Kotlin try catch异常处理i详解

且听真言 2022-11-25 我要评论

一、cancel()无效

当协程任务被取消的时候，它的内部是会产生一个 CancellationException 的。而协程的结构化并发，最大的优势就在于：如果我们取消了父协程，子协程也会跟着被取消。

1.cancel()不被响应

      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
......
程序无法停止

上面的程序无法停止，协程任务的取消，需要互相协作。协程外部取消，协程内部需要做出响应才行。当我们调用 job.cancel() 以后，协程任务已经不是活跃状态了，但代码并没有把 isActive 作为循环条件，因此协程无法真正取消。

可以在协程体中加入状态判断：

 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 循环的条件改成了 while (isActive)，这就意味着，只有协程处于活跃状态的时候，才会继续执行循环体内部的代码。协程的取消需要内部的配合。

2.结构被破坏

协程是结构化的，当我们取消父协程的时候，子协程也会跟着被取消。

但是特殊情况是嵌套创建的子协程并不会跟随父协程一起取消。

 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
......

可以发现，创建子协程的时候，使用了 launch(Job()){}，就打破了原有的协程结构。因为 launch(Job()){}创建的协程的父 Job 是在 launch 当中传入的 Job() 对象。所以调用 parentJob.cancel() 的时候，无法销毁该协程。

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 与它内部的子协程之间都是父子关系，因此它们两个都是会响应协程取消的事件的。不要轻易打破协程的父子结构！

3.未正确处理 CancellationException

对于 Kotlin 提供的挂起函数，可以自动响应协程的取消。

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() 函数可以自动检测当前的协程是否已经被取消，如果已经被取消的话，它会抛出一个 CancellationException，从而终止当前的协程。

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”，这就说明 delay() 确实可以自动响应协程的取消，并且产生 CancellationException 异常。

注意：捕获了 CancellationException 以后没有重新抛出去，就导致子协程无法正常取消。

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，捕获了 CancellationException 以后，要考虑是否应该重新抛出来。

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

可以发现：try-catch 并没有成功捕获异常，程序等待了 100 毫秒左右，最终还是崩溃了。

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

当协程体当中的“1/0”执行的时候，程序已经跳出 try-catch 的作用域了,所以 try-catch失效。

把 try-catch 挪到 launch{} 协程体内部。可以正常捕获到 ArithmeticException 这个异常了。

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() 如果不调用的话，async 当中的异常是否发生？

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

可见，async 当中产生异常，即使不调用 await() 同样是会导致程序崩溃的。

三、SupervisorJob

使用 try-catch 包裹“deferred.await()”，需要配合 SupervisorJob 一起使用。实现“不调用 await() 就不会产生异常而崩溃”。

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 创建一个 scope 以后，用 scope.async{}启动协程后，只要不调用“deferred.await()”，程序就不会因为异常而崩溃。

  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 {}”创建了协程，同时也用 try-catch 包裹“deferred.await()”，这样一来，异常就成功地被捕获了。

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

SupervisorJob() 不是构造函数，它只是一个普通的顶层函数。这个方法返回的对象，是 Job 的子类。SupervisorJob 与 Job 最大的区别就在于，当它的子 Job 发生异常的时候，其他的子 Job 不会受到牵连。

对于普通 Job，出现异常时的应对策略是：由于 parentJob 是一个普通的 Job 对象，当 job1 发生异常之后，它会导致 parentJob 取消，进而导致 job2、job3 也受到牵连。

如果把 parentJob 改为 SupervisorJob，job1 发生异常的的话，就不会影响到其他的 Job 了。

注意：灵活使用 SupervisorJob，控制异常传播的范围。

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处理上述代码中的异常。

  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

定义了一个 CoroutineExceptionHandler，然后把它传入了 scope 当中，就可以捕获其中所有的异常了。

注意点：在特定场景，为什么 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

把自定义的 myExceptionHandler，放到出现异常的 launch 那里传了进去。myExceptionHandler 并不会起作用，异常不会被它捕获。注意：myExceptionHandler 直接定义在发生异常的位置反而不生效，而定义在最顶层却可以生效。因为只在顶层的协程当中才会起作用。也就是说，当子协程当中出现异常以后，它们都会统一上报给顶层的父协程，然后顶层的父协程才会去调用 CoroutineExceptionHandler，来处理对应的异常。所以需要记住：使用 CoroutineExceptionHandler 处理复杂结构的协程异常，它仅在顶层协程中起作用。