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.
Related
I red from the following link:garbage collection from geeksforgeeks
and here it is said that:
The finalize() method is never invoked more than once for any given object.
So, here it is said "more than once" and i'm wondering whether there is possibility that finalize() method is not invoked and garbage collector destroys that object.
Is it possible?
When the GC has found an object with class where finalize() has been overridden it is added to a queue of objects to have finalize() called on them. It is only after the object has been finalized once, that the GC can clean it up. i.e. this would be on a later GC.
e.g. If an object is in tenured space, it might be found until a full collection is performed, and it will only be cleaned up on a full GC after the finalize method has been called.
For further details, this is the Java 11 Javadoc for Object.finalize()
https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Object.html#finalize()
so is there any possibility that garbage collector fully destroyed the object in heap but didn't call the finalize() method on that object?
While the object is in the finalization queue, it can't be removed.
And you all assumed that there is not certainity in the destroying of object by the garbage collector.
It won't be destroyed while there is still a strong reference to it.
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());
}
}
As far as I understand, GC starts with some set of initial objects (stack, static objects) and recursively traverses it building a graph of reachable objects. Then it marks the memory taken by these objects as occupied and assumes all the rest of the memory free.
But what if this 'free' memory contains an object with finalize method? GC has to call it, but I don't see how it can even know about objects that aren't reachable anymore.
I suppose GC can keep track of all 'finalizable' objects while they are alive. If so, does having finalizable objects make garbage collecting more expensive even when they are still alive?
Consider the Reference API.
It offers some references with special semantics to the GC, i.e Weak, Soft, and Phantom references. There’s simply another non-public type of special reference, for objects needing finalization.
Now, when the garbage collector traverses the object graph and encounters such a special reference object, it will not mark objects reachable through this reference as strongly reachable, but reachable with the special semantics. So if an object is only finalizer-reachable, the reference will be enqueued, so that one (or one of the) finalizer thread(s) can poll the queue and execute the finalize() method (it’s not the garbage collector itself calling this method).
In other words, the garbage collector never processes entirely unreachable objects here. To apply a special semantic to the reachability, the reference object must be reachable, so the referent can be reached through that reference. In case of finalizer-reachability, Finalizer.register is called when an object is created and it creates an instance of Finalizer in turn, a subclass of FinalReference, and right in its constructor, it calls an add() method which will insert the reference into a global linked list. So all these FinalReference instances are reachable through that list until an actual finalization happens.
Since this FinalReference will be created right on the instantiation of the object, if its class declares a non-trivial finalize() method, there is already some overhead due to having a finalization requirement, even if the object has not collected yet.
The other issue is that an object processed by a finalizer thread is reachable by that thread and might even escape, depending on what the finalize() method does. But the next time, this object becomes unreachable, the special reference object does not exist anymore, so it can be treated like any other unreachable object.
This would only be a performance issue, if memory is very low and the next garbage collection had to be performed earlier to eventually reclaim that object. But this doesn’t happen in the reference implementation (aka “HotSpot” or “OpenJDK”). In fact, there could be an OutOfMemoryError while objects are pending in the finalizer queue, whose processing could make more memory reclaimable. There is no guaranty that finalization runs fast enough for you’re purposes. That’s why you should not rely on it.
But what if this 'free' memory contains an object with finalize
method? GC has to call it, but I don't see how it can even know about
objects that aren't reachable anymore.
Let's say we use CMS garbage collector. After it successfully marked all live objects in a first phase, it will then scan memory again and remove all dead objects. GC thread does not call finalize method directly for these objects.
During creation, they are wrapped and added to finalizer queue by JVM (see java.lang.ref.Finalizer.register(Object)). This queue is processed in another thread (java.lang.ref.Finalizer.FinalizerThread), finalize method will be called when there are no references to the object. More details are covered in this blog post.
If so, does having finalizable objects make garbage collecting more
expensive even when they are still alive?
As you can now see, most of the time it does not.
The finalise method is called when an object is about to get garbage collected. That means, when GC determines that the object is no longer being referenced, it can call the finalise method on it. It doesn't have to keep track of objects to be finalised.
According to javadoc, finalize
Called by the garbage collector on an object when garbage collection determines that there are no more references to the object.
So the decision is based on reference counter or something like that.
Actually it is possible not to have this method called at all. So it may be not a good idea to use it as destructor.
I encountered this question in an interview with following options:
How to destroy an object in java?
a. System.gc();
b. Runtime.getRuntime.gc();
c. object.delete();
d. object.finalize();
e. Java performs gc by itself, no need to do it manually.
The answer should be e?
what if e was not there? then ?
clearly c is not the answer. a and b will do gc for the whole application(question requires for one object).
I think it is d because finalize() is called just prior to gc(but is it necessary that after finalize gc is invoked ?) or I am wrong ? e must be there to answer this question ?
Answer E is correct answer. If E is not there, you will soon run out of memory (or) No correct answer.
Object should be unreachable to be eligible for GC. JVM will do multiple scans and moving objects from one generation to another generation to determine the eligibility of GC and frees the memory when the objects are not reachable.
To clarify why the other answers can not work:
System.gc() (along with Runtime.getRuntime().gc(), which does the exact same thing) hints that you want stuff destroyed. Vaguely. The JVM is free to ignore requests to run a GC cycle, if it doesn't see the need for one. Plus, unless you've nulled out all reachable references to the object, GC won't touch it anyway. So A and B are both disqualified.
Runtime.getRuntime.gc() is bad grammar. getRuntime is a function, not a variable; you need parentheses after it to call it. So B is double-disqualified.
Object has no delete method. So C is disqualified.
While Object does have a finalize method, it doesn't destroy anything. Only the garbage collector can actually delete an object. (And in many cases, they technically don't even bother to do that; they just don't copy it when they do the others, so it gets left behind.) All finalize does is give an object a chance to clean up before the JVM discards it. What's more, you should never ever be calling finalize directly. (As finalize is protected, the JVM won't let you call it on an arbitrary object anyway.) So D is disqualified.
Besides all that, object.doAnythingAtAllEvenCommitSuicide() requires that running code have a reference to object. That alone makes it "alive" and thus ineligible for garbage collection. So C and D are double-disqualified.
Short Answer - E
Answer isE given that the rest are plainly wrong, but ..
Long Answer - It isn't that simple; it depends ...
Simple fact is, the garbage collector may never decide to garbage collection every single object that is a viable candidate for collection, not unless memory pressure is extremely high. And then there is the fact that Java is just as susceptible to memory leaks as any other language, they are just harder to cause, and thus harder to find when you do cause them!
The following article has many good details on how memory management works and doesn't work and what gets take up by what. How generational Garbage Collectors work and Thanks for the Memory ( Understanding How the JVM uses Native Memory on Windows and Linux )
If you read the links, I think you will get the idea that memory management in Java isn't as simple as a multiple choice question.
Set to null. Then there are no references anymore and the object will become eligible for Garbage Collection. GC will automatically remove the object from the heap.
Here is the code:
public static void main(String argso[]) {
int big_array[] = new int[100000];
// Do some computations with big_array and get a result.
int result = compute(big_array);
// We no longer need big_array. It will get garbage collected when there
// are no more references to it. Since big_array is a local variable,
// it refers to the array until this method returns. But this method
// doesn't return. So we've got to explicitly get rid of the reference
// ourselves, so the garbage collector knows it can reclaim the array.
big_array = null;
// Loop forever, handling the user's input
for(;;) handle_input(result);
}
In java there is no explicit way doing garbage collection. The JVM itself runs some threads in the background checking for the objects that are not having any references which means all the ways through which we access the object are lost. On the other hand an object is also eligible for garbage collection if it runs out of scope that is the program in which we created the object is terminated or ended.
Coming to your question the method finalize is same as the destructor in C++. The finalize method is actually called just before the moment of clearing the object memory by the JVM. It is up to you to define the finalize method or not in your program. However if the garbage collection of the object is done after the program is terminated then the JVM will not invoke the finalize method which you defined in your program.
You might ask what is the use of finalize method?
For instance let us consider that you created an object which requires some
stream to external file and you explicitly defined a finalize method to this object which checks wether the stream opened to the file or not and if not it closes the stream. Suppose, after writing several lines of code you lost the reference to the object. Then it is eligible for garbage collection. When the JVM is about to free the space of your object the JVM just checks have you defined the finalize method or not and invokes the method so there is no risk of the opened stream. finalize method make the program risk free and more robust.
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.