今天碰到了一個很變態的問題，寫了一個很簡單的HelloWord.java，內容如下：

package com.yanjiuyanjiu;

public class HelloWorld {

public static void main(String args[]) {

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

}

}

在eclipse中運行是可以的，但是在命令行下運行總是失敗。我的工程位置為 d:/workspace/HelloWorld/。

嘗試了很多次，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只有這個成功

Hello World!

d:/workspace/HelloWorld/bin/>java -classpath .; com/yanjiuyanjiu/HelloWorld換了個斜杠就不行了

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"

在網上搜索了大半天，大部分說是環境變量，classpath或JDK版本的問題，還有執行時文件名class后綴不要。我一一試過，都沒有解決。最后無意中換了一下命令符的位置，成功了。總結如下：

1. 環境變量，CLASSPATH當然要設置好，執行時不要帶class后綴；

2. 路徑中的斜杠用“/”而不是“/”；

3. 命令符的當前目錄要在包的起點。比如這里應該在 d:/workspace/HelloWorld/bin/>，如果在 d:/workspace/HelloWorld/bin/com/yanjiuyanjiu/>，反而不行，有點“近水樓臺不得月”的意思，不知道為什么，還請高手解釋一下。

總結

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