We Keep Coding

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.

Which reference Finalizer (FinalReference) or Weak/Phantom/Soft Reference have higher priority for GC

When object with non-trivial finalize() method is created, JVM will create Finalizer (FinalReference) with this object as referent. What will happen if this object will be also wrapped by Soft/Weak or Phantom Reference? Would GC try to enqueue the Finalizer (call finalize method on it) at first, and then enqueue the other Reference or the opposite?

I think, your question is not about the time of enqueuing, actually.
Consider the Notification section of the package documentation
Some time after the garbage collector determines that the reachability of the referent has changed to the value corresponding to the type of the reference, it will add the reference to the associated queue.
(Note the “some time after”)
and likewise, all reference types have a statement of the form:
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.
(taken from the WeakReference; Note the “at the same time or at some later time”)
In practice, the garbage collector hands over the discovered references to another thread which does the enqueuing asynchronously. Since an unspecified delay makes the order in which an application will retrieve the references from a queue undeterministic, it’s pointless to ask about an order here.
However, I suppose, you’re actually interested in the other “certain point in time”, when “the garbage collector determines that the reachability of the referent has changed to the value corresponding to the type of the reference” and will decide to clear the references atomically and make them eligible for enqueuing.
When there is a mixture of multiple differently typed reference objects, including a Finalizer reference, but no strong reference, there are two possible scenarios:
There is at least one soft reference and there is no memory pressure. Then, the garbage collector may decide not clear resp. enqueue any reference.
There is no soft reference or the garbage collector decides that memory pressure justifies clearing soft references. Then all soft and weak references are cleared and all soft, weak, and finalizer references are handed over for enqueuing.
Only phantom references stay untouched.
Note that once finalization started, there is no Finalizer reference anymore, but during finalization, new soft or weak reference might get created. So the resulting scenarios are the same as with the optimized handling of objects having a trivial finalize() method. There can be a mixture of soft, weak, and phantom references, without a Finalizer reference. When there is no remaining strong reference, we again have the two possible scenarios:
There is at least one soft reference and there is no memory pressure. Then, the garbage collector may decide not clear resp. enqueue any reference.
There is no soft reference or the garbage collector decides that memory pressure justifies clearing soft references. Then all soft, weak, and phantom references are clearedA and all soft, weak, and phantom references are handed over for enqueuing.
A phantom references are cleared in Java 9 or newer. In previous versions they are only enqueued without being cleared.

Concurrency guarantees of Weak/Soft/Phantom Reference clearing

Besides stronge references, there are three distinct "kinds of *Reference": WeakReference, SoftReference and PhantomReference. There may be others, but they will either extend one those or be irrelevant to my question.
If an object is not strongly reachable during a GC cycle, it gets garbage collected (i.e. finalized and removed from memory). But before it is collected, any *Reference is cleared - depending on reachability conditions, the kind of *Reference to be cleared, memory demand, etc.
I was wondering, if the JLS made any guarantees about whether all *References of the same kind to the same object are cleared atomically/simultaneously, or if there is a slight chance (ie. "race condition"), that some *References get cleared while other *References of the same kind are not, such that an object could potentially be revived (ie. be strongly referenced again) through one of the remaining *References although some *References of the same kind have already been cleared and enqueued.
Question rephrased from a different POV: Is it safe to assume that a cleared *Reference implies that the previously referenced object cannot be revived through a different *Reference of the same kind?

You may refer to the documentation.
SoftReference:
[The garbage collector] may choose to clear atomically all soft references to that object and all soft references to any other softly-reachable objects from which that object is reachable through a chain of strong references.
WeakReference:
[The garbage collector] 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.
The point is moot for PhantomReference because the referent cannot be retrieved from it (get always returns null).

How does a weak hash map know to garbage-collect an object?

I recently found out about the WeakHashMap data structure in Java.
However, I don't understand what it means by it garbage-collects a mapping when it is no longer in ordinary use. How does the data structure know I will no longer use a key in my program? What if I don't refer to a key for a long time?

However, I don't understand what it means by it garbage-collects a mapping when it is no longer in ordinary use.
OK. Under normal circumstances, when the garbage collector runs it will remove objects that your program can no longer use. The technical term is an "unreachable object", and it means that the program execution has no way to get hold of a reference to the object any more. An object that is unreachable, may be collected in the next GC cycle ... or not. Either way, it is no longer the application's concern.
In this case, the WeakHashMap uses a special class called WeakReference to refer to the keys1. A weak reference is an object that acts sort of like an indirect pointer (a pointer to an object holding a pointer). It has the interesting property that the garbage collector is allowed to break the reference; i.e. replace the reference it contains with null. And the rule is that a weak reference to an object will be broken when the GC notices that the object is no longer reachable via a chain of normal (strong) or soft references2.
The phrase "no longer in ordinary use" really means that the key object is no longer strongly or softly reachable; i.e. via a chain of strong and / or soft references.
How does the data structure know I will no longer use a key in my program?
The WeakHashmap doesn't do it. Rather, it is the GC that notices that the key is not strongly reachable.
As part of its normal traversal, the GC will find and mark all strongly reachable objects. Then it goes through all of the WeakReference objects and checks to see if
the objects they refer to have been marked, and breaks them if they have not. (Or something like that ... I've never looked at the actual GC implementation. And it is complicated by the fact that it has to deal with SoftReference and PhantomReference objects as well.)
The only involvement that WeakHashmap has is that:
it created and uses WeakReference objects for the keys, and
it expunges hash table entries whose key WeakReferences have been cleared by the GC.
What if I don't refer to a key for a long time?
The criterion for deciding that a weak reference should be broken is not time based.
But it is possible that timing influences whether not a key is removed. For instance, a key could 1) cease to be strongly reference, 2) be retrieved from the map, and 3) be assigned to a reachable variable making it strongly referenced once again. If the GC doesn't run during the window in which the key is not strongly reachable, the key and its associated value will stay in the map. (Which is what you'd want to happen ...)
1 - Implementation detail: in recent Java releases, the weak references actually refer to the map's internal Entry objects rather than the keys. This allows broken references to be purged from the map more efficiently. Look at the code for details.
2 - Soft references are a kind of reference that the GC is allowed to break if there is a shortage of heap memory.

Java has a system of references where the language can tell your code whether or not some object is still in use. You can use references to detect when some object has been specifically identified as no longer in use or usable, and can then take action accordingly. This tutorial covers references in some depth, in case you're curious how to use them.
Internally, WeakHashMap likely uses these references to automatically detect when a given key cannot be used any more. The implementation can then remove those objects from the hash table so that they no longer take up any space.
Hope this helps!

The JVM garbage collects in this order:
Unreferenced objects
Objects whose only reference is a WeakReference
Objects whose only reference is a SoftReference
Normally, the garbage collector only garbage collects unreferenced objects.
A weak reference to an object does not count as a reference as far as the garbage collector is concerned. The garbage collector may, or may not collect them. Typically, it will not collect them unless memory is running low, but there are no guarantees.
If the JVM is about run out of memory, the garbage collector will collect softly referenced objects. All weak referenced objects will be garbage collected before any soft referenced objects are garbage collected.
From the javadoc of SoftReference:
All soft references to softly-reachable objects are guaranteed to have been cleared before the virtual machine throws an OutOfMemoryError

I read your question as asking about the specific wording "regular use" and so assume you already know about strong and weak references. Regular use refers to the case where a weak hashmap contains keys that are also referenced (strongly) by some other data structure(s). The existence of at least one strong reference to a key is "regular use." The key can't be garbage collected as long as this other data structure references it. When the other data structure is no longer reachable (pointers to it no longer exist), the key also becomes unreachable. It's no longer in regular use because the only reference to it is the weak one in the mapping. The garbage collector can eventually reclaim it, and the mapping disappears.
This comes up when you'd like to extend a type C by subclassing but can't: for example when C is an interface with many implementations. You can work around this problem by using a weak hashmap with key type C and new fields to be added wrapped in a new class E. Whenever you create a C instance, you also create an E instance and add the pair to the map, which is then used to access the new fields during the life of the C intance. When the C instance becomes garbage, the mapping and E instance do also. This is automatic because the hashmap is weak. If it weren't, it would have to be cleaned up manually in the same manner you have to explicitly free storage in languages with no garbage collector.

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.