CompletableFuture 异步执行多个数据库查询

Question

我想并行执行多个数据库查询并将结果存储在地图中。我正在尝试这样做，但是当我访问地图时地图没有完全填充。

我做错了什么吗？

 public Map<MapKeyEnums, Set<String>> doDBCalls(String phoneNumber, long timestamp) {

         Map<MapKeyEnums, Set<String>> instrumentsEdgesMap = new EnumMap<>(MapKeyEnums.class);

         CompletableFuture.supplyAsync(() -> dbReadService.getCall(phoneNumber, PhoneNumber.class, "ABC", timestamp)).
                    thenApply(x -> instrumentsEdgesMap.put(MapKeyEnums.ABC, x));

         CompletableFuture.supplyAsync(() -> dbReadService.getCall(phoneNumber, PhoneNumber.class, "XYZ", timestamp)).
                    thenApply(x -> instrumentsEdgesMap.put(MapKeyEnums.XYZ, x));

         CompletableFuture.supplyAsync(() -> dbReadService.getCall(phoneNumber, PhoneNumber.class, "DEF", timestamp)).
                    thenApply(x -> instrumentsEdgesMap.put(MapKeyEnums.DEF, x));

         return instrumentsEdgesMap;

}

任何帮助将不胜感激，在此先感谢。

Answer 1

您必须等待期货完成，然后才能 return 结果。

试试

    public Map<MapKeyEnums, Set<String>> doDBCalls(String phoneNumber, long timestamp) {

        Map<MapKeyEnums, Set<String>> instrumentsEdgesMap = new EnumMap<>(MapKeyEnums.class);

        CompletableFuture.allOf(
            CompletableFuture.supplyAsync(() -> dbReadService.getCall(phoneNumber, PhoneNumber.class, "ABC", timestamp))
                .thenAccept(x -> instrumentsEdgesMap.put(MapKeyEnums.ABC, x)),

            CompletableFuture.supplyAsync(() -> dbReadService.getCall(phoneNumber, PhoneNumber.class, "XYZ", timestamp))
                .thenAccept(x -> instrumentsEdgesMap.put(MapKeyEnums.XYZ, x)),

            CompletableFuture.supplyAsync(() -> dbReadService.getCall(phoneNumber, PhoneNumber.class, "DEF", timestamp))
                .thenAccept(x -> instrumentsEdgesMap.put(MapKeyEnums.DEF, x)))
        .get(); // wait for completion of all three subtasks

        return instrumentsEdgesMap;
    }

Answer 2

在上面的方法中，supplyAsync 将由 ForkJoinPool 中的 Async 线程执行，但 thenApply 方法始终由调用线程执行。因此，您的查询将运行一个接一个地按顺序进行，这不是异步的

All async methods without an explicit Executor argument are performed using the ForkJoinPool.commonPool() (unless it does not support a parallelism level of at least two, in which case, a new Thread is created to run each task).

示例如下

CompletableFuture.supplyAsync(()->{
        System.out.println(Thread.currentThread().getName());
        return "SupplyAsync";
    }).thenAccept(i->{
    System.out.println(Thread.currentThread().getName()+"--"+i);
    });

输出：

ForkJoinPool.commonPool-worker-3
main--SupplyAsync

因此，如果您希望您的进程是 Async，那么首先使用 supplyAsync 触发所有三个数据库查询，并在 CompletableFuture

中捕获输出

CompletableFuture<Set<String>> first =  CompletableFuture.supplyAsync(() -> dbReadService.getCall(phoneNumber, PhoneNumber.class, "ABC", timestamp));

CompletableFuture<Set<String>> second =  CompletableFuture.supplyAsync(() -> dbReadService.getCall(phoneNumber, PhoneNumber.class, "XYZ", timestamp));

CompletableFuture<Set<String>> third =  CompletableFuture.supplyAsync(() -> dbReadService.getCall(phoneNumber, PhoneNumber.class, "DEF", timestamp));

然后现在创建一个包含其中三个的流，然后将它们收集到 Map

Stream.of(new AbstractMap.SimpleEntry<MapKeyEnums, CompletableFuture<Set<String>>>(MapKeyEnums.ABC, first),
              new AbstractMap.SimpleEntry<MapKeyEnums, CompletableFuture<Set<String>>>(MapKeyEnums.XYZ, second),
              new AbstractMap.SimpleEntry<MapKeyEnums, CompletableFuture<Set<String>>>(MapKeyEnums.DEF, third))
       .forEach(entry->{
           entry.getValue().thenAccept(val-> instrumentsEdgesMap.put(entry.getKey(), val));
       });

CompletableFuture 异步执行多个数据库查询

CompletableFuture to execute multiple DB queries asynchronously

java

parallel-processing

multithreading

java-8

completable-future