CompletableFuture supplyAsync 与 Stream.map()

Question

伙计们！我有一个问题：这段代码有什么作用：

Collection<Contract.class> contracts = fillTheCollectionFromDb();
contracts.stream().filter(condition)
                  .map(contractItem ->
                       CompletableFuture.supplyAsync(() -> 
                           {T result = getAnotherDataFromDb(contractItem); 
                            return result;}, Executor.class)
                  )//end .map
                  .map(CompletableFuture::join).collect(Collectors.toList());

Answer 1

此代码等效于：

Collection<Contract> contracts = fillTheCollectionFromDb();
contracts.stream().filter(condition)
              .map(this::getAnotherDataFromDb)
              .collect(Collectors.toList());

为了使这个程序真正并行，它应该被修改。首先，并行启动对数据库的所有请求：

Collection<Contract> contracts = fillTheCollectionFromDb();
List<CompletableFuture> futures = contracts.stream().filter(condition)
              .map(contractItem ->
                   CompletableFuture.supplyAsync(
                         ()->getAnotherDataFromDb(contractItem),
                       executor)
              )//end .map
              .collect(Collectors.toList());

然后才收集所有结果：

List results = futures.stream
      .map(CompletableFuture::join)
      .collect(Collectors.toList());