为什么 runBlocking 不阻塞调用线程

Question

我正在尝试理解 kotlin 中的 runBlocking。

 println("before runBlocking ${Thread.currentThread().name}")

    runBlocking { // but this expression blocks the main thread
        delay(2000L) // non blocking
        println("inside runBlocking ${Thread.currentThread().name}")
        delay(2000L)
    }

    println("after runBlocking ${Thread.currentThread().name}")

输出

before runBlocking main
inside runBlocking main
after runBlocking main

Kotlin 说

1. runBlocking - Runs a new coroutine 和 blocks the current thread 一直中断直到完成
2. 调用 runBlocking 的主线程会阻塞，直到 runBlocking 内的协程完成。

第 1 点：- 如果 runBlocking 在上面的示例中阻塞了 main 线程。然后在 runBlocking 我如何再次获得main 线程。

第 2 点：- 如果Runs a new coroutine 在上述陈述中为真，那么为什么它没有在runBlocking 内创建新的coroutine。

Answer 1

runBlocking（doc）的签名是

fun <T> runBlocking(
    context: CoroutineContext = EmptyCoroutineContext,
    block: suspend CoroutineScope.() -> T
): T (source)

如果您看到context 参数，它的默认值为EmptyCoroutineContext。因此，当您不传递特定上下文时，默认值是当前线程上的事件循环。由于在运行runBlocking 块之前的当前线程是主线程，所以你在块内运行的任何东西仍然在主线程上。

如果您传递如下的协程上下文，您将在 runBlocking 中的块在不同的线程中运行。

println("before runBlocking ${Thread.currentThread().name}")

runBlocking(Dispatchers.Default) {
    delay(2000L)
    println("inside runBlocking ${Thread.currentThread().name}")
    delay(2000L)
}

println("after runBlocking ${Thread.currentThread().name}")

输出

before runBlocking main
inside runBlocking DefaultDispatcher-worker-1
after runBlocking main

或者，如果您在不传递上下文的情况下启动 runBlocking，但在内部启动协程，如下所示，您会看到它在不同的线程上运行。

println("before runBlocking ${Thread.currentThread().name}")

runBlocking { 
    println("inside runBlocking ${Thread.currentThread().name}")
    delay(2000L) 
    CoroutineScope(Dispatchers.Default).launch {
        println("inside runBlocking coroutineScope ${Thread.currentThread().name}")
    }
    delay(2000L)
}

println("after runBlocking ${Thread.currentThread().name}")

输出

before runBlocking main
inside runBlocking main
inside runBlocking coroutineScope DefaultDispatcher-worker-1
after runBlocking main