For most users, the simplest method is to install the Ubuntu default JDK package, which currently provides OpenJDK 21 on the latest LTS releases: sudo apt update Install the JDK: sudo apt install default-jdk

This happens if you only installed the default-jre or openjdk-XX-jre package.

This will list all running Java applications with their PIDs and fully qualified class names. 2. Capture the Thread Dump

Reload the environment file:

Run sudo apt install default-jdk to ensure the full development kit is installed.

jstack -l <PID> > full_thread_dump.txt

Once installed, jstack allows you to take thread dumps to analyze why a Java application is frozen or using 100% CPU, making it a critical tool for any Ubuntu-based Java production environment.

The process for installing jstack on Ubuntu involves installing a compatible JDK package. The most straightforward method is using the Advanced Package Tool (APT). However, depending on your project's requirements, you may need a different version or distribution.

How to Install jstack on Ubuntu: A Complete Guide The jstack command-line utility is an essential tool for Java developers and system administrators. It prints Java thread stack traces for a given Java process, helping you diagnose performance bottlenecks, deadlocks, and high CPU usage.

This method provides more flexibility, allowing you to install a specific build from Oracle, Adoptium, or another vendor. It's particularly useful for development environments where you need a non-standard version or for servers with strict security policies that restrict internet access.

For the system to recognize the java , javac , and jstack commands, you need to set JAVA_HOME and update the PATH . You can do this for your current user by editing the ~/.bashrc file.

sudo jstack -F 12345

By following these steps, you should now have jstack installed on your Ubuntu system and be able to use it to troubleshoot Java applications.

If your Java process is running as the tomcat or www-data user, you must run jstack as that user using sudo : sudo -u tomcat jstack 12345 > thread_dump.txt Use code with caution.