How do I run Java .class files?

Question

I've compiled a HelloWorld program, and I'm using the command prompt to run it. The .class file is named HelloWorld2.class

The file is located in C:\Users\Matt\workspace\HelloWorld2\bin Here's what I'm getting when I go to command prompt, and type "Java HelloWorld2" :

C:\Users\Matt>Java HelloWorld2
Exception in thread "main" java.lang.NoClassDefFoundError: HelloWorld2
Caused by: java.lang.ClassNotFoundException: HelloWorld2
        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: HelloWorld2.  Program will exit.

I was expecting to see a HelloWorld printed out. What am I doing wrong? I have the JDK installed.

Answer 1

You need to set the classpath to find your compiled class:

java -cp C:\Users\Matt\workspace\HelloWorld2\bin HelloWorld2

Answer 2

To run Java class file from the command line, the syntax is:

java -classpath /path/to/jars <packageName>.<MainClassName>

where packageName (usually starts with either com or org) is the folder name where your class file is present.

For example if your main class name is App and Java package name of your app is com.foo.app, then your class file needs to be in com/foo/app folder (separate folder for each dot), so you run your app as:

$ java com.foo.app.App

^{Note: $ is indicating shell prompt, ignore it when typing}

If your class doesn't have any package name defined, simply run as: java App.

If you've any other jar dependencies, make sure you specified your classpath parameter either with -cp/-classpath or using CLASSPATH variable which points to the folder with your jar/war/ear/zip/class files. So on Linux you can prefix the command with: CLASSPATH=/path/to/jars, on Windows you need to add the folder into system variable. If not set, the user class path consists of the current directory (.).

---

Practical example

Given we've created sample project using Maven as:

$ mvn archetype:generate -DgroupId=com.foo.app -DartifactId=my-app -DarchetypeArtifactId=maven-archetype-quickstart -DinteractiveMode=false 

and we've compiled our project by mvn compile in our my-app/ dir, it'll generate our class file is in target/classes/com/foo/app/App.class.

To run it, we can either specify class path via -cp or going to it directly, check examples below:

$ find . -name "*.class"
./target/classes/com/foo/app/App.class
$ CLASSPATH=target/classes/ java com.foo.app.App
Hello World!
$ java -cp target/classes com.foo.app.App
Hello World!
$ java -classpath .:/path/to/other-jars:target/classes com.foo.app.App
Hello World!
$ cd target/classes && java com.foo.app.App
Hello World!

To double check your class and package name, you can use Java class file disassembler tool, e.g.:

$ javap target/classes/com/foo/app/App.class
Compiled from "App.java"
public class com.foo.app.App {
  public com.foo.app.App();
  public static void main(java.lang.String[]);
}

^{Note: javap won't work if the compiled file has been obfuscated.}

Answer 3

This can mean a lot of things, but the most common one is that the class contained in the file doesn't have the same name as the file itself. So, check if your class is also called HelloWorld2.