CompletableFuture-使用thenapply与使用lambda块的优势

Question

I want to know advantage of using lambda block vs using thenApply() .

I understand that the lambda block always gets executed by same thread executing the function in supplyAsync() whereas thenApply() block can be executed by the thread executing supplyAsync() or by caller thread.

I was also wondering that if thenApply() has to execute only after supplyAsync() function is executed, why would it be ever executed by caller thread.

CompletableFuture.supplyAsync(() -> {return doSomethingAndReturnA();})
                 .thenApply(a -> convertToB(a));

CompletableFuture.supplyAsync(() -> {
    A a = doSomethingAndReturnA();
    convertToB(a);
});

Answer 1

With the lambda block, you have the guarantee that both methods are called in the same thread, without anything else running on that thread in between.

thenApply() is more useful when you already have a CompletionStage / CompletableFuture (eg in a variable or returned by some method call) and you want to have subsequent processing on its result.

It is also useful when you want to have access to the intermediate result (here a ) outside the scope of the lambda, as you could just joint() on the intermediate CompletableFuture to access it.

The more advanced thenApplyAsync() also allows you to process that intermediate result asynchronously, which is difficult in a lambda block.

In some cases you might simply choose one or the other based on readability.