Build a practical debugging habit with thread dumps, heap analysis, profiling, metrics, and JVM diagnostic tools.
Advanced Java · operations
Use the strong answer as your reference, explain it aloud in your own words, then choose how soon you want to review it again.
Interview prompt
Strong answer
Performance work starts with a reproducible workload, a measurable target, and a baseline. Use logs, metrics, thread dumps, heap information, profilers, and JVM tools to identify the bottleneck before changing code. Optimize allocation, contention, I/O, database calls, and algorithms based on evidence.
Key points
Interviewer looks for
Explain your investigation sequence and what evidence would make you change your hypothesis.
Java example
1long started = System.nanoTime();
2service.handle(request);
3long micros = (System.nanoTime() - started) / 1_000;
4logger.info("requestMicros={}", micros);Topic interview questions
These questions are tied to this lesson. Read each question aloud, then compare your explanation with the strong answer.
Strong answer
Define the user-visible target, reproduce the workload, and capture a baseline. Without a measurable comparison, an optimization may improve a local metric while harming production behavior.
Strong answer
It shows thread states, stack traces, locks, and often deadlock information. It helps identify blocked threads, long waits, hot call paths, thread-pool starvation, and contention.
Strong answer
Capture one when retained memory or unexpected object growth needs to be identified. Inspect dominator trees, retained sizes, GC roots, and repeated object graphs rather than only counting allocations.
Strong answer
Latency is the time taken by an individual operation, while throughput is the amount of work completed per unit time. A system can improve throughput while making tail latency worse, so both need targets.
Strong answer
A profiler connects CPU, allocation, lock, and I/O costs to actual call paths under a workload. It directs effort toward the bottleneck instead of optimizing code that is not limiting the system.
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