If your module uses some external library, you will probably use a wrapper module to make classes from that library available to your module at runtime.
A wrapper module is a module that contains no code - really the only significant thing about it is its manifest, which does two significant things, in addition to the standard module unique ID/version/etc.:
Starting with NB 5.0 you can simply use File | New Project | NetBeans Plug-in Modules | Library Wrapper Module Project to make a library wrapper.
So a wrapper module acts as a proxy to turn a library into a NB module. Since you can't modify the NetBeans classpath directly (nor would you want to), this is the way you let your code use third party libraries. It serves the same function that running with java -cp or setting CLASS_PATH would do in a smaller Java application.
There are other options for packaging libraries described in when to use a wrapper module.
If the above was confusing, read the module dependencies FAQ entry.
If you are developing the library yourself, but decide you want to keep the library project separate from any NB module project, you can do so. Just make a plain Java project for the library and build it; and also create a library wrapper module from its JAR output. Here are two ways to hook them up. The first modifies the project so that when the project is built, it copies the jar to the wrapper module. The second modifies the wrapper module so that the wrapper cleans, builds and picks up the jar.
dist.jar=../../foo-wrapper/release/modules/ext/foo.jar
Now you can just build the j2seproject and it will update the wrapper's JAR file. Also code completion on anything that compiles against the foo library should "see" sources in /src/suite/libs/foo/src (so long as the j2seproject is open).
With these changes to a wrapper module, build/clean on the wrapper, or on the module suite that contains the wrapper, also does build/clean on the project.
For this example, my-wrapper is a library wrapper module for the jar file produced by the regular Java project called my-project. my-project and my-wrapper are in the same directory; this only affects relative path specifications and is not a general requirement. This example was created on NetBeans 5.5. If you have jars from multiple projects in a wrapper, then this example is extended by using <antsub> instead of <ant> and a FileSet in the release target's <copy> task.
Only the my-wrapper project needs modification.
<binary-origin>../my-project/dist/my-project.jar</binary-origin>
Make sure a ../src directory (relativly to the jar location) containing the corresponding sources of the library exist if you want go to source functionnality of netbeans works.
extra.module.files=modules/ext/my-project.jar
<property name="original.project.dir" value="../my-project"/>
<property name="original.project.jar"
value="${original.project.dir}/dist/my-project.jar"/>
<target name="release">
<echo message="Building ${original.project.dir}"/>
<ant dir="${original.project.dir}"
target="jar" inheritall="false" inheritrefs="false"/>
<echo message="Done building ${original.project.dir}"/>
<copy todir="${cluster}/modules/ext"
file="${original.project.jar}"/>
</target>
<target name="clean" depends="projectized-common.clean">
<echo message="Cleaning ${original.project.dir}"/>
<ant dir="${original.project.dir}"
target="clean" inheritall="false" inheritrefs="false"/>
<echo message="Done cleaning ${original.project.dir}"/>
</target>