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Java - HashMap and WeakHashMap references used in Application

Just trying to understand something from GC viewpoint
public Set<Something> returnFromDb(String id) {
LookupService service = fromSomewhere();
Map<String,Object> where = new WeakHashMap<>() {}
where.put("id",id);
return service.doLookupByKVPair(where); // where doesn't need to be serializable
}
what I understand is that once this method call leaves the stack, there is no reference to where regardless of using HashMap or WeakHashMap - but since weak reference is weakly reachable wouldn't this be GCd faster? But if the method call leaves the stack, then there is no reachable reference anyway.
I guess the real question that I have is - "Would using WeakHashMap<> here actually matters at all" - I think it's a "No, because the impact is insignificant" - but a second answer wouldn't hurt my knowledge.

When you use a statement like where.put("id",id); you’re associating a value with a String instance created from a literal, permanently referenced by the code containing it. So the weak semantic of the association is pointless, as long as the code is reachable, this specific key object will never get garbage collected.
When the entire WeakHashMap becomes unreachable, the weak nature of the references has no impact on the garbage collection, as unreachable objects have in general. As discussed in this answer, the garbage collection performance mainly depends on the reachable objects, not the unreachable ones.
Keep in mind the documentation:
The relationship between a registered reference object and its queue is one-sided. That is, a queue does not keep track of the references that are registered with it. If a registered reference becomes unreachable itself, then it will never be enqueued. It is the responsibility of the program using reference objects to ensure that the objects remain reachable for as long as the program is interested in their referents.
In other words, a WeakReference has no impact when it is unreachable, as it will be treated like any other garbage, i.e. not treated at all.
When you have a strong reference to a WeakHashMap while a garbage collection is in progress, it will reduce the performance, as the garbage collector has to keep track of the encountered reachable WeakReference instances, to clear and enqueue them if their referent has not been encountered and marked as strongly reachable. This additional effort is the price you have to pay for allowing the earlier collection of the keys and the subsequent cleanup, which is needed to remove the strongly referenced value.
As said, when, like in your example, the key will never become garbage collected, this additional effort is wasted. But if no garbage collection happens while the WeakHashMap is used, there will be no impact, as said, as the collection of an entire object graph happens at once, regardless of what kind of objects are in the garbage.

PhantomReference with null queue

Java allow to write:
new PhantomReference(new Object(), null)
At this case new Object() will be collected?
As I understand, phantom reference is alternative of finalize() method usage.
And after appearing reference in queue, I need to do some additional actions and then run clear()
java doc stays:
It is possible to create a phantom reference with a null queue, but
such a reference is completely useless: Its get method will always
return null and, since it does not have a queue, it will never be
enqueued
What does mean if it will never be enqueued?
As I understand it means that after finalize method invocation rerference will not be added to the referenceQueue. Thus it may lead to:
1. object memory will be cleared at once
2. Object memory will not be cleared
which case correct?

Well, as you noticed yourself, a PhantomReference is not automatically cleared. This implies that as long as you keep a strong reference to the PhantomReference, the referent will stay phantom reachable. As the documentation says: “An object that is reachable via phantom references will remain so until all such references are cleared or themselves become unreachable.”
However, considering when an object is unreachable (now I’m talking about the “phantom references themselves”) can lead to many surprises. Especially as it’s very likely that the reference object, not providing useful operations, will not be subsequently touched anymore.
Since the PhantomReference without a queue will never be enqueued and its get() method will always return null, it is indeed not useful.
So why does the constructor allows to construct such a useless object? Well, the documentation of the very first version (1.2) states that it will throw a NullPointerException if the queue is null. This statement persists until 1.4, then Java 5 is the first version containing the statement that you can construct a PhantomReference without a queue, despite being useless. My guess is, that it always inherited the super class’ behavior of allowing a null queue, contradicting the documentation, and it was noticed so late, that the decision was made to stay compatible and adapt the documentation rather than changing the behavior.
The question, even harder to answer, is why a PhantomReference isn’t automatically cleared. The documentation only says that a phantom reachable object will remain so, which is the consequence of not being cleared, but doesn’t explain why this has any relevance.
This question has been brought up on SO, but the answer isn’t really satisfying. It says “to allow performing cleanup before an object is garbage collected”, which might even match the mindset of whoever made that design decision, but since the cleanup code can’t access the object, it has no relevance whether it is executed before or after the object is reclaimed. As said above, since this rule depends on the reachability of the PhantomReference object, which is subject to optimizing code transformations, it might be even the case that the object is reclaimed together with the PhantomReference instance before the cleanup code completes, without anyone noticing.
I also found a similar question on the HotSpot developer mailing list back in 2013 which also lacks an answer.
There is the enhancement request JDK-8071507 to change that behavior and clear PhantomReferences just like the others, which has the status “fixed” for Java 9, and indeed, its documentation now states that they are cleared like any other reference.
This, unfortunately implies that the answer at the beginning of my post will be wrong starting with Java 9. Then, new PhantomReference(new Object(), null) will make the newly created Object instance immediately eligible for garbage collection, regardless of whether you keep a strong reference to the PhantomReference instance or not.

How does Java GC call finalize() method?

As far as I understand, GC starts with some set of initial objects (stack, static objects) and recursively traverses it building a graph of reachable objects. Then it marks the memory taken by these objects as occupied and assumes all the rest of the memory free.
But what if this 'free' memory contains an object with finalize method? GC has to call it, but I don't see how it can even know about objects that aren't reachable anymore.
I suppose GC can keep track of all 'finalizable' objects while they are alive. If so, does having finalizable objects make garbage collecting more expensive even when they are still alive?

Consider the Reference API.
It offers some references with special semantics to the GC, i.e Weak, Soft, and Phantom references. There’s simply another non-public type of special reference, for objects needing finalization.
Now, when the garbage collector traverses the object graph and encounters such a special reference object, it will not mark objects reachable through this reference as strongly reachable, but reachable with the special semantics. So if an object is only finalizer-reachable, the reference will be enqueued, so that one (or one of the) finalizer thread(s) can poll the queue and execute the finalize() method (it’s not the garbage collector itself calling this method).
In other words, the garbage collector never processes entirely unreachable objects here. To apply a special semantic to the reachability, the reference object must be reachable, so the referent can be reached through that reference. In case of finalizer-reachability, Finalizer.register is called when an object is created and it creates an instance of Finalizer in turn, a subclass of FinalReference, and right in its constructor, it calls an add() method which will insert the reference into a global linked list. So all these FinalReference instances are reachable through that list until an actual finalization happens.
Since this FinalReference will be created right on the instantiation of the object, if its class declares a non-trivial finalize() method, there is already some overhead due to having a finalization requirement, even if the object has not collected yet.
The other issue is that an object processed by a finalizer thread is reachable by that thread and might even escape, depending on what the finalize() method does. But the next time, this object becomes unreachable, the special reference object does not exist anymore, so it can be treated like any other unreachable object.
This would only be a performance issue, if memory is very low and the next garbage collection had to be performed earlier to eventually reclaim that object. But this doesn’t happen in the reference implementation (aka “HotSpot” or “OpenJDK”). In fact, there could be an OutOfMemoryError while objects are pending in the finalizer queue, whose processing could make more memory reclaimable. There is no guaranty that finalization runs fast enough for you’re purposes. That’s why you should not rely on it.

But what if this 'free' memory contains an object with finalize
method? GC has to call it, but I don't see how it can even know about
objects that aren't reachable anymore.
Let's say we use CMS garbage collector. After it successfully marked all live objects in a first phase, it will then scan memory again and remove all dead objects. GC thread does not call finalize method directly for these objects.
During creation, they are wrapped and added to finalizer queue by JVM (see java.lang.ref.Finalizer.register(Object)). This queue is processed in another thread (java.lang.ref.Finalizer.FinalizerThread), finalize method will be called when there are no references to the object. More details are covered in this blog post.
If so, does having finalizable objects make garbage collecting more
expensive even when they are still alive?
As you can now see, most of the time it does not.

The finalise method is called when an object is about to get garbage collected. That means, when GC determines that the object is no longer being referenced, it can call the finalise method on it. It doesn't have to keep track of objects to be finalised.

According to javadoc, finalize
Called by the garbage collector on an object when garbage collection determines that there are no more references to the object.
So the decision is based on reference counter or something like that.
Actually it is possible not to have this method called at all. So it may be not a good idea to use it as destructor.

Java: difference between strong/soft/weak/phantom reference

I have read this article about different types of references in Java (strong, soft, weak, phantom), but I don't really understand it.
What is the difference between these reference types, and when would each type be used?

Java provides two different types/classes of Reference Objects: strong and weak. Weak Reference Objects can be further divided into soft and phantom.
Strong
Weak
soft
phantom
Let's go point by point.
Strong Reference Object
StringBuilder builder = new StringBuilder();
This is the default type/class of Reference Object, if not differently specified: builder is a strong Reference Object. This kind of reference makes the referenced object not eligible for GC. That is, whenever an object is referenced by a chain of strong Reference Objects, it cannot be garbage collected.
Weak Reference Object
WeakReference<StringBuilder> weakBuilder = new WeakReference<StringBuilder>(builder);
Weak Reference Objects are not the default type/class of Reference Object and to be used they should be explicitly specified like in the above example. This kind of reference makes the reference object eligible for GC. That is, in case the only reference reachable for the StringBuilder object in memory is, actually, the weak reference, then the GC is allowed to garbage collect the StringBuilder object. When an object in memory is reachable only by Weak Reference Objects, it becomes automatically eligible for GC.
Levels of Weakness
Two different levels of weakness can be enlisted: soft and phantom.
A soft Reference Object is basically a weak Reference Object that remains in memory a bit more: normally, it resists GC cycle until no memory is available and there is risk of OutOfMemoryError (in that case, it can be removed).
On the other hand, a phantom Reference Object is useful only to know exactly when an object has been effectively removed from memory: normally they are used to fix weird finalize() revival/resurrection behavior, since they actually do not return the object itself but only help in keeping track of their memory presence.
Weak Reference Objects are ideal to implement cache modules. In fact, a sort of automatic eviction can be implemented by allowing the GC to clean up memory areas whenever objects/values are no longer reachable by strong references chain. An example is the WeakHashMap retaining weak keys.

Weak Reference :
A weak reference, simply put, is a reference that isn't strong enough to force an object to remain in memory. Weak references allow you to leverage the garbage collector's ability to determine reachability for you, so you don't have to do it yourself.
Soft Reference :
A soft reference is exactly like a weak reference, except that it is less eager to throw away the object to which it refers. An object which is only weakly reachable (the strongest references to it are WeakReferences) will be discarded at the next garbage collection cycle, but an object which is softly reachable will generally stick around for a while.
Phantom Reference :
A phantom reference is quite different than either SoftReference or WeakReference. Its grip on its object is so tenuous that you can't even retrieve the object -- its get() method always returns null. The only use for such a reference is keeping track of when it gets enqueued into a ReferenceQueue, as at that point you know the object to which it pointed is dead.
This text was extracted from: https://weblogs.java.net/blog/2006/05/04/understanding-weak-references

This article can be super helpful to understand strong, soft, weak and phantom references.
To give you a summary,
If you have a strong reference to an object, then the object can never be collected/reclaimed by GC (Garbage Collector).
If you only have weak references to an object (with no strong references), then the object will be reclaimed by GC in the very next GC cycle.
If you only have soft references to an object (with no strong references), then the object will be reclaimed by GC only when JVM runs out of memory.
We create phantom references to an object to keep track of when the object gets enqueued into the ReferenceQueue. Once you know that you can perform fine-grained finalization. (This would save you from accidentally resurrecting the object as phantom-reference don't give you the referrant). I'd suggest you reading this article to get in-depth detail about this.
So you can say that, strong references have ultimate power (can never be collected by GC)
Soft references are powerful than weak references (as they can escape GC cycle until JVM runs out of memory)
Weak references are even less powerful than soft references (as they cannot escape any GC cycle and will be reclaimed if object have no other strong reference).
Restaurant Analogy
Waiter - GC
You - Object in heap
Restaurant area/space - Heap space
New Customer - New object that wants table in restaurant
Now if you are a strong customer (analogous to strong reference), then even if a new customer comes in the restaurant or what so ever happnes, you will never leave your table (the memory area on heap). The waiter has no right to tell you (or even request you) to leave the restaurant.
If you are a soft customer (analogous to soft reference), then if a new customer comes in the restaurant, the waiter will not ask you to leave the table unless there is no other empty table left to accomodate the new customer. (In other words the waiter will ask you to leave the table only if a new customer steps in and there is no other table left for this new customer)
If you are a weak customer (analogous to weak reference), then waiter, at his will, can (at any point of time) ask you to leave the restaurant :P

The simple difference between SoftReference and WeakReference is provided by Android Developer.
The difference between a SoftReference and a WeakReference is the point of time at which the decision is made to clear and enqueue the reference:
A SoftReference should be cleared and enqueued as late as possible,
that is, in case the VM is in danger of running out of memory.
A WeakReference may be cleared and enqueued as soon as is known to be
weakly-referenced.

The three terms that you have used are mostly related to Object's eligibility to get Garbage collected .
Weak Reference :: Its a reference that is not strong enough to force the object to remain in memory . Its the garbage collector's whims to collect that object for garbage collection.
You can't force that GC not to collect it .
Soft Reference :: Its more or less same like the weak reference . But you can say that it holds the object a bit more strongly than the weak reference from garbage collection.
If the Garbage collectors collect the weak reference in the first life cycle itself, it will collect the soft reference in the next cycle of Garbage collection.
Strong Reference :: Its just opposite to the above two kind of references .
They are less like to get garbage collected (Mostly they are never collected.)
You can refer to the following link for more info :
http://docs.oracle.com/javase/1.4.2/docs/api/java/lang/ref/Reference.html

Strong References
These are your regular object references which we code daily:
Employee emp = new Employee();
The variable “emp” holds a strong reference to an Employee object and objects that are reachable through any chain of strong references are not eligible for garbage collection.
Usually, this is what you want but not always. Now suppose we are fetching lots of employees from database in a collection or map, and we need to do a lot of processing on them regularly, So in order keep performance we will keep them in the cache.
As far as this is good but now we need different data and we don’t need those Employee objects and these are not referenced from anywhere except the cache. Which is causing a memory leak because these objects are not in use but still not eligible for the garbage collection and we cannot remove those objects from cache because we don’t have reference to them?
So here either we need to empty the entire cache manually which is tedious or we could use other kind references e.g. Weak References.
Weak References
A weak reference does not pin an object into memory and will be GC’d in next GC cycle if not referenced from other references. We can use WeakReference class which is provided by Java to create above kind of caches, which will not store objects which are not referenced from somewhere else.
WeakReference<Cache> cache = new WeakReference<Cache>(data);
To access data you need to call cache.get(). This call to get may return null if the weak reference was garbage collected: you must check the returned value to avoid NPEs.
Java provides collections that use weak references e.g., the WeakHashMap class stores keys (not values) as weak references. If the key is GC’d then the value will automatically be removed from the map too.
Since weak references are objects too we need a way to clean them up (they’re no longer useful when the object they were referencing has been GC’d). If you pass a ReferenceQueue into the constructor for a weak reference then the garbage collector will append that weak reference to the ReferenceQueue before they’re finalized or GC’d. You can periodically process this queue and deal with dead references.
Soft References
A SoftReference is like a WeakReference but it is less likely to be garbage collected. Soft references are cleared at the discretion of the garbage collector in response to memory demand. The virtual machine guarantees that all soft references to softly reachable objects will have been cleared before it would ever throw an OutOfMemoryError.
Phantom References
Phantom references are the weakest of all reference types, calling get on them will always return null. An object is phantomly referenced after it has been finalized, but before its allocated memory has been reclaimed, As opposed to weak references which are enqueued before they’re finalized or GC’d Phantom references are rarely used.
So how are they useful? When you construct a phantom reference you must always pass in a ReferenceQueue. This indicates that you can use a phantom reference to see when your object is GC’d.
Hey, so if weak references are enqueued when they’re considered finalize but not yet GC’d we could create a new strong reference to the object in the finalizer block and prevent the object being GC’d. Yep, you can but you probably shouldn’t do this. To check for this case the GC cycle will happen at least twice for each object unless that object is reachable only by a phantom reference. This is why you can run out of heap even when your memory contains plenty of garbage. Phantom references can prevent this.
You can read more on my article Types of References in Java(Strong, Soft, Weak, Phantom).

4 degrees of reference - Strong, Weak, Soft, Phantom
Strong - is a kind of reference, which makes the referenced object not
eligible for GC. builder classes. eg - StringBuilder
Weak - is a reference which is eligible for GC.
Soft - is a kind of reference whose object is eligible for GC until memory is avaiable. Best for image cache. It will hold them till the memory is available.
Phantom - is a kind of reference whose object is directly eligible for GC. Used only to know when an object is removed from memory.
uses:
Allows you to identify when an object is exactly removed from memory.
when finalize() method is overloaded, then GC might not happen in timely fashion for GC eligible objects of the two classes. So phantom reference makes them eligible for GC before finalize(), is why you can get OutOfMemoryErrors even when most of the heap is garbage.
Weak references are ideal to implement the cache modules.

What if a finalizer makes an object reachable?

In Java, finalize is called on an object (that overrides it) when it's about to be garbage collectioned, so when it's unreachable. But what if the finalizer makes the object reachable again, what happens then?

Then the object doesn't get garbage collected, basically. This is called object resurrection. Perform a search for that term, and you should get a bunch of interesting articles. As Jim mentioned, one important point is that the finalizer will only be run once.

The object will not be collected until it gets unreachable again.
According to the JavaDoc, finalize() will not be called again.

If you read the API description carefully, you'll see that the finalizer can make the object reachable again. The object won't be discarded until it is unreachable (again), but finalize() won't be called more than once.

Yeah, this is why you don't use finalizers (Well, one of the many reasons).
There is a reference collection that is made to do this stuff. I'll look it up and post it here in a sec, but I think it's PhantomReference.
Yep, PhantomReference:
Phantom reference objects, which are enqueued after the collector determines that their referents may otherwise be reclaimed. Phantom references are most often used for scheduling pre-mortem cleanup actions in a more flexible way than is possible with the Java finalization mechanism.

It actually does another pass to check and make sure there are no more references to the object. Since it will fail that test on its second pass, you'll end up not freeing the memory for the object.
Because finalize is only called a single time for any given object, the next time through when it has no references, it will just free the memory without calling finalize. Some good information here on finalization.