Isis is build using Maven; details of setting up your development environment can be found here.
To build the source code from the command line, simply go to the root directory and type:
mvn clean install
The first time you do this, you'll find it takes a while since Maven needs to download all of the Isis prerequisites.
Thereafter you can speed up the build by adding the -o (offline flag):
mvn clean install -o
For the most part, though, you may want to rely on an IDE such as Eclipse to build the codebase for you. Both Eclipse and Idea (12.0+) support incremental background compilation.
When using Eclipse, a Maven profile is configured such that Eclipse compiles to target-ide directory rather than the usual target directory. You can therefore switch between Eclipse and Maven command line without one interfering with the other.
Alternatively, you can go to the root of any of the components and build each individually.
For example:
cd core mvn clean install
cd component/objectstore/jdo mvn clean install
cd component/viewer/wicket mvn clean install
A list of all of the Isis components can be found in our release process documentation.