How to Resolve 'Target Process Not Responding' Error with jstack on Ubuntu?

What causes the error 'target process not responding' when using jstack to get a thread dump of Tomcat on Ubuntu?

$ sudo jstack -l 5730

The 'target process not responding' error with jstack typically occurs when the JVM is not initiating properly, or when the process ID (PID) provided does not correspond to a running Java process with the HotSpot VM loaded. Let's explore the causes and solutions to this problem.

$ jstack -l -F 5730

Causes

• The specified PID does not belong to a running Java process.
• The Tomcat server may not have started correctly, resulting in an unresponsive JVM.
• The Environmental variables (like CATALINA_TMPDIR) are incorrectly configured or not set prior to starting Tomcat.
• The 'sudo' permission may restrict access to the JVM socket files.

Solutions

• Verify that you are using the correct PID for the Tomcat process. Use `jps` or `ps aux | grep tomcat` to confirm.
• Ensure the Tomcat server is up and running properly. Check Tomcat's logs for any errors during startup.
• Set the `CATALINA_TMPDIR` before starting Tomcat. Run `export CATALINA_TMPDIR=/tmp` and restart Tomcat with proper permissions.
• Use the `-F` option with jstack to force a thread dump when the process is not responding, though this might lead to incomplete information. Example: `sudo jstack -l -F <PID>`.

Mistake: Running jstack with an incorrect PID due to failing to verify which process is running.

Solution: Always validate the process ID using `jps` or `ps aux` commands.

Mistake: Not checking Tomcat logs for startup issues that may prevent the JVM from running properly.

Solution: Review Tomcat's logs located in `$CATALINA_HOME/logs` for errors during the startup.

Mistake: Misconfigured environment variables leading to issues in finding the JVM socket files.

Solution: Set the environment variable correctly before starting Tomcat, e.g., `export CATALINA_TMPDIR=/tmp`.

Mistake: Using the sudo command without switching users may lead to permission issues.

Solution: Use `sudo -u <username> jstack` to run jstack under the Tomcat user.