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Vavr Try.filter NoSuchElementException

分类：编程问答

如何解决Vavr Try.filter NoSuchElementException

@Value.Immutable
interface TestCliConfiguration extends CliConfiguration {

  default Path getConfigDir() {
    return Try.ofsupplier( this::getClass )
      .map( Class::getClassLoader )
      .map( cl -> cl.getResource( "config" ) )
      .filter( Objects::nonNull )
      .mapTry( URL::toURI )
      .map( Path::of )
      .mapFailure(
        API.Case( API.$( Predicates.instanceOf( URIException.class ) ),Function.identity() ) )
      .get();
  }

}

正在调用 filter

Predicate does not hold for null
java.util.NoSuchElementException: Predicate does not hold for null
    at io.vavr.control.Try.lambda$filterTry$0(Try.java:459)
    at io.vavr.control.Try.filterTry(Try.java:421)
    at io.vavr.control.Try.filterTry(Try.java:459)
    at io.vavr.control.Try.filter(Try.java:395)
    at com.xenoterracide.brix.TestCliConfiguration.getConfigDir(TestCliConfiguration.java:26)
    at com.xenoterracide.brix.ConfigLoader.<init>(ConfigLoader.java:40)
    at com.xenoterracide.brix.ConfigLoaderTest.home(ConfigLoaderTest.java:15)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:566)
    at org.junit.platform.commons.util.ReflectionUtils.invokeMethod(ReflectionUtils.java:688)
    at org.junit.jupiter.engine.execution.MethodInvocation.proceed(MethodInvocation.java:60)
    at org.junit.jupiter.engine.execution.InvocationInterceptorChain$ValidatingInvocation.proceed(InvocationInterceptorChain.java:131)
    at org.junit.jupiter.engine.extension.TimeoutExtension.intercept(TimeoutExtension.java:149)
    at org.junit.jupiter.engine.extension.TimeoutExtension.interceptTestableMethod(TimeoutExtension.java:140)
    at org.junit.jupiter.engine.extension.TimeoutExtension.interceptTestMethod(TimeoutExtension.java:84)
    at org.junit.jupiter.engine.execution.ExecutableInvoker$ReflectiveInterceptorCall.lambda$ofVoidMethod$0(ExecutableInvoker.java:115)
    at org.junit.jupiter.engine.execution.ExecutableInvoker.lambda$invoke$0(ExecutableInvoker.java:105)
    at org.junit.jupiter.engine.execution.InvocationInterceptorChain$InterceptedInvocation.proceed(InvocationInterceptorChain.java:106)
    at org.junit.jupiter.engine.execution.InvocationInterceptorChain.proceed(InvocationInterceptorChain.java:64)
    at org.junit.jupiter.engine.execution.InvocationInterceptorChain.chainAndInvoke(InvocationInterceptorChain.java:45)
    at org.junit.jupiter.engine.execution.InvocationInterceptorChain.invoke(InvocationInterceptorChain.java:37)
    at org.junit.jupiter.engine.execution.ExecutableInvoker.invoke(ExecutableInvoker.java:104)
    at org.junit.jupiter.engine.execution.ExecutableInvoker.invoke(ExecutableInvoker.java:98)
    at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.lambda$invokeTestMethod$6(TestMethodTestDescriptor.java:210)
    at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
    at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.invokeTestMethod(TestMethodTestDescriptor.java:206)
    at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.execute(TestMethodTestDescriptor.java:131)
    at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.execute(TestMethodTestDescriptor.java:65)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$5(NodeTestTask.java:139)
    at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$7(NodeTestTask.java:129)
    at org.junit.platform.engine.support.hierarchical.Node.around(Node.java:137)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$8(NodeTestTask.java:127)
    at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:126)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:84)
    at java.base/java.util.ArrayList.forEach(ArrayList.java:1541)
    at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.invokeAll(SameThreadHierarchicalTestExecutorService.java:38)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$5(NodeTestTask.java:143)
    at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$7(NodeTestTask.java:129)
    at org.junit.platform.engine.support.hierarchical.Node.around(Node.java:137)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$8(NodeTestTask.java:127)
    at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:126)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:84)
    at java.base/java.util.ArrayList.forEach(ArrayList.java:1541)
    at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.invokeAll(SameThreadHierarchicalTestExecutorService.java:38)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$5(NodeTestTask.java:143)
    at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$7(NodeTestTask.java:129)
    at org.junit.platform.engine.support.hierarchical.Node.around(Node.java:137)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$8(NodeTestTask.java:127)
    at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:126)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:84)
    at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.submit(SameThreadHierarchicalTestExecutorService.java:32)
    at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor.execute(HierarchicalTestExecutor.java:57)
    at org.junit.platform.engine.support.hierarchical.HierarchicalTestEngine.execute(HierarchicalTestEngine.java:51)
    at org.junit.platform.launcher.core.EngineExecutionorchestrator.execute(EngineExecutionorchestrator.java:108)
    at org.junit.platform.launcher.core.EngineExecutionorchestrator.execute(EngineExecutionorchestrator.java:88)
    at org.junit.platform.launcher.core.EngineExecutionorchestrator.lambda$execute$0(EngineExecutionorchestrator.java:54)
    at org.junit.platform.launcher.core.EngineExecutionorchestrator.withInterceptedStreams(EngineExecutionorchestrator.java:67)
    at org.junit.platform.launcher.core.EngineExecutionorchestrator.execute(EngineExecutionorchestrator.java:52)
    at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:96)
    at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:75)
    at org.gradle.api.internal.tasks.testing.junitplatform.JUnitPlatformTestClassprocessor$CollectAllTestClassesExecutor.processAllTestClasses(JUnitPlatformTestClassprocessor.java:99)
    at org.gradle.api.internal.tasks.testing.junitplatform.JUnitPlatformTestClassprocessor$CollectAllTestClassesExecutor.access$000(JUnitPlatformTestClassprocessor.java:79)
    at org.gradle.api.internal.tasks.testing.junitplatform.JUnitPlatformTestClassprocessor.stop(JUnitPlatformTestClassprocessor.java:75)
    at org.gradle.api.internal.tasks.testing.SuiteTestClassprocessor.stop(SuiteTestClassprocessor.java:61)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:566)
    at org.gradle.internal.dispatch.Reflectiondispatch.dispatch(Reflectiondispatch.java:36)
    at org.gradle.internal.dispatch.Reflectiondispatch.dispatch(Reflectiondispatch.java:24)
    at org.gradle.internal.dispatch.ContextClassLoaderdispatch.dispatch(ContextClassLoaderdispatch.java:33)
    at org.gradle.internal.dispatch.ProxydispatchAdapter$dispatchingInvocationHandler.invoke(ProxydispatchAdapter.java:94)
    at com.sun.proxy.$Proxy2.stop(UnkNown Source)
    at org.gradle.api.internal.tasks.testing.worker.TestWorker.stop(TestWorker.java:133)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:566)
    at org.gradle.internal.dispatch.Reflectiondispatch.dispatch(Reflectiondispatch.java:36)
    at org.gradle.internal.dispatch.Reflectiondispatch.dispatch(Reflectiondispatch.java:24)
    at org.gradle.internal.remote.internal.hub.MessageHubBackedobjectConnection$dispatchWrapper.dispatch(MessageHubBackedobjectConnection.java:182)
    at org.gradle.internal.remote.internal.hub.MessageHubBackedobjectConnection$dispatchWrapper.dispatch(MessageHubBackedobjectConnection.java:164)
    at org.gradle.internal.remote.internal.hub.MessageHub$Handler.run(MessageHub.java:414)
    at org.gradle.internal.concurrent.ExecutorPolicy$CatchAndRecordFailures.onExecute(ExecutorPolicy.java:64)
    at org.gradle.internal.concurrent.ManagedExecutorImpl$1.run(ManagedExecutorImpl.java:48)
    at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
    at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
    at org.gradle.internal.concurrent.ThreadFactoryImpl$ManagedThreadRunnable.run(ThreadFactoryImpl.java:56)
    at java.base/java.lang.Thread.run(Thread.java:834)

在我在 NullPointerException 上得到 URL::toURI 之前，我做错了什么？

解决方法

在基于 vavr 的转换管道中，您有

cl -> cl.getResource( "config" )

返回 null，这可能由于多种原因发生，详见 documentation for ClassLoader.getResource()。 然后，在下一步中，您尝试使用只允许非空值的谓词过滤 Try 值。 filter 函数是 filterTry 的简写，它

如果这是失败或成功并且值满足谓词，则返回此值。

返回一个新的失败，如果这是一个成功并且该值不满足谓词或在测试谓词时发生异常。返回的 Failure 包装了一个 NoSuchElementException 实例。

在 Try 的情况下，这是预期且正确的行为，因为 Try 是计算结果的容器，计算结果可以是单个值或错误。它的设计根本就没有任何价值（有 Option 类型）。因此，当您从 Try 中过滤掉空值时，它会返回一个新的 Failure(NoSuchElementException)。在最后一步中，当您尝试在此 Failure 上使用 get() 解包值时，没有先检查它是否包含值或错误，就会抛出错误。这有点令人困惑，因为产生的抛出异常的堆栈跟踪指向 filter 调用，而实际上它是由 get 抛出的。

我建议永远不要使用 get()，除非您确实打算抛出异常。如果您想优雅地处理 Success 和 Failure 两种情况，我建议使用替代 getOr... 方法之一或 fold。

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