Spring Boot Caching auto refresh using #PostConstruct - spring-boot

Question

I currently have a Spring Boot based application where there is no active cache. Our application is heavily dependent on key-value configurations which we maintain in an Oracle DB. Currently, without cache, each time I want to get any value from that table, it is a database call. This is, expectedly causing a lot of overhead due to high number of transactions to the DB. Hence, the need for cache arrived.
On searching for caching solutions for SpringBoot, I mostly found links where we are caching object while any CRUD operation is performed via the application code itself, using annotations like #Cacheable, #CachePut, #CacheEvict, etc. but this is not applicable for me. I have a master data of key-value pairs in the DB, any change needs approvals and hence the access is not directly provided to the user, it is made once approved directly in the DB.
I want to have these said key-values to be loaded at startup time and kept in the memory, so I tried to implement the same using #PostConstruct and ConcurrentHashMap class, something like this:
public ConcurrentHashMap<String, String> cacheMap = new ConcurrentHashMap<>();
#PostConstruct
public void initialiseCacheMap() {
List<MyEntity> list = myRepository.findAll();
for(int i = 0; i < list.size(); i++) {
cacheMap.put(list.get(i).getKey(), list.get(i).getValue());
}
}
In my service class, whenever I want to get something, I am first checking if the data is available in the map, if not I am checking the DB.
My purpose is getting fulfilled and I am able to drastically improve the performance of the application. A certain set of transactions were earlier taking 6.28 seconds to complete, which are now completed in mere 562 milliseconds! however, there is just one problem which I am not able to figure out:
#PostConstruct is called once by Spring, on startup, post dependency injection. Which means, I have no means to re-trigger the cache build without restart or application downtime, this is not acceptable unfortunately. Further, as of now, I do not have the liberty to use any existing caching frameworks or libraries like ehcache or Redis.
How can I achieve periodic refreshing of this cache (let's say every 30 minutes?) with only plain old Java/Spring classes/libraries?
Thanks in advance for any ideas!

Answer 1

You can do this several ways, but how you can also achieve this is by doing something in the direction of:
private const val everyThrityMinute = "0 0/30 * * * ?"
#Component
class TheAmazingPreloader {
#Scheduled(cron = everyThrityMinute)
#EventListener(ApplicationReadyEvent::class)
fun refreshCachedEntries() {
// the preloading happens here
}
}
Then you have the preloading bits when the application has started, and also the refreshing mechanism in place that triggers, say, every 30 minutes.
You will require to add the annotation on some #Configuration-class or the #SpringBootApplication-class:
#EnableScheduling