what does Mono.defer() do?

by

It is a bit of an oversimplification but conceptually Reactor sources are either lazy or eager. More advanced ones, like an HTTP request, are expected to be lazily evaluated. On the other side the most simple ones like Mono.just or Flux.fromIterable are eager.

By that, I mean that calling Mono.just(System.currentTimeMillis()) will immediately invoke the currentTimeMillis() method and capture the result. Said result is only emitted by the Mono once it is subscribed to. Subscribing multiple times doesn’t change the value either:

Mono<Long> clock = Mono.just(System.currentTimeMillis());
//time == t0

Thread.sleep(10_000);
//time == t10
clock.block(); //we use block for demonstration purposes, returns t0

Thread.sleep(7_000);
//time == t17
clock.block(); //we re-subscribe to clock, still returns t0

The defer operator is there to make this source lazy, re-evaluating the content of the lambda each time there is a new subscriber:

Mono<Long> clock = Mono.defer(() -> Mono.just(System.currentTimeMillis()));
//time == t0

Thread.sleep(10_000);
//time == t10
clock.block(); //invoked currentTimeMillis() here and returns t10

Thread.sleep(7_000);
//time == t17
clock.block(); //invoke currentTimeMillis() once again here and returns t17

More Related Contents:

  1. block()/blockFirst()/blockLast() are blocking error when calling bodyToMono AFTER exchange()
  2. Can I use SpringMvc and webflux together?
  3. How to correctly read Flux and convert it to a single inputStream
  4. Adding a retry all requests of WebClient
  5. Mono switchIfEmpty() is always called
  6. map vs flatMap in reactor
  7. how to log Spring 5 WebClient call
  8. Correct way of throwing exceptions with Reactor
  9. What’s the point of .switchIfEmpty() getting evaluated eagerly?
  10. Don’t spring-boot-starter-web and spring-boot-starter-webflux work together?
  11. Enable CORS in Spring 5 Webflux?
  12. How to customize SpringWebFlux WebClient JSON deserialization?
  13. How do I remove objects from an array in Java?
  14. Why should I use the keyword “final” on a method parameter in Java?
  15. What is the Java equivalent for LINQ? [closed]
  16. The specified child already has a parent. You must call removeView() on the child’s parent first (Android)
  17. What causes a java.lang.StackOverflowError
  18. Different types of thread-safe Sets in Java
  19. Mockito: InvalidUseOfMatchersException
  20. Android Get Current timestamp?
  21. Html/Javascript debugging in JavaFX WebView
  22. How to represent empty char in Java Character class
  23. Clear prompt text in JavaFX TextField only when user starts typing
  24. Can’t parse hibernate.cfg.xml while offline
  25. Multi-dimensional array transposing
  26. How to add JPanel by clicking JButton?
  27. Finding second occurrence of a substring in a string in Java
  28. Java curve fitting library [closed]
  29. Default Navigation Drawer View to ExpandableListView
  30. org.openqa.selenium.UnhandledAlertException: unexpected alert open

Leave a Comment