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给定代码：

logItemPublisher
    .buffer(
            loggingProperties.getBufferTimeoutMillis(),
            TimeUnit.MILLISECONDS,
            loggingProperties.getBufferSize(),
            logDispatchScheduler
    )
    .onBackpressureDrop(droppedLogsHandler)
    // persist uses Spring RestOperations
    .flatMap(logs -> persist(logs, url)
            .subscribeOn(logDispatchScheduler)
    )
    .subscribe();

在哪里，

logItemPublisher = ReplaySubject.<AbstractHttpLogItem>createWithSize(4 * loggingProperties.getBufferSize())
                .toSerialized();
logDispatchScheduler = new TestScheduler()

我有一个单元测试：

@Test
public void testLogServiceSlow() {
    loggingProperties.setBufferSize(1);
    // rx.ring-buffer.size property stores the size of any in-memory ring buffers that RxJava uses when an
    // Observable cannot keep up with rate of event emissions.
    // default value is 128 on the JVM; RxJava 2.x makes this configurable in flatMap
    System.setProperty("rx.ring-buffer.size", "2");
    // this is what persist does  
    when(restOperations.postForEntity(anyString(), any(HttpEntity.class), eq(Void.class)))
            .thenAnswer(invocation -> {
                Thread.sleep(500);
                return ResponseEntity.ok().build();
            });

    logServiceClient.persistLogs(logs);
    scheduler.advanceTimeBy(2L, TimeUnit.SECONDS);

    System.clearProperty("rx.ring-buffer.size");
    Mockito.verify(restOperations, times(2))
            .postForEntity(Mockito.eq("http://log:9000/log/service"), logsCaptors.capture(), eq(Void.class));
}

测试失败，因为尽管有Thread.sleep，但没有产生背压。我不明白为什么不；内部环形缓冲区应在 2 个项目后填满，其余项目应丢弃。

解决方案

回答我自己的问题：

int ringBufferSize = Optional.ofNullable(System.getProperty("rx.ring-buffer.size"))
    .map(Integer::parseInt)
    .orElse(128);
logItemPublisher
    .buffer(
            loggingProperties.getBufferTimeoutMillis(),
            TimeUnit.MILLISECONDS,
            loggingProperties.getBufferSize(),
            logDispatchScheduler
    )
    .onBackpressureDrop(droppedLogsHandler)
    .observeOn(logDispatchScheduler, ringBufferSize)
    .flatMap(logs -> persist(logs, url))
    .subscribe();

然后在测试中，直接设置即可ringBufferSize = 2，无需设置系统属性rx.ring-buffer.size。