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Google Cloud Platform OpenJDK Docker Image

This repository contains the source for the Google-maintained OpenJDK docker image. This image can be used as the base image for running Java applications on Google App Engine Flexible Environment and Google Kubernetes Engine.

Repository/Tag Details

Supported images/tags include:

• gcr.io/google-appengine/openjdk:8

App Engine Flexible Environment

When using App Engine Flexible, you can use the runtime without worrying about Docker by specifying runtime: java in your app.yaml:

runtime: java
env: flex

The runtime image gcr.io/google-appenine/openjdk:8 will be automatically selected if you are attempting to deploy a JAR (*.jar file).

To select a jdk version, use the runtime_config.jdk field in app.yaml. Supported JDK versions include openjdk8 and openjdk11.

runtime: java
env: flex
runtime_config:
  jdk: openjdk8

If you want to use the image as a base for a custom runtime, you can specify runtime: custom in your app.yaml and then write the Dockerfile like this:

FROM gcr.io/google-appengine/openjdk
COPY your-application.jar $APP_DESTINATION

That will add the JAR in the correct location for the Docker container.

Once you have this configuration, you can use the Google Cloud SDK to deploy this directory containing the 2 configuration files and the JAR using:

gcloud app deploy app.yaml

Kubernetes Engine & other Docker hosts

For other Docker hosts, you'll need to create a Dockerfile based on this image that copies your application code and installs dependencies. For example:

FROM gcr.io/google-appengine/openjdk
COPY your-application.jar $APP_DESTINATION

You can then build the docker container using docker build or Google Cloud Container Builder. By default, the CMD is set to run the application JAR. You can change this by specifying your own CMD or ENTRYPOINT.

Container Memory Limits

The runtime will try to detect the container memory limit by looking at the /sys/fs/cgroup/memory/memory.limit_in_bytes file, which is automatically mounted by Docker. However, this may not work with other container runtimes. In those cases, to help the runtime compute accurate JVM memory defaults when running on Kubernetes, you can indicate memory limit through the Downward API.

To do so add an environment variable named KUBERNETES_MEMORY_LIMIT (This name is subject to change) with the value limits.memory and the name of your container. For example:

apiVersion: v1
kind: Pod
metadata:
  name: dapi-envars-resourcefieldref
spec:
  containers:
    - name: java-kubernetes-container
      image: gcr.io/google-appengine/openjdk
      resources:
        requests:
          memory: "32Mi"
        limits:
          memory: "64Mi"
      env:
        - name: KUBERNETES_MEMORY_LIMIT
          valueFrom:
            resourceFieldRef:
              containerName: java-kubernetes-container
              resource: limits.memory

The Default Entry Point

Any arguments passed to the entry point that are not executable are treated as arguments to the java command:

$ docker run openjdk -jar /usr/share/someapplication.jar

Any arguments passed to the entry point that are executable replace the default command, thus a shell could be run with:

> docker run -it --rm openjdk bash
root@c7b35e88ff93:/# 

Entry Point Features

The entry point for the openjdk8 image is docker-entrypoint.bash, which does the processing of the passed command line arguments to look for an executable alternative or arguments to the default command (java).

If the default command (java) is used, then the entry point sources the setup-env.d/, which looks for supported features to be enabled and/or configured. The following table indicates the environment variables that may be used to enable/disable/configure features, any default values if they are not set:

If not explicitly set, JAVA_OPTS is defaulted to

JAVA_OPTS:=-showversion \
           ${JAVA_TMP_OPTS} \
           ${DBG_AGENT} \
           ${PROFILER_AGENT} \
           ${JAVA_HEAP_OPTS} \
           ${JAVA_GC_OPTS} \
           ${JAVA_USER_OPTS}

The command line executed is effectively (where $@ are the args passed into the docker entry point):

java $JAVA_OPTS "$@"

JVM Shutdown Diagnostics

This feature is not enabled by default.

Sometimes it's necessary to obtain diagnostic information when the JVM is stopped using SIGTERM or docker stop. This may happen on App Engine flexible environment, when the autohealer decides to kill unhealthy VMs that have an app that is unresponsive due to a deadlock or high load and stopped returning requests, including health checks.

To help diagnose such situations the runtime provides support for outputting a thread dump and/or heap info upon JVM shutdown forced by the TERM signal.

The following environment variables should be used to enable app container shutdown reporting (must be set to true or false):

SHUTDOWN_LOGGING_THREAD_DUMP - output thread dump

SHUTDOWN_LOGGING_HEAP_INFO - output heap information

If enabled, the runtime provides a wrapper for the JVM that traps SIGTERM, and runs debugging tools on the JVM to emit the thread dump and heap information to stdout.

If you're running many VMs, sampling is supported by using the environment variable SHUTDOWN_LOGGING_SAMPLE_THRESHOLD which is an integer between 0 and 100. 0 means no VMs report logs, 100 means all VMs report logs. (If this env var is not set, we default to reporting for all VMs).

The Default Command

The default command will attempt to run app.jar in the current working directory. It's equivalent to:

$ docker run openjdk java -jar app.jar
Error: Unable to access jarfile app.jar

The error is normal because the default command is designed for child containers that have a Dockerfile definition like this:

FROM openjdk
ADD my_app_0.0.1.jar app.jar

Development Guide

• See instructions on how to build and test this image.

Contributing changes

• See CONTRIBUTING.md

Licensing

• See LICENSE.md