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How to find if an object is referencing another at runtime

is it possible to check at runtime if an object has a direct or indirect reference to another object?
(I know I can use VisualVm or similar to analyze the HeapDump, but i'd like to automate it at runtime)
I'm working with WeakHashMaps, doing something like this:
public class MyClass {
// the Runnable will be eventually removed if the key is collected by the GC
private static final WeakHashMap<Object, Runnable> map = new WeakHashMap<>();
public static void main(String[] args) {
MyClass a = new MyClass(2);
MyClass b = new MyClass(20);
a = null;// no more Strong references to a
b = null;// no more Strong references to b
System.gc();
for (Runnable r : map.values()) {
r.run();
}
// will print (20), becouse using format() in the lambda cause a Strong
// reference to MyClass (b) and thus the WeakHashMap will never remove b
}
public MyClass(int i) {
if (i < 10) {
map.put(this, () -> {
System.out.println(i);
});
} else {
map.put(this, () -> {
// this is subtle, but calling format() create a strong reference
// between the Runnable and MyClass
System.out.println(format(i));
});
}
}
private String format(Integer i) {
return "(" + i + ")";
}
}
in the code, the two instance of MyClass will add themselves (as key) and a runnable (as value) to the WeakHashMap.
In the first instance (a), the Runnable simply call System.out.println() and when the instance a is no more referenced (a = null) the entry will be removed from the map.
In the second instance (b), the Runnable also call format() an instance function of MyClass. This create a strong reference to b and adding the Runnable to the map will result in a lock condition, where the value is an indirect strong reference to the key preventing the collection by the garbage collector.
Now I know to avoid these conditions (for instance, using a Weakreference inside the lambda), but this is really easy to miss in a real scenario, and will cause a memory leak.
So, prior to adding the pair to the map I'd like to check if the value is somehow referencing the key, and throw an exception if so.
This would be a "debug" task, and will be disabled in production, so I don't care if it is slow or an hack.
--- update ---
I'm trying to deal with WeakListeners, and to avoid them to be immediately collected if not referenced.
So i register them as notifier.addWeakListener(holder, e -> { ... });
and this will add the listener to a WeakHashMap preventing the listener to be collected until holder live.
But any reference to the holder in the listener will create a lock :(
Is there a better way?

The Reflection API gives you access to all fields of a run-time object (and its runtime type, and possibly the Class object). In theory, you could traverse through the tree of you instance's fields (and static fields on the class), the fields' fields etc.
While this is possible, I doubt it would be feasible. You write you don't care about performance, but it may even be too slow for development runs. Which brings us to the Rule 1 of implementing your own cache: Don't do it.

There is already a builtin feature for associations which are automatically cleaned up, ordinary instance fields. I.e
public class MyClass {
public static void main(String[] args) {
MyClass a = new MyClass(2);
MyClass b = new MyClass(20);
WeakReference<MyClass> aRef = new WeakReference<>(a), bRef = new WeakReference<>(b);
a = null;// no more Strong references to a
b = null;// no more Strong references to b
System.gc();
if(aRef.get() == null) System.out.println("a collected");
if(bRef.get() == null) System.out.println("b collected");
}
Runnable r;
public MyClass(int i) {
if (i < 10) {
r = () -> System.out.println(i);
} else {
r = () -> {
// reference from Runnable to MyClass is no problem
System.out.println(format(i));
};
}
}
private String format(Integer i) {
return "(" + i + ")";
}
}
You can put these associated objects into a weak hashmap as keys, to allow them to get garbage collected, which, of course, will only happen when the particular MyClass instance, which still holds a strong reference to it, gets garbage collected:
public class MyClass {
public static void main(String[] args) {
MyClass a = new MyClass(2);
MyClass b = new MyClass(20);
for(Runnable r: REGISTERED) r.run();
System.out.println("cleaning up");
a = null;// no more Strong references to a
b = null;// no more Strong references to b
System.gc();
// empty on common JRE implementations
for(Runnable r: REGISTERED) r.run();
}
static Set<Runnable> REGISTERED = Collections.newSetFromMap(new WeakHashMap<>());
Runnable r;
public MyClass(int i) {
r = i < 10?
() -> System.out.println(i):
() -> {
// reference from Runnable to MyClass is no problem
System.out.println(format(i));
};
REGISTERED.add(r);
}
private String format(Integer i) {
return "(" + i + ")";
}
}
But note that what works smoothly in this simple test setup is nothing you should rely on, especially as you mentioned weak listeners.
In production environments, the garbage collector runs when there are memory needs, which is not connected to application logic, i.e. whether particular actions implemented as listeners should be executed or not. One possible scenario would be that there is always enough memory, so the garbage collector never runs and obsolete listeners keep being executed forever.
But you may encounter problems into the other direction too. Your question suggests that it might be possible to write your listeners (Runnable in the example) in a way that they don’t contain references to the instance whose life time ought to determine the listener’s life time (the MyClass instance). This raises the question, in which way the life times of these objects are connected at all. You have to keep strong references to these key objects, for the sake of keeping these listeners alive, which is error prone too.

WeakReferenced object is not garbage collected after calling System.gc()

I am a fresh new learner of Java. I'm now learning the concept of WeakReference. I came across a problem which probably looks stupid but I just wanna figure out the reason. The problem is: according to Java doc, "Weak reference objects, which do not prevent their referents from being made finalizable, finalized, and then reclaimed."
So I did this small test:
import java.lang.ref.WeakReference;
public class A {
public static void main(String[] args) {
A a = new A();
WeakReference<A> wr = new WeakReference<>(a);
a = null;
A a1 = wr.get();
System.out.println(a);
System.out.println(a1);
try {
System.gc();
Thread.sleep(10000);
} catch (Exception e) {
e.printStackTrace();
}
System.out.println(a1);
}
#Override
protected void finalize( ) {
System.out.println(Thread.currentThread().getName() + ": See ya, nerds!");
}
}
However, I noticed that after GC running, wr.get() could still return object which I expected null, and the method finalize() was not invoked. So what went wrong? Thanks for your kind help in advance! :)

The premise of your test is flawed. System.gc() is only a hint to run the garbage collector. It is frequently ignored.
From the documentation:
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.
(Emphasis mine)
In future, you may use the VM options -verbose:gc and -XX:+PrintGCDetails to see what the garbage collector is doing.
More importantly, you are also very quickly taking the reference out of the weak reference and putting it back into a strong reference:
A a = new A();
WeakReference<A> wr = new WeakReference<>(a);
a = null; // no strong references remain
A a1 = wr.get(); // the instance now has a strong reference again
Unless garbage collection occurs between these exact two instructions, the object will not be garbage collected.
If you remove a1, your code behaved as you would expect when I ran it (though, because of the first part of my answer, your mileage may vary):
class A
{
public static void main(String[] args)
{
A a = new A();
WeakReference<A> wr = new WeakReference<>(a);
a = null;
System.out.println(a);
try {
System.gc(); // instance of A is garbage collected
Thread.sleep(10000);
} catch (Exception e) {
e.printStackTrace();
}
System.out.println(wr.get());
}
#Override
protected void finalize( )
{
System.out.println(Thread.currentThread().getName() + ": See ya, nerds!");
}
}

Firstly, System.gc() does not ensure a garbage collection. Instead, it's just a hint that "It's a good time to run garbage collection".
Secondly, in your code when you put A a1 = wr.get(); before calling System.gc(), it creates a new strong reference to the same object referenced by a, thus even if garbage collection runs, your object will not be garbage collected.
As we have two tasks in hand
Ensure garbage collection
Don't keep any strong reference to the object you want to be garbage collected
Let's do little modification to your code
public class A {
public static void main(String[] args) {
A a = new A();
WeakReference<A> wr = new WeakReference<>(a);
a = null;
// A a1 = wr.get(); Removing this, this does our 2nd task
System.out.println(a);
// System.out.println(a1); Removing this as a1 does not exists anymore
try {
while (null != wr.get()) { // 1st task done, the loop ensures sending the hint until your object collected
System.gc();
// Thread.sleep(10000); it does not have impact
}
} catch (Exception e) {
e.printStackTrace();
}
System.out.println(wr.get()); // Obviously prints null
}
#Override
protected void finalize() {
System.out.println(Thread.currentThread().getName() + ": See ya, nerds!");
}
}

Weak references cleared atomically with placement on reference queue?

Does the Java GC clear a weak reference atomically with placing the weak reference on the ReferenceQueue assigned to that reference? In other words, if a call to WeakReference.get() returns null, will the WeakReference be on the queue?

Lets dissect Javadoc for WeakReference:
Suppose that the garbage collector determines at a certain point in
time that an object is weakly reachable.
At that time it will atomically clear all weak references to that
object and all weak references to any other weakly-reachable objects
from which that object is reachable through a chain of strong and soft
references.
At the same time it will declare all of the formerly weakly-reachable
objects to be finalizable.
At the same time or at some later time it
will enqueue those newly-cleared weak references that are registered
with reference queues.
It looks like the steps are:
Determine if object is weakly reachable.
Clear that object from its WeakReference. WeakReference.get
will return null.
Mark that object finalizable.
Right after or at some other time do enqueue the
WeakReference into the queue (if WeakReference was created with
a queue).
This means that even if WeakReference.get() returns null it is not guarantee that WeakReference.enqueue will be true or ReferenceQueue.poll wont return null.
See https://community.oracle.com/blogs/enicholas/2006/05/04/understanding-weak-references for more.
Reference queues
Once a WeakReference starts returning null, the object it pointed to
has become garbage and the WeakReference object is pretty much
useless. This generally means that some sort of cleanup is
required;WeakHashMap, for example, has to remove such defunct entries
to avoid holding onto an ever-increasing number of deadWeakReferences.
The ReferenceQueue class makes it easy to keep track of dead
references. If you pass a ReferenceQueueinto a weak reference's
constructor, the reference object will be automatically inserted into
the reference queue when the object to which it pointed becomes
garbage. You can then, at some regular interval, process the
ReferenceQueue and perform whatever cleanup is needed for dead
references.
Sample code showing it:
public static class A {
}
public static void main(String[] args) throws Exception{
A a = new A();
ReferenceQueue<A> rq = new ReferenceQueue<A>();
WeakReference<A> aref = new WeakReference<A>(a, rq);
a = null;
//aref.get() should be a, aref.isEnqueued() should return false, rq.poll() should return null
System.out.println( "0: " + aref + " : " + aref.get() + " : " + aref.isEnqueued() + " " + rq.poll() );
Thread.sleep(1000);
System.out.println("Running GC.");
Runtime.getRuntime().gc(); //let GC clear aref
System.out.println("GC ran.");
//aref.get() should be null, aref.isEnqueued() should return false, rq.poll() should return null
System.out.println( "1: " + aref + " : " + aref.get() + " " + aref.isEnqueued() + " " + rq.poll() );
//give some time for GC to enqueue aref
Thread.sleep(1000);
//ref.get() should be null, aref.isEnqueued() should return true, rq.poll() should return aref
System.out.println( "2: " + aref + " : " + aref.get() + " " + aref.isEnqueued() + " " + rq.poll() );
}

Does the Java GC clear a weak reference atomically with placing the weak reference on the ReferenceQueue assigned to that reference?
No. It is queued 'at the same time or some later time'. Nothing atomic about that specification.
In other words, if a call to WeakReference.get() returns null, will the WeakReference be on the queue?
Not necessarily. Note that this isn't really 'in other words' at all: it isn't the same question.

Java WeakHashMap Class

I wanted to test Java WeakHashMap Class functionality and for that matter I wrote the following test:
public class WeakHashMapTest {
public static void main(String args[]) {
Map<String, Object> weakMap = new WeakHashMap<>();
String x = new String("x");
String x1 = new String("x1");
String x2 = new String("x2");
weakMap.put(x, x);
weakMap.put(x1, x1);
weakMap.put(x2, x2);
System.out.println("Map size :" + weakMap.size());
// force all keys be eligible
x=x1=x2=null;
// call garbage collector
System.gc();
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("Map size :" + weakMap.size());
System.out.println("Map :" + weakMap);
}
}
After running the class WeakMapTest I was unpleasantly surprised to get the following output:
map before gc: {x=x, x1=x1, x2=x2}
map after gc: {x=x, x1=x1, x2=x2}
when I expected that the map will be empty.
That is, the garbage collector didn't do its work. But why?

The WeakHashMap will have its keys reclaimed by the garbage collector when they are no longer strongly reachable.
Implementation note: The value objects in a WeakHashMap are held by ordinary strong references. Thus care should be taken to ensure that value objects do not strongly refer to their own keys, either directly or indirectly, since that will prevent the keys from being discarded.
However, because you used the key itself as the value, the value is still strongly reachable, which means that the garbage collector can't reclaim the keys.
However, if you use other objects as values, then the only references to the keys will be the keys themselves.
weakMap.put(x, new Object());
weakMap.put(x1, new Object());
weakMap.put(x2, new Object());
Then, after clearing the variables and calling the garbage collector as you've done already, I get the output:
Map size :3
Map size :0
Map :{}
Even if the call to System.gc() doesn't guarantee the garbage collector runs, it looks like it does run here.

System.gc() is effectively a suggestion to run the garbage collector. There is no guaranteed way to force the garbage collector to run.

Weak reference not getting garbage collected?

I was studying about Weak references. And I understood that all weak references WILL be garbage collected before OutOfMemoryError occurs. I had a simple test something like this (I know catching OOME is not good but just a test) :
Integer weakInt = new Integer(10);
WeakReference<Integer> weakReference = new WeakReference<Integer>(weakInt);
try {
while (weakReference != null) {
String[] generateOutOfMemoryStr = new String[999999999];
}
}
catch (OutOfMemoryError oome) {
System.out.println(weakReference.get());
}
I expected null to be printed because the weak reference SHOULD have been collected but I always get an output of 10.
Please let me know where I am going wrong. May be I understood the concept of weak references wrong?

weakReference itself won't become null... how could it? However, its target can become null.
I suspect you mean:
while (weakReference.get() != null) {
Moreover, I suspect that unless you set weakInt itself to null, that local variable will prevent the Integer object from being garbage collected.
Moreover, I suspect you'll find your loop still won't end - because you're quite possibly asking for more memory than can be allocated even if the Integer is garbage collected.
Here's a program which demonstrates it working, at least on my box:
import java.lang.ref.*;
public class Test {
public static void main(String[] args) {
Integer weakInt = new Integer(10);
WeakReference<Integer> weakReference = new WeakReference<Integer>(weakInt);
weakInt = null;
while (weakReference.get() != null) {
System.out.println("Looping...");
String[] generateOutOfMemoryStr = new String[999999];
}
System.out.println("Weak reference collected");
}
}