public class OsgiJtaPlatform implements JtaPlatform {

    //This might be an option/setting to configure
    private final long                                                   serviceTimeOutMilliseconds = TimeUnit.SECONDS.toMillis(1);

    private static final long                                            serialVersionUID           = 1L;
    private final ServiceTracker<TransactionManager, TransactionManager> transactionManagerServiceTracker;
    private final ServiceTracker<UserTransaction, UserTransaction>       userTransactionServiceTracker;

    public OsgiJtaPlatform(BundleContext bundleContext) {
        transactionManagerServiceTracker = new ServiceTracker<TransactionManager, TransactionManager>(bundleContext, TransactionManager.class, null);
        userTransactionServiceTracker = new ServiceTracker<UserTransaction, UserTransaction>(bundleContext, UserTransaction.class, null);
    }
    
    /**
    * shut down this Platform (call e.g. in activators stop method)
    */
    public void shutdown() {
        transactionManagerServiceTracker.close();
        userTransactionServiceTracker.close();
    }

    @Override
    public TransactionManager retrieveTransactionManager() {
        try {
            TransactionManager service = transactionManagerServiceTracker.waitForService(serviceTimeOutMilliseconds);
            if (service != null) {
                return service;
            }
        } catch (InterruptedException e) {
            throw new TransactionException("Operation was interrupted");
        }
        throw new TransactionException("Can't fetch TransactionManager service within specified timeout");
    }

    @Override
    public UserTransaction retrieveUserTransaction() {
        try {
            UserTransaction service = userTransactionServiceTracker.waitForService(serviceTimeOutMilliseconds);
            if (service != null) {
                return service;
            }
        } catch (InterruptedException e) {
            throw new TransactionException("Operation was interrupted");
        }
        throw new TransactionException("Can't fetch UserTransaction service within specified timeout");
    }

....
}