How does Java application know it is running within a Docker container

Question

I have written a DockerFile for my application primarily to allow it to be run within a NAS machine (via Docker). The web interface allows the user to traverse the filesystem tree looking for Music files, but when using Docker the filesystem tree is irrelevant except for the /Music volume which is the mount point for the users actual Music folder on the NAS.

So I only want to display the /Music folder instead of the whole filesystem tree and to do that the application needs to be aware it is actually running within a Docker rather than an actual native Linux OS.

What is the correct way for the application to know it is in docker, the application is written in Java.

Well ist not a duplicate because this answer specifically asks how to check in Java, which is not addressed by other answer — Paul Taylor, Oct 01 '18 at 07:26

MohammadReza Alagheband · Accepted Answer · 2019-03-14T20:04:37.373

Solution

Check control group of the init process simply by /proc/1/cgroup .

if it is initiated normally all hierarchies have in / value
if it is initiated from docker container they have /docker/<container_id> value.

When running inside docker /proc/1/cgroup has values similar to :

11:perf_event:/docker/897df2a033d6ab07c357c1ac1f75741bd16474487de83c6d4d98518e5ef52249
10:memory:/docker/897df2a033d6ab07c357c1ac1f75741bd16474487de83c6d4d98518e5ef52249
9:cpuset:/docker/897df2a033d6ab07c357c1ac1f75741bd16474487de83c6d4d98518e5ef52249
8:net_cls,net_prio:/docker/897df2a033d6ab07c357c1ac1f75741bd16474487de83c6d4d98518e5ef52249
7:pids:/docker/897df2a033d6ab07c357c1ac1f75741bd16474487de83c6d4d98518e5ef52249
6:cpu,cpuacct:/docker/897df2a033d6ab07c357c1ac1f75741bd16474487de83c6d4d98518e5ef52249
5:blkio:/docker/897df2a033d6ab07c357c1ac1f75741bd16474487de83c6d4d98518e5ef52249
4:freezer:/docker/897df2a033d6ab07c357c1ac1f75741bd16474487de83c6d4d98518e5ef52249
3:hugetlb:/docker/897df2a033d6ab07c357c1ac1f75741bd16474487de83c6d4d98518e5ef52249
2:devices:/docker/897df2a033d6ab07c357c1ac1f75741bd16474487de83c6d4d98518e5ef52249
1:name=systemd:/docker/897df2a033d6ab07c357c1ac1f75741bd16474487de83c6d4d98518e5ef52249

Note: As @JanisKirsteins informed me, If you run your application in amazon ec2 you might want to change the condition to line.contains("/ecs") instead. because in /proc/1/cgroups you will find pattern similar to: /ecs/<uuid>/<uuid>

In Java

public static Boolean isRunningInsideDocker() {

        try (Stream < String > stream =
            Files.lines(Paths.get("/proc/1/cgroup"))) {
            return stream.anyMatch(line -> line.contains("/docker"));
        } catch (IOException e) {
            return false;
        }
    }

Live code checking

outside docker: running outside docker

inside docker : running inside docker

More Info

Worth noting need to wrap with check for file existent, i.e on Windows that would not have this file — Paul Taylor, Oct 01 '18 at 12:04
Dear @PaulTaylor , I updated my answer, when the file does not exist it throws an exception and you can return false from there , I have provided 2 links for live checking of the code: one of them is running inside docker, the other doesn't. — MohammadReza Alagheband, Oct 01 '18 at 13:12
This will miss e.g. applications running in ECS (where it'll be /ecs// instead) — Janis Kirsteins, Mar 13 '19 at 10:33
@JanisKirsteins thanks for having mentioned it, I was not aware of that, i'll update my post. — MohammadReza Alagheband, Mar 14 '19 at 19:51

techtabu · Answer 2 · 2018-09-30T18:47:37.543

-2

When you run the container add volume option.

docker run -v /Music:/Music <image-name>

Right side of : is where you application looking for the directory. I assumed it is /Music.

If you really want to check if your application is running inside the docker, you could check for the existence of file "/.dockerenv". Based on your question, one thing you should know is that when your application is running inside the docker, it does not have access to the filesystem of the host unless you give access via volumes. So, if your real linux machine has a directory /Music, your application has no access to it. You provide access via --volume. When you run application inside docker, you could consider it as it is running in another VM or machine for all practical purposes. That is the main purpose of Docker itself: provide isolation.

edited Sep 30 '18 at 18:47

answered Sep 30 '18 at 16:49

techtabu

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Yes I can do that, but that is not the question. How does my application know it is running inside Docker, it could check for existence of /Music folder but a real linix machine could also have a /Music folder, I was was asking if there was a standard way to identify if running in Docker, i.e does uname tell you this. – Paul Taylor Sep 30 '18 at 18:23
Updated the answer to answer your question. – techtabu Sep 30 '18 at 18:47
1

Aparently according to linked answer /.dockenv is unreliable solution. You are wrong about main use of Docker most Docker apps would be of no use if they could not access the real filessystem. – Paul Taylor Oct 01 '18 at 07:29

How does Java application know it is running within a Docker container

2 Answers2

Live code checking

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