6.15 CompletableFuture

1. From Future to CompletableFuture

A plain Future mainly represents a result that may become available later. CompletableFuture adds a completion-stage model so dependent work can be composed instead of repeatedly blocking on get().

Common entry points are:

CompletableFuture.supplyAsync(() -> load());
CompletableFuture.runAsync(() -> refresh());

2. Chaining Stages

Important operations include:

• thenApply: transform a successful result;
• thenCompose: flatten a dependent asynchronous operation;
• thenCombine: combine independent results;
• allOf / anyOf: coordinate multiple futures;
• exceptionally, handle, whenComplete: observe or recover from failures.

Use thenCompose when a stage itself returns another CompletionStage; otherwise nested futures are easy to create accidentally.

3. Which Thread Runs a Stage?

Do not assume every stage creates a new thread.

Non-Async continuations may run in the thread that completes the previous stage. *Async methods without an explicit executor normally use the default asynchronous execution facility, commonly the ForkJoinPool.commonPool() for ordinary applications.

When workload isolation or blocking behavior matters, pass an explicit Executor.

4. Avoid Hidden Blocking

Calling join() or get() turns an asynchronous pipeline into a blocking boundary. Blocking common-pool workers on slow I/O can also reduce useful parallelism.

Prefer composition through the pipeline and block only at an intentional system boundary.

5. Failure and Cancellation

Exceptions propagate through dependent stages until handled. join() wraps exceptional completion in CompletionException; get() uses checked exceptions.

Cancellation of a CompletableFuture marks that stage as cancelled, but it should not be interpreted as guaranteed interruption or rollback of arbitrary underlying work. Design external side effects with their own cancellation/idempotency semantics.

6. Practical Rule

Use CompletableFuture for dependency graphs of asynchronous work. Use a deliberately chosen executor and explicit timeouts when the work depends on bounded external resources.