How to Attach a Debugger to a Java Application Not Started in Debug Mode

What can I do to debug a Java application that was started without debug arguments?

java -Xdebug -Xrunjdwp:transport=dt_socket,address=1000,server=y,suspend=n

Debugging a Java application that was not initiated with debug options can be challenging, especially in production environments where bugs cannot be easily reproduced. This guide outlines potential strategies for addressing this issue effectively.

// Example command to take a thread dump using jstack
jstack <pid_of_running_jvm> > thread_dump.txt

Causes

• The application was started without the required debug parameters.
• The bug occurs sporadically and cannot be consistently reproduced, making it impossible to restart with the correct parameters.

Solutions

• Use a monitoring tool such as VisualVM, JConsole, or Java Mission Control, which can provide runtime insights without requiring debug options at startup.
• Consider adding logging within your application to capture critical state information before the bug manifests, which can help in understanding the issue.
• Analyze the thread dump of the running Java application using tools like `jstack`. This can give insights into what the application is doing at the moment of the issue.
• If feasible, create a staging environment that closely mirrors the production setup, allowing you to reproduce the issue with debug parameters.

Mistake: Assuming that jdb can be used to attach to a running process without it being started with debug options.

Solution: Understand that jdb requires debug options at startup, and therefore consider alternative tools for analysis.

Mistake: Not implementing sufficient logging prior to a production release.

Solution: Ensure that your application has adequate logging to help trace issues in a production environment where debugging is limited.