Why Your Java App Freezes: Uncovering the Log4j Lock Bug and How to Fix It

We observed a SaaS application that intermittently slowed down, even during low traffic, and restarting only temporarily resolved the issue.

The slowdown was investigated by capturing a thread dump with jstack when the application was sluggish.

Obtaining a thread dump at the moment of failure is crucial!

The dump was uploaded to the online analysis tool fastthread.io , which immediately generated a report.

The report identified that thread http-nio-8080-exec-121 was blocking more than 100 other threads. The dependency graph shows these threads waiting for a lock held by that thread.

Inspecting the stack trace of the blocked thread reveals it has acquired the lock on org.apache.log4j.Logger and is performing other operations.

Other threads, such as http-nio-8080-exec-56, are in BLOCKED state waiting for the same org.apache.log4j.Logger lock.

Because the lock is never released, any thread that tries to log becomes blocked, causing the entire application to stall.

Troubleshooting

We searched for “org.apache.log4j.Logger blocked thread” and found a known Log4j bug (Bug 57714, reported in 2015). The bug causes any thread that attempts to log to block, effectively freezing the application.

Conclusion

Log4j has not been maintained since August 2015. If your application still uses Log4j, upgrade to Log4j 2, which is a completely rewritten framework with many performance improvements. Migrating resolves the blocking issue and prevents similar slowdowns.