我们什么时候使用 launch(SupervisorJob())？

Question

我看过将 SupervisorJob 传递给 CoroutineScope 的教程，以避免在其中一个子程序失败时取消所有协程作业。在run3中，我认为将SupervisorJob传递给launch可以得到相同的结果，但显然不是。它似乎允许在出现异常时重用协程（如果您从 launch 中删除 SupervisorJob，第二个 run2 调用将不会运行协程作业），但它的行为不像 supervisorScope，它的其他子作业可以继续（在示例中是第一个 test1.run 调用）。我想知道在什么场景下我们可以使用这种方式？因为将它传递给 launch 构造函数看起来是合法的。

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传递了 SupervisorJob，而是您传递了 any 类型的 Job。尝试用 + Job() 替换 + SupervisorJob()，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，它会成为已启动协程的父作业，而不是 TestReuseCoroutineAfterException4 中的主作业。因此，协程失败不会取消主作业，并且影响仅限于单个调用。

不鼓励将作业直接传递给 launch，因为它会破坏结构化并发并创建您遇到的奇怪语义。

我们什么时候使用 launch(SupervisorJob())？

When do we use launch(SupervisorJob())?

kotlin

kotlin-coroutines