What is the difference between thenApply and thenApplyAsync of Java CompletableFuture?

What is the difference between thenApply and thenApplyAsync of Java CompletableFuture?

In Java CompletableFuture, both thenApply and thenApplyAsync are methods used for applying a function to the result of a previous CompletableFuture when it completes. However, they differ in terms of how they handle execution context and thread scheduling:

  1. thenApply:

    • thenApply is a synchronous method. It applies the provided function to the result of the previous CompletableFuture in the same thread where the previous computation completed.
    • This method is useful when you have a CPU-bound operation or when you want to ensure that the subsequent operation runs in the same thread context as the previous one.
    • It doesn't introduce additional concurrency or thread switching.

    Example:

    CompletableFuture<Integer> future = CompletableFuture.supplyAsync(() -> 10); CompletableFuture<Integer> result = future.thenApply(x -> x * 2); // Applies the function synchronously

  2. thenApplyAsync:

    • thenApplyAsync is an asynchronous method. It applies the provided function to the result of the previous CompletableFuture, but it may use a different thread or an executor to execute the function.
    • This method is useful when you have an I/O-bound operation or when you want to parallelize computation by utilizing a different thread pool or executor.
    • You can provide an Executor as an argument to specify where the function should be executed. If you don't provide an Executor, it may use the default ForkJoinPool.commonPool().

    Example:

    CompletableFuture<Integer> future = CompletableFuture.supplyAsync(() -> 10); CompletableFuture<Integer> result = future.thenApplyAsync(x -> x * 2); // May use a different thread

In summary, the key difference between thenApply and thenApplyAsync is in their execution behavior:

  • thenApply is synchronous and executes the function in the same thread.
  • thenApplyAsync is asynchronous and may execute the function in a different thread or executor.

The choice between them depends on your specific use case. Use thenApply when you want to continue the computation in the same thread context and use thenApplyAsync when you want to introduce concurrency or specify a different execution context for the next operation.

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