Whereas the synchronous flow of execution fulfills a lot of business use cases, other scenarios need asynchronous behavior. The Java EE environment sets a few constraints to the application in regard to threading. The container manages and pools resources and threads. External concurrency utilities outside of the container are not aware of these threads. Therefore, the application's code is not supposed to start and manage own threads, but to use Java EE functionality to do so. There are several APIs that natively support asynchronicity.