用java -classpath写什么路径？

Question

I have a package with many files and directories (Californium, a CoAP protocol implementation). It comes as a Maven project. I have gone to the directory of the pom.xml file and ran mvn package in the terminal. Everything went fine. But upon going to one of the examples (Helloworldserver.class), and trying to run it with java HelloWorldServer I get:

Error: Could not find or load main class HelloWorldServer .

I look at the classpath by typing echo $CLASSPATH and I get nothing. So I try to set the classpath:

java -classpath org.eclipse.californium.examples

Which generates a description of all the possible options available for the java command, and does NOT set the classpath.

I chose the classpath in the above command because in the HelloWorldServer.java, there is the first line:

package org.eclipse.californium.examples;

I suspect this is wrong, judging by the output of my command. How is the correct way to specify the path? What do I want to point to?

Answer 1

When you built the code using Maven, it will have created a "target" directory, and within that a "classes" directory tree that contains the class files produced by the compiler.

Find that "target/classes" directory or whatever it is called. (It is called "target/classes" in the standard Maven file structure, but the POM file could specify otherwise.)

Now run the application like this:

  $ java -classpath target/classes:<dependency-1>:<dependency-2>:... \
        org.eclipse.californium.examples.HelloWorldServer

where <dependency-1> and <dependency-2> and so on are the pathnames for the dependency JAR files ... as downloaded into your local .m2 repo by Maven.

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The general pattern that the -classpath parameter is a colon-separated ¹ list of directory names, jar/zip file names or jar/zip file patterns. If you run against classes that have not been packaged into a JAR or ZIP file, you need to give the root of the class tree. For instance, if the full name for your HelloWorldServer class is

    org.eclipse.californium.examples.HelloWorldServer

then the target directory tree will look like this:

  ...
  target:
    classes:
      org:
        eclipse:
          californium:
            examples:
              HelloWorldServer.class

The "target/classes" is the correct root directory. The java command will attempt to navigate to the "HelloWorldServer.class" file based on the package naming.

Do not be tempted to do this:

$ java -classpath target/classes/org/eclipse/californium/examples:... \
     HelloWorldServer.class

It won't work.

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^{1 - or a semicolon on Windows.}