Migrating our micro services from Java to Kotlin 2.0
- 1. Migrating our micro services from Java to Kotlin Yet another awesome journey
- 5. Why we migrated from Java to Kotlin 5
- 7. Checked exceptions 7 public static void refundExcessMoney() throws ExternalPspFailureException { // try to refund money throw new ExternalPspFailureException(); } try { refundExcessMoney() } catch (ExternalPspFailureException e) { // log or rethrow }
- 8. Single abstract method types 8 public interface Comparator<T> { public boolean compare(T a, T b); } public class PaymentComparator implements Comparator<PaymentInformation> { @Overwrite public boolean compare(PaymentInformation a, PaymentInformation b) { ... } }
- 9. Wildcard-types 9 // use-site wildcard List<?> paymentRawTypes = new ArrayList<Integer>(); void original(List<Number> paymentRawTypes) { } void withWildcard(List<? extends Number> paymentRawTypes) { } // Kotlin use-site variance var paymentRawTypes = listOf(1, 2, 3)
- 11. Null-safety 11 // non null-able var paymentInformationId: UUID = UUID.randomUUID() paymentInformationId = null // compilation error // null-able var paymentInformationId : UUID? = UUID.randomUUID() paymentInformationId = null // ok
- 12. Extension functions 12 // Java List<OrderId> orderIds = new ArrayList( Arrays.asList(id1, id2, id3) ); return JsonUtils.toJson(orderIds); // Kotlin val orderIds = listOf(id1, id2, id3) return orderIds.toJson()
- 13. Smart casts 13 // Java if (payment instanceof CreditCardPayment) { CreditCardPayment ccPayment = (CreditCardPayment) payment; return ccPayment.getCardHolder(); } // Kotlin if (payment is CreditCardPayment) { return payment.cardHolder }
- 14. Coroutines
- 15. Payment capture flow 15 PAY FEED PSPOM write feed poll status request capture capture poll feed
- 16. fun capturePayment() { val lastKnownEventId = feedsService.getLatestEventId() paymentService.capture() val feedStatus = getFeedStatus(lastKnownEventId) feedStatus.whenError { throw CreditCardCaptureException() } } Request capture 16
- 17. Asynchronous feed reader 17 fun getFeedStatus(lastKnownEventId: UUID?): PaymentFeedStatus { return runBlocking { withTimeout(syncClientTimeout) { var paymentFeedEvent: PaymentFeedEvent? do { paymentFeedEvent = feedsService.getSuccessorEventFor(lastKnownEventId) delay(200) } while (paymentFeedEvent.isNull()) paymentFeedEvent?.getStatus() } } }
- 19. What did we migrate 19
- 20. How did we migrate 20 refactor to idiomatic Kotlin verify using tests fix invalid code automatic code conversion
- 22. Switching to Kotlin frameworks 22 KTOR - asynchronous Kotlin web framework Spring Boot - established Java web framework Often less matured Often less documented No need to forcefully use Kotlin libraries and frameworks
- 24. Review 24 Challenge which we mastered as team Migrating was worth the effort Improved design and new focus on language Quality of life update 10/10 would use Kotlin again
- 25. A heads up 25 Test coverage is a must have Consider your people and eco system Consider language and framework maturity level Amount of new things can be overwhelming
- 26. Outlook 26 Establish Kotlin at METRONOM Help others introduce Kotlin Introduce more native Kotlin frameworks Find new passionate developers
- 27. Follow us on Twitter and Instagram @bjoernwendland @joinmetronomnow #METRONOMians #wearedevs19 #passion #ambition #purpose Hall A Booth A11
- 28. @BjoernWendland 28 Thank you for your attention!
Editor's Notes
- #2: About Metronom ‚The biggest software company you never heard about‘ METRO’s tech unit providing solutions to customers and employees 'Setting the pace in food an technology‘ About Metro German global diversified retail and wholesale/cash and carry group based in Düsseldorf
- #3: About me Joined METRONOM last july as Full Stack Developer Became Kotlin Ambassador at METRONOM over the cause of the last year
- #5: Reasoning behind migrating to Kotlin How to approach the task What we’ve learned in the process
- #6: Why did we choose Kotlin? A developers curiosity Java 2.0 It is close enough to the previous eco system (JVM / Spring Boot/ PostgresDB) Justification: “Setting the pace in food and technology” About the design Started with Java microservices based on proprietary (yet open source) web framework Unhappy with Java’s design Feeling of being outdated and heavy Finally tipping the scale Google announcing Kotlin as officially supported for Android Level of maturity By now Kotlin became the suggested language for Android development
- #7: Checked exceptions Single Abstract Method Types (SAM Types) Wildcard Types
- #8: Benefit: compiler error if exception is not handled Disadvantage: laborious and results in harder to read code Kotlin has unchecked exceptions
- #9: Lambda expressions are considered a SAM type and can be freely converted SAM Type not bad in general, but propper function types are prefered Kotlin has proper function types opposed to SAM-conversions
- #10: Not explicit / lacking information Unsafe (needs casting) Can be confusing Kotlin has use-site variance without wildcards
- #11: Null-safety support in Kotlin typing system Extension functions Smart casts Coroutines
- #12: ATTENTION: Java libraries null-safety can not be infered
- #13: ATTENTION: How to manage extension functions?
- #15: Using Coroutines See how async code is composed in real world code
- #18: runBlocking launches coroutine withTimeout keeps runtime in check and throws exception of needed Read new event Delay:programmer allowes code to be scheduled at this point Infinite loop And return value Still reads like sync code, but allowes to be yielded It handles waiting for a response very greaceful Safe against infinite loops
- #20: Why did we choose Kotlin? A developers curiosity Java 2.0 It is close enough to the previous eco system (JVM / Spring Boot/ PostgresDB) Justification: “Setting the pace in food and technology” About the design Started with Java microservices based on proprietary (yet open source) web framework Unhappy with Java’s design Feeling of being outdated and heavy Finally tipping the scale Google announcing Kotlin as officially supported for Android Level of maturity By now Kotlin became the suggested language for Android development
- #21: Structured process Intellij’s automatic code conversion for class Fix non compiling code / non translatable language constructs Refactor, even partially rewrite methods Verify correctness using unit tests Convert unit tests
- #23: Migrate to Kotlin libraries Goal was a 100% Kotlin micro service Mockk, Asskertk, Klaxon, etc. Add a library / framework next to existing one Phase out previously used Java framework Goal was a 100% Kotlin micro service Spiked KTOR and Spring Boot We decided to use Spring Boot, because it is fully documented and it enforces the project structure KTOR not yet mature enough and lacks documentation also does not enforce a project structure Will be considered again, since it is more lightweight and has better performance
- #24: Finally a look back And one ahead
- #25: Disclaimer Does not only plly to Kotlin
- #27: If anyone is looking for a new job and wants to work with Kotlin
- #30: Structured process Intellij’s automatic code conversion for class Fix non compiling code / non translatable language constructs Refactor, even partially rewrite methods Verify correctness using unit tests Convert unit tests