CompletableFuture - It may be useful to build chain of actions before execution.

下方這隻程式在Java8中應是相當常見:

CompletableFuture.supplyAsync(()->{
	dumpCurrentThreadName("supplyAsync");
	return new Response();
}).whenComplete((response, ex)->{
	dumpCurrentThreadName("whenComplete");
	// process response
});

它的問題是如果supplyAsync中的工作，在執行whenComplete前就結束了；當執行whenComplete時，有可能會使用client thread去執行工作而影響到工作接收的速度。以下是我的執行結果:

ForkJoinPool.commonPool-worker-3: supplyAsync
main: whenComplete

假如你的callback chain很長，遇到這個情況的機率就會越高。

面對這個問題，在Java9中，允許你將實際的執行工作放到callback chain之後:

CompletableFuture<Response> asyncJob = new CompletableFuture<>();
 
asyncJob.whenComplete((response, ex)->{
	dumpCurrentThreadName("whenComplete");
});
 
asyncJob.completeAsync(()->{
	dumpCurrentThreadName("supplyAsync");
	return new Response();
});

輸出結果如下:

ForkJoinPool.commonPool-worker-3: supplyAsync
ForkJoinPool.commonPool-worker-3: whenComplete

至於Java8就科科了。

• Asynchronous API with CompletableFuture - Performance Tips and Tricks

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