The IDE's module system is broken into several layers.

First there is the Open API: {@link org.openide.modules}. Relevant classes are {@link org.openide.modules.ModuleInfo}, giving general information about modules (version, name, etc.) and available through lookup; and {@link org.openide.modules.ModuleInstall}, permitting a module to customize its installation within limits.

In this package the remainder of the system is implemented, but this is still broken into pieces to make it easier to work with.

First {@link org.netbeans.core.modules.Module} is the concrete version of `ModuleInfo`, with additional information such as the full manifest contents, the live classloader in use, and some details of where it came from and its behavioral parameters.

{@link org.netbeans.core.modules.ModuleManager} is the class responsible for keeping track of the set of modules and their interdependencies, and ultimately controls which modules can or cannot be installed and why.

{@link org.netbeans.core.modules.ModuleInstaller} is the interface used by the manager to physically load the contents of the module into the IDE.

{@link org.netbeans.core.modules.Events} permits major events to be logged asynchronously for subsequent formatting and display.

Private helper classes include `ChangeFirer` to fire module manager changes, and `Util` for module-related utility functions. `MultiURLClassLoader` is an optimized classloader used for modules that is able to handle multiple parents, dynamic parent addition and removal, and streamlining of the package search algorithm based on features of the Modules API. Also the exceptions {@link org.netbeans.core.modules.DuplicateException} and {@link org.netbeans.core.modules.InvalidException} are thrown when creating or working with modules.

This layer is largely independent of the workings of the IDE; it uses some Open APIs but only those which can be used in a standalone context. So it makes sense to test these classes in isolation from the others, and without starting the IDE. It is not permitted to use UI and employs no localized messages. (Exception messages need not be localized; problems likely to be encountered by end users who are not module developers should be handled elsewhere are localized.)

The other layer is more dependent on the rest of the IDE core, and the details of the Modules API.

{@link org.netbeans.core.modules.NbInstaller `NbInstaller`} really implements the loading of modules: registering layers, registering the existence of help sets, loading the beans forming manifest sections and making them available, and running `ModuleInstall` methods.

{@link org.netbeans.core.modules.ManifestSection} is used by `NbInstaller` to represent one section in a module manifest during installation, and may be used in other parts of the core (e.g. if sections are registered via lookup for interpretation elsewhere).

{@link org.netbeans.core.modules.ModuleList} keeps track of the set of installed modules on disk, and is the point of contact for reading and writing the list of installed modules, any serialized `ModuleInstall` instances, and autoscanning the modules folder.

{@link org.netbeans.core.modules.ModuleHistory} tracks the origin of a module, whether it was installed before and where, and what version it last was. This information is supplied by `ModuleList` based on its knowledge of the settings on disk, and used by `NbInstaller` to invoke the correct `ModuleInstall` methods.

{@link org.netbeans.core.modules.NbEvents `NbEvents`} performs NetBeans-specific logging of module system events, such as displaying friendly messages in the splash screen, status bar, or console. It can also be a point of contact for performance measurements.

{@link org.netbeans.core.modules.ModuleSystem} is a high-level controller for the whole module system. Core initialization classes needing to start some process in the module system, such as "install newly found modules", should use a method call on this class, which will delegate the details to other module-system objects.

{@link org.netbeans.core.modules.TestModuleDeployer} is a service available via lookup which deploys test modules; accessible separately with a thin API for use by modules such as the API Support.

{@link AutomaticDependencies} is used to parse XML files constrained by `module-auto-deps-1_0.dtd` and automatically update dependencies used by old modules as a result. This is very useful for maintaining backward compatibility across major refactorings.

Also of interest: {@link org.netbeans.core.ui.ModuleBean} provides a Swing-safe UI wrapper for `Module` and `ModuleManager`, used by the Modules node.

Those interested in using the independent parts of the module system as a standalone JAR should run:

    ant -f core/build.xml lib-modules