# Java in Docker pitfalls

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## Why Java is a special case

- JVM ergonomics
- Libraries ergonomics

JVM and libraries decide how much memory and how many threads to use.
This decision is based on a host hardware configuration. They are designed to take full advantage of all available resources.

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## Docker is a leaking abstraction

- Docker uses Linux namespaces and cgroups
- In a docker instance, an application can access a host hardware configuration
- Java doesn't know about Docker, cgroups and namespaces (before April 2017)

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## Memory examples

- What if memory limit less than JVM wants to allocate for heap
- What if memory limit is a little bit more than JVM wants to allocate for heap

JVM allocates memory for
- Heap
- Metaspace
- Code
- GC
- Threads
- Native objects

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## CPU examples

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## Java 9 and Java 8 u131(April 2017)

- CPU limits work out of the box
- for a memory, JVM option is still required

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## Q&A