How do I know how many instances of class are created at any given time in java?

Question

How do I know how many instances of class are created at any given time in java? I have class A and I want to know how many instances are created at any given time? Please let me know the solution

How many instances - in a JVM, on a server, in the whole world? — ppeterka, Sep 18 '13 at 16:39
I tried using counter in constructor but I am looking for other way — user2784473, Sep 18 '13 at 16:40
"Please let me know the solution" -- You haven't stated a problem. — Hot Licks, Sep 18 '13 at 16:41
"Are created" - do you mean how many have been created since the program started running, or how many exist at the moment? — Joni, Sep 18 '13 at 16:41

score 7 · Answer 1 · edited May 23 '17 at 10:31

7

Keep track of it in your constructor with a static variable.

public class A {

    private static int instances = 0;

    public A() {
        instances++;
    }
}

If concurrency is a concern to you:

public class A {

    private static final Object LOCK = new Object();
    private static int instances = 0;

    public A() {
        synchronized(LOCK) {
            instances++;
        }
    }
}

To address what you've said in the comments below your question: if you want to know how many currently exist (i.e., have not been garbage collected) you could try something like the following:

public class A {

    private static final Object LOCK = new Object();
    private static int instances = 0;

    public A() {
        synchronized(LOCK) {
            instances++;
        }
    }

    protected void finalize() throws Throwable {
        synchronized(LOCK) {
            instances--;
        }
    }
}

The finalize() method will be called right before the object is garbage collected. However garbage collection is notoriously unreliable.

Also, as a side note, you could use the AtomicInteger class rather than an int and the synchronized blocks, as @rolfl says below. All upvotes for that idea should go to his answer, should he post one.

edited May 23 '17 at 10:31

Community

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answered Sep 18 '13 at 16:41

asteri

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That's better (LOCK), now you have +1 from me too ;) – Nir Alfasi Sep 18 '13 at 16:44
Is that better than `synchronized(A.class)`? – Marcelo Sep 18 '13 at 16:44
You beat me by 15 seconds ;-) Anyway, you might want to mention the factory pattern as well (as the typical way to deal with instance control). – Vincent van der Weele Sep 18 '13 at 16:45
3

Recommend `AtomicInteger` over the `LOCK` – rolfl Sep 18 '13 at 16:45
@rolfl Nice point! You should add it as an answer. Don't want to take credit for your idea. :) You'd have my +1. – asteri Sep 18 '13 at 16:46
1

Also call [`super.finalize()`](http://stackoverflow.com/a/11379181/995891) (within a `finally` block) – zapl Sep 18 '13 at 17:22

score 0 · Answer 2 · answered Sep 18 '13 at 16:41

0

That's actually not such a bad question, you can control the number of instances by using sychronize and a counter, this design pattern is called instance-controlled class and it's one of the first things that are mentioned in "effective Java" by Joshua Bloch (chapter 2)

answered Sep 18 '13 at 16:41

Nir Alfasi

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Of course, there's no way to count the instances that are collected. – Hot Licks Sep 18 '13 at 16:42
@HotLicks sorry but I didn't understand your comment (the collected part). – Nir Alfasi Sep 18 '13 at 16:43
One can attempt to subtract the garbage-collected objects with a `finalize` method, but it's highly unreliable. So the total number in existence at any one point in time is not knowable without using "instruments". – Hot Licks Sep 18 '13 at 19:58
@HotLicks but that's another question (how many LIVE objects are there) - the original question was how many instances were created ;) – Nir Alfasi Sep 18 '13 at 20:08
How many instances *ARE* created. That can be interpreted either way. And later he says "How many instances exist at the moment". – Hot Licks Sep 18 '13 at 20:31
@HotLicks yeps, I guess you're right - it can be interpreted either way – Nir Alfasi Sep 18 '13 at 20:37

score 0 · Answer 3 · answered Sep 18 '13 at 16:41

There's no direct way, but if you're really interested in keeping track of this information you could create a private static int instanceCounter field and increment it in your constructor(s). You would also need a finalize method which would decrement the counter whenever an instance is garbage collected.

score 0 · Answer 4 · answered Sep 18 '13 at 16:42

0

you could implement a counter when it inits

answered Sep 18 '13 at 16:42

sinfere

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How do I know how many instances of class are created at any given time in java?

4 Answers4