Does it help GC to null local variables in Java

Question

I was \'forced\' to add myLocalVar = null; statement into finally clause just before leaving method. Reason is to help GC. I was told I will get SMS\'s during n

Answer 1

Nulling local variables can indeed help in some edge cases. This doesn't apply to situation in original question, but is educational anyway... Let's consider this program:

public class Main {
    public static void main(String[] args) {
       {
           Main local = new Main();

           // inner = null;
       }

       while (true) {
           // long running method
       }
    }
}

If inner = null; is commented out, object in local variable cannot be garbage-collected during while loop. Reason is that Java Virtual Machine doesn't know about scopes like this. All it has is:

D:\workspaces\workspace-3.4\test\src>javap -verbose -c Main
public class Main extends java.lang.Object
  minor version: 0
  major version: 50
  Constant pool:
const #1 = Method       #4.#11; //  java/lang/Object."":()V
const #2 = class        #12;    //  Main
const #3 = Method       #2.#11; //  Main."":()V
const #4 = class        #13;    //  java/lang/Object
const #5 = Asciz        ;
const #6 = Asciz        ()V;
const #7 = Asciz        Code;
const #8 = Asciz        main;
const #9 = Asciz        ([Ljava/lang/String;)V;
const #10 = Asciz       StackMapTable;
const #11 = NameAndType #5:#6;//  "":()V
const #12 = Asciz       Main;
const #13 = Asciz       java/lang/Object;

{
public Main();
  Code:
   Stack=1, Locals=1, Args_size=1
   0:   aload_0
   1:   invokespecial   #1; //Method java/lang/Object."":()V
   4:   return

public static void main(java.lang.String[]);
  Code:
   Stack=2, Locals=2, Args_size=1
   0:   new     #2; //class Main
   3:   dup
   4:   invokespecial   #3; //Method "":()V
   7:   astore_1
   8:   goto    8
  StackMapTable: number_of_entries = 1
   frame_type = 8 /* same */

}

There is no information about scope of local variable. So from JVM's point of view, above program is equivalent to:

public class Main
{

    public Main() { }

    public static void main(String args[])
    {
        Main main1 = new Main();
        do
            ;
        while(true);
    }
}

(Generated by JAD decompiler)

Conclusion: there IS some rationale in nulling local variables in very special cases like this. But if method is going to finish soon (like in my original question), it doesn't help.

This was inspired by comment from Zdenek Tronicek on java-cz mailing list (in czech language, sorry)