JVM never exit even the main thread finished

public class Benchmark
{
    
    private static long ITER = 1000000000L;

    public static double testHeap()
    {
        CacheService<Long, String> cacheService = new DirectMemory<Long, String>()
                        .setNumberOfBuffers( 100 )
                        .setSize( 10000 )
                        .setInitialCapacity( 100000 )
                        .setConcurrencyLevel( 1 )
                        .setSerializer( new MessagePackSerializer() )
                        .newCacheService();
        
        final String str = "helloworld";

        // warm up
        for(long l = 0; l < ITER; l++) {
            cacheService.put(l, str);
        }
        cacheService.clear();
        long start = System.currentTimeMillis();
        for(long l = 0; l < ITER; l++) {
            cacheService.put(l, str);
        }
        return ((double)  ITER / (System.currentTimeMillis() - start )) * 1000;
    }
    

    public static void main( String[] args )
    {
        ITER = Long.parseLong( args[0] );
        System.gc();
        System.out.println("testHeap: ops=" + testHeap());
    }

}