英文:
Error caused on runtime of java class file
问题
我如何在不知道开发者使用的特定包的情况下编译和运行Java文件...
我得到了一个由他人编写的Java文件文件夹，我正在尝试编译和运行这些Java文件，我可以通过运行 javac *.java 来编译这些文件，但是如果我尝试通过调用 java MainClass 来运行主类，我会得到一个错误，像这样：错误：找不到或加载主类 MainClass，原因：java.lang.NoClassDefFoundError: SomePackageName/MainClass（名称错误：MainClass）
我不知道 SomePackageName 是什么，它是由开发者定义的，我无法控制。
所以问题是，如果我只知道主类，我该如何编译和运行这些Java文件？
英文:
How do I compile and run java files if you don't know the specific package used by the developer...
I got a folder of java files written by someone else, and I'm trying to compile and run the java files, I can compile the files by running javac *.java, but if I try to run the main class by calling java MainClass, I get an error like so: Error could not find or load main class MainClass, Caused by: java.lang.NoClassDefFoundError: SomePackageName/MainClass (wrong name: MainClass)
I have no idea whats the SomePackageName its defined by the developer and I have no control over that.
so the question is how do I compile and run the java files if all I know is which is the main class?
答案1
得分: 2
- 完全限定类名的格式为：
com.foo.Bar，其中com.foo是包名（例如，定义 Bar 类的文件以package com.foo;开头），而 Bar 是类名（例如，文件包含行public class Bar {）。 - 在运行
java并指定类路径+类名时，传递的是类名，而不是文件名。因此，要运行此文件，您会写成java -cp /path/to/base com.foo.Bar。 - 完全限定名称在运行时（类文件）和编译时（Java 文件）都必须与目录结构匹配。因此，在这里，您应该有类似
/Users/Foghunt/projects/myproj/src/com/foo/Bar.java的路径，并且编译后应该有类似/Users/Foghunt/projects/myproj/build/com/foo/Bar.class的路径。 - “基本目录” 是包含第一个包目录的目录。因此，源文件的基本目录是
/Users/Foghunt/projects/myproj/src，编译结果的基本目录是/Users/Foghunt/projects/myproj/build。 - 基本目录需要在类路径中。类路径条目是文件路径。因此，要运行这个东西，您会写成
java -cp /Users/Foghunt/projects/myproj/build com.foo.Bar。 javac不介意您是否费心正确地定义事物（例如，在/Users/Foghunt/projects/myproj/src/com/foo中运行javac *.java是可行的），但如果涉及多个包，情况会很复杂。- 通常情况下，您应该使用构建系统。如果您继承了这个项目，请在项目根目录中检查是否有名为
build.xml、pom.xml或build.gradle的文件。如果存在这些文件之一，请使用网络搜索引擎（build.xml-> 搜索java ant，pom.xml-> 搜索java maven，build.gradle-> 搜索java gradle），并使用它来构建项目，不要自行运行javac。
有了这些知识，您现在可以分析、构建和运行任何 Java 项目了。
英文:
- Fully qualified class names are of the form:
com.foo.Bar, wherecom.foois the package (e.g. the file defining the Bar class starts with the linepackage com.foo;), and Bar is the name (e.g. the file contains the linepublic class Bar {). - When running
javaand specifying classpath+classname, you pass a class name. Not a file name. So to run this file you'd writejava -cp /path/to/base com.foo.Bar. - fully qualified names must match directory structure both at runtime (class files) and compile time (java files). So, here you should have say
/Users/Foghunt/projects/myproj/src/com/foo/Bar.javaand after compilation you should have e.g./Users/Foghunt/projects/myproj/build/com/foo/Bar.class - The 'base dir' is the one that contains the first package directory. So, the base dir for the source file is
/Users/Foghunt/projects/myproj/srcand for the compiled result is/Users/Foghunt/projects/myproj/build. - The base dir needs to be in the classpath. Classpath entries are file paths. So, to run this thing you'd write
java -cp /Users/Foghunt/projects/myproj/build com.foo.Bar. javacdoesn't mind if you don't go through the trouble of properly defining things (e.g. runningjavac *.javain/Users/Foghunt/projects/myproj/src/com/foodoes work), but it gets complicated quickly if you have multiple packages.- In general you should be using build systems. If you've inherited this project, check for a file named
build.xmlorpom.xmlorbuild.gradlein the root of the project. If it is there, use a web search engine (build.xml-> search forjava ant,pom.xml->java maven,build.gradle->java gradle), and use that to build the project, don't runjavacyourself.
There you go. Armed with that knowledge you can now analyse, build, and run any java project.
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