Related
Is it compulsory that finalize()method will be called every time we call System.gc() explicitly in Java?
1. How to Handle Java Finalization's Memory-Retention Issues
I want to understand above link
2. I understand that finalize() will not be called every time we call System.gc() explicitly, But Now the question is why it happens that sometimes it get called and sometime not? I think I have made my question more valuable now.
In this example finalize method() may be called or may not be called. Its totally depends on the JVM that when finalize method will be called. If you run this program sometimes you will get the output sometimes not.. (-,-) So try your luck
class Person{
public int a;
public void finalize(){
//System.out.println("finalize called"+this.hashCode());
System.out.println("finalize called"+this.a);
}
public static void main(String[] args){
Person f1=new Person();
f1.a=10;
Person f2=new Person();
f1=null;
f2=null;
System.gc();
}}
No.
System.gc
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.
finalize
The finalize method is never invoked more than once by a Java virtual
machine for any given object.
No. It isn't guaranteed that GC will ever occur, let alone in response to System.gc(), let alone that finalizers will ever be run in consequence.
now the question is why it happens that sometimes it get called and sometime not
Because GC and finalization may run as a result of System.gc(): nothing to stop it, so it happens, and if not, not, because there is no guarantee.
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();
}
}
}
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();
}
}
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.
I've been reading through a lot of the rookie Java questions on finalize() and find it kind of bewildering that no one has really made it plain that finalize() is an unreliable way to clean up resources. I saw someone comment that they use it to clean up Connections, which is really scary since the only way to come as close to a guarantee that a Connection is closed is to implement try (catch) finally.
I was not schooled in CS, but I have been programming in Java professionally for close to a decade now and I have never seen anyone implement finalize() in a production system ever. This still doesn't mean that it doesn't have its uses, or that people I've worked with have been doing it right.
So my question is, what use cases are there for implementing finalize() that cannot be handled more reliably via another process or syntax within the language?
Please provide specific scenarios or your experience, simply repeating a Java text book, or finalize's intended use is not enough, as is not the intent of this question.
You could use it as a backstop for an object holding an external resource (socket, file, etc). Implement a close() method and document that it needs to be called.
Implement finalize() to do the close() processing if you detect it hasn't been done. Maybe with something dumped to stderr to point out that you're cleaning up after a buggy caller.
It provides extra safety in an exceptional/buggy situation. Not every caller is going to do the correct try {} finally {} stuff every time. Unfortunate, but true in most environments.
I agree that it's rarely needed. And as commenters point out, it comes with GC overhead. Only use if you need that "belt and suspenders" safety in a long-running app.
I see that as of Java 9, Object.finalize() is deprecated! They point us to java.lang.ref.Cleaner and java.lang.ref.PhantomReference as alternatives.
finalize() is a hint to the JVM that it might be nice to execute your code at an unspecified time. This is good when you want code to mysteriously fail to run.
Doing anything significant in finalizers (basically anything except logging) is also good in three situations:
you want to gamble that other finalized objects will still be in a state that the rest of your program considers valid.
you want to add lots of checking code to all the methods of all your classes that have a finalizer, to make sure they behave correctly after finalization.
you want to accidentally resurrect finalized objects, and spend a lot of time trying to figure out why they don't work, and/or why they don't get finalized when they are eventually released.
If you think you need finalize(), sometimes what you really want is a phantom reference (which in the example given could hold a hard reference to a connection used by its referand, and close it after the phantom reference has been queued). This also has the property that it may mysteriously never run, but at least it can't call methods on or resurrect finalized objects. So it's just right for situations where you don't absolutely need to close that connection cleanly, but you'd quite like to, and the clients of your class can't or won't call close themselves (which is actually fair enough - what's the point of having a garbage collector at all if you design interfaces that require a specific action be taken prior to collection? That just puts us back in the days of malloc/free.)
Other times you need the resource you think you're managing to be more robust. For example, why do you need to close that connection? It must ultimately be based on some kind of I/O provided by the system (socket, file, whatever), so why can't you rely on the system to close it for you when the lowest level of resource is gced? If the server at the other end absolutely requires you to close the connection cleanly rather than just dropping the socket, then what's going to happen when someone trips over the power cable of the machine your code is running on, or the intervening network goes out?
Disclaimer: I've worked on a JVM implementation in the past. I hate finalizers.
A simple rule: never use finalizers.
The fact alone that an object has a finalizer (regardless what code it executes) is enough to cause considerable overhead for garbage collection.
From an article by Brian Goetz:
Objects with finalizers (those that
have a non-trivial finalize() method)
have significant overhead compared to
objects without finalizers, and should
be used sparingly. Finalizeable
objects are both slower to allocate
and slower to collect. At allocation
time, the JVM must register any
finalizeable objects with the garbage
collector, and (at least in the
HotSpot JVM implementation)
finalizeable objects must follow a
slower allocation path than most other
objects. Similarly, finalizeable
objects are slower to collect, too. It
takes at least two garbage collection
cycles (in the best case) before a
finalizeable object can be reclaimed,
and the garbage collector has to do
extra work to invoke the finalizer.
The result is more time spent
allocating and collecting objects and
more pressure on the garbage
collector, because the memory used by
unreachable finalizeable objects is
retained longer. Combine that with the
fact that finalizers are not
guaranteed to run in any predictable
timeframe, or even at all, and you can
see that there are relatively few
situations for which finalization is
the right tool to use.
The only time I've used finalize in production code was to implement a check that a given object's resources had been cleaned up, and if not, then log a very vocal message. It didn't actually try and do it itself, it just shouted a lot if it wasn't done properly. Turned out to be quite useful.
I've been doing Java professionally since 1998, and I've never implemented finalize(). Not once.
The accepted answer is good, I just wanted to add that there is now a way to have the functionality of finalize without actually using it at all.
Look at the "Reference" classes. Weak reference, Phantom Reference & Soft Reference.
You can use them to keep a reference to all your objects, but this reference ALONE will not stop GC. The neat thing about this is you can have it call a method when it will be deleted, and this method can be guaranteed to be called.
As for finalize:
I used finalize once to understand what objects were being freed. You can play some neat games with statics, reference counting and such--but it was only for analysis, but watch out for code like this (not just in finalize, but that's where you are most likely to see it):
public void finalize() {
ref1 = null;
ref2 = null;
othercrap = null;
}
It is a sign that somebody didn't know what they were doing. "Cleaning up" like this is virtually never needed. When the class is GC'd, this is done automatically.
If you find code like that in a finalize it's guaranteed that the person who wrote it was confused.
If it's elsewhere, it could be that the code is a valid patch to a bad model (a class stays around for a long time and for some reason things it referenced had to be manually freed before the object is GC'd). Generally it's because someone forgot to remove a listener or something and can't figure out why their object isn't being GC'd so they just delete things it refers to and shrug their shoulders and walk away.
It should never be used to clean things up "Quicker".
I'm not sure what you can make of this, but...
itsadok#laptop ~/jdk1.6.0_02/src/
$ find . -name "*.java" | xargs grep "void finalize()" | wc -l
41
So I guess the Sun found some cases where (they think) it should be used.
class MyObject {
Test main;
public MyObject(Test t) {
main = t;
}
protected void finalize() {
main.ref = this; // let instance become reachable again
System.out.println("This is finalize"); //test finalize run only once
}
}
class Test {
MyObject ref;
public static void main(String[] args) {
Test test = new Test();
test.ref = new MyObject(test);
test.ref = null; //MyObject become unreachable，finalize will be invoked
System.gc();
if (test.ref != null) System.out.println("MyObject still alive!");
}
}
====================================
result:
This is finalize
MyObject still alive!
=====================================
So you may make an unreachable instance reachable in finalize method.
finalize() can be useful to catch resource leaks. If the resource should be closed but is not write the fact that it wasn't closed to a log file and close it. That way you remove the resource leak and give yourself a way to know that it has happened so you can fix it.
I have been programming in Java since 1.0 alpha 3 (1995) and I have yet to override finalize for anything...
You shouldn't depend on finalize() to clean up your resources for you. finalize() won't run until the class is garbage collected, if then. It's much better to explicitly free resources when you're done using them.
To highlight a point in the above answers: finalizers will be executed on the lone GC thread. I have heard of a major Sun demo where the developers added a small sleep to some finalizers and intentionally brought an otherwise fancy 3D demo to its knees.
Best to avoid, with possible exception of test-env diagnostics.
Eckel's Thinking in Java has a good section on this.
Be careful about what you do in a finalize(). Especially if you are using it for things like calling close() to ensure that resources are cleaned up. We ran into several situations where we had JNI libraries linked in to the running java code, and in any circumstances where we used finalize() to invoke JNI methods, we would get very bad java heap corruption. The corruption was not caused by the underlying JNI code itself, all of the memory traces were fine in the native libraries. It was just the fact that we were calling JNI methods from the finalize() at all.
This was with a JDK 1.5 which is still in widespread use.
We wouldn't find out that something went wrong until much later, but in the end the culprit was always the finalize() method making use of JNI calls.
Hmmm, I once used it to clean up objects that weren't being returned to an existing pool.
They were passed around a lot, so it was impossible to tell when they could safely be returned to the pool. The problem was that it introduced a huge penalty during garbage collection that was far greater than any savings from pooling the objects. It was in production for about a month before I ripped out the whole pool, made everything dynamic and was done with it.
When writing code that will be used by other developers that requires some sort of "cleanup" method to be called to free up resources. Sometimes those other developers forget to call your cleanup (or close, or destroy, or whatever) method. To avoid possible resource leaks you can check in the finalize method to ensure that the method was called and if it wasn't you can call it yourself.
Many database drivers do this in their Statement and Connection implementations to provide a little safety against developers who forget to call close on them.
Edit: Okay, it really doesn't work. I implemented it and thought if it fails sometimes that's ok for me but it did not even call the finalize method a single time.
I am not a professional programmer but in my program I have a case that I think to be an example of a good case of using finalize(), that is a cache that writes its content to disk before it is destroyed. Because it is not necessary that it is executed every time on destruction, it does only speed up my program, I hope that it i didn't do it wrong.
#Override
public void finalize()
{
try {saveCache();} catch (Exception e) {e.printStackTrace();}
}
public void saveCache() throws FileNotFoundException, IOException
{
ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream("temp/cache.tmp"));
out.writeObject(cache);
}
It can be handy to remove things that have been added to a global/static place (out of need), and need to be removed when the object is removed. For instance:
private void addGlobalClickListener() {
weakAwtEventListener = new WeakAWTEventListener(this);
Toolkit.getDefaultToolkit().addAWTEventListener(weakAwtEventListener, AWTEvent.MOUSE_EVENT_MASK);
}
#Override
protected void finalize() throws Throwable {
super.finalize();
if(weakAwtEventListener != null) {
Toolkit.getDefaultToolkit().removeAWTEventListener(weakAwtEventListener);
}
}
The accepted answer lists that closing a resource during finalize can be done.
However this answer shows that at least in java8 with the JIT compiler, you run into unexpected issues where sometimes the finalizer is called even before you finish reading from a stream maintained by your object.
So even in that situation calling finalize would not be recommended.
iirc - you can use finalize method as a means of implementing a pooling mechanism for expensive resources - so they don't get GC's too.
As a side note:
An object that overrides finalize() is treated specially by the garbage collector. Usually, an object is immediately destroyed during the collection cycle after the object is no longer in scope. However, finalizable objects are instead moved to a queue, where separate finalization threads will drain the queue and run the finalize() method on each object. Once the finalize() method terminates, the object will at last be ready for garbage collection in the next cycle.
Source: finalize() deprecated on java-9
The resources (File, Socket, Stream etc.) need to be closed once we are done with them. They generally have close() method which we generally call in finally section of try-catch statements. Sometimes finalize() can also be used by few developers but IMO that is not a suitable way as there is no guarantee that finalize will be called always.
In Java 7 we have got try-with-resources statement which can be used like:
try (BufferedReader br = new BufferedReader(new FileReader(path))) {
// Processing and other logic here.
} catch (Exception e) {
// log exception
} finally {
// Just in case we need to do some stuff here.
}
In the above example try-with-resource will automatically close the resource BufferedReader by invoking close() method. If we want we can also implement Closeable in our own classes and use it in similar way. IMO it seems more neat and simple to understand.
Personally, I almost never used finalize() except in one rare circumstance: I made a custom generic-type collection, and I wrote a custom finalize() method that does the following:
public void finalize() throws Throwable {
super.finalize();
if (destructiveFinalize) {
T item;
for (int i = 0, l = length(); i < l; i++) {
item = get(i);
if (item == null) {
continue;
}
if (item instanceof Window) {
((Window) get(i)).dispose();
}
if (item instanceof CompleteObject) {
((CompleteObject) get(i)).finalize();
}
set(i, null);
}
}
}
(CompleteObject is an interface I made that lets you specify that you've implemented rarely-implemented Object methods like #finalize(), #hashCode(), and #clone())
So, using a sister #setDestructivelyFinalizes(boolean) method, the program using my collection can (help) guarantee that destroying a reference to this collection also destroys references to its contents and disposes any windows that might keep the JVM alive unintentionally. I considered also stopping any threads, but that opened a whole new can of worms.