Caching Currency-Conversion Rates in Spring Boot With @Cacheable and Scheduled Eviction

package com.example.fx.service;

import org.springframework.cache.annotation.CacheEvict;
import org.springframework.cache.annotation.Cacheable;
import org.springframework.scheduling.annotation.Scheduled;
import org.springframework.stereotype.Service;
import org.springframework.web.client.RestClient;

import java.math.BigDecimal;

@Service
public class CurrencyRateService {

    private final RestClient restClient;

    public CurrencyRateService(RestClient.Builder builder) {
        this.restClient = builder.baseUrl("https://api.example-fx.com").build();
    }

    @Cacheable(value = "fxRates", key = "#from + '-' + #to", unless = "#result == null")
    public BigDecimal getRate(String from, String to) {
        return fetchRateFromProvider(from, to);
    }

    @CacheEvict(value = "fxRates", allEntries = true)
    @Scheduled(fixedRate = 300_000)
    public void refreshAllRates() {
        // Cache cleared proactively; next reads repopulate lazily.
    }

    private BigDecimal fetchRateFromProvider(String from, String to) {
        RateResponse response = restClient.get()
                .uri("/latest?base={base}&symbols={sym}", from, to)
                .retrieve()
                .body(RateResponse.class);
        if (response == null || response.rate() == null) {
            return null;
        }
        return response.rate();
    }

    public record RateResponse(String base, String symbol, BigDecimal rate) {
    }
}

package com.example.fx.web;

import com.example.fx.service.CurrencyRateService;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.RestController;

import java.math.BigDecimal;
import java.math.RoundingMode;

@RestController
public class ConversionController {

    private final CurrencyRateService rateService;

    public ConversionController(CurrencyRateService rateService) {
        this.rateService = rateService;
    }

    @GetMapping("/convert")
    public ResponseEntity<ConversionResult> convert(
            @RequestParam String from,
            @RequestParam String to,
            @RequestParam BigDecimal amount) {

        BigDecimal rate = rateService.getRate(from.toUpperCase(), to.toUpperCase());
        if (rate == null) {
            return ResponseEntity.status(502).build();
        }
        BigDecimal converted = amount.multiply(rate).setScale(2, RoundingMode.HALF_UP);
        return ResponseEntity.ok(new ConversionResult(from, to, amount, rate, converted));
    }

    public record ConversionResult(String from, String to, BigDecimal amount,
                                   BigDecimal rate, BigDecimal result) {
    }
}