Java 找不到主类 HelloWorld

Could not find main class HelloWorld

提问by Newbie

I installed Java 1.7.0 in the following folder C:\Program Files\Java. My operating system is Windows XP(Version 2002) with Service pack 3.

我在以下文件夹中安装了 Java 1.7.0 C:\Program Files\Java。我的操作系统是带有 Service Pack 3 的 Windows XP（2002 版）。

The environment variables which I set are:

我设置的环境变量是：

• CLASSPATH: C:\Program Files\Java\jdk1.7.0\jre\lib\rt.jar;

• Path :C:\Program Files\Java\jdk1.7.0\bin;

• JAVA_HOME: C:\Program Files\Java;

• 类路径：C:\Program Files\Java\jdk1.7.0\jre\lib\rt.jar;

• 小路 ：C:\Program Files\Java\jdk1.7.0\bin;

• JAVA_HOME:C:\Program Files\Java;

I have presented here the class names which are in my system.

我在这里展示了我系统中的类名。

Next I wrote a program, HelloWorld.java:

接下来我写了一个程序，HelloWorld.java：

import java.io.*;

class HelloWorld
{
    public static void main(String[] args) 
    {
        System.out.println("Hello World!");
    }
}

When I am compiling using javac HelloWorld.javait is compiling fine.

当我使用javac HelloWorld.java它进行编译时，它编译得很好。

But after I issue java HelloWorldI am encountering the below error:

但是在我发出后，我java HelloWorld遇到了以下错误：

Error: Could not find main class HelloWorld
Exception in thread "main" java.lang.NoClassDefFoundError: HelloWorld
        at sun.launcher.LauncherHelper.checkAndLoadMain(LauncherHelper.java:198)

Caused by: java.lang.ClassNotFoundException: HelloWorld
        at java.net.URLClassLoader.run(URLClassLoader.java:299)
        at java.net.URLClassLoader.run(URLClassLoader.java:288)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(URLClassLoader.java:287)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:422)
        at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:325)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:355)
        at sun.launcher.LauncherHelper.checkAndLoadMain(LauncherHelper.java:195)

After a bit of searching around, I found that may be something wrong in the environment variable. I tried to play with that but no luck.

经过一番搜索，我发现环境变量可能有问题。我试着玩这个，但没有运气。

I even RESTARTED the machine and then again I tried to run but with same fate.

我什至重新启动了机器，然后我再次尝试运行，但命运相同。

采纳答案by Sean Owen

You are not setting a classpath that includes your compiled class! javacan't find any classes if you don't tell it where to look.

您没有设置包含已编译类的类路径！java如果你不告诉它在哪里看，就找不到任何课程。

java -cp [compiler outpur dir] HelloWorld

Incidentally you do not need to set CLASSPATH the way you have done.

顺便说一句，您不需要像以前那样设置 CLASSPATH。

回答by unbeli

Tell it where to look for you class: it's in ".", which is the current directory:

告诉它在哪里寻找你的类：它在“.”中，这是当前目录：

java -classpath . HelloWorld

No need to set JAVA_HOMEor CLASSPATHin this case

无需设置JAVA_HOME或CLASSPATH在这种情况下

回答by Andreas Dolk

JAVA_HOMEis not necessary if you start java and javac from the command line. But JAVA_HOMEshould point to the real jdk directory, C:\Program Files\Java\jdk1.7.0in your case.

JAVA_HOME如果从命令行启动 java 和 javac，则不需要。但在你的情况下JAVA_HOME，应该指向真正的 jdk 目录C:\Program Files\Java\jdk1.7.0。

I'd never use the CLASSPATHenvironment variable outside of build scripts, especially not global defined. The -cpflag is better. But in your case, as you do not need additional libraries (rt.jardoesn't count), you won't need a classpath declaration. A missing -cpis equivalent to a -cp .and that's what you need here)

我永远不会在CLASSPATH构建脚本之外使用环境变量，尤其是没有全局定义的环境变量。该-cp标志是更好的。但是在您的情况下，由于您不需要额外的库（rt.jar不算数），因此您不需要类路径声明。一个缺失-cp相当于一个-cp .，这就是你在这里需要的）

The HelloWorldclass needs to be declared as public. This actually may be the cause for your problems.(I was pretty sure, that a source file needs one public class... or was it one public class at most?)

本HelloWorld类需要声明为public。这实际上可能是导致您出现问题的原因。（我很确定，源文件需要一个公共类......或者最多是一个公共类？）

回答by Jorn

You either want to add "." to your CLASSPATH to specify the current directory, or add it manually at run time the way unbeli suggested.

您要么想添加“。” 到您的 CLASSPATH 以指定当前目录，或者按照 unbeli 建议的方式在运行时手动添加它。

回答by user85421

Java is not finding where your compiled class file (HelloWorld.class) is. It uses the directories and JAR-files in the CLASSPATHenvironment variable for searching if no -cpor -classpathoption is given when running java.exe.

Java 没有找到您编译的类文件 (HelloWorld.class) 的位置。它使用CLASSPATH环境变量中的目录和 JAR 文件来搜索运行 java.exe 时是否给出了-cp或-classpath选项。

You don't need the rt.jar in the CLASSPATH, these was only needed for older versions of Java. You can leave it undefined and the current working directory will be used, or just add .(a single point), separated by ';', to the CLASSPATHvariable to indicate the current directory:

您不需要 rt.jar 中的 rt.jar，CLASSPATH只有旧版本的 Java 才需要这些。您可以不定义它并使用当前工作目录，或者只是添加.（一个点），以“;”分隔，CLASSPATH以指示当前目录的变量：

CLASSPATH:.;C:\...\some.jar

类路径：.;C:\...\some.jar

Alternatively you can use the -cpor -classpathoption:

或者，您可以使用-cp或-classpath选项：

java -cp . HelloWorld

And, as Andreas wrote, JAVA_HOMEis not needed by Java, just for some third-party tools like ant (but should point to the correct location).

而且，正如 Andreas 所写的，JAVA_HOMEJava 不需要，只需要一些第三方工具，如 ant（但应该指向正确的位置）。

回答by BluesPower

Just remove your "classpath" from you environment variable. Then try running:

只需从您的环境变量中删除您的“类路径”。然后尝试运行：

java HelloWorld 

This should work fine.

这应该可以正常工作。

回答by d9daniel

I had the same problem. Perhaps, the problem is that you have compiled and executed the class with different Java versions.

我有同样的问题。也许，问题在于您使用不同的 Java 版本编译并执行了该类。

Make sure the version of the compiler is the same as the command "java":

确保编译器的版本与命令“java”相同：

javac -version

java -version

In Linux, use

在 Linux 中，使用

sudo update-alternatives --config java

须藤更新替代品--config java

to change the version of Java.

更改Java的版本。

回答by user3329068

put .; at classpath value in beginning..it will start working...it happens because it searches the class file in classpath which is mentioned in path variable.

放 。; 在开始时的类路径值......它将开始工作......它发生是因为它在路径变量中提到的类路径中搜索类文件。

回答by Anoop Reddy

I have also faced same problem....

我也遇到了同样的问题......

Actually this problem is raised due to the fact that your program .classfiles are not saved in that directory. Remove your CLASSPATH from your environment variable (you do no need to set classpath for simple Java programs) and reopen cmd prompt, then compile and execute.

实际上，由于您的程序.class文件未保存在该目录中，因此引发了此问题。从您的环境变量中删除您的 CLASSPATH（您不需要为简单的 Java 程序设置类路径）并重新打开 cmd 提示符，然后编译并执行。

If you observe carefully your .classfile will save in the same location. (I am not an expert, I am also basic programer if there is any mistake in my sentences please ignore it :-))

如果您仔细观察，您的.class文件将保存在同一位置。（我不是专家，我也是初级程序员，如果我的句子有任何错误，请忽略它:-)）

回答by Chandan Prakash Sharma

It looks that you had done all setup properly but there might be one area where it might be causing problem

看起来您已正确完成所有设置，但可能有一个区域可能导致问题

Check the value of your "CLASSPATH" variable and make sure at the end you kept ;.

检查“CLASSPATH”变量的值，并确保最后保留了;。

Note: ;is for end separator .is for including existing path at the end

注意： ; 用于结束分隔符 。用于在末尾包含现有路径