Java Docker Container having OutOfMemoryError

I'm deploying a Java Spring-Boot application as a Docker container to servers having 16 GB of memory.

The Docker server version is 19.03.12 on a Ubuntu 18.04.4 LTS. My Docker image has openjdk-11 as the Java runtime installed. The entrypoint is defined as:

ENTRYPOINT ["java", "-Djava.security.egd=file:/dev/./urandom", "-jar", "app.jar"]

I start the container without any restrictions to CPU and memory, but the application dies always with an OutOfMemoryError when the Heap space reaches 8 GB. There are running some other containers, but the system has still around 3-4 GB of free memory.

Why is the JVM inside the container somehow limited to use 8 GB of memory, although is 'sees' the entire 16 GB? What do I have to do that the JVM is allowed to consume as much memory the system provides?

You can use command line switches to adjust the heap size available for the JVM. For example, to set a 12GB maximum heap which leaves 4GB for the operating system and other JVM functions, add the command line argument -Xmx 12G.

There is no way to tell Java it can use as much memory as it wants, or as much as the operating system has installed - see https://stackoverflow.com/questions/3358328/why-does-the-sun-jvm-have-a-fixed-upper-limit-for-memory-usage-xmx

You are assuming that JVM gives your application to the entirety of available memory. It does not. In fact the amount of memory is deliberately capped - the exact value depends on the system. You can read more about determinig how much memory is actually available to your app here.

As for how to manually manage how much emory your app is able to access read this article. It has a nice explanation of JVM parameters.