今天碰到了一个很变态的问题，写了一个很简单的HelloWord.java，内容如下：

package com.yanjiuyanjiu;

public class HelloWorld {

public static void main(String args[]) {

System.out.println("Hello World!");

}

}

在eclipse中运行是可以的，但是在命令行下运行总是失败。我的工程位置为 d:/workspace/HelloWorld/。

尝试了很多次，如下

d:/workspace/HelloWorld/bin/com/yanjiuyanjiu/>java HelloWorld

Exception in thread "main" java.lang.NoClassDefFoundError: HelloWorld (wrong name: com/yanjiuyanjiu/HelloWorld)

at java.lang.ClassLoader.defineClass1(Native Method)

at java.lang.ClassLoader.defineClassCond(Unknown Source)

at java.lang.ClassLoader.defineClass(Unknown Source)

at java.security.SecureClassLoader.defineClass(Unknown Source)

at java.net.URLClassLoader.defineClass(Unknown Source)

at java.net.URLClassLoader.access$000(Unknown Source)

at java.net.URLClassLoader$1.run(Unknown Source)

at java.security.AccessController.doPrivileged(Native Method)

at java.net.URLClassLoader.findClass(Unknown Source)

at java.lang.ClassLoader.loadClass(Unknown Source)

at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)

at java.lang.ClassLoader.loadClass(Unknown Source)

Could not find the main class: HelloWorld.  Program will exit.

Exception in thread "main"

d:/workspace/HelloWorld/bin/com/yanjiuyanjiu/>java -classpath .; HelloWorld

Exception in thread "main" java.lang.NoClassDefFoundError: HelloWorld (wrong name: com/yanjiuyanjiu/HelloWorld)

at java.lang.ClassLoader.defineClass1(Native Method)

at java.lang.ClassLoader.defineClassCond(Unknown Source)

at java.lang.ClassLoader.defineClass(Unknown Source)

at java.security.SecureClassLoader.defineClass(Unknown Source)

at java.net.URLClassLoader.defineClass(Unknown Source)

at java.net.URLClassLoader.access$000(Unknown Source)

at java.net.URLClassLoader$1.run(Unknown Source)

at java.security.AccessController.doPrivileged(Native Method)

at java.net.URLClassLoader.findClass(Unknown Source)

at java.lang.ClassLoader.loadClass(Unknown Source)

at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)

at java.lang.ClassLoader.loadClass(Unknown Source)

Could not find the main class: HelloWorld.  Program will exit.

Exception in thread "main"

d:/workspace/HelloWorld/bin/com/yanjiuyanjiu/>cd..

d:/workspace/HelloWorld/bin/com/>cd..

d:/workspace/HelloWorld/bin/>java -classpath .; com/yanjiuyanjiu/HelloWorld只有这个成功

Hello World!

d:/workspace/HelloWorld/bin/>java -classpath .; com/yanjiuyanjiu/HelloWorld换了个斜杠就不行了

Exception in thread "main" java.lang.NoClassDefFoundError: com/yanjiuyanjiu/HelloWorld (wrong name: com/yanjiuyanjiu/HelloWorld)

at java.lang.ClassLoader.defineClass1(Native Method)

at java.lang.ClassLoader.defineClassCond(Unknown Source)

at java.lang.ClassLoader.defineClass(Unknown Source)

at java.security.SecureClassLoader.defineClass(Unknown Source)

at java.net.URLClassLoader.defineClass(Unknown Source)

at java.net.URLClassLoader.access$000(Unknown Source)

at java.net.URLClassLoader$1.run(Unknown Source)

at java.security.AccessController.doPrivileged(Native Method)

at java.net.URLClassLoader.findClass(Unknown Source)

at java.lang.ClassLoader.loadClass(Unknown Source)

at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)

at java.lang.ClassLoader.loadClass(Unknown Source)

Could not find the main class: com/yanjiuyanjiu/HelloWorld.  Program will exit.

Exception in thread "main"

d:/workspace/HelloWorld/bin/>java -classpath ./com/yanjiuyanjiu/; HelloWorld

Exception in thread "main"java.lang.NoClassDefFoundError: HelloWorld (wrong name: com/yanjiuyanjiu/HelloWorld)

at java.lang.ClassLoader.defineClass1(Native Method)

at java.lang.ClassLoader.defineClassCond(Unknown Source)

at java.lang.ClassLoader.defineClass(Unknown Source)

at java.security.SecureClassLoader.defineClass(Unknown Source)

at java.net.URLClassLoader.defineClass(Unknown Source)

at java.net.URLClassLoader.access$000(Unknown Source)

at java.net.URLClassLoader$1.run(Unknown Source)

at java.security.AccessController.doPrivileged(Native Method)

at java.net.URLClassLoader.findClass(Unknown Source)

at java.lang.ClassLoader.loadClass(Unknown Source)

at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)

at java.lang.ClassLoader.loadClass(Unknown Source)

Could not find the main class: HelloWorld.  Program will exit.

Exception in thread "main"

在网上搜索了大半天，大部分说是环境变量，classpath或JDK版本的问题，还有执行时文件名class后缀不要。我一一试过，都没有解决。最后无意中换了一下命令符的位置，成功了。总结如下：

1. 环境变量，CLASSPATH当然要设置好，执行时不要带class后缀；

2. 路径中的斜杠用“/”而不是“/”；

3. 命令符的当前目录要在包的起点。比如这里应该在 d:/workspace/HelloWorld/bin/>，如果在 d:/workspace/HelloWorld/bin/com/yanjiuyanjiu/>，反而不行，有点“近水楼台不得月”的意思，不知道为什么，还请高手解释一下。