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I am new to Java and I am not sure how apt is my question title. Please suggest any better title.

I have got a Java API which has many executable modules which do various things related to parsing MS Outlook's .pst files. And my problem is that I am getting this exception when I run the execute command to execute a .class file of one module of the API(java -classpath /opt/Java/libs/JPST/lib/jpst.jar /opt/Java/libs/JPST/examples/GetInboxMessages/bin/Example)-

Exception in thread "main" java.lang.NoClassDefFoundError: /opt/Java/libs/JPST/examples/GetInboxMessages/bin/Example
Caused by: java.lang.ClassNotFoundException: .opt.Java.libs.JPST.examples.GetInboxMessages.bin.Example

        at java.net.URLClassLoader$1.run(URLClassLoader.java:202)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:307)
        at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:248)
Could not find the main class: /opt/Java/libs/JPST/examples/GetInboxMessages/bin/Example.  Program will exit.

So far ...
You can check my previous question where I concluded that I was correctly running the compile and execute commands. But I am still getting the above mentioned `exception.

I asked the API's developer who says that I should get Eclipse or Netbeans IDE. He said that the

examples folder inside the API's folder structure is entire Eclipse workspace.

and also said that

It is easier to compile and run all examples.

And now ...
So, I have setup Eclipse Helios for Java. Can anyone tell me how is it easy to compile and run all the examples (I think the modules inside the examples folder) using the IDE. I have earlier worked with Eclipse for PHP but never for Java.

Folder structure of the API
I need to execute a .class file of one of those modules (many module folders are present inside examples directory). There are many such modules present inside the examples folder of the API. Each of these modules folders contain a bin folder (containing .class file) and src folder (containing .java file) . I need to execute one such .class file.

There is a lib directory containing a .jar file (which is needed to be referenced by my target .class file). The lib directory is at the same level as the examples directory.

Solution
I have verified that the solution given by Jonathan works (verified only without-using-elipse case). To make it more clear the classpath needs to include all the class locations in both compile and execute commands. So, if you need to compile and execute an Example.java file with a dependency Parent.jar do this:-

Compile

javac -classpath /path/to/Parent.jar/file/Parent.jar:/path/to/Example.java/file/ Example.java

Execute

java -classpath /path/to/Parent.jar/file/Parent.jar:/path/to/Example.class/file Example

And note the separator - should be : for linux and ; for windows.

Thanks,
Sandeepan

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Sandeepan Nath
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2 Answers2

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When Eclipse starts it should ask you which workspace to use. Select the examples folder and, if it really is an eclipse workspace, you should see all the projects already set up (it may take some time to build everything depending on the size of the projects and the speed of your machine). Then simply find the class you want and run it. If everything is set up correctly it should just work. If not you may have to go back to the developer for more details.

But I don't think you need Eclipse to run that one class. Looking at the stacktrace you provided at the beginning of your question it looks like it failed to run because you haven't set your classpath properly. Try:

java -classpath /opt/Java/libs/JPST/lib/jpst.jar:/opt/Java/libs/JPST/examples/GetInboxMessages/bin Example

To very briefly explain: The classpath lists the places where Java looks for the classes it needs. You specify the class to run relative to the classpath, not as a file path. Since the class you are trying to run in in the folder "/opt/Java/libs/JPST/examples/GetInboxMessages/bin" this folder must be on the classpath (note, I am assuming a unix based system, if you are using windows use ';' to separate entries not ':'). Then simply specify the class to run by itself relative to the classpath (which in this case is just the class name by itself).

Please be aware this is a very basic overview, things get a more complex if you start using packages (folders that would appear below the bin folder). This is one advantage of using a program like Eclipse since it will handle a lot of this for you.

Jonathan
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  • Great answer! Ignoring the eclipse settings part for now. Your tips to make it run without using eclipse worked perfectly fine. I was not setting the classpaths properly. I was assuming that only `jpst.jar` was to be added to the `classpath` because it was a dependency but like you have pointed out that as well as the `Example.class` file's path also needs to be added in the classpath. Great quality of answer. Keep up the good work! – Sandeepan Nath Jan 20 '11 at 06:36
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Sounds like a classpath issue.

  • Right click on your project in eclipse.

  • Go to buildPath > configure build path

  • Make sure the jar file is referenced in the libraries tab and if it isn't add it.
mR_fr0g
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  • Thanks for your answer. I did that. There are 4 tabs in "Java Build Path" menu. In the Libraries menu, that `jar` file is present. In the "Order and Export" menu the checkbox against that `.jar` file is not checked. – Sandeepan Nath Jan 19 '11 at 13:48