具有可变变量的通量可见性问题

Question

我有一个简单的Spring控制器，它有一个Spring服务作为依赖项。在服务类中，我有一个名为flag的int类型的静态volatile字段。当我通过控制器调用createFlux((方法时，标志被设置为5，然后创建一个新的Flux，它每秒检查一次标志，并根据标志值打印一条消息。由于delayElements方法的语义，代码将并行执行。之后，如果我调用changeFlag((方法，它会更改标志的值，并且由于标志变量是可变的，我希望打印的消息会更改，但这并没有发生。

这是代码：

@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));
}
}

这是控制台中的输出：

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

从输出中可以看出，即使flag的值更改为3，它也会继续打印消息Start。过了一段时间，打印的信息就变了。我想有一些缓存或类似的东西，但volatile变量没有被缓存。

问题是——这是一个bug还是我遗漏了什么？

Answer 1

生成使用者会立即为一堆元素执行。您可以通过在generate:之后添加日志来轻松确认

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

打印：

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

CCD_ 2方法基于来自下游的需求来发射元素。它首先生成32个元素并对它们进行缓冲。当下游开始处理元素时，以及当缓冲区大小降至阈值以下时，它会发出更多的元素。