How does a java app get rid of non-garbage ?

Question

Not sure if I should rather post to "New to Java", but it's related to JNI as well:
I have a dynamic library, a corresponding Java singleton class and some other classes declaring native methods in that dll.
"All" is fine for the moment, except that eventually a test application does not terminate when exiting main().
I had the fear it has to do with some static Java objects which are never gc'ed and I built a test scenario like this:
public class Test1 {
     public static void main(String[] args) {
          boolean explicitGC = false;
          createObj();
          createObj();
          createObj();
          createObj();
          if (B.getIt() == null)
               System.out.println("All is gone");
          else
               System.out.println("leftover: "+B.getIt().toString());
          // B.getIt().destroy();
          if (explicitGC) {
               Runtime rt = Runtime.getRuntime();
               rt.gc();
               rt.runFinalization();
          if (B.getIt() == null)
               System.out.println("All is gone");
          else
               System.out.println("leftover: "+B.getIt().toString());
     static void createObj() {
          A myA= new A();
          System.out.println("Created " + myA.toString());
class A {
     B b;
     public A() {
          b=B.getSingle(this);
     public void finalize() {
          System.out.println(this.toString() +" finalizing");
     public String toString() {
          return "A@"+hashCode()+" referencing "+b.toString();
class B {
     private static B myself;
     private A a;
     private B(A _a) {
          a = _a;
          try {
               System.load("c:\\path\\MyNative.dll");
          } catch (UnsatisfiedLinkError e) {
               e.printStackTrace();
     public static B getSingle(A creator){
          if (myself==null) {
               myself = new B(creator);
          return myself;
     public static B getIt() {
          return myself;
     public String toString() {
          return "[B created by A@"+ a.hashCode()+"]";
     public void destroy() {
          System.out.println( toString()+" being destroyed now");
          myself = null;
     public void finalize() {
          destroy();
} <Sorry for the long code.>
This test does not show any problem, although the static B.myself is never destroyed explicitly, and neither the creating B.a object, if I remove the
B.getIt().destroy(); and the block contolled by explicitGC .
Now, it's a miracle for me, how JVM manages this nicely in this test app, and even unloads the DLL automatically.
My original problem is just slightly more complicated:
class B is extended by
     private native long Create(String url, String usr, String pwd);
     private native void Destroy(long obj);
                private static long obj;
     private B(A _a) {
          obj = Create(myURL, usr, pwd);
     public void destroy() {
          Destroy(obj);
     }Intention was to clean the native objects whenever the corresponding Java singleton object disappears.
A wise rule says "Leave the GC do its work alone, never rely on finalize() being called", but what else could be a solution?
Of course, in my test app I can add the B.getIt().destroy();
but in "real life" things are hidden like in main() above, and an end user of class A can easily forget (and won't understand why he should) call B.destroy()
It's perhaps a general design question about native singletons and how to get rid of them as nicely as Java does it.
Perhaps an explanation how it works in pure Java does help me already.
Do you need more explanations to get the problem? Any feedback is welcome.
Thanks!

Answer 1

jschell wrote:
eventually a test application does not terminate when exiting main(). Then a java thread or C thread is still running. Stop it from running.Thanks. So it's not related to static (native) objects but to some background (native) thread?
Well, but my issue is how to get notice of the final cleanup.
If I get the chance to run my native Destroy() method, all is fine.
However, the sample code - which can be exercised without a native lib - shows that it's easy (or possible, at least) to never run into the finalize() method of a static object and any other object it references.
Do I have to build my own reference counting?
What would be the most robust mechanism for this task: How to get notice of an application ending?
Sorry, now it's rather "New to Java" than JNI.
But I rather don't crosspost, OK ? ;)