Can I run from command line program created by Ecl

Question

Using Eclipse I have created a SWT Hello World program. I was able to run this program from Eclipse and it worked fine.

In the "/home/myname/workspace/HelloWorldSWT" I found two files: HelloWorldSWT.java and HelloWorldSWT.class. I wanted to execute the corresponding program from the command line. First I tried to type "java HelloWorld" and I got the following error message:

Exception in thread "main" java.lang.NoClassDefFoundError: HelloWorld
   at gnu.java.lang.MainThread.run(libgcj.so.90)
Caused by: java.lang.ClassNotFoundException: HelloWorld not found in gnu.gcj.runtime.SystemClassLoader{urls=[file:./], parent=gnu.gcj.runtime.ExtensionClassLoader{urls=[], parent=null}}
   at java.net.URLClassLoader.findClass(libgcj.so.90)
   at gnu.gcj.runtime.SystemClassLoader.findClass(libgcj.so.90)
   at java.lang.ClassLoader.loadClass(libgcj.so.90)
   at java.lang.ClassLoader.loadClass(libgcj.so.90)
   at gnu.java.lang.MainThread.run(libgcj.so.90)

I also tried this "java -cp /home/roman/workspace/ HelloWorld.HelloWorld". As the result I got the following error message:

Exception in thread "main" java.lang.NoClassDefFoundError: loaded class HelloWorld.HelloWorld was in fact named HelloWorld
   at java.lang.VMClassLoader.defineClass(libgcj.so.90)
   at java.lang.ClassLoader.defineClass(libgcj.so.90)
   at java.security.SecureClassLoader.defineClass(libgcj.so.90)
   at java.net.URLClassLoader.findClass(libgcj.so.90)
   at gnu.gcj.runtime.SystemClassLoader.findClass(libgcj.so.90)
   at java.lang.ClassLoader.loadClass(libgcj.so.90)
   at java.lang.ClassLoader.loadClass(libgcj.so.90)
   at gnu.java.lang.MainThread.run(libgcj.so.90)

Does anybody know what I am doing wrong? Thank you in advance for any help.

Answer 1

I found another quick and dirty solution if you just want to see some output on the command line. But this is not a good practice for the long term!

1. Remove the package declaration in your code (Eclipse will complain and give you a red cross, ignore that first), e.g. package hello;

2. Save.

3. Open your command line and navigate until you are in the src folder (use cd).

4. Compile the java file, e.g. javac HelloWorld.java

5. Execute the java class file, e.g. java HelloWorld

It should work, as long as you are in the correct directory without any package declarations in the code! The package declarations do cause issues..and Eclipse is set up to make sure everything works so that's why it's fine to have the package declarations there. An alternative workaround is to really go and set the classpath on your own (which always confuses me every time I have to do it, which I rarely do).

Answer 2

Go to the Debug perspective, and select the program you just ran (where it says Terminated, exit value... in the Debug tab) Right click, and choose Properties, there you can see the whole command line command that was launched by eclipse.

You can run this same command in the same directory eclipse did (see in Run Configurations, Arguments, Working directory) and it will work.

Answer 3

You have not set classpath correctly then.

I think it is usually more convenient to have the IDE build a jar-file with all non-jre libs included and execute that from the command-line.

java -jar myprogram.jar

What is the use of the program written in Eclipse if it can be easily executed only within the Eclipse?

That's really a good question, and deployment of java applications is really an art in itself. Both compilation and execution is - if not complicated - at least cumbersome when performed by hand. But Eclipse and other tools like Netbeans can help you perform these tasks in an easy way, and even package up your program for you so that others that do not use these tools also can execute the programs easily.

Answer 4

You must add the directory where eclipse is storing the HelloWorldSWT.class file in the classpath. It is defined when the project is created, like "bin", "build" or "classes". Check the project properties or search the HelloWorldSWT.class file.

Assuming it is the build directory inside the HelloWorldSWT workspace and that your class is in no package (default package), the command should be:

java -cp /home/roman/workspace/HelloWorldSWT/build HelloWorldSWT

or just change to that directory and use:

java -cp . HelloWorldSWT

EDIT:
The -cp . means that only the actual directory is used for finding the class files. You must also add all libraries (JARs or other directories) used by your program (databse, SWT, ...).
For more details you can have a look at the documentation: How Classes are Found, Setting the Classpath and java comand

Answer 5

This is the problem which is caused because the JVM is not able to find the HelloWorld class as you have the class name as HelloWorldSWT with main method in it. try with

java HelloWorldSWT

Answer 6

If you want to run the program with command line arguments from within Eclipse you can go to Run->Run Configurations, which will bring up a window with the program you're running as well as a box where you can enter arguments.