How does annotation processors behave in multithreaded code?

Annotation processors in Java are primarily designed to operate in a single-threaded environment during the compilation process. When annotations are processed, the Java compiler invokes annotation processors in a sequential manner, which means that each processor runs to completion before the next one begins. This ensures that the order of execution is maintained and that no data races occur during the annotation processing phase.

However, if you use annotation processors that spawn threads to perform background tasks, you need to be cautious. You must manage concurrency properly, as multiple threads interacting with shared resources can lead to issues like race conditions, deadlocks, or inconsistent state.

It is important to note that while the annotation processing framework itself executes processors in a single-threaded manner, the logic within those processors can still be made concurrent if designed carefully. Always ensure that shared resources are properly synchronized, or use thread-safe collections to avoid multithreading complications.