Kotlin try catch异常处理i详解
目录
- 一、cancel()无效
- 1.cancel()不被响应
- 2.结构被破坏
- 3.未正确处理 CancellationException
- 二、try-catch不起作用
- 三、SupervisorJob
- 四、CoroutineExceptandroidionHandler
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 =$i") } } delay(2000L) job.cancel() job.join() println("END") } Log
i =1
i =2i =3i =4i =5i =6i =7i =8i =9i =10i =11i =12......程序无法停止
上面的程序无法停止,协程任务的取消,需要互相协作。协程外部取消,协程内部需要做出响应才行。当我们调用 job.cancel() 以后,协程任务已经不是活跃状态了,但代码并没有把 isActive 作为循环条件,因此协程无法真正取消。
可以在协程体中加入状态判断:
runblocking { val job = launch(Dispatchers.Default) { var i = 0 while (isActive) { Thread.sleep(500L) i++ println("i =$i") } } delay(2000L) job.cancel() job.join() println("END") }
Log
i =1i =2i =3i =4END 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:1First i:1First i:2Second i:2Second i:3First i:3Second i:4First i:4EndFirst i:5First i:6First i:7First i:8First i:9First i:10First i:11First i:12First i:13First i:14First 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:1First i:2Second i:2First i:3Second i:3First i:4Second i:4End
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 = $i") } } } delay(2000L) parentJob.cancel() parentJob.join() println("End") }
First i = 1
Second i = 1First i = 2Second i = 2First i = 3Second i = 3End Process finished with exit code 0
delay() 函数可以自动检测当前的协程是否已经被取消,如果已经被取消的话,它会抛出一个 CancellationException,从而终止当前的协程。
runBlocking { val parentJob = launch(Dispatchers.Default) { launch { var i = 0 while (true) { try { delay(500L) } catch (e: CancellationException) { println("Catch CancellationException") throw e } i++ println("First i =$i") } } launch { var i = 0 while (true) { delay(500L) i++ println("Second i = $i") } } } delay(2000L) parentJob.cancel() parentJob.join() println("END") }
Log:
First i =1Second i = 1First i =2Second i = 2First i =3Second i = 3Catch CancellationExceptionEND Process finished with exit code 0
try-catch 包裹了 delay() 以后,打印出“Catch CancellationException”,这就说明 delay() 确实可以自动响应协程的取消,并且产生 CancellationException 异常。
注意:捕获了 CancellationException 以后没有重新抛出去,就导致子协程无法正常取消。
runBlocking { val parentJob = launch(Dispatchers.Default) { launch { var i = 0 while (true) { try { delay(500L) } catch (e: CancellationException) { println("Catch CancellationException") //throw e } i++ println("First i =$i") } } launch { var i = 0 while (true) { delay(500L) i++ println("Second i = $i") } } } delay(2000L) parentJob.cancel() parentJob.join() println("END") }
......
First i =656179Catch CancellationExceptionFirst i =656180Catch CancellationExceptionFirst i =656181Catch CancellationExceptionFirst i =656182Catch CancellationExceptionFirst i =656183Catch 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 zeroEnd 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:$e") } println("End") }
atch:java.lang.ArithmeticException: / by zero
EndException in thread "main" java.lang.ArithmeticException: / by zero at com.example.myapplication.testcoroutinue.TestTryCatchKt$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$default(Unknown Source) at com.example.myapplication.testcoroutinue.TestTryCatchKt.testTryCatch11(TestTryCatch.kt:272) at com.example.myapplication.testcoroutinue.TestTryCatchKt.main(TestTryCatch.kt:16) at com.example.myapplication.testcoroutinue.TestTryCatchKt.main(TestTryCatch.kt) Process finished with exit code 1
await() 如果不调用的话,async 当中的异常是否编程发生?
runBlocking { var deffered = async { delay(100L) 1 / 0 } delay(500L) println("End") }
Exception in thread "main" java.lang.ArithmeticException: / by zero
at com.example.myapplication.testcoroutinue.TestTryCatchKt$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$default(Unknown Source) at com.example.myapplication.testcoroutinue.TestTryCatchKt.testTryCatch12(TestTrphpyCatch.kt:287) at com.example.myapplication.testcoroutinue.TestTryCatchKt.main(TestTryCatch.kt:16) at com.example.myapplication.testcoroutinue.TestTryCatchKt.main(TestTryCatch.kt) Process finished with exit code 1
可见,async 当中产生异常,即使不调用 await() 同样是会导致程序崩溃的。
三、SupervisorJob
使用 try-catch 包裹“deferred.await()”,需要配合 SupervisorJob 一起使用。实现“不调用 await() 就不会产生异常而崩溃”。
unBlocking { val scope = CoroutineScope(SupervisorJob()) scope.async { delay(100L) 1 / 0 } delay(开发者_Go入门500L) println("End") }
Log
End Process finished with exit code 0
使用 SupervisorJob 创建一个 scope 以后,用 scope.async{}启动协程后,只要不调用“deferred.await()”,程序就不会因为异常而崩溃。
runBlocking { val coroutineScope = CoroutineScope(SupervisorJob()) val deferred = coroutineScope.async { delay(100L) 1 / 0 } try { deferred.await() } catch (e: Exception) { println("Catch:$e") } delay(500L) println("End") }
Log
Catch:java.lang.ArithmeticException: / by zeroEnd 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$2.invokeSuspend(TestTryCatch.kt:338) at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33) &nbPqXsniZBjsp; at kotlinx.coroutines.DispatchedTaskKt.resume(DispatchedTask.kt:234) at kotlinx.coroutines.DispatchedTaskKt.dispatch(DispatchedTask.kt:166) at kotlinx.coroutines.CancellableContinuationImpl.dispatchResume(CancellableContinuationImpl.kt:397) at kotlinx.coroutines.CancellableContinuationImpl.resumeImpl(CancellableContinuationImpl.kt:431) at kotlinx.coroutines.CancellableContinuationImpl.resumeImpl$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 -> print编程客栈ln("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 zeroEND 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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