How to run a .class file that is part of a package

问题:

I keep getting errors when I make my .class part of a package and try to run it from cmd.

Here's the code that works after using javac and then java:

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

and then the code that does not work:

package com;

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

giving me this error after trying to run the program via the command: java HelloWorld:

Exception in thread "main" java.lang.NoClassDefFoundError: HelloWorld (wrong nam
e: com/HelloWorld)
    at java.lang.ClassLoader.defineClass1(Native Method)
    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$100(Unknown Source)
    at java.net.URLClassLoader$1.run(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)
    at sun.launcher.LauncherHelper.checkAndLoadMain(Unknown Source)

Here's what I've tried doing so far:

java -cp . HelloWorld
java -cp . com.HelloWorld
java -cp . com/HelloWorld
java HelloWorld
java com.HelloWorld
java com/HelloWorld

Keep in mind that javac returns with no errors and that simply removing package com; solves the problem. Sometimes in other scenarios I get an error that says the main class file cannot be found or something along those lines.

What am I doing wrong?

回答1:

Suppose you did cd C:/projects and HelloWorld.class is in C:/projects/com, then just type:

java com.HelloWorld

回答2:

Packages are directly related to the expected directory location of the file.

That is, if you have a source file with the package directive of com, it is expected that the file will live in the com directory.

In your HelloWorld example, it would be expected that the HelloWorld.java file would be stored in the com directory, like com\HelloWorld.java

When you compile the file, it will create a class file called HelloWorld.class in the com directory, like com\HelloWorld.class

This way, when Java goes looking for the com.HelloWorld class, it would actually be searching it's class path and appending com\HelloWorld.class to it until it finds your class file or runs out of class path

Example

So, I copied your HelloWorld.java (with package) example to C:\java\com\HelloWord.java

From the command line, I changed to the C:\java directory...

C:\java>dir com
 Volume in drive C is OS
 Volume Serial Number is ####-####

 Directory of C:\java\com

09/08/2013  01:55 PM    <DIR>          .
09/08/2013  01:55 PM    <DIR>          ..
09/08/2013  01:55 PM               135 HelloWorld.java

Then I compiled the HelloWorld.java

C:\java>javac com\HelloWorld.java

Then I ran it...

C:\java>java com.HelloWorld
Hello World!

---

You might like to have a read through Packages tutorial

回答3:

The syntax is:

java -classpath /path/to/package-folder <packageName>.<MainClassName>

So you may try: java com.HelloWorld which would expect com/HelloWorld.class file to be present as classpath by default points to the current directory (if not specified).

In case you're in different folder, try specifying classpath:

$ CLASSPATH=/path/to/package-folder/ java com.HelloWorld
Hello World!
$ java -cp /path/to/package-folder/ com.HelloWorld
Hello World!
$ cd /path/to/package-folder/ && java com.HelloWorld
Hello World!

For further explanation, check: How do I run Java .class files?

回答4:

Run the program from the parent directory of the com directory.

java com.HelloWorld

回答5:

Suppose the file locates at C:/projects/com/HelloWorld and you can try the following ways.

1.java -cp c:/projects com.HelloWorld

2.cd c:/projects
 java com.HelloWorld
(The example 2 is not working in many situation,such as java process.)

if there is no package declaration and there will be a little change.

1.java -cp c:/projects/com HelloWorld

2.cd c:/projects/com
 java HelloWorld
(It's not good with the same reason.)

alternatively,relative path will be ok but has some risk. Last remember put the class file in end of cmd.

回答6:

When you compile the java code, use -d, in your case, it would be

javac -d . com.HelloWorld.java

After the above command, java compiler generate a folder named "com", under the "com" folder, you will see your HelloWorld.class

Then under the same folder as you run javac, run the following command

java com.HelloWorld

回答7:

Try to use absolute directory or put your HelloWorld.class into ..\last_directory\com

1. java -cp .......\last_directory com.HelloWorld
2. java -cp .......\last_directory HelloWorld(with created com)

回答8:

Create a folder named com under Java folder and put the HelloWorld.java into com folder. Then run again javac and java.

回答9:

You do not need a -cp flag while running a java class , -cp needed while running a class or a main program from a binary (jar) file. While running a main program from a command line you need to make sure you have the class in the same folder structure as package name in the java file, ex.

home/user/foo java com.test.className   

here classNmae class file and exist under home/user/foo/com/test

hope it helps.

回答10:

There could be such a problem, when you are executing 'java' command directly from folder of your '*.class' files. This command should be executed from project 'parent' directory. All is well explained in the following article:

http://javarevisited.blogspot.de/2015/04/error-could-not-find-or-load-main-class-helloworld-java.html

回答11:

You should compile it first by typing this command in CMD, for exemple your file is in c:\ directory :

C:\com>javac HelloWorld.java

After that you can run the result by typing:

c:\com>java HelloWorld