Ensure that code within CompletableFuture callback executes after

问题

Say I have this:

public void foo(){
     CompletableFuture.delayedExecutor(1, TimeUnit.MILLISECONDS).execute(() -> {                 
         doSomethingA();                
      });       
      doSomethingB();        
}

Is there any guarantee that doSomethingB(); will always run before doSomethingA();? Something tells me with thread pre-emption, it's possible, although unlikely, that doSomethingA() could be run first?

回答1:

No, there is no guarantee, when on a machine which has multiple processors, that doSomethingB() would always execute before doSomethingA(). Though it will likely happen 99.999999999% of the time.

I noticed in some of your comments a reference to an "Event Loop". In Java there is no native concept of an Event Loop like there is in, say, Node and Javascript. If you have 4 threads, with four processors, and you submit four tasks, all four tasks will run in parallel.

来源：https://stackoverflow.com/questions/54560757/ensure-that-code-within-completablefuture-callback-executes-after