Does using final for variables in Java improve garbage collection?

Question

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.

Solution 3

No, it is emphatically not true.

Remember that final does not mean constant, it just means you can't change the reference.

final MyObject o = new MyObject();
o.setValue("foo"); // Works just fine
o = new MyObject(); // Doesn't work.

There may be some small optimisation based around the knowledge that the JVM will never have to modify the reference (such as not having check to see if it has changed) but it would be so minor as to not worry about.

Final should be thought of as useful meta-data to the developer and not as a compiler optimisation.

Solution 4

Some points to clear up:

• Nulling out reference should not help GC. If it did, it would indicate that your variables are over scoped. One exception is the case of object nepotism.

• There is no on-stack allocation as of yet in Java.

• Declaring a variable final means you can't (under normal conditions) assign a new value to that variable. Since final says nothing about scope, it doesn't say anything about it's effect on GC.

Solution 5

Well, I don't know about the use of the "final" modifier in this case, or its effect on the GC.

But I can tell you this: your use of Boxed values rather than primitives (e.g., Double instead of double) will allocate those objects on the heap rather than the stack, and will produce unnecessary garbage that the GC will have to clean up.

I only use boxed primitives when required by an existing API, or when I need nullable primatives.

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