如何在 Java 8 中多次使用 thenCompose 的结果？

Question

I have a series of thenCompose calls, similar to

myObject.updateDB(payload)
      .thenCompose(__ -> getUserID(payload.ID()))
      .thenCompose(id -> getProfile(id))
      .thenCompose(userProfile -> updateSomething(userProfile))
      .thenCompose(__ -> notifyUser(id))
      .thenAccept(__ -> doSomething())
      .exceptionally(t -> doSomethingElse());

The getUserID call returns a CompletionStage<String> which I use in the next call for getProfile . I need the same id again for the notifyUser call. How to make it available there? The IDE is showing

Cannot resolve symbol id.

Answer 1

The issue with your current code is that by the time you reach .thenCompose(__ -> notifyUser(id)) , the variable id is not in scope anymore.

A simple solution in this case would be to invoke multiple thenCompose directly on the CompletionStage returned by getProfile :

myObject.updateDB(payload)
  .thenCompose(__ -> getUserID(payload.ID()))
  .thenCompose(id -> 
      getProfile(id)
          .thenCompose(userProfile -> updateSomething(userProfile))
          .thenCompose(__ -> notifyUser(id))
  )
  // rest of chain calls

Answer 2

I think, your code becomes simpler, if you don't insist on using thenCompose for every step:

myObject.updateDB(payload)
    .thenCompose(__ -> getUserID(payload.ID()))
    .thenAccept(id -> {
        updateSomething(getProfile(id).join());
        notifyUser(id);
    })
    .thenRun(() -> doSomething())
    .exceptionally(t -> doSomethingElse());

If having each step effectively sequential is your requirement, you can simply use join :

myObject.updateDB(payload)
    .thenCompose(__ -> getUserID(payload.ID()))
    .thenAccept(id -> {
        updateSomething(getProfile(id).join()).join();
        notifyUser(id).join();
    })
    .thenRun(() -> doSomething())
    .exceptionally(t -> doSomethingElse());

Considering that the whole chain is effectively sequential, you may just write it straight-forward:

myObject.updateDB(payload)
    .thenRun(() -> {
        YourUserIDType id = getUserID(payload.ID()).join();
        updateSomething(getProfile(id).join()).join();
        notifyUser(id).join();
        doSomething();
    })
    .exceptionally(t -> doSomethingElse());