HardExperience level: 8-15 years1 min read

What causes a ClassLoader leak on application redeploy, and how do you fix it?

Reviewed by Gurusankar M.

Asked inAmazonMicrosoftDeloitte
#classloader#memory leak#redeploy#tomcat#metaspace#production
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Short Answer

On undeploy the old web-app ClassLoader should be GC'd. If anything outside the app (a JVM-wide thread, ThreadLocal, JDBC driver, or static cache) still references a class loaded by it, the whole ClassLoader (and all its classes) is pinned — leaking Metaspace each redeploy until OOM.

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Coffee Chat Question

“What causes a ClassLoader leak on application redeploy, and how do you fix it?”

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Mind Map Answer

Pin sources
ThreadLocals, JVM threads, JDBC drivers
Also
static caches, shutdown hooks, MBeans
Symptom
Metaspace grows per redeploy → OOM

Think Beyond the Expected

What If?

How do you actually find what's pinning the old ClassLoader?

Heap-dump after a couple of redeploys and use MAT's 'Path to GC Roots' on the leaked WebappClassLoader instances — it shows the exact reference chain (e.g. a Thread's ThreadLocal or a static driver list) holding it.

Real World

The textbook case: an app registers a JDBC driver but never deregisters it, so the JVM's DriverManager (a system class) keeps a strong reference to the app's driver class — pinning the whole app ClassLoader on every redeploy.

Interviewer's Expectation

Keywords they're listening for:

  • per-webapp classloader
  • external strong ref pins it
  • ThreadLocal/driver/thread/static causes
  • Metaspace growth
  • MAT path-to-GC-roots

Common Mistakes

  • Not deregistering JDBC drivers / stopping threads on shutdown
  • Library ThreadLocals left set on container threads
  • Static caches holding app-loaded classes

Best Practices

  • Clean up in ServletContextListener.contextDestroyed (threads, drivers, ThreadLocals)
  • Monitor Metaspace; alert on per-redeploy growth
  • Prefer full restarts over hot redeploys in production

Follow-up Questions

  1. Why does deregistering the JDBC driver on shutdown matter?
  2. How is this different from a normal heap leak?
  3. How does Metaspace differ from the old PermGen?

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Plain-language foundations

I'm preparing for a software engineering interview and want to understand this from scratch, as a beginner.

Topic: JVM/Class loading (Core Java)
Interview question: "What causes a ClassLoader leak on application redeploy, and how do you fix it?"

Please:
1. Explain the core idea in simple, plain language, using an everyday analogy.
2. Define any technical terms you use.
3. Walk through one small, concrete example.
4. Finish with a single sentence I can easily remember.

Keep the tone friendly and assume I'm new to this topic.
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