Chapter 4, Lightweight Java EE already showed the necessity of fast turnarounds. This principle of 12-factor applications requires technology that enables velocity and elasticity. In order to rapidly scale up, the software should startup in a matter of seconds, making it possible to tackle a growing workload.

Application shutdowns should gracefully finish in-flight requests and properly close all open connections and resources. Especially requests and transactions that are executed while the shutdown signal occurs should be finished properly not to maliciously abort client use cases. In a Unix process approach shutdown signals are sent as SIGTERM signals. Linux containers are stopped in the same way, giving the container process a chance to shutdown properly. When building container images, developers should pay attention that the process handles Unix signals properly, resulting in a graceful shutdown of the application server when it receives a SIGTERM signal.

Java EE supports both fast startups and graceful shutdowns. As shown previously, modern application servers start up and deploy applications in a matter of seconds.

Since the application servers manage beans, resources, pooling, and threading, they take care of closing the resources properly at JVM shutdown. The developers don't need to take care of this aspect themselves. Beans that manage custom resources or handles that need to be closed, use pre-destroy methods to implemented proper closing. The following shows a client control using a JAX-RS client handle which is closed on server shutdown:

@ApplicationScoped
public class CoffeePurchaser {

    private Client client;

    ...

    @PreDestroy
    public void closeClient() {
        client.close();
    }
}

The platform guarantees that the pre-destroy methods of all managed beans are called once the application server shuts down.