Count Instances of Class

Question

I am looking for a way to programatically get the number of instances of a certain class type in .NET and Java.

Say for example I have class Foo. I want to be able to, in the same process, get a current count of all instance of Foo.
However I cannot modify Foo, so a static int with counting is out. Also I cannot just add all instances I make to some static list and count that. I want to be able to just say:

System.GC.numberOf< Foo >()

or something.

I was looking through the garbage collectors but I could not find any relevant methods.

there are similar questions but the answers always involve modifying the class with some sort of counter. I want to count instances of classes I do not define or control — Charles, Apr 12 '11 at 19:10
The keep track of the number of objects you create in code you do control. — Security Hound, Apr 12 '11 at 19:16
I have used tools to count instances of objects in remote processes, very useful for tracking down leaks. I am hoping there is a way to replicate this logic in the same program for testing purposes — Charles, Apr 12 '11 at 19:23

score 5 · Answer 1 · answered Apr 12 '11 at 19:39

If you can't modify the class directly (perhaps because it is a built-in class?), could you create a wrapper, or a subclass that inherits the original?

public class subFoo extends foo
{
    protected static int count = 0;

    public subFoo() 
    {
        count++;
        super();
    }

    protected void finalize() throws Throwable
    {
        count--;
        super.finalize();
    } 

    public static int getInstanceCount()
    {
        return count;
    }
}

This example is Java and may have some syntax issues 'cause I'm a little rusty.

Of course, you'd have to be sure to redeclare all your foo as subFoo throughout the rest of your code.

This did cross my mind, but unfortunately what I am dealing with are objects generated by a third party library. I want to count these objects while running automated tests to make sure they are being properly released. +1 for reading the question and offering a good solution though! — Charles, Apr 12 '11 at 19:47

score 2 · Answer 2 · answered Apr 12 '11 at 19:29

2

Do you have control of how the Java VM is being run? If so, you can write a quick and dirty debugger agent... http://download.oracle.com/javase/1.5.0/docs/guide/jvmti/jvmti.html#writingAgents

See the events "VM Object Allocation" and "Object Free"

answered Apr 12 '11 at 19:29

Dilum Ranatunga

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score 2 · Accepted Answer · answered Apr 12 '11 at 20:49

2

Another somewhat exotic way to do it would be to use aspect-oriented techniques to instrument the constructor(s) of the class(es) in question. Take a look at AspectJ, for example.

answered Apr 12 '11 at 20:49

mazaneicha

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Going to shoot for this solution since I can probably instrument .net classes too in some other similar way. – Charles Apr 15 '11 at 14:43

score 1 · Answer 4 · edited May 23 '17 at 12:22

1

As already commented, there are similar questions in SO.
One hack you can use - a big one IMO - is to change the Object class: see this answer

Resume:

copy the source of Object
add counting to its constructor (finalize)
add method to read the count
prepend the directory with the compiled class to the boot classpath (-Xbootclasspath)

edited May 23 '17 at 12:22

Community

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answered Apr 12 '11 at 20:06

user85421

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score 0 · Answer 5 · answered Apr 12 '11 at 19:18

0

Quick and dirty: if you can't change the class, maybe you can just put a counter in another part of the project, incrementing it when you instantiate said class?

answered Apr 12 '11 at 19:18

Lupuss

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Count Instances of Class

5 Answers5