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Will garbage collector run when no java program is running? [duplicate]

I need to know when the finalize() method is called in the JVM. I created a test class which writes into a file when the finalize() method is called by overriding it. It is not executed. Can anybody tell me the reason why it is not executing?

The finalize method is called when an object is about to get garbage collected. That can be at any time after it has become eligible for garbage collection.
Note that it's entirely possible that an object never gets garbage collected (and thus finalize is never called). This can happen when the object never becomes eligible for gc (because it's reachable through the entire lifetime of the JVM) or when no garbage collection actually runs between the time the object become eligible and the time the JVM stops running (this often occurs with simple test programs).
There are ways to tell the JVM to run finalize on objects that it wasn't called on yet, but using them isn't a good idea either (the guarantees of that method aren't very strong either).
If you rely on finalize for the correct operation of your application, then you're doing something wrong. finalize should only be used for cleanup of (usually non-Java) resources. And that's exactly because the JVM doesn't guarantee that finalize is ever called on any object.

In general it's best not to rely on finalize() to do any cleaning up etc.
According to the Javadoc (which it would be worth reading), it is:
Called by the garbage collector on an object when garbage collection determines that there are no more references to the object.
As Joachim pointed out, this may never happen in the life of a program if the object is always accessible.
Also, the garbage collector is not guaranteed to run at any specific time. In general, what I'm trying to say is finalize() is probably not the best method to use in general unless there's something specific you need it for.

protected void finalize() throws Throwable {}
every class inherits the finalize() method from
java.lang.Object
the method is called by the garbage collector when it determines
no more references to the object
exist
the Object finalize method performs no actions but it may be overridden by
any class
normally it should be overridden to clean-up non-Java resources ie closing
a file
if overridding finalize() it is good programming practice to use a
try-catch-finally statement and to
always call super.finalize(). This
is a safety measure to ensure you do
not inadvertently miss closing a
resource used by the objects calling
class
protected void finalize() throws Throwable {
try {
close(); // close open files
} finally {
super.finalize();
}
}
any exception thrown by finalize() during garbage collection halts the
finalization but is otherwise ignored
finalize() is never run more than once on any object
quoted from: http://www.janeg.ca/scjp/gc/finalize.html
You could also check this article:
Object finalization and cleanup

The Java finalize() method is not a destructor and should not be used to handle logic that your application depends on. The Java spec states there is no guarantee that the finalize method is called at all during the livetime of the application.
What you problably want is a combination of finally and a cleanup method, as in:
MyClass myObj;
try {
myObj = new MyClass();
// ...
} finally {
if (null != myObj) {
myObj.cleanup();
}
}
This will correctly handle the situation when the MyClass() constructor throws an exception.

Check out Effective Java, 2nd edition page 27.
Item 7: Avoid finalizers
Finalizers are unpredictable, often dangerous, and generally unnecessary. never do anything time-critical in a finalizer. never
depend on a finalizer to update critical persistent state.
To terminate a resource, use try-finally instead:
// try-finally block guarantees execution of termination method
Foo foo = new Foo(...);
try {
// Do what must be done with foo
...
} finally {
foo.terminate(); // Explicit termination method
}

When is the finalize() method called in Java?
The finalize method will be called after the GC detects that the object is no longer reachable, and before it actually reclaims the memory used by the object.
If an object never becomes unreachable, finalize() will never be called on it.
If the GC doesn't run then finalize() may never be called. (Normally, the GC only runs when the JVM decides that there is likely to enough garbage to make it worthwhile.)
It may take more than one GC cycle before the GC determines that a specific object is unreachable. (Java GCs are typically "generational" collectors ...)
Once the GC detects an object is unreachable and finalizable, it is places on a finalization queue. Finalization typically occurs asynchronously with the normal GC.
(The JVM spec actually allows a JVM to never run finalizers ... provided that it doesn't reclaim the space used by the objects. A JVM that was implemented this way would be crippled / useless, but it this behavior is "allowed".)
The upshot is that it is unwise to rely on finalization to do things that have to be done in a definite time-frame. It is "best practice" not to use them at all. There should be a better (i.e. more reliable) way to do whatever it is you are trying to do in the finalize() method.
The only legitimate use for finalization is to clean up resources associated with objects that have been lost by application code. Even then, you should try to write the application code so that it doesn't lose the objects in the first place. (For example, use Java 7+ try-with-resources to ensure that close() is always called ...)
I created a test class which writes into a file when the finalize() method is called by overriding it. It is not executed. Can anybody tell me the reason why it is not executing?
It is hard to say, but there are a few possibilities:
The object is not garbage collected because it is still reachable.
The object is not garbage collected because the GC doesn't run before your test finishes.
The object is found by the GC and placed in the finalization queue by the GC, but finalization isn't completed before your test finishes.

Since there is an uncertainity in calling of finalize() method by JVM (not sure whether finalize() which is overridden would be executed or not), for study purposes the better way to observe what happens when finalize() is called, is to force the JVM to call garbage collection by command System.gc().
Specifically, finalize() is called when an object is no longer in use. But when we try to call it by creating new objects there is no certainty of its call. So for certainty we create a null object c which obviously has no future use, hence we see the object c's finalize call.
Example
class Car {
int maxspeed;
Car() {
maxspeed = 70;
}
protected void finalize() {
// Originally finalize method does nothing, but here we override finalize() saying it to print some stmt
// Calling of finalize is uncertain. Difficult to observe so we force JVM to call it by System.gc(); GarbageCollection
System.out.println("Called finalize method in class Car...");
}
}
class Bike {
int maxspeed;
Bike() {
maxspeed = 50;
}
protected void finalize() {
System.out.println("Called finalize method in class Bike...");
}
}
class Example {
public static void main(String args[]) {
Car c = new Car();
c = null; // if c weren`t null JVM wouldn't be certain it's cleared or not, null means has no future use or no longer in use hence clears it
Bike b = new Bike();
System.gc(); // should clear c, but not b
for (b.maxspeed = 1; b.maxspeed <= 70; b.maxspeed++) {
System.out.print("\t" + b.maxspeed);
if (b.maxspeed > 50) {
System.out.println("Over Speed. Pls slow down.");
}
}
}
}
Output
Called finalize method in class Car...
1 2 3 4 5 6 7 8 9
10 11 12 13 14 15 16 17 18 19
20 21 22 23 24 25 26 27 28 29
30 31 32 33 34 35 36 37 38 39
40 41 42 43 44 45 46 47 48 49
50 51Over Speed. Pls slow down.
52Over Speed. Pls slow down.
53Over Speed. Pls slow down.
54Over Speed. Pls slow down.
55Over Speed. Pls slow down.
56Over Speed. Pls slow down.
57Over Speed. Pls slow down.
58Over Speed. Pls slow down.
59Over Speed. Pls slow down.
60Over Speed. Pls slow down.
61Over Speed. Pls slow down.
62Over Speed. Pls slow down.
63Over Speed. Pls slow down.
64Over Speed. Pls slow down.
65Over Speed. Pls slow down.
66Over Speed. Pls slow down.
67Over Speed. Pls slow down.
68Over Speed. Pls slow down.
69Over Speed. Pls slow down.
70Over Speed. Pls slow down.
Note - Even after printing upto 70 and after which object b is not being used in the program, there is uncertainty that b is cleared or not by JVM since "Called finalize method in class Bike..." is not printed.

finalize will print out the count for class creation.
protected void finalize() throws Throwable {
System.out.println("Run F" );
if ( checkedOut)
System.out.println("Error: Checked out");
System.out.println("Class Create Count: " + classCreate);
}
main
while ( true) {
Book novel=new Book(true);
//System.out.println(novel.checkedOut);
//Runtime.getRuntime().runFinalization();
novel.checkIn();
new Book(true);
//System.runFinalization();
System.gc();
As you can see. The following out put show the gc got executed first time when the class count is 36.
C:\javaCode\firstClass>java TerminationCondition
Run F
Error: Checked out
Class Create Count: 36
Run F
Error: Checked out
Class Create Count: 48
Run F

Having wrestled with finalizer methods lately (in order to dispose connection pools during testing), I have to say that finalizer lacks many things. Using VisualVM to observe as well as using weak references to track the actual interaction I found that following things are true in a Java 8 environment (Oracle JDK, Ubuntu 15):
Finalize is not called immediately the Finalizer (GC part) individually owns the reference elusively
The default Garbage Collector pools unreachable objects
Finalize is called in bulk pointing to an implementation detail that there is a certain phase the garbage collector frees the resources.
Calling System.gc() often does not result in objects being finalized more often, it just results in the Finalizer getting aware of an unreachable object more rapidly
Creating a thread dump almost always result in triggering the finalizer due to high heap overhead during performing the heap dump or some other internal mechanism
Finalization seams to be bound by either memory requirements (free up more memory) or by the list of objects being marked for finalization growing of a certain internal limit. So if you have a lot of objects getting finalized the finalization phase will be triggered more often and earlier when compared with only a few
There were circumstances a System.gc() triggered a finalize directly but only if the reference was a local and short living. This might be generation related.
Final Thought
Finalize method is unreliable but can be used for one thing only. You can ensure that an object was closed or disposed before it was garbage collected making it possible to implement a fail safe if objects with a more complex life-cylce involving a end-of-life action are handled correctly. That is the one reason I can think of that makes it worth in order to override it.

An Object becomes eligible for Garbage collection or GC if its not reachable from any live threads or any static refrences in other words you can say that an object becomes eligible for garbage collection if its all references are null. Cyclic dependencies are not counted as reference so if Object A has reference of object B and object B has reference of Object A and they don't have any other live reference then both Objects A and B will be eligible for Garbage collection.
Generally an object becomes eligible for garbage collection in Java on following cases:
All references of that object explicitly set to null e.g. object = null
Object is created inside a block and reference goes out scope once control exit that block.
Parent object set to null, if an object holds reference of another object and when you set container object's reference null, child or contained object automatically becomes eligible for garbage collection.
If an object has only live references via WeakHashMap it will be eligible for garbage collection.

finalize method is not guaranteed.This method is called when the object becomes eligible for GC. There are many situations where the objects may not be garbage collected.

Sometimes when it is destroyed, an object must make an action. For example, if an object has a non-java resource such as a file handle or a font, you can verify that these resources are released before destroying an object. To manage such situations, java offers a mechanism called "finalizing". By finalizing it, you can define specific actions that occur when an object is about to be removed from the garbage collector.
To add a finalizer to a class simply define the finalize() method. Java execution time calls this method whenever it is about to delete an object of that class. Within the finalize method() you specify actions to be performed before destroying an object.
The garbage collector is periodically searched for objects that no longer refer to any running state or indirectly any other object with reference. Before an asset is released, the Java runtime calls the finalize() method on the object. The finalize() method has the following general form:
protected void finalize(){
// This is where the finalization code is entered
}
With the protected keyword, access to finalize() by code outside its class is prevented.
It is important to understand that finalize() is called just just before the garbage collection. It is not called when an object leaves the scope, for example. It means you can not know when, or if, finalize() will be executed. As a result, the program must provide other means to free system resources or other resources used by the object. You should not rely on finalize() for normal running of the program.

Recent news from JDK 18
According to JEPS 421 delivered on openjdk 18 the finalization and therefore the functionality of finalize() method will be marked as deprecated(forRemoval=true) meaning the permanent removal would follow in some later version after jdk 18.
As from jdk 18 a new command-line option --finalization=disabled disables finalization mechanism everywhere even for declarations inside the jdk itself.
This is also relevant to this question here as the reason it is planned for removal, is some major flaws it contains. One of those flaws is that a very long time may pass between the moment an object becomes unreachable and the moment its finalizer is called. It is also true that the GC provides no guarantee that any finalizer will ever be called.

Class where we override finalize method
public class TestClass {
public TestClass() {
System.out.println("constructor");
}
public void display() {
System.out.println("display");
}
#Override
public void finalize() {
System.out.println("destructor");
}
}
The chances of finalize method being called
public class TestGarbageCollection {
public static void main(String[] args) {
while (true) {
TestClass s = new TestClass();
s.display();
System.gc();
}
}
}
when the memory is overloaded with dump objects the gc will call finalize method
run and see the console, where you dont find the finalize method being called frequently, when the memory is getting overloaded then the finalize method will be called.

finalize() is called just before garbage collection. It is not called when an object goes out of scope. This means that you cannot know when or even if finalize() will be executed.
Example:
If your program end before garbage collector occur, then finalize() will not execute. Therefore, it should be used as backup procedure to ensure the proper handling of other resources, or for special use applications, not as the means that your program uses in its normal operation.

Java allows objects to implement a method called finalize()
that might get called.
finalize() method gets called if the garbage collector tries to
collect the object.
If the garbage collector doesn't run, the method doesn't get called.
If the garbage collector fails to collect the object and tries to run
it again, the method doesn't get called in the second time.
In practice, you are highly unlikely to use it in real projects.
Just keep in mind that it might not get called and that it definitely
won't be called twice. The finalize() method could run zero or one
time.
In the following code, finalize() method produces no output when we
run it since the program exits before there is any need to run the
garbage collector.
Source

As pointed out in https://wiki.sei.cmu.edu/confluence/display/java/MET12-J.+Do+not+use+finalizers,
There is no fixed time at which finalizers must be executed because time of execution depends on the Java Virtual Machine (JVM). The only guarantee is that any finalizer method that executes will do so sometime after the associated object has become unreachable (detected during the first cycle of garbage collection) and sometime before the garbage collector reclaims the associated object's storage (during the garbage collector's second cycle). Execution of an object's finalizer may be delayed for an arbitrarily long time after the object becomes unreachable. Consequently, invoking time-critical functionality such as closing file handles in an object's finalize() method is problematic.

Try runiing this Program for better understanding
public class FinalizeTest
{
static {
System.out.println(Runtime.getRuntime().freeMemory());
}
public void run() {
System.out.println("run");
System.out.println(Runtime.getRuntime().freeMemory());
}
protected void finalize() throws Throwable {
System.out.println("finalize");
while(true)
break;
}
public static void main(String[] args) {
for (int i = 0 ; i < 500000 ; i++ ) {
new FinalizeTest().run();
}
}
}

What does "finalize objects on finalization queue" do? [duplicate]

My understanding of finalization is this:
To clean up or reclaim the memory that an object occupies, the Garbage collector comes into action. (automatically is invoked?)
The garbage collector then dereferences the object. Sometimes, there is no way for the garbage collector to access the object. Then finalize is invoked to do a final clean up processing after which the garbage collector can be invoked.
Is this an accurate description of finalization?

The garbage collector is working automatically in the background (although it can be explicitly invoked, but the need for this should be rare). It basically cleans up only objects which are not referenced by other objects (granted, the full picture is more complicated, but this is the basic idea). So it does not change any references in any live objects. If an object can not be accessed from any live object, this means that it can be safely garbage collected.
Finalization was meant to clean up resources acquired by the object (not memory, but other resources, e.g. file handles, ports, DB connections etc.). However, it did not really work out :-(
it is unpredictable when finalize() will be called
in fact, there is no guarantee that finalize() will be called ever!
So even if it were guaranteed to be called, it would not be a good place to release resources: by the time it is called to free up all the DB connections you have opened, the system may have run out of free connections completely, and your app does not work anymore.

From this article:
Any instances of classes that
implement the finalize() method are
often called finalizable objects. They
will not be immediately reclaimed by
the Java garbage collector when they
are no longer referenced. Instead, the
Java garbage collector appends the
objects to a special queue for the
finalization process. Usually it's
performed by a special thread called a
"Reference Handler" on some Java
Virtual Machines. During this
finalization process, the "Finalizer"
thread will execute each finalize()
method of the objects. Only after
successful completion of the
finalize() method will an object be
handed over for Java garbage
collection to get its space reclaimed
by "future" garbage collection.
You are free to do virtually anything
in the finalize() method of your
class. When you do that, please do not
expect the memory space occupied by
each and every object to be reclaimed
by the Java garbage collector when the
object is no longer referenced or no
longer needed. Why? It is not
guaranteed that the finalize() method
will complete the execution in timely
manner. Worst case, it may not be even
invoked even when there are no more
references to the object. That means
it's not guaranteed that any objects
that have a finalize() method are
garbage collected.
Also, this article from Sun has some nice diagrams explaining the process.

Nope. The finalize() method is run only if the garbage collector attempts to reclaim your object.
Any memory used by your object will (usually, I can't think of an exception) automatically be connected to your object and cleaned up along with it. Finalization, therefore, isn't meant for freeing memory, but rather any other resources your object may be associated with. For example, this could be used to close open files or database connections, or perhaps run some low-level code interfacing with the operating system to release some system-level resources.

Actually, here's the behavior of the finalize() method:
Once the Garbage collector runs (the VM decides it needs to free up memory, you cannot force it to run) and decided to collect the memory from this object (which means there are NO references pointing to it anymore, from reachable objects at least), just before it deletes the memory occupied by it, it runs the method finalize() on the object. You can be sure that if garbage collected, the object will run finalize() just before it disappears, but you cannot be sure that it will get GC'ed at all so you shouldn't rely on the method to do any sanitizing at all. You should run sanitizing statements inside finally {} blocks and not use finalize() as it is not guaranteed to run.
Furthermore, some people have done performance tests and showed that the finalize method somewhat slows down creation/destruction of the object. I cannot remember the source so treat this info as not very reliable. :)

Finalization is used to clean up resources, which cannot be freed by the garbage collector. For example, consider a program which allocates (via some native API) resources directly from the OS. This usually yields some kind of "handle" (a UNIX file descriptor or Windows HANDLE, or something similar):
class Wrapper {
private long handle;
private Handle(long h) {
handle = h;
}
private static native long getHandleFromOS();
static Wrapper allocate() {
return new Handle(getHandleFromOS());
}
}
So, what happens, if your code allocates an instance of class Wrapper? Well the class allocates some kind of OS specific resource and keeps a reference to it (the handle) in a member variable. But what happens, when the last Java reference to a wrapper instance is lost? Now, the garbage collector will (at some point) reclaim the space of the now defunct wrapper instance. But what happens to the OS resource allocated by the wrapper? It will be leaked in the above scenario, which is a bad thing, if it is a costly resource, such as a file descriptor.
In order to allow your code to clean up in such a scenario, there is the finalize method.
class Wrapper {
private long handle;
private Handle(long h) {
handle = h;
}
protected void finalize() {
returnHandleToOS(handle);
}
private static native long getHandleFromOS();
private static native void returnHandleToOS(long handle);
static Wrapper allocate() {
return new Handle(getHandleFromOS());
}
}
Now, when the GC reclaims the space of a wrapper instance, the finalizer makes sure, that the resource is properly returned to the OS.
This sounds all nice, but as others have already pointed out, the downside is, that finalization is inherently unreliable: you do not know when the finalizer will be run. Worse: there are no guarantees that it will be run at all. So ist best to provide an dispose mechanism and use finalization only as safety-net in case, the clients of your class forget to properly dispose their references:
class Wrapper {
private long handle;
private Handle(long h) {
handle = h;
}
protected void finalize() {
if( handle != 0 ) returnHandleToOS(handle);
}
public void dispose() {
returnHandleToOS(handle);
handle = 0;
}
private static native long getHandleFromOS();
private static native void returnHandleToOS(long handle);
static Wrapper allocate() {
return new Handle(getHandleFromOS());
}
}

finalize() method not getting called despite WeakReference [duplicate]

This question already has answers here:
Why is finalize not being called?
(4 answers)
Closed 6 years ago.
I am currently having an issue with objects of a class of mine not having their finalize() method called, despite only being stored in a ConcurrentHashMap as values, and the Object's UUID as the keys for the map.
Here is an example I have created (it isn't the one used in my actual project but this example also does not have the objects' finalize() methods called.
TestMain.class //The main class handling the HashMap
public class TestMain {
public static ConcurrentHashMap<String, WeakReference<Test>> map = new ConcurrentHashMap<String, WeakReference<Test>>();
public static void main(String args[]) {
map.put("Key1", new WeakReference<Test>(new Test("test 1", 2))); //This "Test" Object shouldn't be accessible anywhere besides the HashMap anymore, and therefore I want it to be "finalized" by the garbage collector.
System.out.println("DONE");
}
}
Test.class //The Object Class
public class Test {
public String string;
public int intt;
public Test(String string, int intt) {
this.string = string;
this.intt = intt;
}
#Override
protected void finalize() throws Throwable {
System.out.println("FINALIZED TEST CLASS: " + string);
super.finalize();
}
}
When the main method is run, all that is printed is "DONE" when I expected "FINALIZED TEST CLASS: Test 1" to also be printed prior to that. Is there something wrong with my code, or am I misunderstanding how finalize() works?
Also, I wish for this to work as I explain because I need the objects with the finalize() methods to be erased from the "map" Map and from memory as there will be several hundred of the object generated during my project's run duration, so I want the objects no longer in use to be cleared to make room for more new ones.
EDIT: I have seen elsewhere on the internet that people say the program might terminate before the garbage collector has a chance to dispose of the said objects. Therefore, I have attempted to let my program not terminate by adding a simple while(true){} loop and waited for a couple of minutes, and still did not see the message of the object being "finalized".
Any help is much appreciated, thank you.

The most likely explanation is that the GC never ran.
If the GC doesn't run, then objects won't be garbage collected.
If objects are not collected, they won't be finalized.
have attempted to let my program not terminate by adding a simple while(true){} loop and waited for a couple of minutes,
Waiting for the GC is not going to help. Normally it only runs when it >>needs to<< run; i.e. when one of the heap spaces exceeds a threshold of fullness.
You can force the GC to run by calling System.gc() ... but that is BAD PRACTICE. It will make your application inefficient and non-responsive.
It is also unwise to try to try to do anything in finalizers that has to be done in a timely fashion. You should design your code so that:
it doesn't matter if an object never gets finalized at all, and
it doesn't matter when an object gets finalized.
Why? Because you have little control over when objects get finalized ... and none at all if you don't call System.gc() ... which you should not do; see above.
In practice, finalizers are a feature of Java that should be used rarely, if at all. The only valid use-cases involve releasing external resources associated with resource objects that the application has not tidied up. Even then, it is better for the application to use "try with resources" (or a finally block) to clean up the resources explicitly.

As rightly mentioned in comments, there is no guarantee of finalize method being called.
The Java programming language does not guarantee which thread will
invoke the finalize method for any given object. It is guaranteed,
however, that the thread that invokes finalize will not be holding any
user-visible synchronization locks when finalize is invoked. If an
uncaught exception is thrown by the finalize method, the exception is
ignored and finalization of that object terminates.
Now coming back to your concern of manually disposing these objects after they are no longer accessible, you need not handle that, Java Garbage collector would handle that for you with it's automatic memory management. If you want to accelerate garbage collection, read on G1 Garbage collector.

What is difference between System.gc() and finalize() method in java?

I am confuse in between system.gc() and finalize() method of java.
We can't force to collect garbage object to JVM. We are allow to write both methods in our java code then if both are used for garbage collection, then what is point in providing two methods for garbage collection by java?
Please tell me the exact working of both methods and internally how it works?

System.gc() kindly asks the sytem to perform a garbage collection. Javadoc says:
Runs the garbage collector.
You can not control how "hard" the garbage collector will work. How the garbage collector work internally is VM-specific and a research topic on its own. But there are usually "full" garbage collection and some smaller "incremental" collection going on. So consider System.gc as a request, but there's not guaranteed that garbage collection of your object will happen.
Object.finalize() can be overriden to specify what to do when a given object is garbage collected (actually what to do just before it is garbage collected). Javadoc says:
Called by the garbage collector on an object when garbage collection
determines that there are no more references to the object.
Classical use of finalizer are to de-allocate system resources when an object is garbage collected, e.g. release file handles, temporary files, etc.
Do not use finalizer to perform actions when the JVM exits. For this purpose use a shutdown hook that you register with Runtime.getRuntime().addShutdownHook(Thread).

System.gc() forces the garbage collector to run, while the Finalize() method of your object defines what garbage collector should do when collecting this specific object.
Roughly speaking, it is like this:
class MyClass {
public UnmanagedResource resource;
void Finalize() {
FreeUnmanagedResource(resource);
}
}
MyClass[] data = new MyClass[1000];
for(int i=0; i<1000; i++) {
data[i] = new MyClass();
}
//do some work
data = null;
System.gc(); //Note that it doesn't guarantee you that all MyClass instances will be actually collected at this point.

system.gc() method notifies the JVM that the garbage collector can run now to clear the memory by deleting unused objects. As per the java doc:
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.
finalize() method will not trigger garbage collector instead it will be called while the garbage collector about the destroy the object. It provides the instructions to clear the object properly.

The answers here are great, just wanted to elaborate small point about the finalize() method: you should never use it.
It's execution is not guaranteed at all and eventually the usage of finalize() adds performance penalty.
As written in Effective Java by Joshua Bloch:
Finalizers are unpredictable, often dangerous, and generally
unnecessary. never do anything time-critical in a finalizer. never
depend on a finalizer to update critical persistent state.
And:
Oh, and one more thing: there is a severe performance penalty for
using finalizers. On my machine, the time to create and destroy a
simple object is about 5.6 ns. Adding a finalizer increases the time
to 2,400 ns. In other words, it is about 430 times slower to create
and destroy objects with finalizers.
Prefer using the following resource terminating try-finally pattern, which are guaranteed to run:
Bar bar = new Bar(...);
try {
// Your code here
} finally {
bar.releaseResource(); // You implementation. For example close connection
}

The System.gc() method garbage collects the objects that are created by a new keyword, whereas the finalize() method is used when we want to garbage collect the objects that are not created using a new keyword. So, I guess this ans. your Q

Garbage collection is used to reclaim the allocated memory of object and Finalize() is called by garbage collector before reclaiming the memory Finalize() method mostly used to do some operation before deleting the object

Garbage collector is invoked by calling the System.gc() method on any class. But it does not guarantees that all the nullified objects will be garbage collected before jvm shuts down. So basically gc() method is garbage collecting only those objects which are created using new keyword. Objects are created by any other logic than this will be garbage collected by explicitly calling finalize() method before gc calls the garbage collector.

There are many classes that contain the finalize() method, I'll assume you mean the one in the Object class. System.gc() is what you call when you want Java's garbage compiler to run. It will run by itself every few minutes, but you can ask it to go whenever you want. When the garbage collector runs, it calls the finalize() method on each object that has no more reference. Basically, System.gc() cleans up memory and uses finalize() to get rid of the individual objects.

Why is finalize not being called?

I have couple of questions regarding garbage collector in java.
Q1.As far as I understand, finalize() gets called when object is out of scope and JVM is about to collect garbage. I thought finalize() method is called automatically by garbage collector, but it does not seems to work in this case. What is the explanation? Why is the need for me to explicitly call finalize() method?
public class MultipleConstruct {
int x,y;
public MultipleConstruct(int x)
{
this.x= x;
y=5;
System.out.println("ONE");
}
#Override
protected void finalize() throws Throwable {
// TODO Auto-generated method stub
super.finalize();
System.out.println("FINALIZED");
}
public static void main(String[] args) throws Throwable {
MultipleConstruct construct = new MultipleConstruct(3);
}
}
Q2. Also, when is garbage collector invoked? I understand gc is a daemon thread and invoked by JVM depending on heap size remaining. Does that mean, JVM waits for the program to use threshold limit of resources and then notify the gc to sweep garbage objects.
EDIT: How does gc resolved circular references?

There is a lot to finalize() method which is frankly a lot to write, but in short:
An object is in the finalized state if it is still unreachable after its finalize method, if any, has been run. A finalized object is awaiting deallocation. Note that the VM implementation controls when the finalizer is run. You are almost always better off doing your own cleanup instead of relying on a finalizer. Using a finalizer can also leave behind critical resources that won't be recovered for an indeterminate amount of time.
In your case the reason it does not print is that you do not know when the finalizer thread will call the finalize() method. What is happening is that the program is terminating before anything can get printed. To check it:
edit the code inside main code by( NOTE: this does not guarrantee nor should you should ever rely on it but still it does prints some time)
for(int i =0;i<1000000;i++)
{
MultipleConstruct construct = new MultipleConstruct(3);
construct = null;
}
There are a lot of disadvantages of using a finalize() right from taking more time in object construction to possibility of memory leakage and memory starvation. If you strongly refer to the same object inside the finalize() then it is never called the second time and thus can leave system in undesired state etc etc etc...
The only place where you should use finalize() is as a safety net to dispose any resources like InputStream uses it to close (which again there is no guarrantee that it will will br run when your program is still alive). Another place to use it is while using natives where garbage collector has no control.
For more info visit:
http://jatinpuri.com/?p=106

q1) finalize method is called when the object is being garbage collected, thus, if no GC is being performed, your finalizer may not be called. You need to call super simply to preserve the behavior provided by Object implementation.
q2) the exact moment in which GC is performed depends on a lot of factors like: which JVM you are using, tuning parameters, amount of free heap, etc. So it does not only rely on a used heap threshold. You can also ask for a GC to be performed through System.gc() but you have no guarantee about if and when it will be actually executed.
You can find some details on how to configure GC in http://java.sun.com/performance/reference/whitepapers/tuning.html

it gets called eventually or not at all
basically the GC scans the heap for everything that is not reachable and runs the finalizer on those (after which it needs to prove again it is not reachable for it to be freed)
however it can take a while (effectively undefined and actually dependent on program behavior) for the GC to find it which is why you shouldn't really rely on it to dispose of critical data
edit: as for circular references it distinguishes between objects with a finalize method and objects without one
for an object to be freed (deleted from main memory) it may not be reachable by any code (this includes finalizers that still need to run)
when 2 objects with finalizers are eligible to get the finalizers run the GC arbitrarily selects one object and runs the finalizer on it and then it can run the other object
note that a finalizer can run while the fields of the objects may or may not be finalized already

finalize() method is called automatically during garbage collection. System.gc() method forcibly calls garbage collector. but we will have to destroy object before.
example:
public class Sample
{
public Sample()
{
System.out.println("Object created");
}
#Override
public void finalize()
{
System.out.println("Object Destroyed");
}
public static void main(String args[])
{
Sample x=new Sample();
Sample y=new Sample();
x=null;
y=null;
System.gc();
}
}