volatile變數的通量可見性問題

我有一個簡單的 Spring 控制器，它有一個 Spring 服務作為依賴項。在服務類中，我有一個名為 flag 的 int 型別的靜態 volatile 欄位。當我通過控制器呼叫 createFlux() 方法時，標志設定為 5，然后創建一個新的 Flux，它每秒檢查標志，并根據標志值列印一條訊息。由于 delayElements 方法語意，代碼將并行執行。之后，如果我呼叫 changeFlag() 方法，該方法會更改標志的值，并且由于標志變數是易失的，我希望列印的訊息會發生變化，但這不會發生。

這是代碼：

@RestController
public class MyController {

  @Autowired private MyService myService;

  @GetMapping("createFlux")
  public void createFlux() {
    myService.createFlux();
  }

  @GetMapping("changeFlag")
  public void changeFlag() {
    myService.changeFlag();
  }
}


@Service
public class MyService {

  private static volatile int flag = 3;

  public void changeFlag() {
    flag = 3;
    System.out.println("############# Flag = "   flag);
  }

  public void createFlux() {
    flag = 5;
    System.out.println("Flag = "   flag);

    Flux.generate(sink -> {
      if (flag == 3) {
        sink.next("Stop");
      } else {
        sink.next("Start");
      }
    }).delayElements(Duration.ofSeconds(1)).subscribe(s -> System.out.println(Thread.currentThread().getName()   " : "   s));
  }
}

這是控制臺中的輸出：

Flag = 5
parallel-1 : Start
parallel-2 : Start
parallel-3 : Start
parallel-4 : Start
############# Flag = 3
parallel-5 : Start
parallel-6 : Start
parallel-7 : Start
parallel-8 : Start
parallel-1 : Start
parallel-2 : Start
parallel-3 : Start
parallel-4 : Start
parallel-5 : Start
parallel-6 : Start
parallel-7 : Start
parallel-8 : Start
parallel-1 : Start
parallel-2 : Start
parallel-3 : Start
parallel-4 : Start
parallel-5 : Start
parallel-6 : Start
parallel-7 : Start
parallel-8 : Start
parallel-1 : Start
parallel-2 : Start
parallel-3 : Start
parallel-4 : Start
parallel-5 : Start
parallel-6 : Start
parallel-7 : Start
parallel-8 : Start
parallel-1 : Stop
parallel-2 : Stop
parallel-3 : Stop
parallel-4 : Stop
parallel-5 : Stop
parallel-6 : Stop

從輸出可以看出，即使 flag 的值更改為 3，它也會繼續列印訊息 Start。一段時間后，列印的訊息也發生了變化。我想有一些快取或類似的東西，但是 volatile 變數沒有被快取。

問題是 - 這是一個錯誤還是我錯過了什么？

uj5u.com熱心網友回復：

為一堆元素立即執行生成消費者。您可以通過在以下內容之后添加日志來輕松確認generate：

    Flux.generate(...).doOnNext(e -> log.info("executed: {}", e))

印刷：

2022-01-20 13:31:50,346  INFO parallel-1  - Flag = 5
2022-01-20 13:31:50,349  INFO parallel-1  - executed: Start
2022-01-20 13:31:50,351  INFO parallel-1  - executed: Start
2022-01-20 13:31:50,351  INFO parallel-1  - executed: Start
2022-01-20 13:31:50,351  INFO parallel-1  - executed: Start
2022-01-20 13:31:50,351  INFO parallel-1  - executed: Start
2022-01-20 13:31:50,351  INFO parallel-1  - executed: Start
2022-01-20 13:31:50,351  INFO parallel-1  - executed: Start
2022-01-20 13:31:50,351  INFO parallel-1  - executed: Start
2022-01-20 13:31:50,351  INFO parallel-1  - executed: Start
2022-01-20 13:31:50,351  INFO parallel-1  - executed: Start
2022-01-20 13:31:50,351  INFO parallel-1  - executed: Start
2022-01-20 13:31:50,351  INFO parallel-1  - executed: Start
2022-01-20 13:31:50,351  INFO parallel-1  - executed: Start
2022-01-20 13:31:50,351  INFO parallel-1  - executed: Start
2022-01-20 13:31:50,351  INFO parallel-1  - executed: Start
2022-01-20 13:31:50,351  INFO parallel-1  - executed: Start
2022-01-20 13:31:50,351  INFO parallel-1  - executed: Start
2022-01-20 13:31:50,351  INFO parallel-1  - executed: Start
2022-01-20 13:31:50,351  INFO parallel-1  - executed: Start
2022-01-20 13:31:50,351  INFO parallel-1  - executed: Start
2022-01-20 13:31:50,351  INFO parallel-1  - executed: Start
2022-01-20 13:31:50,351  INFO parallel-1  - executed: Start
2022-01-20 13:31:50,351  INFO parallel-1  - executed: Start
2022-01-20 13:31:50,351  INFO parallel-1  - executed: Start
2022-01-20 13:31:50,351  INFO parallel-1  - executed: Start
2022-01-20 13:31:50,351  INFO parallel-1  - executed: Start
2022-01-20 13:31:50,351  INFO parallel-1  - executed: Start
2022-01-20 13:31:50,351  INFO parallel-1  - executed: Start
2022-01-20 13:31:50,352  INFO parallel-1  - executed: Start
2022-01-20 13:31:50,352  INFO parallel-1  - executed: Start
2022-01-20 13:31:50,352  INFO parallel-1  - executed: Start
2022-01-20 13:31:50,352  INFO parallel-1  - executed: Start

generate方法根據下游的需求發出元素。它首先生成 32 個元素并緩沖它們。當下游開始處理元素并且緩沖區大小低于閾值時，它會發出更多元素。

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