Question

I have a simple example. The example loads an ArrayList<Integer> from a file f containing 10000000 random integers.

doLog("Test 2");
{
    FileInputStream fis = new FileInputStream(f);
    ObjectInputStream ois = new ObjectInputStream(fis);
    List<Integer> l = (List<Integer>) ois.readObject();
    ois.close();
    fis.close();
    doLog("Test 2.1");
    //l = null; 
    doLog("Test 2.2");
}
doLog("Test 2.3");
System.gc();
doLog("Test 2.4");

When I have l = null, I get this log:

Test 2                          Used Mem = 492 KB   Total Mem = 123 MB
Test 2.1                        Used Mem = 44 MB    Total Mem = 123 MB
Test 2.2                        Used Mem = 44 MB    Total Mem = 123 MB
Test 2.3                        Used Mem = 44 MB    Total Mem = 123 MB
Test 2.4                        Used Mem = 493 KB   Total Mem = 123 MB

But when I remove it, I get this log instead.

Test 2                          Used Mem = 492 KB   Total Mem = 123 MB
Test 2.1                        Used Mem = 44 MB    Total Mem = 123 MB
Test 2.2                        Used Mem = 44 MB    Total Mem = 123 MB
Test 2.3                        Used Mem = 44 MB    Total Mem = 123 MB
Test 2.4                        Used Mem = 44 MB    Total Mem = 123 MB

Used Memory is calculated by: runTime.totalMemory() - runTime.freeMemory()

Question: In case where l = null; is present, is there a memory leak?
l is inaccessible, so why can’t it be freed?

Answer 1

There is no memory leak in the above code.

As soon as you leave the code block enclosed in {}, the variable l falls out of scope, and the List is a candidate for garbage collection, regardless of if you set it to null first or not.

However, after the code block and until the return of the method, the List is in a state called invisible. While this is true, the JVM is unlikely to automatically null out the reference and collect the List‘s memory. Therefore, explicitly setting l = null can help the JVM collect the memory before you do your memory calculations. Otherwise, it will happen automatically when the method returns.

You will probably get different results for different runs of your code, since you never know exactly when the garbage collector will run. You can suggest that you think it should run using System.gc() (and it might even collect the invisible List even without setting l = null), but there are no promises. It is stated in the javadoc for System.gc():

Calling the gc method suggests that the Java Virtual Machine expend
effort toward recycling unused objects in order to make the memory
they currently occupy available for quick reuse. When control returns
from the method call, the Java Virtual Machine has made a best effort
to reclaim space from all discarded objects.