在运行时找不到 jar 中的类，但用于编译 [英] Class in jar not found at runtime, but was used to compile

问题描述

从 ant 文件构建这个项目后，我收到一个包含我构建的所有类的 jar.当我尝试运行此 jar 时，出现以下错误:

After I build this project from an ant file, I recieve a jar that contains all of the classes I built. When I try to run this jar, I get the following error:

Exception in thread "main" java.lang.NoClassDefFoundError: javax/media/j3d/SceneGraphObject

此错误表示无法找到我正在使用的 java3d 中的一个 jar，特别是 j3dcore.jar.但是，当通过 ant 构建编译到类文件中时，这个 jar 位于类路径上.

This error indicates that a one of the jars, specifically the j3dcore.jar from java3d, I am using can not be found. However, this jar is on the classpath when compiling through the ant build into the class files.

为什么在运行时找不到这个类，而在编译时却找到了?运行 jar 以及在 ant build 中更改它时，我是否必须在我的 shell 中手动更改我的类路径?

Why can this class not be found at runtime, but it is found at compile time? Do I have to manually change my classpath in my shell when running the jar as well as change it in the ant build?

如果我使用 java -cp j3d/*.jar -jar idv.jar

我收到错误错误:无法找到或加载主类j3d.j3dutils.jar

推荐答案

我是否必须在运行 jar 时手动更改我的 shell 中的类路径以及在 ant 构建中更改它?

Do I have to manually change my classpath in my shell when running the jar as well as change it in the ant build?

是的，绝对.在编译时使类可用不会将该类嵌入到您的输出或类似的东西中.它只是让编译器可以使用它(找出存在哪些方法等).

Yes, absolutely. Making a class available at compile-time doesn't embed the class into your output or anything like that. It just makes it available to the compiler (to find out what methods are present etc).

如果我使用 java -cp j3d/*.jar -jar idv.jar 将 jar 添加到我的类路径中

If I add the jars to my classpath using java -cp j3d/*.jar -jar idv.jar

是的，它会 - 因为它被扩展为:

Yes, it would - because that's being expanded into:

java -cp j3d/foo.jar j3d/bar.jar ... -jar idv.jar

鉴于 本文档:

It's not clear to me whether -cp is meant to work at all with -jar, given this documentation:

使用此选项时，JAR 文件是所有用户类的来源，其他用户类路径设置将被忽略.

When you use this option, the JAR file is the source of all user classes, and other user class path settings are ignored.

一种选择是 在 jar 文件的清单中设置类路径本身.例如:

Class-Path: j3d/foo.jar j3d/bar.jar

另一种方法是暂时忽略 -jar 命令行选项，并使用:

Another would be to ignore the -jar command-line option for now, and use:

java -cp j3d/*:idv.jar your.class.name.Here

注意 * 而不是 *.jar，如文档所示:

Note the * rather than *.jar, as documented:

为了特别方便，包含基名 * 的类路径元素被认为等效于指定目录中所有扩展名为 .jar 或 .JAR 的文件的列表(java 程序无法区分两者之间的区别)调用).

As a special convenience, a class path element containing a basename of * is considered equivalent to specifying a list of all the files in the directory with the extension .jar or .JAR (a java program cannot tell the difference between the two invocations).

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