This question already has answers here:
When is the finalize() method called in Java?
(18 answers)
Closed 8 years ago.
Why execution of finalize() isn't guaranteed at all in Java? Is finalize() method shouldn't be used???
Consider below program.
class Test{
protected void finalize()
{
System.out.println("Will i execute?");
}
public static void main(String args[])
{
Test t=new Test();
}
}
There is an empty output when this program runs. We know that finalize() is used to cleanup any external resources before the object becomes eligible for garbage collection & finalize() will be called by JVM. Inside finalize() we will specify those actions that must be performed before an object is destroyed. Is finalize() method evil??
Javadoc link, because many quotes will follow.
We know that finalize() is used to cleanup any external resources before the object becomes eligible for garbage collection
No. Once the object becomes eligible for garbage collection, then finalize() can get invoked. finalize could theoretically make the object no longer eligible for garbage collection and the garbage collector would then skip it. As stated
After the finalize method has been invoked for an object, no further
action is taken until the Java virtual machine has again determined
that there is no longer any means by which this object can be accessed
by any thread that has not yet died
Why execution of finalize() isn't guaranteed at all in Java?
It is guaranteed to run. As the Javadoc states
The general contract of finalize is that it is invoked if and when the
Java™ virtual machine has determined that there is no longer any means
by which this object can be accessed by any thread that has not yet
died, except as a result of an action taken by the finalization of
some other object or class which is ready to be finalized.
What isn't guaranteed is when or if garbage collection will occur.
Should i use finalize() or not in java?
That depends on your use case. Start by reading the javadoc and understand the implications.
finalize() is Called by the garbage collector on an object when garbage collection determines that there are no more references to the object. .
You could use System.gc() to explicitly call for garbage collection and check if your statement gets printed.
But the JVM has the liberty to ignore your request (depends on the JVM implementation actually..). Only when the JVM's internal state tells it that there is too much garbage and it needs to be collected, then it will run GC (and the line will I execute) will be printed.
If I understand your question, you can use the System.gc() to request garbage collection with something like this,
#Override
protected void finalize() {
System.out.println("I will execute.");
}
public static void main(String args[]) {
Test t = new Test();
t = null; // <-- make it eligible for gc.
System.gc(); // <-- request gc.
System.runFinalization(); // <-- run finalization(s).
System.out.println("Exit"); // <-- exit.
}
Depending on your JVM, you might find the order of output swaps if you comment out System.runFinalization(); -
public static void main(String args[]) {
Test t = new Test();
t = null; // <-- make it eligible for gc.
System.gc(); // <-- request gc.
// System.runFinalization(); // <-- run finalization(s).
System.out.println("Exit"); // <-- exit.
}
Before removing an object from memory Garbage collection thread invokes finalize () method of that object and gives an opportunity to perform any sort of cleanup operations.
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.
public Test(){
System.out.println("Object created");
}
protected void finalize(){
System.out.println("Will i execute?");
}
public static void main(String args[]){
Test test = new Test();
test = null;
new Test();
System.gc();
}
finalize() of any particular class is called by jvm just before collecting the object of that class as a garbage.
Now next question is when an object of a class is collected as garbage?
Answer is:
When there is no more objects reference to that object or simply no more object reference is pointing to that allocated memory space or an object has only live references via WeakHashMap it will be eligible for garbage collection.
Read more here.
As an option we can use System.gc() or Runtime.gc() that will give a request to jvm to consider the garbage collection. But whether your request will be listened or ignored by jvm that depends upon following factors:
Internal implementation of JVM & its Garbage Collection algorithm
Java Heap size
Free memory available in Heap
Related
I found it in many places that the finalize() method in java is called when the garbage collector or System.gc() has successfully retained the memory consumed by the redundant object with no more references to it. Also found that this method is called not more than a single time. I am not new to java but also not pretty much experienced. I may have a wrong understanding of it but let's say a piece of code
public class Solution {
#Override
protected void finalize(){
System.out.print("method called");
}
public static void main(String... args){
Solution obj1= new Solution();
Solution obj2 = new Solution();
Solution obj3 = new Solution();
System.gc();
obj1=obj2;
System.gc();
obj3=null;
System.gc();
}
}
Here, the finalize method is called twice because the memory heap becomes eligible for garbage cleaning two times. So, I am a bit confused whether I know the whole thing right or if it is supposed to behave the way it's behaving.
No. The finalize() method will only be called once by the GC on an object. The JVM sets a flag in the object header (I think) to say that it has been finalized, and won't finalize it again.
The javadoc states this explicitly:
" The finalize method is never invoked more than once by a Java virtual machine for any given object. "
Of course, there is nothing to stop an object method from calling this.finalize() any number of times.
Note that finalize() is deprecated in Java 9 and later for reasons stated in the javadoc. It is recommended that you switch to using one of the following instead:
AutoCloseable + try with resources
Cleaner
PhantomReference
Someone commented thus:
finalize() is called for every Object that is collected.
This is not true for a couple of reasons.
The javadoc explicitly states that there are no guarantees that finalize will ever be called. The thing that is guaranteed is that it will be called (once) before an object's storage is reclaimed. That is a weaker statement than the statement that the comment makes.
One scenario where garbage collected objects may not be finalized is if the JVM exits soon after a GC run.
Another (pathological) scenario occurs when a classes finalize method never returns1. When an instance of that class is finalized, the finalizer thread will get stuck. When all finalizer threads are stuck in that way, no more finalizable objects can be finalized2.
If the Object::finalize is not overridden in a class, the JVM will skip the finalization step for that class.
1 - This could be due to an infinite loop, or because the finalize() method gets stuck waiting on a lock or waiting for an internal or external "event" that never happens. Note also that "never" could mean "not for a long time" in this context. The overall impact can be the same.
2 - The objects will sit in the finalization queue indefinitely. This is a memory leak.
This question already has answers here:
Can java finalize an object when it is still in scope?
(2 answers)
Closed 3 years ago.
As far as I know, a method's local variable is located in a stack frame in an executing thread and a reference type of a local variable only has a objects' reference, not the object itself. All of objects in JVM are located in a heap space.
I want to know that objects referenced by local variables in a method being executed are never garbage collected until the end of the method execution. (without using java.lang.ref.WeakReference and SoftReference.)
Are they garbage collected? or never? Is there compiler's optimization to this type of stuff?
(If they are never garbage collected, this means it may be needed to assign null to variables no longer used when executing big methods which take long time.)
As elaborated in Can java finalize an object when it is still in scope?, local variables do not prevent the garbage collection of referenced objects. Or, as this answer puts it, scope is a only a language concept, irrelevant to the garbage collector.
I’ll cite the relevant part of the specification, JLS §12.6.1 again:
A reachable object is any object that can be accessed in any potential continuing computation from any live thread.
Further, I extended the answer’s example to
class A {
static volatile boolean finalized;
Object b = new Object() {
#Override protected void finalize() {
System.out.println(this + " was finalized!");
finalized = true;
}
#Override public String toString() {
return "B#"+Integer.toHexString(hashCode());
}
};
#Override protected void finalize() {
System.out.println(this + " was finalized!");
}
#Override public String toString() {
return super.toString() + " with "+b;
}
public static void main(String[] args) {
A a = new A();
System.out.println("Created " + a);
for(int i = 0; !finalized; i++) {
if (i % 1_000_000 == 0)
System.gc();
}
System.out.println("finalized");
}
}
Created A#59a6e353 with B#6aaa5eb0
B#6aaa5eb0 was finalized!
finalized
A#59a6e353 with B#6aaa5eb0 was finalized!
which demonstrates that even the method with the variable in scope may detect the finalization of the referenced object. Further, being referenced from a heap variable doesn’t necessarily prevent the garbage collection either, as the B object is unreachable, as no continuing computation can access it when the object containing the reference is unreachable too.
It’s worth emphasizing that even using the object does not always prevent its garbage collection. What matters, is whether the object’s memory is needed for the ongoing operation(s) and not every access to an object’s field in source code has to lead to an actual memory access at runtime. The specification states:
Optimizing transformations of a program can be designed that reduce the number of objects that are reachable to be less than those which would naively be considered reachable. […]
Another example of this occurs if the values in an object's fields are stored in registers. The program may then access the registers instead of the object, and never access the object again. This would imply that the object is garbage.
This is not only a theoretical option. As discussed in finalize() called on strongly reachable object in Java 8, it may even happen to objects while a method is invoked on them, or in other words, the this reference may get garbage collected while an instance method is still executing.
The only ways to prevent an objects garbage collection for sure, are synchronization on the object if the finalizer also does synchronization on the object or calling Reference.reachabilityFence(object), a method added in Java 9. The late addition of the fence method demonstrates the impact of the optimizers getting better from version to version on the issue of earlier-than-wanted garbage collection. Of course, the preferred solution is to write code that does not depend on the time of garbage collection at all.
It is not quite true that all of the objects are in heap space; but it is generally true. Java has been extended to have stack-local objects, provided the JVM can detect that the object will live only as long as the stack frame.
Now for the objects on the heap, which have a local reference in a method. While the method is being processed, the stack frame associated with the method run contains the local variable references. As long as the reference can be used (which includes being still in the stack frame) the object will not be garbage collected.
Once the reference has been destroyed, and the object can no longer be reached by the running program (because there's no references that can reach it), then the garbage collector will collect it.
The following snippet from RuntimeUtil.java from jlibs guarantees GC that garbage collection is done.
Since, it also uses System.gc(), i dont understand how can they guarantee that it will happen 100% of the times.
Following is the snippet:
/**
* This method guarantees that garbage collection is
* done unlike <code>{#link System#gc()}</code>
*/
public static void gc(){
Object obj = new Object();
WeakReference ref = new WeakReference<Object>(obj);
obj = null;
while(ref.get()!=null)
System.gc();
}
Its related with Strong Reference & Weak Reference with respect to Garbae Collection.
A strong reference is an ordinary Java reference, the kind you use every day.
If an object is reachable via a chain of strong references (strongly reachable), it is not eligible for garbage collection. As you don't want the garbage collector destroying objects you're working on, this is normally exactly what you want.
A weak reference, simply put, is a reference that isn't strong enough to force an object to remain in memory. Weak references allow you to leverage the garbage collector's ability to determine reachability
The gc() method in the pointed class works on this concept.
/**
* This method guarantees that garbage collection is
* done unlike <code>{#link System#gc()}</code>
*/
public static void gc(){
Object obj = new Object();
WeakReference ref = new WeakReference<Object>(obj);
obj = null;
while(ref.get()!=null)
System.gc();
}
Once a WeakReference starts returning null, the object it pointed to has become garbage and the WeakReference object is pretty much useless. This generally means that some sort of cleanup is required;
That's why they guarantee that it will happen 100% of the times.
From what I can see it should work as they
1) create an object
2) get a "weak" reference to it
3) null the reference to it, in order to mark it for garbage collection
4) and wait with the while loop until it actually disappears
A single call to System.gc() does not guarantee all objects that are eligible for garbage collection are re-claimed. See How to cause soft references to be cleared in Java?
In this case, the garbage collection is run many times till a an weak object ref is returning null. I doubt efficacy of this approach. There will be many other objects that may be garbage collected. To me it (jlib) does not have any thing to learn from.
For example the code in the library written in 2009. (extracted)
/**
* This method guarantees that garbage colleciton is
* done after JVM shutdown is initialized
*/
public static void gcOnExit(){
Runtime.getRuntime().addShutdownHook(new Thread(){
#Override
public void run(){
gc();
}
});
}
Why to you need gc when shutdown is called? The library quoted in question is pure garbage.
How to prevent an object from getting garbage collected?
Are there any approaches by finalize or phantom reference or any other approaches?
I was asked this question in an interview. The interviewer suggested that finalize() can be used.
Hold a reference. If your object is getting collected prematurely, it is a symptom that you have a bug in the design of your application.
The garbage collector collects only objects to which there is no reference in your application. If there is no object that would naturally reference the collected object, ask yourself why it should be kept alive.
One usecase in which you typically have no references, but want to keep an object is a singleton. In this case, you could use a static variable. One possible implementation of a singleton would look like this:
public class Singleton {
private static Singleton uniqueInstance;
private Singleton() {
}
public static synchronized Singleton getInstance() {
if (uniqueInstance == null) {
uniqueInstance = new Singleton();
}
return uniqInstance;
}
}
Edit: Technically, you can store a reference somewhere in your finalizer. This will prevent the object from being collected until the collector determines again that there are no more references. The finalizer will only be called at most once, however, so you must ensure that your object (including its superclasses) need not be finalized after the first collection. I would advise you, however, not to use this technique in actual programs. (It will leave colleagues like me yelling WTF!? ;)
protected void finalize() throws Throwable {
MyObjectStore.getInstance().store(this);
super.finalize(); // questionable, but you should ensure calling it somewhere.
}
The trick answer your interviewer was looking for is probably that he wants you to know that you can prevent garbage collection from removing an object by forcing a memory leak.
Obviously, if you keep a reference to the object in some long-lived context, it won't be collected, but that's not what the OP's recruiter asked about. That's not something which happens in the finalize method.
What you can do to prevent garbage collection from within the finalize method is to write an infinite loop, in which you call Thread.yield();(presumably to keep an empty loop from being optimized away):
#Override
protected void finalize() throws Throwable {
while (true) {
Thread.yield();
}
}
My reference here is an article by Elliot Back, in which describes forcing a memory leak by this method.
Just another way in which finalize methods are evil.
The best way is to use Unsafe, although ByteBuffer might be a possible workaround for some cases.
Also search for the keyword "off-heap" memory.
Unsafe
Advantages over ByteBuffer:
allows objects to be represented directly, without for serialization and thus faster
no bounds checking, so faster
explicit deallocation control
can allocate more than the JVM limit
It is not however easy to get working. The method is described in the following articles:
http://mishadoff.com/blog/java-magic-part-4-sun-dot-misc-dot-unsafe/
https://highlyscalable.wordpress.com/2012/02/02/direct-memory-access-in-java/
http://java.dzone.com/articles/understanding-sunmiscunsafe
They all consist of the following steps:
we need a sizeof operator, which Unsafe does not have. How to make one was asked at: In Java, what is the best way to determine the size of an object?. The best options is likely the instrument API, but that requires you to create a Jar and use special command line options...
once we have sizeof, allocate enough memory with Unsafe#allocateMemory, which is basically a malloc and returns an address
create a regular on heap object, copy it to the allocated memory with Unsafe#copyMemory. To do this, you need to the address of the on-heap object, and the size of the object
set an Object to point to the allocated memory, then cast the Object to your class.
It does not seem possible to set the address of a variable directly with Unsafe, so we need to wrap the object into an array or wrapper object, and use Unsafe#arrayBaseOffset or Unsafe#objectFieldOffset.
once you are done, free the allocated memory with freeMemory
If I ever get this to not segfault I will post an example :-)
ByteBuffer
Advantages over Unsafe:
stable across Java versions while Unsafe may break
does bound checking, so safer than... Unsafe, which allows for memory leaks and SIGSEGV
JLS says:
The contents of direct buffers may reside outside of the normal garbage-collected heap.
Example of usage with primitives:
ByteBuffer bb = ByteBuffer.allocateDirect(8);
bb.putInt(0, 1);
bb.putInt(4, 2);
assert bb.getInt(0) == 1;
assert bb.getInt(4) == 2;
// Bound chekcs are done.
boolean fail = false;
try {
bb.getInt(8);
} catch(IndexOutOfBoundsException e) {
fail = true;
}
assert fail;
Related threads:
Difference between "on-heap" and "off-heap"
If there is still a reference to the object, it won't get garbage collected. If there aren't any references to it, you shouldn't care.
In other words - the garbage collector only collects garbage. Let it do its job.
I suspect what you might be referring to is if your finalize method stashes away a reference to the object being finalized. In this case (if my reading of the Java Language Spec is correct) the finalize method will never be re-run, but the object will not yet be garbage collected.
This is not the sort of thing one does in real life, except possibly by accident!
This sounds like one of those interview-only-time-you'll-see-it questions. finalize() is run when your object is getting garbage collected, so it'd be pretty perverse to put something in there to prevent collection. Normally you just hold a reference and that's all you need.
I'm not even sure what would happen if you'd create a new reference for something in the finalizer - since the garbage collector's already decided to collect it would you then end up with a null ref? Seems like a poor idea, in any case. e.g.
public class Foo {
static Foo reference;
...
finalize (){
reference = this;
}
}
I doubt this would work, or it might work but be dependant on the GC implenetation, or be "unspecified behavior". Looks evil, though.
The key point is if we set the real reference variable pointing to the object null,although we have instance variables of that class pointing to that object not set to null.
The object is automatically eligible for garbage collection.if save the object to GC, use this code...
public class GcTest {
public int id;
public String name;
private static GcTest gcTest=null;
#Override
protected void finalize() throws Throwable {
super.finalize();
System.out.println("In finalize method.");
System.out.println("In finalize :ID :"+this.id);
System.out.println("In finalize :ID :"+this.name);
gcTest=this;
}
public static void main(String[] args) {
GcTest myGcTest=new GcTest();
myGcTest.id=1001;
myGcTest.name="Praveen";
myGcTest=null;
// requesting Garbage Collector to execute.
// internally GC uses Mark and Sweep algorithm to clear heap memory.
// gc() is a native method in RunTime class.
System.gc(); // or Runtime.getRuntime().gc();
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("\n------- After called GC () ---------\n");
System.out.println("Id :"+gcTest.id);
System.out.println("Name :"+gcTest.name);
}
}
Output :
In finalize method.
In finalize :ID :1001
In finalize :ID :Praveen
------- After called GC () --------
Id :1001
Name :Praveen
I wonder if what they're going for is the pattern with resource pools (e.g. for network/db connections, or threads) where you use finalize to return a resource to the pool so that the actual object holding the resource isn't GC'ed.
Stupid example, in Java-like pseudocode and missing any kind of synchronization:
class SlowResourceInternal {
private final SlowResourcePool parent;
<some instance data>
returnToPool() {
parent.add(this);
}
}
class SlowResourceHolder {
private final SlowResourceInternal impl;
<delegate actual stuff to the internal object>
finalize() {
if (impl != null) impl.returnToPool();
}
}
I believe there is a pattern out there for this. Not sure if it the factory pattern. But you have one object that creates all your objects and holds a reference to them. When you are finished with them, you de-reference them in the factory, making the call explicit.
We have three ways to achieve same -
1) Increasing the Heap -Eden space size .
2) Create Singleton class with Static reference .
3) Override finalize() method and never let that object dereference.
There are 3 ways to prevent an Object from Garbage Collection as following:-
Increase the Heap Size of JVM
// Xms specifies initial memory to be allocated
// and Xmx specifies maximum memory can be allocated
java -Xms1024m -Xmx4096m ClassFile
Use a SingleTon Class Object as #Tobias mentioned
public class MySingletonClass {
private static MySingletonClass uniqueInstance;
// marking constructor as private
private MySingletonClass() {
}
public static synchronized MySingletonClass getInstance() {
if (uniqueInstance == null) {
uniqueInstance = new Singleton();
}
return uniqInstance;
}
}
We can override finalize method. That is last method executed on an object. Hence, it will remain in memory.
// using finalize method
class MyClassNotGc{
static MyClassNotGc staticSelfObj;
pubic void finalize() {
// Putting the reference id
//Object reference saved.
//The object won't be collected by the garbage collector
staticSelfObj = this;
}
}
If I call finalize() on an object from my program code, will the JVM still run the method again when the garbage collector processes this object?
This would be an approximate example:
MyObject m = new MyObject();
m.finalize();
m = null;
System.gc()
Would the explicit call to finalize() make the JVM's garbage collector not to run the finalize() method on object m?
According to this simple test program, the JVM will still make its call to finalize() even if you explicitly called it:
private static class Blah
{
public void finalize() { System.out.println("finalizing!"); }
}
private static void f() throws Throwable
{
Blah blah = new Blah();
blah.finalize();
}
public static void main(String[] args) throws Throwable
{
System.out.println("start");
f();
System.gc();
System.out.println("done");
}
The output is:
start
finalizing!
finalizing!
done
Every resource out there says to never call finalize() explicitly, and pretty much never even implement the method because there are no guarantees as to if and when it will be called. You're better off just closing all of your resources manually.
One must understand the Garbage Collector(GC) Workflow to understand the function of finalize. calling .finalize() will not invoke the garbage collector, nor calling system.gc. Actually, What finalize will help the coder is to declare the reference of the object as "unreferenced".
GC forces a suspension on the running operation of JVM, which creates a dent on the performance. During operation, GC will traverse all referenced objects, starting from the root object(your main method call). This suspension time can be decreased by declaring the objects as unreferenced manually, because it will cut down the operation costs to declare the object reference obsolete by the automated run. By declaring finalize(), coder sets the reference to the object obsolete, thus on the next run of GC operation, GC run will sweep the objects without using operation time.
Quote: "After the finalize method has been invoked for an object, no further action is taken until the Java virtual machine has again determined that there is no longer any means by which this object can be accessed by any thread that has not yet died, including possible actions by other objects or classes which are ready to be finalized, at which point the object may be discarded. " from Java API Doc on java.Object.finalize();
For detailed explanation, you can also check: javabook.computerware
The finalize method is never invoked more than once by a JVM for any given object. You shouldn't be relying on finalize anyway because there's no guarantee that it will be invoked. If you're calling finalize because you need to execute clean up code then better to put it into a separate method and make it explicit, e.g:
public void cleanUp() {
.
.
.
}
myInstance.cleanUp();