Kotlin协程-try-catch基础

Posted 2022-12-02 且听真言

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在 Kotlin 协程当中，我们通常把异常分为两大类，一类是取消异常（CancellationException），另一类是其他异常。在 Kotlin 协程当中，这两种异常的处理方式是不一样的。

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。

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