Java Performance Tuning – Interview Refresher for Performance Engineers - Part 1

Java is one of the most common platforms for enterprise applications.

For performance engineers, understanding Java threads, concurrency, JVM internals, and tuning options is critical.

This post is a refresher on the most important Java performance tuning topics you need for interviews.

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🔹 1. Threads & Concurrency

Q1. Difference between process and thread

• Process: independent execution unit with its own memory space.
• Thread: lightweight execution unit within a process, shares memory. 💡 Interview Angle: Thread-level issues (locks, contention) often cause performance bottlenecks.

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Q2. Thread States

• NEW → RUNNABLE → BLOCKED → WAITING → TIMED_WAITING → TERMINATED 💡 Understand what causes BLOCKED vs WAITING → often asked in concurrency interviews.

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Q3. Thread Pools (ExecutorService)

ExecutorService pool = Executors.newFixedThreadPool(10);
pool.submit(() -> System.out.println("Task executed"));
pool.shutdown();

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• Prevents overhead of creating/destroying threads.
• Use bounded pools to avoid unbounded thread creation.

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Q4. Common Threading Issues

• Deadlocks
• Starvation
• Livelocks
• Race conditions 💡 Interview Angle: Explain with real examples — e.g., DB connection pool deadlocks.

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Q5. Concurrency Utilities

• ReentrantLock vs synchronized
• ConcurrentHashMap vs HashMap
• CountDownLatch, CyclicBarrier, Semaphore

👉 Example:

java
ConcurrentHashMap<String, String> cache = new ConcurrentHashMap<>();
cache.put("k1", "v1");


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🔹 2. JVM & Memory Model

Q6. JVM Memory Areas

• Heap → Young Gen, Old Gen
• Stack → per-thread method frames
• Metaspace → class metadata
• Code Cache → JIT compiled code

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Q7. Garbage Collectors

• Serial GC → single-threaded
• Parallel GC → multi-threaded
• CMS (deprecated) → concurrent mark-sweep
• G1 GC → balances latency & throughput
• ZGC / Shenandoah → low-latency

💡 Interview Angle: Be ready to answer “Which GC would you pick for low latency apps?” → G1/ZGC.

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Q8. GC Tuning Flags

bash
-XX:+UseG1GC
-XX:MaxGCPauseMillis=200
-Xms2g -Xmx2g


• Always set Xms = Xmx to avoid resizing overhead.
• Tune pause times for SLAs.

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Q9. OutOfMemory Errors

• Java heap space → insufficient heap.
• GC overhead limit exceeded → too much time in GC.
• Metaspace → class loading issues.

💡 Tools: jmap, jstack, jconsole, VisualVM.

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🔹 3. Performance Tuning Concepts

Q10. Object Creation

• Avoid unnecessary objects inside loops.
• Use StringBuilder instead of String concatenation.

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Q11. Caching

• In-memory caches (ConcurrentHashMap, Guava, Caffeine).
• Distributed caches (Redis, Hazelcast). 💡 Performance engineers should know how caching reduces DB calls.

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Q12. Connection Pools

• Use HikariCP, DBCP.
• Tune pool size = ((Core count * 2) + Effective Spindle Count).

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Q13. Synchronization Overhead

• Avoid global locks.
• Prefer lock-free structures (AtomicInteger, ConcurrentHashMap).

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Q14. Thread Dump Analysis

Command:

bash
jstack <pid>


Look for:

• Deadlocks
• High CPU threads (runnable state)
• Long GC pauses

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🔹 4. Key Java Performance Questions (Interview Style)

Q15. How do you detect a memory leak in Java?

• Monitor heap usage over time → if it grows endlessly = leak.
• Use tools: Eclipse MAT, VisualVM.

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Q16. What’s the difference between == and .equals()?

• == → reference comparison.
• .equals() → value comparison. 💡 Important in performance tuning when dealing with collections.

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Q17. Why is synchronized slow?

• Causes context switching and blocking.
• Alternatives: ReentrantLock, Atomic classes.

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Q18. How to reduce GC overhead?

• Reuse objects (object pools).
• Use primitive collections (Trove, FastUtil).
• Tune heap sizes and GC algorithm.

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Q19. What is false sharing?

• Multiple threads update variables on the same CPU cache line → performance degradation. 💡 Fix: use padding (@Contended annotation).

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Q20. What profiling tools do you use?

• JDK tools: jmap, jstack, jconsole, VisualVM
• APM tools: Dynatrace, AppDynamics, New Relic
• Others: YourKit, JProfiler

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✅ Final Takeaway

For Java performance tuning interviews, focus on:

• Threads & Concurrency → thread pools, locks, race conditions
• JVM Memory Model → heap, GC, tuning flags
• Performance Concepts → caching, connection pools, synchronization overhead
• Troubleshooting → thread dumps, memory leaks, profiling tools

If you can connect these concepts to real issues you solved in performance testing, you’ll stand out as a performance engineer, not just a tester.

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