What is the best way to clean up an Object in Java?

Question

We don't have any destructor in Java as we have in C++.

Q1. How should we clean up any Object in java.

Q2. Is there any alternative to a finally block.

Q3. Sometimes we have to call explicitely initialization/termination of a third party code from our class eg

public classs MyClass{
    public MyClass(){
        ThirdPartyInitialize();
    }

    protected void finalize(){
        ThirdPartyTerminate();
    }
}

Is this the correct way?

Answer 1

You generally cannot "clean up" Java objects yourself. The garbage collector decides when to clean objects up. You can indicate when you are done with an object reference by setting it to null , but generally just letting it go out of scope is good enough. Either way, you still have no control over when it gets garbage collected.

The finally block is intended for performing actions whether an exception is thrown from a try block or not, and is the best place to perform clean up. Generally you only would clean up non-object resources like open streams.

finalize() is not guaranteed to be called because the garbage collector is not guaranteed to be called before your program exits. It is not really like a C++ destructor because C++ destructors are always called and you can rely on them being called. You cannot rely on finalize() being called.

So 1) use finally blocks to release non-object resources 2) let the garbage collector clean up object-resources 3) you can hint to the garbage collector that you are done with an object by setting it to null if it is used in a long-running method.

Answer 2

You could also add a shutdown hook to your program, if your program is shutting down too:

//add shutdown hook
Runtime.getRuntime().addShutdownHook(new Thread() {
    public void run() {
        ThirdPartyTerminate();
    }
});

Answer 3

Two syntax sugar options:

1) There is a @Cleanup annotation in Lombok that mostly resembles C++ destructors ( more ):

@Cleanup
ResourceClass resource = new ResourceClass();

2) There is also try-with-resources statement. For example:

try (BufferedReader br = new BufferedReader(new FileReader(path))) {
  System.out.println(br.readLine());
}

Answer 4

The best way to clean up after objects is to just drop the object.

Finally blocks can be abstracted using the Execute Around idiom.

Finalisers should be avoided. They probably wont get called immediately. The clean up should happen anyway. They are relatively slow. You might want to add one as a safety-net if you are writing a low-level resource wrapper (for, say, a file handle) if performance is not critical.

Answer 5

这是关于终结者的一个很好的见解。

Answer 6

You can clean up objects by removing references to it, when you no longer need it. You dont have to do it explicitly. An explicit cleanup would require setting the reference to null, thereby providing a hint to the garbage collector that the object can be collected. This hint works because of internal reference counts for each object, that are maintained internally. For instance

C a = new C(); //create an object of class C and assign it to 'a'
a = new C(); //create another object of class C and assign it to 'a'. The older object is no longer referred to. It is now eligible for GC.

There are better alternatives to finalize() depending on how much finalize() is helping your situation.

Often, the best practice is to provide a method like close() or disposeResources() in the API which will allow the caller to help the API clean up itself. For instance, the java.sql.Connection class does this . This is better than the finalize() method, since the JVM will call the finalize() method. Often the finalize() method will be run by a low priority thread in the JVM, leading to certain strange errors.

In the case of the Connection class, waiting for the JVM to perform finalization does prove costly in a lot of applications, since a database can accept only so many connections at a time. Therefore, most programmers will call close() explicitly on a Connection object.

In your case, it should translate into something similar (and in this context, the finally block is guaranteed to run always)

try
{
 ThirdPartyInitialize();
 // use third party component in this try block
}
finally
{
 ThirdPartyTerminate();
}

This is similar to the how the Connection class is also used in most situations.

Answer 7

您可以在Java 9中使用java.lang.ref.Cleaner ，

Answer 8

The finalize is used similar to a destructor, but, if you use the try...finally block for resources then you can open a resource and in the finally block you close the resource.

The finally block is always called, when the block is exited, either normally or through an exception being thrown.

Finalize is risky for resource management, as you don't know when it will be called, and if it closes an object that also has finalize then it can take a while.

The finally block is the better approach.

Answer 9

Your objects in Java really never need to be "Cleaned Up", GC Just works. Nearly every time I've seen someObject=null in code, it's been someone who didn't know what they were doing. There IS a theoretical case for this, but it's really an edge case and generally better handled in other ways like the recently added try with resource.

If you have an external resource that needs to be cleaned up when an object is no longer used, that is another matter.

There are "Reference" classes that will hold a special type of Reference to your class--it will not stop garbage collection, but can notify you when the class is garbage collected (Through a callback if you like). Look up WeakReference, PhantomReference, etc.

These are reliable and work in a much more deterministic way than a actual "finalize" method would because the callback is outside your class, so you don't end up executing a method in some pre-deleted or half-deleted state and the problems that could cause.