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How does Java solve retain cycles in garbage collection?

I know that a retain cycle (at least in Objective-C and Swift) is when two objects claim ownership of one another (they have references to each other). And in Objective-C we can solve the issue by declaring one of them weak.
From what I have read and understood, the Java GC is not affected by retain cycles, and we do not have to worry about weak references. How does it solve it?

The Java (JVM) garbage collector works by looking for "reachable" objects - from the root(s) of the object tree. If they can't be reached (if they have no outside object references) then entire object graphs can be discarded.
Essentially it just just traverses the tree from root(s) to leaf nodes and marks all objects it encounters. Any memory not taken up by marked objects in the heap is swept (marked as free). This is called mark and sweep. img src
This can't be done easily in objective-c because it uses reference counting, not mark and sweep which has it's flaws
The reason there can be no retain cycles is because if they aren't linked to the "tree" anywhere, they aren't marked and can be discarded.

The garbage collector looks for reachable objects, starting from the roots (typically: variables on the call stack or global variables). So if two objects reference each other but are not otherwise reachable they won't be flagged as "live" and will be collected.

As the name suggests, Garbage Collection refers to removing of objects
which are no longer in use. It is a well known fact that irrespective
of their scope objects, Java stores objects in heap. Thus, if we keep
on creating objects without clearing the heap, our computers might run
out of heap space and we get ‘Out of Memory’ error. Garbage Collection
in Java is a mechanism which is controlled and executed by the Java
Virtual Machine (JVM) to release the heap space occupied by the
objects which are no more in use. In contrast to C++, garbage
collection in java relives the developer from the Memory Management
related activities. The JVM executes this process with the help of a
demon thread called the ‘Garbage Collector’. The garbage collector
thread first invokes the finalize method of the object. This performs
the cleanup activity on the said object. As a developer we cannot
force the JVM to run the garbage collector thread. Though there are
methods e.g Runtime.gc () or System.gc(), but none of these assures
the execution of garbage collector thread. These methods are used to
send garbage collection requests to the JVM. It is up to the Java
Virtual machine when it will initiate the garbage collection process.
Take a look at this stuff
How Garbage Collection works in Java

In basic terms, Garbage Collection works by walking the object graphs from a number of predefined roots. Anything not accessible from those roots is garbage, therefore one object referencing another is irrelevant unless either can be accessed from one or more roots.
It's all explained in more detail in How Garbage Collection Really Works.

The behavior of a tracing garbage collector may be viewed as analogous to that of a bowling alley pinsetter, which automatically sweeps up all pins that have been knocked over without disrupting pins that are still standing. Rather than trying to identify knocked-over pins, the pinsetter grabs all of the pins that are still standing, lifts them off the alley, and then runs a sweeper bar over the alley surface, removing wholesale any pins that might happen to be there without knowing or caring where they are.
A tracing GC works by visiting a certain set of "rooted" object references (which are regarded as always "reachable") and objects that are reachable via references held in reachable objects. The GC will mark such objects and protect their contents somehow. Once all such objects have been visited, the system will then visit some "special" objects (e.g. lists of weak or phantom references, or references to objects with finalizers) and others which are reachable from them but weren't reachable from ordinary rooted references, and then regard any storage which hasn't been guarded as eligible for reuse.
The system will need to specially treat objects that were reachable from special objects but weren't reachable from ordinary ones, but otherwise won't need to care about "ordinary" objects that become eligible for collection. If an object doesn't have a finalizer and isn't targeted by a weak or phantom reference, the GC may reuse its associated storage without ever bothering to look at any of it. There's no need for the GC to worry about the possibility that a group of objects that aren't reachable via any rooted references might hold references to each other because the GC wouldn't bother examining of those references even if they existed.

When will an object reference get garbage collected?

Someone told me only when a reference be set as null, the garbage collector will collect it. However I think the garbage collector will collect all out of scope references, which have never been set as null.
Anyone can told me when a reference will be claimed by the garbage collector?

The GC collects objects, not references.
The GC will collect an object some time after it has no more live references. (the GC is non-deterministic)

What that someone probably meant was that you can let GC collect an object before it goes out of scope by setting the variable that holds a reference to that object to null. This technique has been of value in the past for some very rare cases (for example, before a long-running loop that does not reference an object). The compiler technology these days renders this idea virtually useless, because compilers are smart enough to detect these conditions, and act accordingly.

GC will removes an object from memory "when it wants". You can try to run System.gc() but it's just a hint for GC that it should run. Whne GC runs, it finds non-referenced objects (or objects with weak references etc. only). GC runs frequency depdens on memory space. It runs most often in eden space. By default GC runs in eden space when it gets full, but you can tune up your JVM.
Why are you bothering GC in the first place? What's your problem?

Java "dead" objects not being garbage collected

I know that during garbage collection in Java, objects that don't have any more references to them are marked as "dead" so that they can be deleted from memory by the garbage collector.
My question is if, during a garbage collection phase, all of the "dead" objects get deleted from memory or some of them survive? Why would a "dead" object survive a garbage collection phase?
LATER EDIT
Thank you for all of your answers. I can deduce that the main reason why "dead" objects would not be deleted is due to timing or spacing limitations of the way the Garbage Collector operates.
However, supposing that the Garbage Collector can reach all of the "dead" objects, I was wondering if there is a way to declare, reference, use, dereference, etc.. an object such that somehow it would skip the deletion phase even though it is "dead". I was thinking maybe objects belonging to classes which have static methods or inner classes or something like that may be kept in memory for some reason, even though they have no references to them.
Is such a scenario possible?
Thank you

My question is if, during a garbage collection phase, all of the "dead" objects get deleted from memory or some of them survive? Why would a "dead" object survive a garbage collection phase?
All current HotSpot GCs are generational collectors. Quoting from Wikipedia:
"It has been empirically observed that in many programs, the most recently created objects are also those most likely to become unreachable quickly (known as infant mortality or the generational hypothesis). A generational GC (also known as ephemeral GC) divides objects into generations and, on most cycles, will place only the objects of a subset of generations into the initial white (condemned) set. Furthermore, the runtime system maintains knowledge of when references cross generations by observing the creation and overwriting of references. When the garbage collector runs, it may be able to use this knowledge to prove that some objects in the initial white set are unreachable without having to traverse the entire reference tree. If the generational hypothesis holds, this results in much faster collection cycles while still reclaiming most unreachable objects."
What this means for your question is that most GC cycles collect only garbage objects in young generations. A garbage object in the oldest generation can survive multiple GC cycles ... until the old generation is finally collected. (And in the new G1 GC, apparently the old generation is collected a bit at a time ... which can delay reclamation even further.)
Other causes for (notionally) unreachable objects to survive include:
Unreachable objects with (unexecuted) finalizers are attached to a finalization queue by the garbage collector for processing after the GC has finished.
Objects that are softly, weakly or phantom referenced are actually still reachable, and are handled by their respective reference queue managers after the GC has finished.
Objects that are reachable by virtue of JNI global references, etcetera. (thanks #bestss)
Various hidden references exist that relate instances, their classes and their classloaders.
There is a hidden reference from an inner instance to its outer instance.
There is a hidden reference from a class to the intern'd String objects that represent its string literals.
However, these are all consequences of the definition of reachability:
"A reachable object is any object that can be accessed in any potential continuing computation from any live thread." - JLS 12.6.1
It is also worth noting that the rules for the GC have an element of conservativeness about them. They say that a reachable object won't be deleted, but they don't say that an object that is (strictly) unreachable will be deleted. This allows for cases where an object cannot be accessed but the runtime system is unable to figure that out.
Your followup question:
However, supposing that the Garbage Collector can reach all of the "dead" objects, I was wondering if there is a way to declare, reference, use, dereference, etc.. an object such that somehow it would skip the deletion phase even though it is "dead".
"Dead" is not a well-defined term. If the garbage collector can reach the objects, they are by definition reachable. They will not be deleted while they are still reachable.
If they are both dead AND reachable (whatever "dead" means!) then the fact that they are reachable means they won't be deleted.
What you are proposing doesn't make sense.
I was thinking maybe objects belonging to classes which have static methods or inner classes or something like that may be kept in memory for some reason, even though they have no references to them. Is such a scenario possible?
Static methods don't have references ... unless they happen to be on the call stack. Then the local variables may contain references just like any other method call. Normal reachability rules apply.
Static fields are GC roots, for as long as the class itself exists. Normal reachability rules apply.
Instances of inner classes are no different to instance of other classes from a GC perspective. There can be a reference to an outer class instance in an inner class instance, but that leads to normal reachability.
In summary, there are some unexpected "causes" for reachability, but they are all a logical consequence of the definition of reachability.

As the System.gc() javadoc says
When control returns from the method
call, the Java Virtual Machine has
made a best effort to reclaim space
from all discarded objects.
From which you can infer that a call to the garbage collector does not insure that all unused object will be reclaimed. As the garbage collection can completely differ between implementation, no definitive answer can be given. There is even java implementations without any garbage collection.

One potential explanation for an unreachable object not being collected is time. As of Java 1.5 the amount of time the JVM spends garbage collecting can be limited using on of the following options...
-XX:MaxGCPauseMillis
-XX:GCTimeRatio=<nnn>
Both options are explained in detail here

There are dead objects in "young" generation and there are dead objects in "old" generation. If GC being performed in "minor GC", only dead objects from young generation will be collected.
Additionally, you can use finalize() method to stop VM from collecting your object by throwing exception from finalize() (at least, this is how I understand Object.finalize() javadoc: Any exception thrown by the finalize method causes the finalization of this object to be halted, but is otherwise ignored).

The behaviour of the garbage collector is not fully specified. If a particular implementation choose not to collect certain objects, it is allowed to do so. This could be done to avoid spending large periods of time in the garbage collector, which could have detrimental effects to the operation of the application.

Imagine you had a collection which contained millions of small objects, most of which were not referenced anywhere else. If the only references to that collection was cleared, would you want the GC to spend a long time cleaning out those millions of small objects, or would you want it to do so over the course of several calls? In most cases, the latter would be better for the application.

When does Java's garbage collection free a memory allocation?

I have created an object in Java, Named FOO. FOO contains a large amount of data.. I don't know say for a ten mega byte text file that I have pulled into ram for manipulation.(This is just an example)
This is clearly a huge amount of space and I want to deallocate it from memory. I set FOO to NULL.
Will this free up that space in memory automatically?
or
Will the memory taken by the loaded text file be around until automatic garbage collection?

When you set the reference of any object to null, it becomes available for garbage collection. It still occupies the memory until the garbage collector actually runs. There are no guarantees regarding when GC will run except that it will definitely run and reclaim memory from unreachable objects before an OutOfMemoryException is thrown.
You can call System.gc() to request garbage collection, however, that's what it is - a request. It is upto GC's discretion to run.
Using a WeakReference can help in some cases. See this article by Brian Goetz.

Actually the object is not named FOO. FOO is the name of a variable which is not the object; the variable contains a reference to the object. There could be several distinct variables containing references to the same object.
The garbage collector works by automatically detecting unreachable objects: these are objects which the application cannot use anymore because it has irretrievably forgotten where they are (the application may possibly access any object for which it has a reference to, including the references stored in field in objects it can access, and so on).
When you set FOO = null, assuming that FOO contained at that point the last reachable reference to the object, then the memory is released immediately, in the following sense: at the very clock cycle at which null is set in FOO, the object becomes unreachable. Therefore, the garbage collector will notice that unreachable object and reclaim the corresponding memory block; that is, the GC will do that the next time it can be bothered to run. Of course, the actual bits which constitute the object may linger a bit in memory; but that block is nonetheless "free" since the memory allocator will automatically run the GC when free memory is tight. From the application point of view, the object is as good as dead and the corresponding memory is free since that memory will be reused the next time the application needs it. The whole thing is automatic.
Things are a bit more complex with regards to the operating system. If an unreachable object is free memory from the application point of view, it is still, as far as the OS is concerned, a block of RAM dedicated to the running process. That block of RAM may be given back to the OS only when the GC (which is, at the OS level, a part of the process) actually runs, notices that the object is unreachable, and condescends to give the block back to the OS. When the GC runs heavily depends on the GC technology and how the application allocates objects; also, some GC will never give back the block the OS at all (the GC knows that the block it free, the memory allocator will reuse it at will, but not other processes).
System.gc() is a hint to the VM, so that it runs the GC now. Formally, it is only a hint, and the VM is free to ignore it. In practice, it runs the GC, unless the VM was instructed not to obey such commands (with Sun's JVM, this is a matter of a specific command-line flag). Even if the GC runs, it does not necessarily give back the memory to the operating system. System.gc() is not terribly useful.

Setting foo = null; does not mean that foo will be garbage collected immediately. Instead, it will be collected when the GC next runs, if it can be. When foo is collected, any objects for which it holds the sole reference will also be eligible for collection and therefore collected.

Note that even calling System.gc() does not guarantee that that JVM will do it right away.
System.gc() is just a request and there is no guarantee that it's effect immediately.

There's no guarantee that JVM will do it right away, you can try to force it by using System.gc()

The garbage collector will free the memory after you "destroy" the reference. i.3 Setting the object reference to null. You can use forced garbage collection option but you should use it with care. The Garbage collector is designed to use an optimized schedule so calling the System.gc() may ruin the rhythem and possibly have less performance due to unnecessary task switching.
Alternatively you can think about a way that allows you to not to load large amounts of data into memory. If you can gain that by improving your code that would be much better.

can any unused object escape from Garbage Collector?

Is there any possibility that a object which is not referenced anywhere and still existing on heap. I mean is there a possibility that a unused object getting escaped from garbage collector and be there on the heap until the end of the application.
Wanted to know because if it is there, then while coding i can be more cautious.

If an object is no longer referenced, it does still exist on the heap, but it is also free to be garbage-collected (unless we are talking Class objects, which live in PermGen space and never get garbage-collected - but this is generally not something you need to worry about).
There is no guarantee on how soon that will be, but your application will not run out of memory before memory from those objects is reclaimed.
However, garbage collection does involve overhead, so if you are creating more objects than you need to and can easily create less, then by all means do so.
Edit: in response to your comment, if an object is truly not referenced by anything, it will be reclaimed during garbage collection (assuming you are using the latest JVM from Sun; I can't speak toward other implementations). The reason why is as follows: all objects are allocated contiguously on the heap. When GC is to happen, the JVM follows all references to "mark" objects that it knows are reachable - these objects are then moved into another, clean area. The old area is then considered to be free memory. Anything that cannot be found via a reference cannot be moved. The point is that the GC does not need to "find" the unreferenced objects. If anything, I would be more worried about objects that are still referenced when they are not intended to be, which will cause memory leaks.

You should know that, before a JVM throws an out-of-memory exception, it will have garbage collected everything possible.

If an instance is no longer referenced, it is a possible candidate for garbage collection. This means, that sooner or later it can be removed but there are no guaranties. If you do not run out of of memory, the garbage collector might not even run, thus the instance my be there until the program ends.
The CG system is very good at finding not referenced objects. There is a tiny, tiny chance that you end up keeping a weird mix of references where the garbage collector can not decide for sure if the object is no longer referenced or not. But this would be a bug in the CG system and nothing you should worry about while coding.

It depends on when and how often the object is used. If you allocate something then deallocate (i.e., remove all references to it) it immediately after, it will stay in "new" part of the heap and will probably be knocked out on the next garbage collection run.
If you allocate an object at the beginning of your program and keep it around for a while (if it survives through several garbage collections), it will get promoted to "old" status. Objects in that part of the heap are less likely to be collected later.
If you want to know all the nitty-gitty details, check out some of Sun's gc documentation.

Yes; imagine something like this:
Foo foo = new Foo();
// do some work here
while(1) {};
foo.someOp(); // if this is the only reference to foo,
// it's theoreticaly impossible to reach here, so it
// should be GC-ed, but all GC systems I know of will
// not Gc it
I am using definition of: garbage = object that can never be reached in any execution of the code.

Garbage collection intentionally makes few guarantees about WHEN the objects are collected. If memory never gets too tight, it's entirely possible that an unreferenced object won't be collected by the time the program ends.

The garbage collector will eventually reclaim all unreachable objects. Note the "eventually": this may take some time. You can somewhat force the issue with System.gc() but this is rarely a good idea (if used without discretion, then performance may decrease).
What can happen is that an object is "unused" (as in: the application will not use it anymore) while still being "reachable" (the GC can find a path of references from one of its roots -- static fields, local variables -- to the object). If you are not too messy with your objects and structures then you will not encounter such situations. A rule of thumb would be: if the application seems to take too much RAM, run a profiler on it; if thousands of instances of the same class have accumulated without any apparent reason, then there may be some fishy code somewhere. Correction often involves explicitly setting a field to null to avoid referencing an object for too long.

This is theoretically possible (there is no guarantee the GC will always find all objects), but should not worry you for any real application - it usually does not happen and certainly does not affect a significant chunk of memory.

In theory, the garbage collector will find all unused objects. There could, of course, be bugs in the garbage collector…
That said, "In theory there is no difference between theory and practice, in practice, there is." Under some, mostly older, garbage collectors, if an object definition manages to reach the permanent generation, then it will no longer be garbage collected under any circumstances. This only applied to Class definitions that were loaded, not to regular objects that were granted tenured status.
Correspondingly, if you have a static reference to an object, that takes up space in the "regular" object heap, this could conceivably cause problems, since you only need to hold a reference to the class definition from your class definition, and that static data cannot be garbage collected, even if you don't actually refer to any instances of the class itself.
In practice though, this is a very unlikely event, and you shouldn't need to worry about it. If you are super concerned about performance, then creating lots of "long-lived" objects, that is, those that escape "escape-analysis", will create extra work for the garbage collector. For 99.99% of coders this is a total non-issue though.

My advice - Don't worry about it.
Reason - It is possible for a non-referenced object to stay on the heap for some time, but it is very unlikely to adversely affect you because it is guaranteed to be reclaimed before you get an out of memory error.

In general, all objects to which there are no live hard references, will be garbage-collected. This is what you should assume and code for. However, the exact moment this happens is not predictable.
Just for completeness, two tricky situations [which you are unlikely to run into] come into my mind:
Bugs in JVM or garbage collector code
So called invisible references - they rarely matter but I did have to take them into account one or two times during the last 5 years in a performance-sensitive application I work on