97 Things Every Java Programmer Should Know

Chapter 4. Be Aware of Your Container Surroundings

David Delabassee

There is a danger to containerizing legacy Java applications as is, with their legacy Java Virtual Machine (JVM), because the ergonomics of those older JVMs will be fooled when running inside Docker containers.

Containers have become the de facto runtime packaging mechanism. They provide many benefits: a certain level of isolation, improved resource utilization, the ability to deploy applications across different environments, and more. Containers also help reduce the coupling between an application and the underlying platform, as that application can be packaged into a portable container. This technique is sometimes used to modernize legacy applications. In the case of Java, a container embeds a legacy Java application along with its dependencies, including an older version of the JVM used by that application.

The practice of containerizing legacy Java applications with their environments can certainly help keep older applications running on modern, supported infrastructure by decoupling them from older, unsupported infrastructure. But the potential benefits of such a practice come with their own set of risks due to the JVM ergonomics.

JVM ergonomics enables the JVM to tune itself by looking at two key environmental metrics: the number of CPUs and the available memory. With these metrics, the JVM determines important ...