Question

If I'm not mistaken, under certain situations a lambda in Java is generated as an anonymous class instance. For example, in this code the lambda needs to capture a variable from the outside:

final int local = 123456;
list.forEach(x -> System.out.println(x + local));

Does it means that the garbage collector will claim the lambda as an object?

Answer 1

Yes, instances that represent lambda expressions are garbage collected just like any other object. Even the classes that implement the functional interface can be collected, if the class that contains the lambda expression gets eligible for collection.

The reason that the above lambda is not collected is the result of an optimization by the lambda bootstrap method. Normally, a bootstrap method binds a static factory method to the invokedynamic call site. That factory takes all required variables of the enclosing methods and a reference to the declaring instance, if required. The factory then creates an instance of the class that represents the lambda and stores all values in fields of this instance.

However, if a lambda expression does not have any state, the bootstrap method creates a singleton which is then directly bound to the lambda expression's call site. Naturally, this singleton cannot be collected before the enclosing class is.

If you want to read more about how invokedynamic and lambda expressions work, I recently wrote a summary: http://mydailyjava.blogspot.no/2015/03/dismantling-invokedynamic.html?m=1

This is the current implementation of HotSpot, the bootstrapper can change in the future.