Thread and Heap Dumps in Docker Containers

Introduction

Sometimes we need to debug our docker applications with the use of Thread/Heap dump, below is a simple and quick way to do it from docker container

Environment

• Vagrant: (Ubuntu 18.04)
• Example of docker container: java-example

Thread dump

Following steps will allow take thread dumps from a docker container, and then copy saved file into to host

• Run example application:

root@vagrant:/home/vagrant# docker run -d bitnami/java-example:0.0.1

• Run the below command and go into the container.

root@vagrant:/home/vagrant# docker ps
CONTAINER ID        IMAGE                        COMMAND                  CREATED             STATUS              PORTS               NAMES
bf342a25fc61        bitnami/java-example:0.0.1   "java -jar jenkins.w…"   2 seconds ago       Up 1 second         8080/tcp            competent_dubinsky
root@vagrant:/home/vagrant# docker exec -it bf342a25fc61 bash

• Find PID of java process (First column is user that run the process, second column PID)

root@bf342a25fc61:/app# ps -fea|grep -i java
root         1     0  2 19:02 ?        00:00:06 java -jar jenkins.war
root        43    26  0 19:06 pts/0    00:00:00 grep -i java

• Run the below command to get the thread dump. (Change PID to number of process)

root@bf342a25fc61:/app# jstack PID > thread.tdump

• Exit from the docker container and download thethread.tdump from the docker container to host

root@bf342a25fc61:/app# exit
root@vagrant:/home/vagrant# docker ps
CONTAINER ID        IMAGE                        COMMAND                  CREATED             STATUS              PORTS               NAMES
bf342a25fc61        bitnami/java-example:0.0.1   "java -jar jenkins.w…"   8 minutes ago       Up 7 minutes        8080/tcp            competent_dubinsky
root@vagrant:/home/vagrant# docker cp bf342a25fc61:/app/thread.tdump .
root@vagrant:/home/vagrant# ls
file_with_password.txt  new_file.txt  thread.tdump

Heap Dump

Following steps will allow take heap dumps from a docker container, and then copy saved file into to host

• Run example application:

docker run -d bitnami/java-example:0.0.1

• Run the below command and go into the container.

root@vagrant:/home/vagrant# docker ps
CONTAINER ID        IMAGE                        COMMAND                  CREATED             STATUS              PORTS               NAMES
68e2cd8a3415        bitnami/java-example:0.0.1   "java -jar jenkins.w…"   3 seconds ago       Up 1 second         8080/tcp            nice_lehmann
root@vagrant:/home/vagrant# docker exec -it 68e2cd8a3415 bash

• Find PID of java process (First column is user that run the process, second column PID)

root@68e2cd8a3415:/app# ps -fea|grep -i java
root         1     0 10 19:14 ?        00:00:05 java -jar jenkins.war
root        32    26  0 19:14 pts/0    00:00:00 grep -i java

• Run the below command to get the heap dump. (Change PID to number of process)

root@68e2cd8a3415:/app# jmap -dump:live,format=b,file=heap.hprof 1
Dumping heap to /app/heap.hprof ...
Heap dump file created
root@68e2cd8a3415:/app#

• Exit from the docker container and download heap.hprof from the docker container to host.

root@68e2cd8a3415:/app# exit
exit
root@vagrant:/home/vagrant# docker cp 68e2cd8a3415:/app/heap.hprof .
root@vagrant:/home/vagrant# ls
heap.hprof thread.tdump