Given an webapp running with a JRE-based docker image in Kubernetes cluster, example: tomcat:9.0-jre11-temurin, then getting the thread dump is not easy as the JRE-based docker image does not include the tools like jstack, jmap...
We can use jattach tools to get our task done. TLDR; jattach is a compact tool written in C by Andrei Pangin.
First we need to download the tool to container:
Open the shell in the container:
kubectl -n default exec -it your-tomcat-pod -c your-tomcat-container -- bash
Download the jattach to the working dir
cd /working-dir
curl -L -O https://github.com/apangin/jattach/releases/download/v2.2/jattach
chmod +x jattach
From your terminal, create thread dump and store the result in the output file
kubectl -n default exec your-tomcat-pod \
-c your-tomcat-container -- \
/working-dir/jattach 1 threaddump > thread-dump.txt
Then you can use jstack.review tool to analyze the thread dump.
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