Java compilation/runtime classpath problems

Question

I am trying to build a Java SNMP client. I have a folder on my Ubuntu desktop called snmpclient. Inside that folder I have the main class Client.java and the snmp.jar library, which is used by the main class.

I managed to compile it sucessfully using the following command on the terminal:

~/Desktop$ javac snmpclient/Client.java -classpath ./snmpclient/snmp.jar

Then I tried to run it with this command:

~/Desktop$ java snmpclient.Client -classpath ./snmpclient/snmp.jar

But I am getting a "java.lang.ClassNotFoundException" error, saying it can't find the classes of the snmp library. I unzipped the jar file to make sure the classes I am using are all there, and they are.

Any idea on how I can solve this?

Does your Client.java have package snmpclient; instruction in the top? Give us the error text also, it helps a lot. — maksimov
– maksimov, Commented Jun 19, 2012 at 13:30
Yep I do have the package outlined on the Client.java class. I'll paste the errors right now. — Daniel Scocco
– Daniel Scocco, Commented Jun 19, 2012 at 13:32
Yes, I was thinking along the same lines just now: where your classes get compiled to, you need to tell Java about this location. — maksimov
– maksimov, Commented Jun 19, 2012 at 13:37

Brian Agnew · Accepted Answer · 2012-06-19 13:34:25Z

1

I would rearrange your args thus:

~/Desktop$ java -classpath ./snmpclient/snmp.jar snmpclient.Client

such that your classpath preceeds the class to run. Note that your classpath defaults to the current directory if you don't specify -classpath, so your full invocation should be:

~/Desktop$ java -classpath ./snmpclient/snmp.jar:. snmpclient.Client

to specify the root directory where your classes reside (that's the dot), plus the SNMP jar file.

The -classpath arg consists of jar files and paths to directories separated by colons. See here for more info on setting the classpath.

edited Jun 19, 2012 at 13:34

answered Jun 19, 2012 at 13:28

Brian Agnew

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4 Comments

Daniel Scocco Over a year ago

That second line worked like a charm. I didn't understand yet what the ":." do after the classpath though. Could you explain? Thanks a lot.

Brian Agnew Over a year ago

I've amended the answer appropriately. Let me know if no good

Daniel Scocco Over a year ago

Let me see if I got this: Without the colon the command won't work cause it will interpret everything as the classpath. With the colon it manages to separate what's the classpath and what's the class you want to invoke. Is that it? Anyway thanks, it helped a lot.

Brian Agnew Over a year ago

You need to specify the path to your classes (that's the dot) and the jar file that contains the 3rd party classes (that's the jar file). You can specify multiple entries in the classpath (e.g. many .jar files plus a couple of directories etc.)

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