“无法找到或加载主类”使用 cmd 提示符运行 java 程序时出错

"Could not find or load main class" Error while running java program using cmd prompt

提问by user241702

I am running a simple "HelloWorld" Program. I get this error in the command prompt:

我正在运行一个简单的“HelloWorld”程序。我在命令提示符中收到此错误：

Could not find or load main class HelloWorld.

无法找到或加载主类HelloWorld。

I have set the CLASSPATHand PATHvariable in the system. In the cmdprompt, I am running from the directory where I have saved HelloWorldprogram. I can see the class name and the file name are same and also .classfile created in the same directory. What else could be the problem?

我已经在系统中设置了CLASSPATH和PATH变量。在cmd提示中，我从保存HelloWorld程序的目录运行。我可以看到类名和文件名是相同的，并且.class在同一目录中创建了文件。还有什么问题？

My sample program looks like this:

我的示例程序如下所示：

package org.tij.exercises;
public class HelloWorld {
  public static void main(String[] args) {
    // TODO Auto-generated method stub
    System.out.println("Hello World!!");
  }
}

回答by Kakarot

When the Main class is inside a package then you need to run it as follows :

当 Main 类位于包内时，您需要按如下方式运行它：

java <packageName>.<MainClassName>

In your case you should run the program as follows :

在您的情况下，您应该按如下方式运行程序：

java org.tij.exercises.HelloWorld 

回答by youngzy

What's your CLASSPATHvalue?

你的CLASSPATH价值是什么？

It may look like this:

它可能看起来像这样：

 .;%JAVA_HOME%\lib\dt.jar;%JAVA_HOME%\lib\tools.jar 

I guess your value does not contain this .;.

我猜你的价值不包含这个.;。

So, ADD IT.
When you done , restartCMD

所以，添加它。
完成后，重新启动CMD

That may works.

这可能有效。

---

---

For example the file HelloWorld.javais in path: D:\myjavatest\org\yz\testand its packageis: org.yz.test.

例如，文件HelloWorld.java在 path: 中D:\myjavatest\org\yz\test，它package是：org.yz.test。

Now, you're in path D:\myjavatest\on the CMD line.
Type this to compile it:

现在，您在D:\myjavatest\CMD 行的路径中。
输入这个来编译它：

javac org/yz/test/HelloWorld.java

Then, type this to run it:

然后，键入以下内容以运行它：

java org.yz.test.HelloWorld

You may get what you want.

你可能会得到你想要的。

回答by user241702

I removed binfrom the CLASSPATH. I found out that I was executing the javacommand from the directory where the HelloWorld.javais located, i.e.:

我bin从CLASSPATH. 我发现我是java从所在目录执行命令的HelloWorld.java，即：

C:\Users\xyz\Documents\Java\javastudy\src\org\tij\exercises>java HelloWorld

C:\Users\xyz\Documents\Java\javastudy\src\org\tij\exercises>java HelloWorld

So I moved back to the main directory and executed:

所以我移回主目录并执行：

java org.tij.exercises.HelloWorld

java org.tij.exercises.HelloWorld

and it worked, i.e.:

它起作用了，即：

C:\Users\xyz\Documents\Java\javastudy\src>java org.tij.exercises.HelloWorld

C:\Users\xyz\Documents\Java\javastudy\src>java org.tij.exercises.HelloWorld

Hello World!!

你好，世界！！

回答by Vishrant

Execute your Java program using java -d . HelloWorldcommand.

使用java -d . HelloWorld命令执行您的 Java 程序。

This command works when you have declared package.

当您声明 package.json 时，此命令有效。

.represent current directory/.

.代表当前目录/。

回答by Adnan

One reason for this error might be

此错误的一个原因可能是

Could not find or load main class <class name>

无法找到或加载主类 <class name>

Maybe you use your class name as different name and save the class name with another name you can save a java source file name by another name than class name. For example:

也许您使用您的类名作为不同的名称并用另一个名称保存类名，您可以使用类名以外的其他名称保存java源文件名。例如：

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

you can save as Hello.javabut,

你可以另存为Hello.java但是，

To Compile : javac Hello.java

编译 ： javac Hello.java

This will auto generate A.class file at same location.

这将在同一位置自动生成 A.class 文件。

Now To Run        : java A

现在运行： java A

回答by decoder

I faced the same problem and tried everything mentioned here. The thing was I didn't refresh my project in eclipse after class creation . And once I refreshed it things worked as expected.

我遇到了同样的问题并尝试了这里提到的所有内容。问题是我没有在创建类后在 Eclipse 中刷新我的项目。一旦我刷新它，事情就会按预期进行。

回答by outlooker

I had the same problem, mine was a little different though I did not have a package name. My problem was the Class Path for example:

我有同样的问题，虽然我没有包名，但我的有点不同。我的问题是类路径，例如：

C:\Java Example>java -cp . HelloWorld 

The -cpoption for Java and from what I can tell from my experience (not much) but I encountered the error about 20 times trying different methods and until I declared the class Path I was receiving the same error. Vishrant was correct in stating that . represents current directory.

-cpJava的选项以及我从我的经验中可以看出的内容（不多），但是我尝试不同的方法时遇到了大约 20 次错误，直到我声明了类路径，我才收到相同的错误。Vishrant 的说法是正确的。代表当前目录。

If you need more information about the java options enter java -?or java -helpI think the options are not optional.

如果您需要有关 java 选项的更多信息，请输入java -?或者java -help我认为这些选项不是可选的。

I just did some more research I found a website that goes into detail about CLASSPATH. The CLASSPATHmust be set as an environment variable; to the current directory <.>. You can set it from the command line in windows:

我只是做了一些更多的研究，我找到了一个详细介绍CLASSPATH. 在CLASSPATH必须被设置为环境变量; 到当前目录 <.>。您可以从 Windows 中的命令行设置它：

// Set CLASSPATH to the current directory '.'
prompt> set CLASSPATH=.

When you add a new environment setting you need to reboot before enabling the variable. But from the command prompt you can set it. It also can be set like I mentioned at the beginning. For more info, and if your using a different OS, check: Environment Variables.

添加新环境设置时，您需要在启用变量之前重新启动。但是从命令提示符您可以设置它。它也可以像我开头提到的那样设置。有关更多信息，如果您使用不同的操作系统，请检查：环境变量。

回答by kenorb

Since you're running it from command prompt, you need to make sure your classpathis correct. If you set it already, you need to restart your terminal to re-load your system variables.

由于您是从命令提示符运行它，因此您需要确保您的类路径是正确的。如果您已经设置了它，则需要重新启动终端以重新加载系统变量。

If -classpathand -cpare not used and CLASSPATHis not set, the current directory is used (.), however when running .classfiles, you need to be in the folder which consist Java package namefolders.

如果-classpath和-cp不使用CLASSPATH且未设置，则使用当前目录（.），但是运行.class文件时，您需要在包含Java包名称文件夹的文件夹中。

So having the .classfile in ./target/classes/com/foo/app/App.class, you've the following possibilities:

因此，将.class文件放入 中./target/classes/com/foo/app/App.class，您有以下可能性：

java -cp target/classes com.foo.app.App
CLASSPATH=target/classes java com.foo.app.App
cd target/classes && java com.foo.app.App

You can check your classpath, by printing CLASSPATHvariable:

您可以通过打印CLASSPATH变量来检查您的类路径：

• Linux: echo $CLASSPATH
• Windows: echo %CLASSPATH%

• Linux： echo $CLASSPATH
• 视窗： echo %CLASSPATH%

which has entries separated by :.

其中条目由:.

See also: How do I run Java .class files?

另请参阅：如何运行 Java .class 文件？

回答by pev.hall

I had a similar problem when running java on win10

我在 win10 上运行 java 时遇到了类似的问题

instead of

代替

$ java ./hello
Error: Could not find or load main class ..hello

Run

跑

$ java hello
Hello, World

回答by Maria

faced the same problem. solved by following these steps

面临同样的问题。按照以下步骤解决

• go to directory containing the package 'org.tij.exercises' (e.g: in eclipse it may be your src folder)
• use java org.tij.exercises.HelloWorld

• 转到包含“org.tij.exercises”包的目录（例如：在 eclipse 中，它可能是您的 src 文件夹）
• 用 java org.tij.exercises.HelloWorld