Wrong way to synchronize factory method

ServiceFactoryImpl_Buggy.java

public class ServiceFactoryImpl_Buggy<T extends Service> {
    protected Map<String, T> serviceMap = new HashMap<String, T>();

    public T getService( String name ) {
        T result = serviceMap.get( name );

        if ( null == result ) {
            synchronized ( this ) {
                result = createService( name );
                serviceMap.put( name, result );
            }
        }

        return result;
    }

    private T createService( String name ) {
        ...
    }
}

ServiceFactoryImpl_Correct.java

public class ServiceFactoryImpl_Correct<T extends Service> {
    protected Map<String, T> serviceMap = new HashMap<String, T>();
    protected final ReadWriteLock serviceMapLock = new ReentrantReadWriteLock();

    public T getService( String name ) {
        T result;
        serviceMapLock.readLock().lock();
        try {
            result = serviceMap.get( name );
        } finally {
            serviceMapLock.readLock().unlock();
        }

        if ( result == null ) {
            serviceMapLock.writeLock().lock();
            try {
                result = serviceMap.get( name );
                if (result == null) {
                    result = createService( name );
                    serviceMap.put( name, result );
                }
            } finally {
                serviceMapLock.writeLock().unlock();
            }
        }

        return result;
    }

    private T createService( String name ) {
        return null;
    }
}