如何在运行时查找堆/堆栈中的对象有多少个强引用

Question

Team,

Is there possible in java, to know how many active/strong references for a object currently available ?

For example in the below code; Object of class A can be hold by many classes in the project. But i want to print that in the monitor thread.

public class A {
  public static A a = new A();
  public static A getInstance() {
     return a;
  }

  private A() {
     new Monitor(this).start();
  }


  class Monitor extends Thread {
      A refA;
      public Monitor(A ref) {
         this.refA = ref;
      }
      public void run () {

      //TODO Print how many references currently available for Object A referenced by refA;
      //Sure It will be minimum one. (which is "a" in this class A)   
      } 

   }
}

Please don't give much importance to this example program. My question is how to find how many strong references available to an object in the heap/stack? Only good thing is we have one strong reference in hand for that object.

If it is not possible in java; can i pass this strong reference to C language; and from C language can i able to do that?

I just wonder how the Profilers/tools are able to do this? Please help.

Answer 1

No you can't get the exact count without changing the class or branch tools on the VM (which can hardly be made in production due to the impact on performances).

Using the ref package , you can be notified if an object is about to be garbaged (and act at this time) but there is no count available (and not always one handled by the VM).

Answer 2

You can perform a heap dump and analyse it to find the number of references to any object.

What is your requirement for doing this and what will you do with the information as I suspect there is an easier/better way to achieve what you want.

---

Based on WeakHashMap

/**
 * Reference queue for cleared WeakEntries
 */
private final ReferenceQueue<Connection> queue = new ReferenceQueue<>();

List<WeakReference<Connection>> usedConnections = ....

// when you have a new connection

Connection connection = ....
usedConnections.add(new WeakReference(connection, queue));

// checking the queue for discarded objects.

    // remove null references from usedConnections

    for (Connection x; (x = queue.poll()) != null; ) {
        synchronized (queue) {
           x.close();
        }
     }

Answer 3

You may try out somthing like this

Class A
{
   static int instanceCount = 0;
   public A()
   {
      instanceCount++;
   }

   protected finalize()
   {
      instanceCount--;
   }

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

I believe this is the closest you can get to conting references of a class using code. Hope it helps ...