JVM

Summary and answers for 24 JVM interview questions - Juejin.

Troubleshooting memory saturation, CPU/thread problems, deadlocks, JVM tuning, and related references.

Reference counting, reachability analysis, GC Roots, concrete garbage collection algorithms, and references.

The class lifecycle: loading, linking, initialization, use, unloading, class loaders, parent delegation, and related questions.

JVM memory regions: shared heap and method area, per-thread stack and program counter, and PermGen/Metaspace.

Goals of tuning, overall production steps, JMeter load-test tuning, and CMS tuning flow.

Mark-sweep, mark-compact, copying, and generational collection.

When initialization is triggered, initialization rules, and examples for a single class and an inheritance hierarchy.

Historical rules and flow for object allocation in the JVM.

Generational collection and historical Serial, Parallel, CMS, and G1 collectors, including CMS/G1 details and comparison.

When a custom class loader is needed, how to implement one with findClass or loadClass, an example, and a reference.

What escape analysis is, how to enable it, optimizations for non-escaping objects, lock elimination, scalar replacement, stack allocation, and references.

Matching young- and old-generation collectors, JVM parameters, and how to choose.

Code, bytecode, explanation, and a proposed solution for object-creation instruction reordering.

Garbage-collection concepts, when GC happens, what gets collected, what GC does, and references.

Object composition, object headers, instance data, padding, and a JOL experiment.

Strong, soft, weak, and phantom references, examples, and WeakHashMap.

Oracle JDK and OpenJDK use direct pointers: fast access, but pointers need to be updated if GC moves surviving objects.

Why OOM occurs and examples for heap, Metaspace, stack overflow, GC overhead, direct buffer memory, and native-thread exhaustion.

Built-in JDK tools such as jps, jinfo, jstack, jmap, jstat, and jhat, plus JMX, MAT, Java Mission Control, BTrace, flame graphs, YourKit, JProfiler, and GC log tools.

Which JVM parameters to tune, JVM parameter types, how to inspect them, common options, examples, and a typical configuration.

Parallel GC logs, tools, and references.