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Kotlin?try?catch異常處理i詳解_Android

作者：且聽真言 ? 更新時間： 2022-12-23 編程語言

一、cancel()無效

當協程任務被取消的時候，它的內部是會產生一個 CancellationException 的。而協程的結構化并發，最大的優勢就在于：如果我們取消了父協程，子協程也會跟著被取消。

1.cancel()不被響應

      val job = launch(Dispatchers.Default) {
            var i = 0
            while (true) {
                Thread.sleep(500L)
                i++
                println("i =$i")
            }
        }
        delay(2000L)
        job.cancel()
        job.join()
        println("END")
    }
Log

i =1
i =2
i =3
i =4
i =5
i =6
i =7
i =8
i =9
i =10
i =11
i =12
......
程序無法停止

上面的程序無法停止，協程任務的取消，需要互相協作。協程外部取消，協程內部需要做出響應才行。當我們調用 job.cancel() 以后，協程任務已經不是活躍狀態了，但代碼并沒有把 isActive 作為循環條件，因此協程無法真正取消。

可以在協程體中加入狀態判斷：

 runBlocking {
        val job = launch(Dispatchers.Default) {
            var i = 0
            while (isActive) {
                Thread.sleep(500L)
                i++
                println("i =$i")
            }
        }
        delay(2000L)
        job.cancel()
        job.join()
        println("END")
    }

Log
?
i =1
i =2
i =3
i =4
END
?
Process finished with exit code 0

把 while 循環的條件改成了 while (isActive)，這就意味著，只有協程處于活躍狀態的時候，才會繼續執行循環體內部的代碼。協程的取消需要內部的配合。

2.結構被破壞

協程是結構化的，當我們取消父協程的時候，子協程也會跟著被取消。

但是特殊情況是嵌套創建的子協程并不會跟隨父協程一起取消。

 runBlocking {
        val parentJob = launch(fixedDispatcher) {
            launch(Job()) {
                var i = 0
                while (isActive) {
                    Thread.sleep(500L)
                    i++
                    println("First i:$i")
                }
            }
            launch {
                var i = 0
                while (isActive) {
                    Thread.sleep(500L)
                    i++
                    println("Second i:$i")
                }
            }
        }
        delay(2000L)
        parentJob.cancel()
        parentJob.join()
        println("End")
    }

Log
?
Second i:1
First i:1
First i:2
Second i:2
Second i:3
First i:3
Second i:4
First i:4
End
First i:5
First i:6
First i:7
First i:8
First i:9
First i:10
First i:11
First i:12
First i:13
First i:14
First i:15
......

可以發現，創建子協程的時候，使用了 launch(Job()){}，就打破了原有的協程結構。因為 launch(Job()){}創建的協程的父 Job 是在 launch 當中傳入的 Job() 對象。所以調用 parentJob.cancel() 的時候，無法銷毀該協程。

可以按如下修改：

 runBlocking {
        val parentJob = launch(fixedDispatcher) {
            launch {
                var i = 0
                while (isActive) {
                    Thread.sleep(500L)
                    i++
                    println("First i:$i")
                }
            }
            launch {
                var i = 0
                while (isActive) {
                    Thread.sleep(500L)
                    i++
                    println("Second i:$i")
                }
            }
        }
        delay(2000L)
        parentJob.cancel()
        parentJob.join()
        println("End")
    }

First i:1
Second i:1
First i:2
Second i:2
First i:3
Second i:3
First i:4
Second i:4
End

parentJob 與它內部的子協程之間都是父子關系，因此它們兩個都是會響應協程取消的事件的。不要輕易打破協程的父子結構！

3.未正確處理 CancellationException

對于 Kotlin 提供的掛起函數，可以自動響應協程的取消。

例如：

    runBlocking {
        val parentJob = launch(Dispatchers.Default) {
            launch {
                var i = 0
                while (true) {
                    delay(500L)
                    i++
                    println("First i = $i")
                }
            }
            launch {
                var i = 0
                while (true) {
                    delay(500L)
                    i++
                    println("Second i = $i")
                }
            }
        }
        delay(2000L)
        parentJob.cancel()
        parentJob.join()
        println("End")
    }

First i = 1
Second i = 1
First i = 2
Second i = 2
First i = 3
Second i = 3
End
?
Process finished with exit code 0

delay() 函數可以自動檢測當前的協程是否已經被取消，如果已經被取消的話，它會拋出一個 CancellationException，從而終止當前的協程。

runBlocking {
        val parentJob = launch(Dispatchers.Default) {
            launch {
                var i = 0
                while (true) {
                    try {
                        delay(500L)
                    } catch (e: CancellationException) {
                        println("Catch CancellationException")
                        throw e
                    }
                    i++
                    println("First i =$i")
                }
            }
            launch {
                var i = 0
                while (true) {
                    delay(500L)
                    i++
                    println("Second i = $i")
                }
            }
        }
        delay(2000L)
        parentJob.cancel()
        parentJob.join()
        println("END")
    }

Log:
?
First i =1
Second i = 1
First i =2
Second i = 2
First i =3
Second i = 3
Catch CancellationException
END
?
Process finished with exit code 0

try-catch 包裹了 delay() 以后，打印出“Catch CancellationException”，這就說明 delay() 確實可以自動響應協程的取消，并且產生 CancellationException 異常。

注意：捕獲了 CancellationException 以后沒有重新拋出去，就導致子協程無法正常取消。

runBlocking {
        val parentJob = launch(Dispatchers.Default) {
            launch {
                var i = 0
                while (true) {
                    try {
                        delay(500L)
                    } catch (e: CancellationException) {
                        println("Catch CancellationException")
                        //throw e
                    }
                    i++
                    println("First i =$i")
                }
            }
            launch {
                var i = 0
                while (true) {
                    delay(500L)
                    i++
                    println("Second i = $i")
                }
            }
        }
        delay(2000L)
        parentJob.cancel()
        parentJob.join()
        println("END")
    }

......
First i =656179
Catch CancellationException
First i =656180
Catch CancellationException
First i =656181
Catch CancellationException
First i =656182
Catch CancellationException
First i =656183
Catch CancellationException
.....

所以，捕獲了 CancellationException 以后，要考慮是否應該重新拋出來。

二、try-catch不起作用

runBlocking {
        try {
            launch {
                delay(100L)
                1 / 0
            }
        } catch (e: Exception) {
            println("catch: $e")
        }
        delay(500L)
        println("End")
    }

Log
Exception in thread "main" java.lang.ArithmeticException: / by zero
?? ?at com.example.myapplication.testcoroutinue.TestTryCatchKt$testTryCatch8$1$1.invokeSuspend(TestTryCatch.kt:225)
?? ?at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
?? ?at kotlinx.coroutines.DispatchedTaskKt.resume(DispatchedTask.kt:234)
?? ?at kotlinx.coroutines.DispatchedTaskKt.dispatch(DispatchedTask.kt:166)
?? ?at kotlinx.coroutines.CancellableContinuationImpl.dispatchResume(CancellableContinuationImpl.kt:397)
?? ?at kotlinx.coroutines.CancellableContinuationImpl.resumeImpl(CancellableContinuationImpl.kt:431)
?? ?at kotlinx.coroutines.CancellableContinuationImpl.resumeImpl$default(CancellableContinuationImpl.kt:420)
?? ?at kotlinx.coroutines.CancellableContinuationImpl.resumeUndispatched(CancellableContinuationImpl.kt:518)
?? ?at kotlinx.coroutines.EventLoopImplBase$DelayedResumeTask.run(EventLoop.common.kt:494)
?? ?at kotlinx.coroutines.EventLoopImplBase.processNextEvent(EventLoop.common.kt:279)
?? ?at kotlinx.coroutines.BlockingCoroutine.joinBlocking(Builders.kt:85)
?? ?at kotlinx.coroutines.BuildersKt__BuildersKt.runBlocking(Builders.kt:59)
?? ?at kotlinx.coroutines.BuildersKt.runBlocking(Unknown Source)
?? ?at kotlinx.coroutines.BuildersKt__BuildersKt.runBlocking$default(Builders.kt:38)
?? ?at kotlinx.coroutines.BuildersKt.runBlocking$default(Unknown Source)
?? ?at com.example.myapplication.testcoroutinue.TestTryCatchKt.testTryCatch8(TestTryCatch.kt:221)
?? ?at com.example.myapplication.testcoroutinue.TestTryCatchKt.main(TestTryCatch.kt:15)
?? ?at com.example.myapplication.testcoroutinue.TestTryCatchKt.main(TestTryCatch.kt)
Process finished with exit code 1

可以發現：try-catch 并沒有成功捕獲異常，程序等待了 100 毫秒左右，最終還是崩潰了。

使用async

runBlocking {
        var deffered: Deferred<Any>? = null
        try {
            deffered = async {
                delay(100L)
                1 / 0
            }
        } catch (e: ArithmeticException) {
            println("Catch:$e")
        }
        deffered?.await()
        println("End")
    }

Exception in thread "main" java.lang.ArithmeticException: / by zero
?? ?at com.example.myapplication.testcoroutinue.TestTryCatchKt$testTryCatch9$1$1.invokeSuspend(TestTryCatch.kt:242)
?? ?at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
?? ?at kotlinx.coroutines.DispatchedTaskKt.resume(DispatchedTask.kt:234)
?? ?at kotlinx.coroutines.DispatchedTaskKt.dispatch(DispatchedTask.kt:166)
?? ?at kotlinx.coroutines.CancellableContinuationImpl.dispatchResume(CancellableContinuationImpl.kt:397)
?? ?at kotlinx.coroutines.CancellableContinuationImpl.resumeImpl(CancellableContinuationImpl.kt:431)
?? ?at kotlinx.coroutines.CancellableContinuationImpl.resumeImpl$default(CancellableContinuationImpl.kt:420)
?? ?at kotlinx.coroutines.CancellableContinuationImpl.resumeUndispatched(CancellableContinuationImpl.kt:518)
?? ?at kotlinx.coroutines.EventLoopImplBase$DelayedResumeTask.run(EventLoop.common.kt:494)
?? ?at kotlinx.coroutines.EventLoopImplBase.processNextEvent(EventLoop.common.kt:279)
?? ?at kotlinx.coroutines.BlockingCoroutine.joinBlocking(Builders.kt:85)
?? ?at kotlinx.coroutines.BuildersKt__BuildersKt.runBlocking(Builders.kt:59)
?? ?at kotlinx.coroutines.BuildersKt.runBlocking(Unknown Source)
?? ?at kotlinx.coroutines.BuildersKt__BuildersKt.runBlocking$default(Builders.kt:38)
?? ?at kotlinx.coroutines.BuildersKt.runBlocking$default(Unknown Source)
?? ?at com.example.myapplication.testcoroutinue.TestTryCatchKt.testTryCatch9(TestTryCatch.kt:237)
?? ?at com.example.myapplication.testcoroutinue.TestTryCatchKt.main(TestTryCatch.kt:16)
?? ?at com.example.myapplication.testcoroutinue.TestTryCatchKt.main(TestTryCatch.kt)
Process finished with exit code 1

當協程體當中的“1/0”執行的時候，程序已經跳出 try-catch 的作用域了,所以 try-catch失效。

把 try-catch 挪到 launch{} 協程體內部。可以正常捕獲到 ArithmeticException 這個異常了。

runBlocking {
        var deffered: Deferred<Any>? = null
        deffered = async {
            try {
                delay(100L)
                1 / 0
            } catch (e: ArithmeticException) {
                println("Catch:$e")
            }
        }
        deffered.await()
        println("End")
    }

Log
?
Catch:java.lang.ArithmeticException: / by zero
End
?
Process finished with exit code 0

注意：不要用 try-catch 直接包裹 launch、async。

使用 try-catch 包裹“deferred.await()”。

例：

runBlocking {
        var deffered = async {
            delay(100L)
            1 / 0
        }
        try {
            deffered.await()
        } catch (e: Exception) {
            println("Catch:$e")
        }
        println("End")
    }

atch:java.lang.ArithmeticException: / by zero
End
Exception in thread "main" java.lang.ArithmeticException: / by zero
?? ?at com.example.myapplication.testcoroutinue.TestTryCatchKt$testTryCatch11$1$deffered$1.invokeSuspend(TestTryCatch.kt:275)
?? ?at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
?? ?at kotlinx.coroutines.DispatchedTaskKt.resume(DispatchedTask.kt:234)
?? ?at kotlinx.coroutines.DispatchedTaskKt.dispatch(DispatchedTask.kt:166)
?? ?at kotlinx.coroutines.CancellableContinuationImpl.dispatchResume(CancellableContinuationImpl.kt:397)
?? ?at kotlinx.coroutines.CancellableContinuationImpl.resumeImpl(CancellableContinuationImpl.kt:431)
?? ?at kotlinx.coroutines.CancellableContinuationImpl.resumeImpl$default(CancellableContinuationImpl.kt:420)
?? ?at kotlinx.coroutines.CancellableContinuationImpl.resumeUndispatched(CancellableContinuationImpl.kt:518)
?? ?at kotlinx.coroutines.EventLoopImplBase$DelayedResumeTask.run(EventLoop.common.kt:494)
?? ?at kotlinx.coroutines.EventLoopImplBase.processNextEvent(EventLoop.common.kt:279)
?? ?at kotlinx.coroutines.BlockingCoroutine.joinBlocking(Builders.kt:85)
?? ?at kotlinx.coroutines.BuildersKt__BuildersKt.runBlocking(Builders.kt:59)
?? ?at kotlinx.coroutines.BuildersKt.runBlocking(Unknown Source)
?? ?at kotlinx.coroutines.BuildersKt__BuildersKt.runBlocking$default(Builders.kt:38)
?? ?at kotlinx.coroutines.BuildersKt.runBlocking$default(Unknown Source)
?? ?at com.example.myapplication.testcoroutinue.TestTryCatchKt.testTryCatch11(TestTryCatch.kt:272)
?? ?at com.example.myapplication.testcoroutinue.TestTryCatchKt.main(TestTryCatch.kt:16)
?? ?at com.example.myapplication.testcoroutinue.TestTryCatchKt.main(TestTryCatch.kt)
?
Process finished with exit code 1

await() 如果不調用的話，async 當中的異常是否發生？

runBlocking {
        var deffered = async {
            delay(100L)
            1 / 0
        }
        delay(500L)
        println("End")
    }

Exception in thread "main" java.lang.ArithmeticException: / by zero
?? ?at com.example.myapplication.testcoroutinue.TestTryCatchKt$testTryCatch12$1$deffered$1.invokeSuspend(TestTryCatch.kt:290)
?? ?at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
?? ?at kotlinx.coroutines.DispatchedTaskKt.resume(DispatchedTask.kt:234)
?? ?at kotlinx.coroutines.DispatchedTaskKt.dispatch(DispatchedTask.kt:166)
?? ?at kotlinx.coroutines.CancellableContinuationImpl.dispatchResume(CancellableContinuationImpl.kt:397)
?? ?at kotlinx.coroutines.CancellableContinuationImpl.resumeImpl(CancellableContinuationImpl.kt:431)
?? ?at kotlinx.coroutines.CancellableContinuationImpl.resumeImpl$default(CancellableContinuationImpl.kt:420)
?? ?at kotlinx.coroutines.CancellableContinuationImpl.resumeUndispatched(CancellableContinuationImpl.kt:518)
?? ?at kotlinx.coroutines.EventLoopImplBase$DelayedResumeTask.run(EventLoop.common.kt:494)
?? ?at kotlinx.coroutines.EventLoopImplBase.processNextEvent(EventLoop.common.kt:279)
?? ?at kotlinx.coroutines.BlockingCoroutine.joinBlocking(Builders.kt:85)
?? ?at kotlinx.coroutines.BuildersKt__BuildersKt.runBlocking(Builders.kt:59)
?? ?at kotlinx.coroutines.BuildersKt.runBlocking(Unknown Source)
?? ?at kotlinx.coroutines.BuildersKt__BuildersKt.runBlocking$default(Builders.kt:38)
?? ?at kotlinx.coroutines.BuildersKt.runBlocking$default(Unknown Source)
?? ?at com.example.myapplication.testcoroutinue.TestTryCatchKt.testTryCatch12(TestTryCatch.kt:287)
?? ?at com.example.myapplication.testcoroutinue.TestTryCatchKt.main(TestTryCatch.kt:16)
?? ?at com.example.myapplication.testcoroutinue.TestTryCatchKt.main(TestTryCatch.kt)
?
Process finished with exit code 1

可見，async 當中產生異常，即使不調用 await() 同樣是會導致程序崩潰的。

三、SupervisorJob

使用 try-catch 包裹“deferred.await()”，需要配合 SupervisorJob 一起使用。實現“不調用 await() 就不會產生異常而崩潰”。

unBlocking {
        val scope = CoroutineScope(SupervisorJob())
        scope.async {
            delay(100L)
            1 / 0
        }
        delay(500L)
        println("End")
    }

Log
?
End
?
Process finished with exit code 0

使用 SupervisorJob 創建一個 scope 以后，用 scope.async{}啟動協程后，只要不調用“deferred.await()”，程序就不會因為異常而崩潰。

  runBlocking {
        val coroutineScope = CoroutineScope(SupervisorJob())
        val deferred = coroutineScope.async {
            delay(100L)
            1 / 0
        }
        try {
            deferred.await()
        } catch (e: Exception) {
            println("Catch:$e")
        }
        delay(500L)
        println("End")
    }

Log
Catch:java.lang.ArithmeticException: / by zero
End
?
Process finished with exit code 0

使用“coroutineScope.async {}”創建了協程，同時也用 try-catch 包裹“deferred.await()”，這樣一來，異常就成功地被捕獲了。

public fun SupervisorJob(parent: Job? = null) : CompletableJob 
                    = SupervisorJobImpl(parent)
public interface CompletableJob : Job {
    public fun complete(): Boolean
    public fun completeExceptionally(exception: Throwable): Boolean
}

SupervisorJob() 不是構造函數，它只是一個普通的頂層函數。這個方法返回的對象，是 Job 的子類。SupervisorJob 與 Job 最大的區別就在于，當它的子 Job 發生異常的時候，其他的子 Job 不會受到牽連。

對于普通 Job， 出現異常時的應對策略是：由于 parentJob 是一個普通的 Job 對象，當 job1 發生異常之后，它會導致 parentJob 取消，進而導致 job2、job3 也受到牽連。

如果把 parentJob 改為 SupervisorJob，job1 發生異常的的話，就不會影響到其他的 Job 了。

注意：靈活使用 SupervisorJob，控制異常傳播的范圍。

四、CoroutineExceptionHandler

runBlocking {
        val coroutineScope = CoroutineScope(coroutineContext)
        coroutineScope.launch {
            async {
                delay(100L)
            }
            launch {
                delay(100L)
                1/0
            }
        }
        delay(1000L)
        println("END")
    }

Exception in thread "main" java.lang.ArithmeticException: / by zero
?? ?at com.example.myapplication.testcoroutinue.TestTryCatchKt$testTryCatch15$1$1$2.invokeSuspend(TestTryCatch.kt:338)
?? ?at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
?? ?at kotlinx.coroutines.DispatchedTaskKt.resume(DispatchedTask.kt:234)
?? ?at kotlinx.coroutines.DispatchedTaskKt.dispatch(DispatchedTask.kt:166)
?? ?at kotlinx.coroutines.CancellableContinuationImpl.dispatchResume(CancellableContinuationImpl.kt:397)
?? ?at kotlinx.coroutines.CancellableContinuationImpl.resumeImpl(CancellableContinuationImpl.kt:431)
?? ?at kotlinx.coroutines.CancellableContinuationImpl.resumeImpl$default(CancellableContinuationImpl.kt:420)
?? ?at kotlinx.coroutines.CancellableContinuationImpl.resumeUndispatched(CancellableContinuationImpl.kt:518)
?? ?at kotlinx.coroutines.EventLoopImplBase$DelayedResumeTask.run(EventLoop.common.kt:494)
?? ?at kotlinx.coroutines.EventLoopImplBase.processNextEvent(EventLoop.common.kt:279)
?? ?at kotlinx.coroutines.BlockingCoroutine.joinBlocking(Builders.kt:85)
?? ?at kotlinx.coroutines.BuildersKt__BuildersKt.runBlocking(Builders.kt:59)
?? ?at kotlinx.coroutines.BuildersKt.runBlocking(Unknown Source)
?? ?at kotlinx.coroutines.BuildersKt__BuildersKt.runBlocking$default(Builders.kt:38)
?? ?at kotlinx.coroutines.BuildersKt.runBlocking$default(Unknown Source)
?? ?at com.example.myapplication.testcoroutinue.TestTryCatchKt.testTryCatch15(TestTryCatch.kt:329)
?? ?at com.example.myapplication.testcoroutinue.TestTryCatchKt.main(TestTryCatch.kt:16)
?? ?at com.example.myapplication.testcoroutinue.TestTryCatchKt.main(TestTryCatch.kt)
?
Process finished with exit code 1

使用CoroutineExceptionHandler處理上述代碼中的異常。

  runBlocking {
        val coroutineExceptionHandler = CoroutineExceptionHandler { _, throwable ->
            println("Catch: $throwable")
        }
        val coroutineScope = CoroutineScope(coroutineContext + Job() + coroutineExceptionHandler)
        coroutineScope.launch {
            async {
                delay(100L)
            }
            launch {
                delay(100L)
                1 / 0
            }
        }
        delay(1000L)
        println("END")
    }

Log
Catch: java.lang.ArithmeticException: / by zero
END
?
Process finished with exit code 0

定義了一個 CoroutineExceptionHandler，然后把它傳入了 scope 當中，就可以捕獲其中所有的異常了。

注意點：在特定場景，為什么 CoroutineExceptionHandler 不起作用？

runBlocking {
        val coroutineExceptionHandler = CoroutineExceptionHandler { _, throwable ->
            println("Catch: $throwable")
        }
        val coroutineScope = CoroutineScope(coroutineContext)
        coroutineScope.launch {
            async {
                delay(100L)
            }
            launch(coroutineExceptionHandler) {
                delay(100L)
                1 / 0
            }
        }
        delay(1000L)
        println("END")
    }

Exception in thread "main" java.lang.ArithmeticException: / by zero
?? ?at com.example.myapplication.testcoroutinue.TestTryCatchKt$testTryCatch17$1$1$2.invokeSuspend(TestTryCatch.kt:383)
?? ?at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
?? ?at kotlinx.coroutines.DispatchedTaskKt.resume(DispatchedTask.kt:234)
?? ?at kotlinx.coroutines.DispatchedTaskKt.dispatch(DispatchedTask.kt:166)
?? ?at kotlinx.coroutines.CancellableContinuationImpl.dispatchResume(CancellableContinuationImpl.kt:397)
?? ?at kotlinx.coroutines.CancellableContinuationImpl.resumeImpl(CancellableContinuationImpl.kt:431)
?? ?at kotlinx.coroutines.CancellableContinuationImpl.resumeImpl$default(CancellableContinuationImpl.kt:420)
?? ?at kotlinx.coroutines.CancellableContinuationImpl.resumeUndispatched(CancellableContinuationImpl.kt:518)
?? ?at kotlinx.coroutines.EventLoopImplBase$DelayedResumeTask.run(EventLoop.common.kt:494)
?? ?at kotlinx.coroutines.EventLoopImplBase.processNextEvent(EventLoop.common.kt:279)
?? ?at kotlinx.coroutines.BlockingCoroutine.joinBlocking(Builders.kt:85)
?? ?at kotlinx.coroutines.BuildersKt__BuildersKt.runBlocking(Builders.kt:59)
?? ?at kotlinx.coroutines.BuildersKt.runBlocking(Unknown Source)
?? ?at kotlinx.coroutines.BuildersKt__BuildersKt.runBlocking$default(Builders.kt:38)
?? ?at kotlinx.coroutines.BuildersKt.runBlocking$default(Unknown Source)
?? ?at com.example.myapplication.testcoroutinue.TestTryCatchKt.testTryCatch17(TestTryCatch.kt:371)
?? ?at com.example.myapplication.testcoroutinue.TestTryCatchKt.main(TestTryCatch.kt:16)
?? ?at com.example.myapplication.testcoroutinue.TestTryCatchKt.main(TestTryCatch.kt)
?
Process finished with exit code 1

把自定義的 myExceptionHandler，放到出現異常的 launch 那里傳了進去。myExceptionHandler 并不會起作用，異常不會被它捕獲。注意：myExceptionHandler 直接定義在發生異常的位置反而不生效，而定義在最頂層卻可以生效。因為只在頂層的協程當中才會起作用。也就是說，當子協程當中出現異常以后，它們都會統一上報給頂層的父協程，然后頂層的父協程才會去調用 CoroutineExceptionHandler，來處理對應的異常。所以需要記住：使用 CoroutineExceptionHandler 處理復雜結構的協程異常，它僅在頂層協程中起作用。

原文鏈接：https://blog.csdn.net/zhangying1994/article/details/127610104

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