CoroutineScope配SupervisorJob与supervisorScope的行为差异疑问

我在学习Kotlin协程时，了解到SupervisorJob()可以避免子协程取消牵连其他子协程——coroutineScope里某个子协程失败会取消所有子协程，但supervisorScope只会取消失败的那个。不过我尝试给CoroutineScope添加SupervisorJob作为上下文时，没得到预期结果：本来想让getData1()和getData3()的结果正常打印，结果getData2()抛出异常后所有协程都被取消了。

相关代码如下：

fun main() = runBlocking {

    val exceptionHandler = CoroutineExceptionHandler { coroutineContext, throwable ->
        println(throwable)
    }

    val customScope = CoroutineScope(SupervisorJob() + exceptionHandler)


    customScope.launch {
        launch {
            getData1().also { println(it) }
        }
        launch {
            getData2().also { println(it) }
        }
        launch {
            getData3().also { println(it) }
        }
     }.join()

}


private suspend fun getData1(): String? {
    delay(1000)
    return "data 1"
}

private suspend fun getData2(): String? {
    delay(300)
    throw RuntimeException("While getting Data 2 exception happened")
}

private suspend fun getData3(): String? {
    delay(800)
    return "data 3"
}

问题原因 #

你创建的customScope虽然用了SupervisorJob()，但在customScope.launch内部启动的三个子协程，它们的父Job是这个外层launch生成的普通Job，而非customScope的SupervisorJob()。SupervisorJob的隔离作用仅对其直接子协程生效，内层launch会继承外层launch的Job上下文，所以当getData2()抛出异常时，外层launch的Job会触发所有子协程取消，导致getData1()和getData3()也被终止。

解决方案 #

有两种方式可以实现预期的隔离效果：

方式一：用supervisorScope包裹内层协程 #

将三个内层launch放到supervisorScope中，让它们直接处于supervisorScope的作用域下，单个子协程失败不会影响其他协程：

fun main() = runBlocking {

    val exceptionHandler = CoroutineExceptionHandler { coroutineContext, throwable ->
        println(throwable)
    }

    val customScope = CoroutineScope(SupervisorJob() + exceptionHandler)

    customScope.launch {
        supervisorScope {
            launch {
                getData1().also { println(it) }
            }
            launch {
                getData2().also { println(it) }
            }
            launch {
                getData3().also { println(it) }
            }
        }
     }.join()
}

private suspend fun getData1(): String? {
    delay(1000)
    return "data 1"
}

private suspend fun getData2(): String? {
    delay(300)
    throw RuntimeException("While getting Data 2 exception happened")
}

private suspend fun getData3(): String? {
    delay(800)
    return "data 3"
}

方式二：让内层协程直接继承SupervisorJob上下文 #

修改内层launch的上下文，让它们直接继承customScope的SupervisorJob，而非外层launch的普通Job：

fun main() = runBlocking {

    val exceptionHandler = CoroutineExceptionHandler { coroutineContext, throwable ->
        println(throwable)
    }

    val customScope = CoroutineScope(SupervisorJob() + exceptionHandler)

    customScope.launch {
        // 直接继承customScope的上下文，包含SupervisorJob
        launch(coroutineContext) {
            getData1().also { println(it) }
        }
        launch(coroutineContext) {
            getData2().also { println(it) }
        }
        launch(coroutineContext) {
            getData3().also { println(it) }
        }
     }.join()
}

private suspend fun getData1(): String? {
    delay(1000)
    return "data 1"
}

private suspend fun getData2(): String? {
    delay(300)
    throw RuntimeException("While getting Data 2 exception happened")
}

private suspend fun getData3(): String? {
    delay(800)
    return "data 3"
}

验证结果 #

修改后运行代码，会输出如下内容，符合预期：

java.lang.RuntimeException: While getting Data 2 exception happened
data 3
data 1

内容的提问来源于stack exchange，提问作者nasibeyyubov