当我运行Java应用程序时,我得到了一个NoClassDefFoundError。造成这种情况的典型原因是什么?


当前回答

我通过禁用所有模块的preDexLibraries来解决我的问题:

dexOptions {
        preDexLibraries false
        ...

其他回答

每个人都在这里谈论一些Java配置的东西,JVM的问题等,在我的情况下,错误与这些主题完全无关,有一个非常琐碎和容易解决的原因:我在我的控制器(Spring Boot应用程序)的端点有一个错误的注释。

虽然这可能是由于编译时和运行时之间的类路径不匹配造成的,但这并不一定是真的。

在这种情况下,记住两到三个不同的异常是很重要的:

java.lang.ClassNotFoundException This exception indicates that the class was not found on the classpath. This indicates that we were trying to load the class definition, and the class did not exist on the classpath. java.lang.NoClassDefFoundError This exception indicates that the JVM looked in its internal class definition data structure for the definition of a class and did not find it. This is different than saying that it could not be loaded from the classpath. Usually this indicates that we previously attempted to load a class from the classpath, but it failed for some reason - now we're trying to use the class again (and thus need to load it, since it failed last time), but we're not even going to try to load it, because we failed loading it earlier (and reasonably suspect that we would fail again). The earlier failure could be a ClassNotFoundException or an ExceptionInInitializerError (indicating a failure in the static initialization block) or any number of other problems. The point is, a NoClassDefFoundError is not necessarily a classpath problem.

我发现,当使用运行时发现的不兼容的类版本编译代码时,有时会得到NoClassDefFound错误。我记得的具体实例是apache axis库。在我的运行时类路径上实际上有2个版本,它正在挑选过时和不兼容的版本,而不是正确的版本,导致NoClassDefFound错误。这是在一个命令行应用程序中,我正在使用类似的命令。

set classpath=%classpath%;axis.jar

我能够让它拿起正确的版本使用:

set classpath=axis.jar;%classpath%;

下面的技巧帮助了我很多次:

System.out.println(TheNoDefFoundClass.class.getProtectionDomain().getCodeSource().getLocation());

其中nodeffoundclass是可能会“丢失”的类,这是由于您的程序使用相同库的旧版本的首选项。当客户端软件被部署到一个占主导地位的容器中,并配备了自己的类加载器和大量最流行的库的古老版本时,这种情况最常发生。

更新[https://www.infoq.com/articles/single-file-execution-java11/]:

在Java SE 11中,可以选择启动单个源代码文件 直接,不需要中间编译。为了方便大家, 所以像你这样的新手不需要运行javac + Java(当然, 让他们感到困惑)。