Top 25 Garbage Collection Interview Questions for Experienced Developers

Top 25 Garbage Collection Interview Questions for Experienced Developers Work through these 15 questions out loud. Open any question for the full model answer, the trap to avoid, and the follow-up a panel usually asks.

1. What is Metaspace and how do class loading issues cause memory leaks?

   Production set MaxMetaspaceSize 256m with monitoring. Investigate Metaspace OOM: heap dump + classloader histogram. Avoid generating unlimited dynamic proxies. OSGi/script...

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2. JVM container awareness and memory settings for Kubernetes?

   Container limit 1Gi: JAVA_TOOL_OPTIONS=-XX:MaxRAMPercentage=75.0 -XX:InitialRAMPercentage=50.0. Monitor container memory vs heap used in Actuator metrics. Leave headroom for...

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3. A REST API is slow only in production, not locally. How do you debug it?

   Enable Micrometer timers and Hibernate statistics or datasource-proxy in staging with prod-like data. Check Hikari `connections-active`, JVM GC pauses, and Feign/RestClient...

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4. API returns correct data but response time is inconsistent. Why?

   Tail latency often from stop-the-world GC, connection pool contention, or cold cache. Monitor p99 separately from mean. ```yaml spring.datasource.hikari.maximum-pool-size: 10...

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5. Your Spring Boot app fails only when deployed in Docker. Why?

   Container `-Xmx256m` with 256Mi limit causes OOMKilled. Missing `DB_HOST` defaults to localhost inside container. ```dockerfile ENV JAVA_OPTS="-XX:MaxRAMPercentage=75.0" ```...

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6. Why is ThreadLocal problematic with virtual threads?

   Libraries storing per-request state in ThreadLocal allocate one entry per virtual thread — catastrophic memory with 100k concurrent requests. Prefer ScopedValue for request...

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7. Explain the happens-before relationship in the Java Memory Model.

   Thread A writes x=1 without sync; Thread B reads x may see 0 forever. volatile x or synchronized block creates happens-before. Classic DCL singleton needs volatile instance...

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8. What are GraalVM native image considerations for Spring Boot?

   Use spring-boot-maven-plugin native profile + GraalVM 21. Run integration tests on native binary. Register reflection for Jackson/custom serializers. Trade longer CI builds for...

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9. Your Spring Boot service crashes under high traffic. What could be the reasons?

   Common causes: too-small heap (`-Xmx`), Hikari pool smaller than concurrent requests, blocking calls on limited threads, missing backpressure. Fix: tune...

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10. Your application memory usage keeps increasing over time. What do you suspect?

    Suspect memory leak if heap used grows after GC without traffic drop. Capture heap dump on OOM (`-XX:+HeapDumpOnOutOfMemoryError`), compare dominator tree. Common in Spring:...

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11. Explain immutability and why String is immutable.

    Immutable objects cannot change after build — thread-safe without synchronization. String immutability allows intern pool, caching hashCode, and security when used as...

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12. Explain garbage collection basics in the JVM.

    New objects allocate in Eden; minor GC copies survivors to survivor spaces, then old gen. When old gen fills, major/full GC runs — longer pauses. Tune -Xmx, choose G1 for...

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13. How do CompletableFuture and virtual threads compare for concurrency?

    Use virtual threads when business logic is naturally blocking — call DB, call HTTP, return. Use CompletableFuture when composing multiple independent async results with...

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14. What are weak references and WeakHashMap use cases?

    Attach metadata to objects without extending their lifetime — canonical mapping while object lives. When key GCs, entry drops. Do not rely on timing. For API response caching...

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15. Compare G1, ZGC, and Shenandoah garbage collectors.

    Default G1 fine for most Spring Boot services — tune -XX:MaxGCPauseMillis=200. Payment API p99 SLA <10ms GC pause: try -XX:+UseZGC on Java 21 large heap. Benchmark with JMH and...

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