kotlin - 我们什么时候使用launch(Supervisor Job())？

Question

我已经看到了将SupervisorJob传递给CoroutineScope的教程，以避免在子例程之一失败时所有协程作业被取消。
在run3中，我认为将SupervisorJob传递给launch可以获得相同的结果，但显然没有。似乎允许协程在出现异常的情况下被重用(如果您从SupervisorJob中删除launch，则第二个run2调用将不会运行协程作业)，但是它的行为不像supervisorScope，后者的其他子级作业可以继续(在示例中，第一个test1.run调用)。我想知道在什么情况下可以使用这种方式？因为将它传递给launch构造函数看起来很合法。

package coroutine.exceptions

import kotlinx.coroutines.*

fun log(msg: String) = println("$msg (${Thread.currentThread().name})")

val logExceptionHandler = CoroutineExceptionHandler { _, e ->
    log(e.localizedMessage)
}

fun main() = runBlocking {

    TestReuseCoroutineAfterException4("test1").run {
        run1(true)
        delay(100)
        println()

        run1(false)
        delay(100)
    }

    println("================================================================")

    TestReuseCoroutineAfterException4("test2").run {
        run2(true)
        delay(100)
        println()

        run2(false)
        delay(100)
    }

    println("================================================================")

    TestReuseCoroutineAfterException4("test3").run {
        run3(true)
        delay(100)
        println()

        run3(false)
        delay(100)
        println()
    }

    log("finished")
}

class TestReuseCoroutineAfterException4(
    private val testName: String
) : CoroutineScope by CoroutineScope(CoroutineName(testName)) {

    // by passing a Job, we can let the exception propagate to this coroutine scope instead of the
    // root one, which allows us to reuse the root scope.
    fun run1(throwException: Boolean) = launch(logExceptionHandler + Job()) {

        val logPrefix = "$testName.run1:"

        coroutineScope {

            launch {

                launch {
                    if (throwException)
                        throw RuntimeException("$logPrefix throw exception")
                    else
                        log("$logPrefix done (job#1-1)")
                }.join()

                launch {
                    log("$logPrefix done (job#1-2)")
                }.join()


                log("$logPrefix done (job#1)")

            }.join()

            launch {
                log("$logPrefix done (job#2)")
            }.join()
        }
    }

    suspend fun run2(throwException: Boolean) {

        val logPrefix = "$testName.run2:"

        supervisorScope {

            launch(logExceptionHandler) {

                launch {
                    if (throwException)
                        throw Exception("$logPrefix throw exception")
                    else
                        log("$logPrefix done (job#1-1)")
                }.join()

                launch {
                    log("$logPrefix done (job#1-2)")
                }.join()


                log("$logPrefix done (job#1)")

            }.join()

            // this will be run.
            launch {
                log("$logPrefix done (job#2)")
            }.join()
        }

    }

    fun run3(throwException: Boolean) {

        val logPrefix = "$testName.run3:"

        launch(logExceptionHandler + SupervisorJob()) {

            launch {

                launch {
                    if (throwException)
                        throw Exception("$logPrefix throw exception")
                    else
                        log("$logPrefix done (job#1-1)")
                }.join()

                launch {
                    log("$logPrefix done (job#1-2)")
                }.join()


                log("$logPrefix done (job#1)")

            }.join()

            // this will still be run.
            launch {
                log("$logPrefix done (job#2)")
            }.join()
        }

    }

}

输出

test1.run1: throw exception (DefaultDispatcher-worker-2 @test1#2)

test1.run1: done (job#1-1) (DefaultDispatcher-worker-2 @test1#7)
test1.run1: done (job#1-2) (DefaultDispatcher-worker-2 @test1#8)
test1.run1: done (job#1) (DefaultDispatcher-worker-2 @test1#6)
test1.run1: done (job#2) (DefaultDispatcher-worker-2 @test1#9)
================================================================
test2.run2: throw exception (main @coroutine#10)
test2.run2: done (job#2) (main @coroutine#12)

test2.run2: done (job#1-1) (main @coroutine#14)
test2.run2: done (job#1-2) (main @coroutine#15)
test2.run2: done (job#1) (main @coroutine#13)
test2.run2: done (job#2) (main @coroutine#16)
================================================================
test3.run3: throw exception (DefaultDispatcher-worker-2 @test3#18)

test3.run3: done (job#1-1) (DefaultDispatcher-worker-4 @test3#22)
test3.run3: done (job#1-2) (DefaultDispatcher-worker-4 @test3#23)
test3.run3: done (job#1) (DefaultDispatcher-worker-4 @test3#21)
test3.run3: done (job#2) (DefaultDispatcher-worker-4 @test3#24)

finished (main @coroutine#1)

Process finished with exit code 0

Answer 1

if you remove the SupervisorJob from launch, second run2 call won't run the coroutine job

此行为的原因不是您要传递 SupervisorJob，而是要向其传递任何类型的 Job。尝试用 + SupervisorJob()替换 + Job()，第二次调用 run2()将执行协程。

主要区别在于，当您将显式作业传递给 launch时，它将成为已启动协程的父级作业，而不是 TestReuseCoroutineAfterException4中的主作业。因此，协程失败不会取消主作业，并且效果只限于一次调用。

不建议将工作直接传递给 launch，因为这会破坏结构化并发并创建您遇到的怪异语义。