The run and console actions provide a means for running user code in the same virtual machine as sbt. This page describes the problems with doing so, how sbt handles these problems, what types of code can use this feature, and what types of code must use a forked JVM. Skip to User Code if you just want to see when you should use a forked JVM.
User code can call System.exit, which normally shuts down the JVM. Because the run and console actions run inside the same JVM as sbt, this also ends the build and requires restarting sbt.
User code can also start other threads. Threads can be left running after the main method returns. In particular, creating a GUI creates several threads, some of which may not terminate until the JVM terminates. The program is not completed until either System.exit is called or all non-daemon threads terminate.
User code is run with a custom SecurityManager that throws a custom SecurityException when System.exit is called. This exception is caught by sbt. sbt then disposes of all top-level windows, interrupts (not stops) all user-created threads, and handles the exit code. If the exit code is nonzero, run and console complete unsuccessfully. If the exit code is zero, they complete normally.
sbt makes a list of all threads running before executing user code. After the user code returns, sbt can then determine the threads created by the user code. For each user-created thread, sbt replaces the uncaught exception handler with a custom one that handles the custom SecurityException thrown by calls to System.exit and delegates to the original handler for everything else. sbt then waits for each created thread to exit or for System.exit to be called. sbt handles a call to System.exit as described above.
A user-created thread is one that is not in the system thread group and is not an AWT implementation thread (e.g. AWT-XAWT, AWT-Windows). User-created threads include the AWT-EventQueue-* thread(s).
Given the above, when can user code be run with the run and console actions?
The user code cannot rely on shutdown hooks and at least one of the following situations must apply for user code to run in the same JVM:
The requirements on threading and shutdown hooks are required because the JVM does not actually shut down. So, shutdown hooks cannot be run and threads are not terminated unless they stop when interrupted. If these requirements are not met, code must run in a forked JVM.
The feature of allowing System.exit and multiple threads to be used cannot completely emulate the situation of running in a separate JVM and is intended for development. Program execution should be checked in a forked JVM when using multiple threads or System.exit.