jdk8 npe_JDK 14中更好的NPE消息

深藏阁楼爱情的钟 2023-02-25 04:59 58阅读 0赞

jdk8 npe

我的2019年3月博客文章“ [Java会出现更好的默认NullPointerException消息吗？][Java_NullPointerException] ”的撰写是为了获得更好的[NullPointerException][]消息的*JEP **草案***尚未针对特定的JDK版本。 此后，该JEP草案成为[JEP][] [14的][14] [目标][Link 1] [JEP 358（“ Helpful NullPointerExceptions”）][JEP] 。 更好的是，初始实现（ [JDK-8218628][] ） [已经在JDK 14分支中][JDK 14] ，并且可以在[JDK 14 Early Access Builds][] **Build 20（2019/10/23）中使用** 。

在本文中，我将针对JDK 14 Early Access Build 20运行先前文章中介绍的[示例代码][Link 2] ，以演示现在提供的其他详细信息。 要查看为故意引入导致`NullPointerException`的各种情况而编写的示例代码，请参阅前面的文章或查看[GitHub上][Link 2]的[源代码][Link 2] 。

下载[JDK 14 Early Access Build 20][]并按其路径指向该文件后，运行`java -version`时会看到以下内容：

openjdk version "14-ea" 2020 - 03 - 17  OpenJDK Runtime Environment (build 14 -ea+ 20 - 879 )  OpenJDK 64 -Bit Server VM (build 14 -ea+ 20 - 879 , mixed mode, sharing)

正确配置了**JDK 14 Early Access Build 20之后** ，我重新构建了[前面提到][Link 2]的[源代码][Link 2] ，然后使用[Java启动器][Java]重新运行了该代码，而没有任何新选项。 该输出（如下所示）与先前JDK版本的输出没有实质性差异。

=========================================  | # | # 1 : Element [ null boolean array | ] on : Element [ 0 ] on  =========================================  java.lang.NullPointerException 
      at dustin.examples.npe.NpeDemo.demonstrateFirstExampleIndexAccessOnNullBooleanArray(Unknown Source) 
      at dustin.examples.npe.NpeDemo.demonstrateJdk8218628Examples(Unknown Source) 
      at dustin.examples.npe.NpeDemo.main(Unknown Source)  =================================  | # | # 2 : .length on null boolean [] |  =================================  java.lang.NullPointerException 
      at dustin.examples.npe.NpeDemo.demonstrateSecondExampleLengthOnNullBooleanArray(Unknown Source) 
      at dustin.examples.npe.NpeDemo.demonstrateJdk8218628Examples(Unknown Source) 
      at dustin.examples.npe.NpeDemo.main(Unknown Source)  =======================================  | # | # 3 : Assigning float : Assigning to null float [] |  =======================================  java.lang.NullPointerException 
      at dustin.examples.npe.NpeDemo.demonstrateThirdExampleAssigningValueToElementOfNullFloatArray(Unknown Source) 
      at dustin.examples.npe.NpeDemo.demonstrateJdk8218628Examples(Unknown Source) 
      at dustin.examples.npe.NpeDemo.main(Unknown Source)  ======================================  | # | # 4 : Accessing field on null object | : Accessing field on object |  ======================================  java.lang.NullPointerException 
      at dustin.examples.npe.NpeDemo.demonstrateFourthExampleAccessInstanceFieldOfNullObject(Unknown Source) 
      at dustin.examples.npe.NpeDemo.demonstrateJdk8218628Examples(Unknown Source) 
      at dustin.examples.npe.NpeDemo.main(Unknown Source)  ===================  | # | # 5 : throw null ; | ; |  ===================  java.lang.NullPointerException 
      at dustin.examples.npe.NpeDemo.demonstrateFifthExampleThrowingConstantNull(Unknown Source) 
      at dustin.examples.npe.NpeDemo.demonstrateJdk8218628Examples(Unknown Source) 
      at dustin.examples.npe.NpeDemo.main(Unknown Source)  ================================================  | # | # 6 : Method invocation on null instance field | : Method invocation on  ================================================  java.lang.NullPointerException 
      at dustin.examples.npe.NpeDemo.demonstrateSixthExampleMethodInvocationOnNullInstanceField(Unknown Source) 
      at dustin.examples.npe.NpeDemo.demonstrateJdk8218628Examples(Unknown Source) 
      at dustin.examples.npe.NpeDemo.main(Unknown Source)  =============================================  | # | # 7 : () on null instance field | () on synchronized () on instance field |  =============================================  java.lang.NullPointerException 
      at dustin.examples.npe.NpeDemo.demonstrateSeventhExampleSynchronizedNullInstanceField(Unknown Source) 
      at dustin.examples.npe.NpeDemo.demonstrateJdk8218628Examples(Unknown Source) 
      at dustin.examples.npe.NpeDemo.main(Unknown Source)  ==========================================================================  | >>> Null Lost in Long Series of Method Invocations in Single Statement |  ==========================================================================  java.lang.NullPointerException 
      at dustin.examples.npe.NpeDemo.demonstrateNullLostInSeriesOfMethodInvocationsInSingleStatement(Unknown Source) 
      at dustin.examples.npe.NpeDemo.main(Unknown Source)  =======================================================  | >>> Null Lost in Dereferenced Constructor Arguments |  =======================================================  java.lang.NullPointerException 
      at dustin.examples.npe.NpeDemo.demonstrateNullLostInConstructorAcceptingMultiplePotentiallyNullArgumentsDereferenced(Unknown Source) 
      at dustin.examples.npe.NpeDemo.main(Unknown Source)  ==================================================  | >>> Null Lost in Dereferenced Method Arguments |  ==================================================  java.lang.NullPointerException 
      at dustin.examples.npe.NpeDemo.demonstrateNullLostInMethodAcceptingMultiplePotentiallyNullArgumentsDereferenced(Unknown Source) 
      at dustin.examples.npe.NpeDemo.main(Unknown Source)

如以上输出所示，即使使用新的JDK 14 Early Access Build 20，当我正常运行应用程序时，也看不到有关`NullPointerException`的任何新的详细信息。 我包含了此输出，以表明需要一个特殊的标志来启用更详细的`NullPointerException` ，并使比较不带额外细节的输出更加方便。 下一个输出清单显示了将Java启动器传递给标志`-XX:+ShowCodeDetailsInExceptionMessages`时提供的其他详细信息：

=========================================  | # | # 1 : Element [ null boolean array | ] on : Element [ 0 ] on  =========================================  java.lang.NullPointerException: Cannot load from byte / boolean array because java.lang.NullPointerException: Cannot load from array because "<local1>" is null 
      at dustin.examples.npe.NpeDemo.demonstrateFirstExampleIndexAccessOnNullBooleanArray(Unknown Source) 
      at dustin.examples.npe.NpeDemo.demonstrateJdk8218628Examples(Unknown Source) 
      at dustin.examples.npe.NpeDemo.main(Unknown Source)  =================================  | # | # 2 : .length on null boolean [] |  =================================  java.lang.NullPointerException: Cannot read the array length because "<local1>" is null 
      at dustin.examples.npe.NpeDemo.demonstrateSecondExampleLengthOnNullBooleanArray(Unknown Source) 
      at dustin.examples.npe.NpeDemo.demonstrateJdk8218628Examples(Unknown Source) 
      at dustin.examples.npe.NpeDemo.main(Unknown Source)  =======================================  | # | # 3 : Assigning float : Assigning to null float [] |  =======================================  java.lang.NullPointerException: Cannot store to array because "<local1>" is null array because java.lang.NullPointerException: Cannot store to float array because 
      at dustin.examples.npe.NpeDemo.demonstrateThirdExampleAssigningValueToElementOfNullFloatArray(Unknown Source) 
      at dustin.examples.npe.NpeDemo.demonstrateJdk8218628Examples(Unknown Source) 
      at dustin.examples.npe.NpeDemo.main(Unknown Source)  ======================================  | # | # 4 : Accessing field on null object | : Accessing field on object |  ======================================  java.lang.NullPointerException: Cannot read field "nullInstanceField" because "<local1>" is null 
      at dustin.examples.npe.NpeDemo.demonstrateFourthExampleAccessInstanceFieldOfNullObject(Unknown Source) 
      at dustin.examples.npe.NpeDemo.demonstrateJdk8218628Examples(Unknown Source) 
      at dustin.examples.npe.NpeDemo.main(Unknown Source)  ===================  | # | # 5 : throw null ; | ; |  ===================  java.lang.NullPointerException: Cannot throw exception because "null" is null 
      at dustin.examples.npe.NpeDemo.demonstrateFifthExampleThrowingConstantNull(Unknown Source) 
      at dustin.examples.npe.NpeDemo.demonstrateJdk8218628Examples(Unknown Source) 
      at dustin.examples.npe.NpeDemo.main(Unknown Source)  ================================================  | # | # 6 : Method invocation on null instance field | : Method invocation on  ================================================  java.lang.NullPointerException: Cannot invoke "String.isEmpty()" because "this.nullInstanceField" is null 
      at dustin.examples.npe.NpeDemo.demonstrateSixthExampleMethodInvocationOnNullInstanceField(Unknown Source) 
      at dustin.examples.npe.NpeDemo.demonstrateJdk8218628Examples(Unknown Source) 
      at dustin.examples.npe.NpeDemo.main(Unknown Source)  =============================================  | # | # 7 : () on null instance field | () on synchronized () on instance field |  =============================================  java.lang.NullPointerException: Cannot enter synchronized block because "this.nullInstanceField" is null 
      at dustin.examples.npe.NpeDemo.demonstrateSeventhExampleSynchronizedNullInstanceField(Unknown Source) 
      at dustin.examples.npe.NpeDemo.demonstrateJdk8218628Examples(Unknown Source) 
      at dustin.examples.npe.NpeDemo.main(Unknown Source)  ==========================================================================  | >>> Null Lost in Long Series of Method Invocations in Single Statement |  ==========================================================================  java.lang.NullPointerException: Cannot invoke "dustin.examples.npe.DysfunctionalLocation$Province.getCity()" because the return value of "dustin.examples.npe.DysfunctionalLocation$Nation.getProvince()" is null 
      at dustin.examples.npe.NpeDemo.demonstrateNullLostInSeriesOfMethodInvocationsInSingleStatement(Unknown Source) 
      at dustin.examples.npe.NpeDemo.main(Unknown Source)  =======================================================  | >>> Null Lost in Dereferenced Constructor Arguments |  =======================================================  java.lang.NullPointerException: Cannot invoke "java.lang.Long.longValue()" because "<local6>" is null 
      at dustin.examples.npe.NpeDemo.demonstrateNullLostInConstructorAcceptingMultiplePotentiallyNullArgumentsDereferenced(Unknown Source) 
      at dustin.examples.npe.NpeDemo.main(Unknown Source)  ==================================================  | >>> Null Lost in Dereferenced Method Arguments |  ==================================================  java.lang.NullPointerException: Cannot invoke "java.lang.Long.longValue()" because "<local6>" is null 
      at dustin.examples.npe.NpeDemo.demonstrateNullLostInMethodAcceptingMultiplePotentiallyNullArgumentsDereferenced(Unknown Source) 
      at dustin.examples.npe.NpeDemo.main(Unknown Source)

JEP 358解释了如何使用此标志查看其他`NullPointerException`详细信息：“可以使用新的`boolean`命令行选项`-XX { + | - } ShowCodeDetailsInExceptionMessages` `-XX { + | - } ShowCodeDetailsInExceptionMessages` 。 该选项将首先具有默认值' `false` '，以便不打印消息。 它旨在在以后的版本中默认情况下在异常消息中启用代码详细信息。” 如我们所见，此功能默认情况下最初处于关闭状态，但是有计划在将来启用更详细的`NullPointerException`消息。

[最近的][Link 3]一条[Tweet][Link 3]提出了一个问题：“如果字节码不包含变量名，它将如何工作？” 这个问题通过提供一个具体的例子来继续：“假设我们有像`Object a = ....; a.getName(); //NPE`这样的代码`Object a = ....; a.getName(); //NPE` `Object a = ....; a.getName(); //NPE` `Object a = ....; a.getName(); //NPE` NPE会`Object a = ....; a.getName(); //NPE`哪种消息？” 尽管前面显示的一系列测试中都包含了一个示例，但我认为我会在此处针对该问题提供更集中的示例。 下一个代码清单（也在[GitHub上提供][GitHub] ）显示了改编自Tweet中使用的示例的代码。

package dustin.examples.npe;  /** 
      * Simple demonstration to answer Tweet-ed question 
      * "How it will work if bytecode doesn't contain variable names?" 
      * ( https://twitter.com/2doublewhiskey/status/1180365953240055809 ). 
      */  public class TwoDoubleWhiskeyTweetExample  { 
        public static void main( final String[] arguments) 
        { 
           final Person person = null ; 
           person.getName(); //NPE 
        }    
        public static class Person 
        { 
           private String name; 
           public Person( final String newName) 
           { 
              name = newName; 
           } 
           public String getName() 
           { 
              return name; 
           } 
        }  }

下一个屏幕快照显示了使用JDK 14 Early Access Build 20（不带`java`启动器标志`-XX:+ShowCodeDetailsInExceptionMessages`运行此简单应用程序的结果。

![format_png][]

如屏幕快照所示，在JDK 14 Early Access Build 20中使用`-XX:+ShowCodeDetailsInExceptionMessages`标志可提供与此简单`NullPointerException`示例相关的其他详细信息：“无法调用” dustin.examples.npe.TwoDoubleWhiskeyTweetExample $ **Person.getName（）** ”，因为“ **<local1>** ”为空”

[GitHub上][GitHub 1]提供了一个更简单，甚至更接近Tweet-ed问题中[提供][GitHub 1]的原始示例的示例。

[JEP 358][JEP] （“ **有用的NullPointerExceptions** ”）可能不像新JDK版本中的其他一些[JEP][]那样浮华，但最终可能是[每天为Java开发人员][Java 1]提供的[价值][Java 1]要比其一些浮华的同行更高。 有许多示例将对您有所帮助，JEP本身以及本文引用的我的[代码示例中][Link 4]都阐明了许多示例情况。

> 翻译自: [https://www.javacodegeeks.com/2019/10/better-npe-messages-in-jdk-14.html][https_www.javacodegeeks.com_2019_10_better-npe-messages-in-jdk-14.html]

jdk8 npe

[Java_NullPointerException]: https://marxsoftware.blogspot.com/2019/03/better-nullpointerexception-message.html
[NullPointerException]: https://docs.oracle.com/en/java/javase/13/docs/api/java.base/java/lang/NullPointerException.html
[JEP]: https://openjdk.java.net/jeps/358
[14]: https://openjdk.java.net/projects/jdk/14/
[Link 1]: https://twitter.com/OpenJDK/status/1179533128475365378
[JDK-8218628]: https://bugs.openjdk.java.net/browse/JDK-8218628
[JDK 14]: https://hg.openjdk.java.net/jdk/jdk/log?rev=reverse%28%22jdk-14%2B19%22%3A%3A%22jdk-14%2B20%22-%22jdk-14%2B19%22%29&revcount=1000
[JDK 14 Early Access Builds]: https://jdk.java.net/14/
[Link 2]: https://github.com/dustinmarx/javademos/tree/master/src/dustin/examples/npe
[JDK 14 Early Access Build 20]: https://twitter.com/OpenJDK/status/1187641081917300736
[Java]: https://docs.oracle.com/en/java/javase/13/docs/specs/man/java.html
[Link 3]: https://twitter.com/2doublewhiskey/status/1180365953240055809
[GitHub]: https://github.com/dustinmarx/javademos/blob/master/src/dustin/examples/npe/TwoDoubleWhiskeyTweetExample.java
[format_png]: /images/20230209/e9399fe6d49940f6bab9c214cc7b09ac.png
[GitHub 1]: https://github.com/dustinmarx/javademos/blob/master/src/dustin/examples/npe/TwoDoubleWhiskeyTweetExample2.java
[Java 1]: https://twitter.com/openjdk/status/1154888870028173314?lang=en
[Link 4]: https://github.com/dustinmarx/javademos/blob/master/src/dustin/examples/npe/NpeDemo.java
[https_www.javacodegeeks.com_2019_10_better-npe-messages-in-jdk-14.html]: https://www.javacodegeeks.com/2019/10/better-npe-messages-in-jdk-14.html