deleting objects? how to free a class (JVM)

[Sungron]

Okay, I have allocated new class from C code...

jobject obj = env->AllocObject(objClass)

But I got no idea what should I do when "obj" is no longer needed, how to tell jvm to free it? Are there any "delete" operators in Java or "delete" functions in jdk?

[Fubarable]

Welcome to Java. This is one of the places where Java is vastly different from C/C++. You don't delete or free objects in Java. Instead you should use them in the most limited scope possible. When they are no longer needed, the JVM frees them from memory for you.

[Turtle]

Furbarable is right.

When there is no reference to an object it will automatically be "garbage collected" (and this is achieved with limited scope)

Assigning null to all references for an object will also achieve this.
ie.

Object a = new Object();
Object b = a;

a = null; // won't be garbage collected yet because of b reference
b = null; // will be "eligable" for garbage collection

But exactly when the garbage collection process happens depends on the JVM.

It is possible to add something akin to a destructor by providing a finalize method. But it is better practice to explicitly do your own clean up work, as you don't know when the JVM will free up un-referenced objects.

Happy coding :-)

[Sungron]

Huh, I heard about it....
but it seems strange, how can Java know when object created from C code is no longer needed?

First I wanted to create new Java object everytime when I alloc new class in my application, but now it seem silly.
Ill allocate 100~ objects on application init and reuse them many times... instead of "deleting" them I may call method "ClearUp()".... hmm, is this a good solution? (if you know what I mean...)


EDIT: - I just spotted second answer.

When there is no reference to an object it will automatically be "garbage collected" (and this is achieved with limited scope)

thats what I am worrying about...
There is (and there will be) NO references to objects created from C code using "AllocObject".
I wanted to do something like this...
C++ code

foo::foo()
{ //creates new class in C++ and new object in JVM
 jo = javaEnv->AllocObject(javaClass);
}
void foo::do_somthing()
{ //call Java method on our object
javaEnv->CallFloatMethodV(jo,.........);
}
foo::~foo()
{//we're deleting the foo, so "jo" is no longer needed
javaEnv->FreeObject(jo); //this function does not exists
}

edit2: okay, let my silly post grow a bit

what about:

public class GarbageHeap
{	
	GarbageHeap me;
      public void newME()
      {
		me = this;
	}
      public void deleteME()
         {
		me = null;
	}
          public void thinkME()
       {
           System.out.print("I am still alive and well");
	}	
}

this class should persist untill removeME get called?

[Turtle]

Hi Sungron,

Eek, I didn't realize you were talking about JNI. I don't know much about this.

After a quick google:
Fields and Methods
JNI Functions

Perhaps, DeleteLocalRef is what you're looking for?

Best of luck

[Fubarable]

Ah, if you are talking about JNI, then you yourself are responsible for delocating memory and objects from within your C code. I don't believe that this can be done by Java, except for making a native method that you've created call via JNI that does this cleanup (again within C code).

[Sungron]

Perhaps, DeleteLocalRef is what you're looking for?

DeleteLocalRef... DeleteLocalRef? Oh, thats it! I just wonder, why I didnt spot this before! Am I blind? Or just work too much....
Sorry for this true spam... and thanks for help :)

[Turtle]

Good :-)

You've got me wondering how a class's members are handled now.

ie, would a variation on the singleton pattern to remove the only instance work?

class S {
  private static S instance = null;
  public static S getInstance() {
    if (instance == null) {
      instance = new S();
    }
    return instance;
  }
  public static void removeInstance() {
    instance = null;
  }
}

Just a curiousity. I'm glad you've got the solution.