CompletableFuture常用方法示例

1. runAsync 接收前一步传递的数据，无返回值

ThreadPoolExecutor pool = new ThreadPoolExecutor(2, 4, 3,
                TimeUnit.SECONDS, new ArrayBlockingQueue<Runnable>(3),
                new ThreadPoolExecutor.DiscardOldestPolicy());
CompletableFuture.runAsync(() -> System.out.println("Hello completableFuture!"), pool);

2. supplyAsync 接收前一步传递的数据，处理后返回结果，返回结果可以不同与前一步数据类型

int poiId = 111;
CompletableFuture<String> future = CompletableFuture.supplyAsync(() -> {
  PoiDTO poi = poiService.loadById(poiId);
  return poi.getName();
});
//阻塞等待结果
String poiName = future.get();

3.thenApply 将上一阶段完成的结果作为当前阶段的入参，有返回值

CompletableFuture<Boolean> future = CompletableFuture.supplyAsync(() -> {
  PoiDTO poi = poiService.loadById(poiId);
  return poi.getMainCategory();
}).thenApply((s) -> isMainPoi(s)); 

3. thenAccept 消费前一阶段的结果，无返回值

CompletableFuture<Void> future = CompletableFuture.supplyAsync(() -> "Hello")
                           .thenAccept((s) -> System.out.println(s + " world"));

4. thenRun 当上一阶段完成后，执行本阶段的任务

CompletableFuture<Void> future = CompletableFuture.supplyAsync(() -> "Hello")
                           .thenRun(() -> System.out.println("do other things. 比如异步打印日志或发送消息"));

5. thenCombine 合并另外一个任务，两个任务都完成后，执行BiFunction，入参为两个任务结果，返回新结果

CompletableFuture<List<DealGroupDTO>> getDeal(List<Integer> poiIds){
  return CompletableFuture.supplyAsync(() ->  poiService.queryPoiIds(poiIds));
}
//thenCompose
CompletableFuture<List<DealGroupDTO>> resultFuture = poiFuture.thenCompose(poiIds -> getDeal(poiIds));
resultFuture.get();

CompletableFuture<String> future = CompletableFuture.supplyAsync(() -> "Hello")
  .thenCombine(CompletableFuture.supplyAsync(() -> "world"), (s1, s2) -> s1 + s2);
//future.get()

6. 组合多个 CompletableFuture 需要多个异步任务都完成再进行后续处理，可以使用allOf，get 方法一样阻塞的等待任务完成

CompletableFuture<Void> poiIDTOFuture = CompletableFuture
 .supplyAsync(() -> poiService.loadPoi(poiId))
  .thenAccept(poi -> {
    model.setModelTitle(poi.getShopName());
    //do more thing
  });

CompletableFuture<Void> productFuture = CompletableFuture
 .supplyAsync(() -> productService.findAllByPoiIdOrderByUpdateTimeDesc(poiId))
  .thenAccept(list -> {
    model.setDefaultCount(list.size());
    model.setMoreDesc("more");
  });
  CompletableFuture.allOf(poiIDTOFuture, productFuture, future3, ...).join();

7. exceptionally 方法可以处理异步任务的异常，在出现异常时，给异步任务链一个从错误中恢复的机会

Integer age = -1;
CompletableFuture<Void> maturityFuture = CompletableFuture.supplyAsync(() -> {
  if(age < 0) {
    throw new IllegalArgumentException("Age can not be negative");
  }
  if(age > 18) {
    return "Adult";
  } else {
    return "Child";
  }
}).exceptionally(ex -> {
  System.out.println("Oops! We have an exception - " + ex.getMessage());
  return "Unknown!";
}).thenAccept(s -> System.out.print(s));

8. handler 方法也可以处理异常，并且无论是否发生异常它都会被调用

Integer age = -1;

CompletableFuture<String> maturityFuture = CompletableFuture.supplyAsync(() -> {
    if(age < 0) {
        throw new IllegalArgumentException("Age can not be negative");
    }
    if(age > 18) {
        return "Adult";
    } else {
        return "Child";
    }
}).handle((res, ex) -> {
    if(ex != null) {
        System.out.println("Oops! We have an exception - " + ex.getMessage());
        return "Unknown!";
    }
    return res;
});