Does using final for variables in Java improve garbage collection?

Today my colleagues and me have a discussion about the usage of the final keyword in Java to improve the garbage collection.

For example, if you write a method like:

public Double doCalc(final Double value)
{
   final Double maxWeight = 1000.0;
   final Double totalWeight = maxWeight * value;
   return totalWeight;  
}

Declaring the variables in the method final would help the garbage collection to clean up the memory from the unused variables in the method after the method exits.

Is this true?

Solution 1:

Here's a slightly different example, one with final reference-type fields rather than final value-type local variables:

public class MyClass {

   public final MyOtherObject obj;

}

Every time you create an instance of MyClass, you'll be creating an outgoing reference to a MyOtherObject instance, and the GC will have to follow that link to look for live objects.

The JVM uses a mark-sweep GC algorithm, which has to examine all the live refereces in the GC "root" locations (like all the objects in the current call stack). Each live object is "marked" as being alive, and any object referred to by a live object is also marked as being alive.

After the completion of the mark phase, the GC sweeps through the heap, freeing memory for all unmarked objects (and compacting the memory for the remaining live objects).

Also, it's important to recognize that the Java heap memory is partitioned into a "young generation" and an "old generation". All objects are initially allocated in the young generation (sometimes referred to as "the nursery"). Since most objects are short-lived, the GC is more aggressive about freeing recent garbage from the young generation. If an object survives a collection cycle of the young generation, it gets moved into the old generation (sometimes referred to as the "tenured generation"), which is processed less frequently.

So, off the top of my head, I'm going to say "no, the 'final' modifer doesn't help the GC reduce its workload".

In my opinion, the best strategy for optimizing your memory-management in Java is to eliminate spurious references as quickly as possible. You could do that by assigning "null" to an object reference as soon as you're done using it.

Or, better yet, minimize the size of each declaration scope. For example, if you declare an object at the beginning of a 1000-line method, and if the object stays alive until the close of that method's scope (the last closing curly brace), then the object might stay alive for much longer that actually necessary.

If you use small methods, with only a dozen or so lines of code, then the objects declared within that method will fall out of scope more quickly, and the GC will be able to do most of its work within the much-more-efficient young generation. You don't want objects being moved into the older generation unless absolutely necessary.

Solution 2:

Declaring a local variable final will not affect garbage collection, it only means you can not modify the variable. Your example above should not compile as you are modifying the variable totalWeight which has been marked final. On the other hand, declaring a primitive (double instead of Double) final will allows that variable to be inlined into the calling code, so that could cause some memory and performance improvement. This is used when you have a number of public static final Strings in a class.

In general, the compiler and runtime will optimize where it can. It is best to write the code appropriately and not try to be too tricky. Use final when you do not want the variable to be modified. Assume that any easy optimizations will be performed by the compiler, and if you are worried about performance or memory use, use a profiler to determine the real problem.