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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();
}
}
}

Call destructor on close objects

i want to call destructor, when i close object, but i dont know how or when,.. I try whit finalize but its not calling. I dont know why
There is example code, i try with debugger but its help me..
class tralala(){
int i=0;
tralala(){
i=i+1;}
protected void finalize( ){
i=i-1;
}}
But it isnt call i dont know why. If anyone have got a suggestion plis help me. I am in the stuck at the moment, but i am not shure if java has got destructors.

Method finalize( ) "сalled by the garbage collector on an object when garbage collection determines that there are no more references to the object". So it will be called after garbage collector observe it to delete, but not immediately when object leave the scope.
I rly can't understand what do you mean as "close object"? You should find some other architecture solution to do what you want: maybe just call your method or raise some custom event with supplied handler when object leave scope ("close object").

I think you are trying to implement similar behavior like destructors. As destructors are not available into Java, I think you are trying to do with finalize.
But 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 your programs).
Read more about When is the finalize() method called in Java? and Why not to use finalize() method in java
And read How to implement object counter in Java SO QA for your solution. This is used for object counter. You can use same logic.

There is no destructors in java
class Tralala(){
int i=0;
Tralala(){
i=i+1;}
protected void reset( ){
i=i-1;
}}
call reset method when ever you want to destruct.
As a side note:There are lots of corrections in your code like naming conventions, formatting and also providing modifiers.
And I'm not getting, when finalize called, your instance is no more. Why you are resetting at that time,Since after that NO MORE OPERATIONS ON THAT OBJECT.

JAVA does not have destructors. JAVA has Garbage collectors. Garbage Collector runs as a thread and automatically destroys instances which do not have any active references.

it depends on your definition of closing the class.
you could implement the interface Closable so that eclipse could warn you about not closing the instance when needed.
it's different from finalize since you need to call it by yourself.
you could call close() in the finalize function in order to make sure it will get called when the instance is GC'ed .

How finalize method works with phantom reference in java

Hi I have one doubt about phantom reference. What I understand the finalize method is called just before when object are going for garbage collection. But some time if object are not eligible for garbage collection then finalize method will not execute.
Now talking about phantom reference when this finalize method will called.
Is finalize always called in phantom reference.
I am very much confuse about this. Please help me.

Finalizers are never guaranteed to be called, whether there is a phantom reference or not. Don't rely on finalizers for any critical part of your code because there is no guarantee that they will be called in a timely manner or in fact at all.
Many people advocate that you simply should never use finalizers at all because they are incredibly difficult to use correctly.

When object becomes available only through phantom reference then after the first GC finalize() method is invoked and after the second GC the reference is enqueued. If after that phantom reference is cleaned (or becomes unavailable itself) then the memory is cleared after the third GC.

Finalize will always be called, but not neccessarely, when you expect it. It may happen, that the call will only be made at the JVM shutdown (assuming you don't simply kill the program). You should not rely on finalize() in order to do significant work. But it is also good practice to implement a usefull finalize() and include a call to super.finalize() too.

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.

What happened internally (JVM) when System.gc() or finalize() method called?

What happened internally (JVM) when System.gc() or finalize() method called?
Is this really collect garbage or reduce performance ?

Exactly what happens when you call System.gc() is JVM dependent. The JVM looks at this call as a suggestion that it might be a good time to run the garbage collector, so don't depend on it.
An object's finalize() method is run on an object by the garbage collector when the JVM determines that there are no more reachable references to that object. You shouldn't call it in your code. Keep in mind that finalize() is not called at the time when your program loses that final reference, but at a future time when the garbage collector is run, so don't depend on it happening at a specific point in time (or at all).

If you want to know garbage collection internals, you should read Hotspot Memory Management Whitepaper from Sun.

By invoking System.gc() the garbage collector is run, as the name says. At this point, when the objects are really being removed, the finalize() is called on these objects before they vanish.
The last question shouldn't have an "or" in it. It is garbage collection and reduced performance.
Usually you shouldn't care about gc at all because it does a really good job for you. There are usage scenarios where you want to get rid of a lot of objects at a certain point in time; Then it is feasible.

System.gc() cleans up memory, and uses finalize() to get rid of the individual objects.

finalize() is a means to execute final bit of code just before the object is ready for garbage collection ( when the object has no strong reference to it).
So when should it be used?
Only in the present two cases:
As a safety to make sure that some service is closed down or some
desired final change is made. For example InputStream class uses it
to close down the i/o stream. That is for example you have made an
instance of BufferedInputStream. Manually after using it one is
suppose to close() it. But because one can forget to do that,
finalize() act’s as a safety net to close() the stream.
While
using Native’s. Because garbage collector has no control over native
objects, finalize() can be used as a means to reclaim it.
Apart from the above two cases never use it. To understand why? we need to understand the functioning and lifecycle of an object.
Introduction: There is a separate daemon thread called as finalizer thread which is responsible for calling finalize() method . Finalization queue is the queu where objects which are ready to be called finalize() method are placed.
When an Object is created, JVM checks if the object has a finalize() method. If it has then it internally notes that this particular object has finalize() method.
When an object is ready for garbage collection, then the garbage collector thread checks if this particular object has finalize() from table mentioned in (1).
2a) If it doesn’t then it is sent for garbage collection.
2b) It is has, then it is added to the finalization queue. And it removes the entry of the object from the table (1).
Finalizer thread keeps polling the queue. For every object in the queue, its finalize() method is called. After calling the finalize() cycle from (2) is again repeated. If this object still has no strong reference, then sent for GC. If
it has then ALWAYS (2a) is called because the entry was deleted in (2b)
Basically finalize() method is only called once.
So what’s the issue with the above cycle?
From (1). Its take extra time in object creation. Memory allocation in Java is 5x to 10x faster than malloc/calloc etc. All the time gained is lost in the procedure of noting the object in the table etc. I once tried it. Create 100000 objects in a loop and measure the time taken for program to terminate in 2 cases: One with no finalize(), Second with finalize(). Found it to be 20% faster.
From (2b): Memory Leakage and Starvation. If the object in the queue has references to a lot of memory resources, then all those objects wont get freed unless this object is ready for GC.If all the objects are heavy weight objects, then there can be a shortage.
From (2b): Because finalize() is called only once, what if in teh finalize() you had a strong reference to “this” object. Next time the object’s finalie() is never called hence can leave the object in an inconsistent state.
If inside finalize() an exception is thrown, it is ignored.
You do not know when finalize() is called as you have no control over when GC is called. Sometimes it might happen that you are printing the value’s in finalize() but the output is never shown, because your program might have got terminated by the time finalize() is called.
Hence avoid using it. Instead create a method say dispose() which will close the necessory resources or for final log etc.
the complete post on it. I hope this clears.

Yes System.gc(); will trigger finalize() method if required.
public class TestGarbageCollection {
public static void main(String[] args) {
while (true) {
TestClass s = new TestClass();
s.display();
System.gc();
}
}
}
public class TestClass {
public TestClass() {
System.out.println("constructor");
}
public void display() {
System.out.println("display");
}
#Override
public void finalize() {
System.out.println("destructor");
}
}
This will trigger finalize() method. whether you override finalize or not finalize method of local class or Object's finalize will be called.