While the transaction is not provided in @PostConstruct methods, it's possible to use it "the standard way" via ApplicationContext.getBean():

@Transactional(readOnly = true)
public class MyServiceImpl implements MyService {

    @Autowired
    private MyDao myDao;
    private CacheList cacheList;

    @Autowired 
    private ApplicationContext appContext;

    @PostConstruct
    public void init() {
        appContext.getBean(MyService.class).onInit();
    }

    @Transactional(readOnly = true)
    public void onInit() {
        this.cacheList = new CacheList();
        this.cacheList.reloadCache(this.myDao.getAllFromServer());
    }

    ...
}