classpath - running a java program from the command line

Question

My code compiled fine with the following command:

javac -cp "../lib/*" AvroReader.java

(lib is where i put my jar files)

At run time I get a ClassNotFoundException on the following line:

DatumReader<?> dtmrdr = new GenericDatumReader();

It says it can't find org.apache.avro.generic.GenericDatumReader even though I've imported it.

Why is this happening?

Jugular

How are you running the program? You have to provide the classpath with all your dependencies when you run your application as well. – Anesthesia

I ran it by running java Generator . Generator is a class that calls a method in AvroReader.java – Jugular

Answer 1

Importing has nothing to do with loading classes or setting CLASSPATH.

Try this:

java -cp .;../lib/* Generator

Using the dot '.' as the first entry in the CLASSPATH assumes that the Generator.class file exists in the directory from which you're running java, and /lib is one level up from that directory. Adjust as needed if both of these are not correct.

Svoboda

I'm not sure I understand what you mean by "using . as the first entry in the CLASSPATH" – Jugular

Look very carefully at the compilation string I posted. See the dot just before the semi-colon? That's the step you missed that caused the JVM to not be able to find your Generator.class. – Svoboda

@DaoLam - The argument after -cp is the classpath. It is a list of directories separated by ; characters. The first item is . which means the current directory. The second item is ../lib/* which means everything in the ../lib directory. Duffymo is telling you that you need to include the current directory (the location of Generator.class) to your classpath. – Ayah

@duffymo: you don't have to be so harsh & discouraging. The fact that I'm here shows that I'm not a pro at Java yet and I'm still learning. After reading jahroy's comment, I realized what it means and I already know it from before. My brain was probably just dead at the time I posted that comment. – Jugular

Thanks though. It works but there's something I don't understand. After reading other posts (before you posted yours), I added the current directory to the classpath (which is supposed to be the same as "."), but it didn't work. Why is that? – Jugular

On Mac OS the file separator is colon(:) and need to enclose directories in quotes. So do java -cp .:"../lib/*" Generator – Circle

Yes. Nobody seemed to mind that missing detail when I answered this five years ago. Surely there are better uses for your time here than writing comments like these. – Svoboda

Answer 2

You should run the program including again the same cp:

java -cp "lib directory where i put all the jars" MainClassOfYourApplication

After you compiled it with:

javac -cp "lib directory where i put all the jars" AvroReader.java

More applied to your example:

First step(compile all the needed java files): javac -cp "path/to/jars/*" AvroReader.java //here you should include all the java files not yet compiled but which you need to run your app
Second step: java -cp "path/to/jars/*" package.subpackage1.subpackage2.Generator

Renown

You can't just use a directory of jars; you'd need a wildcard. A directory classpath entry is only for class files, AFAIK--a pointer to docs stating otherwise would be helpful. I'm looking at the 1.7 classpath docs. – Zofiazoha

I'm calling a method from AvroReader in another class. This is how I ran it: javac -cp "jar directory" AvroReader.java javac Generator.java java Generator I tried what you suggested java -cp "jar directory" Generator, but now it's saying that it can't find Generator. – Jugular

@Dave Newton: Yea I actually have a wildcard. It's basically something like "../lib/*" Sorry I wasn't being clear in my original post. – Jugular

@DaoLam - If you're just calling java Generator it won't work because you're not specifying a classpath. Your comment seems to contradict the info in your question, however... – Ayah

@Ayah Can you be more specific on how to run it? Thank you! :) – Jugular

@DaoLam - Read this answer, it tells you what to do: You need to include the classpath when you run the program AND when you compile it. – Ayah

Answer 3

To compile and execute java file on Linux System with external jar files :

javac -cp jar_file1.jar:jar_file2:jar_file3.jar:. java_program_name.java

java -cp new_mail_api.jar:activation.jar:additional.jar:.java_program_name

To compile and execute java file on Windows System with external jar files :

javac -cp jar_file1.jar;jar_file2;jar_file3.jar;. java_program_name.java

java -cp new_mail_api.jar;activation.jar;additional.jar;.java_program_name

In Unix of Linux, Java Classpath contains names of the directory with colon “:” separated, On Windows Java Classpath will be semicolon “;” separated while if you defined java classpath in Manifest file those will be space separated.

For more knowledge about Classpath, visit https://javarevisited.blogspot.com/2011/01/how-classpath-work-in-java.html

Acrogen