## Application registration

Applications that use the application framework maintain their own data. This includes options, output components (via an output provider), streams, etc. Only a single application can be registered for each thread. Only once the application terminates and automatically unregisters itself, can a new application register itself in that thread. To run multiple applications in parallel, simply run them on different threads.

### Multi-threaded applications

All data stored for the application is wrapped in the `AppEnvData` class, and stored by the `AppEnv` class, on a per-thread basis. If your application uses multiple threads, you need to register each thread with the application framework. Use the `AppEnv.registerThread` method for this. This method requires the current application environment data as parameter, which may be obtained by using the `AppEnv.getData` method. To avoid managed memory leaks, always unregister threads once they are no longer used, by using the `AppEnv.unregisterThread` method.

### Unit tests

If unit tests use methods that depend on the application being registered, then the unit test will need to register an application. Examples of method using the application framework are methods that use options, or produce output via the application framework. Especially for unit tests, the `AppEnv.registerSimple` method can be used to register a dummy application. This method uses a default application environment, without an actual application, registers a default stream output provider, sets the output mode to errors and warnings only (no normal or debug output), and disables development mode. It can be used in a unit test class as follows:

    /***/ @BeforeAll
    public static void oneTimeSetUp() {
        AppEnv.registerSimple();
    }

    /***/ @AfterAll
    public static void oneTimeTearDown() {
        AppEnv.unregisterApplication();
    }

If any options are used, they will need to be available as well. For instance, one could add the following to the `oneTimeSetUp` method, or at the start of the actual unit test method:

    Options.set(SomeOption.class, <value>);

### Running an application from another application

As noted above, only a single application can be registered for a single thread. To start one application from another application, simply run the second application in a fresh thread. In the new thread, do the following:

- Construct the child application, using a constructor with the `AppStreams` argument, to pass along the streams from the parent application.

- Set the current working directory to the current working directory of the parent application.

- Obtain the Eclipse IDE console (if any) from the parent application, and couple it the child application.

- Run the child application.

After the child application thread finishes, make sure you:

- Restore the coupling between the Eclipse IDE console (if any) and the parent application.

- If the child application finished due to a termination request, request termination for the parent application.

- Decide what to do with the exit code of the child application. If it is non-zero, you’ll probably want to terminate the parent application.

To make it easier to follow this approach, the `ChildAppStarter.exec` methods can be used.