Comprehensive JVM Garbage Collection Interview Questions and Answers

1. Headline Interview: JVM heap partition from GC perspective

Answer: The Java heap is divided into the old generation and the young generation. The young generation further consists of the Eden space and two Survivor spaces (S0 and S1).

Why split the heap?

Over 90% of objects are short‑lived; placing newly created objects in Eden and long‑lived objects in the old generation improves garbage‑collection performance.

Why further subdivide the young generation?

Eden fills quickly; when it is full, a Minor GC moves surviving objects to Survivor spaces, reducing the frequency of Full GCs. Survivor spaces act as a buffer between Eden and the old generation.

Do Survivor spaces undergo GC?

Yes, but they are reclaimed implicitly during Eden’s GC; the default ratio of Eden:S0:S1 is 8:1:1.

Why not just one Eden and one Survivor?

The copy algorithm uses two Survivor spaces to avoid fragmentation and to ensure that live objects are copied into a contiguous region.

2. Meituan Interview: JVM garbage‑collection algorithms

The article lists seven algorithms:

1. Reachability analysis (mark phase)

Starts from GC roots (stack, static fields, etc.) and marks all reachable objects.

2. Mark‑Sweep (young‑gen)

Stops the world, marks reachable objects, then sweeps unmarked memory, which can cause fragmentation.

3. Copying algorithm (young‑gen)

Copies live objects to a new space, leaving the old space free; avoids fragmentation but requires extra memory.

4. Mark‑Compact (old‑gen)

Marks live objects, then compacts them to one end of the heap, eliminating fragmentation.

5. Generational collection

Uses different algorithms for young (copying) and old (mark‑sweep or mark‑compact) generations.

6. Incremental collection

Interleaves GC work with application threads to reduce pause times, at the cost of higher overall overhead.

7. Region‑based (G1) collection

Divides the heap into equal‑sized regions and collects them independently, allowing predictable pause times.

3. Didi Interview: Types of GC collectors and their pros/cons

Young‑generation collectors

Serial : Single‑threaded, uses copying; pauses all application threads.

ParNew : Multithreaded version of Serial.

ParallelScavenge : Targets overall throughput rather than minimal pause.

Old‑generation collectors

SerialOld : Single‑threaded, uses mark‑compact.

ParallelOld : Multithreaded, also uses mark‑compact.

CMS (Concurrent Mark‑Sweep) : Aims for short pauses; consists of initial mark, concurrent mark, remark, and concurrent sweep phases.

Heap‑wide collector

G1 : Introduced in JDK 7, default in JDK 9; partitions the heap into regions and performs incremental collection.

4. Bytedance Interview: Coordination between JVM memory regions

The runtime data area includes Method Area, Java Heap, Java Stacks, Native Method Stacks, and Program Counter. Class loaders, execution engine, and native libraries reside outside the runtime data area.

How do heap, stack, and method area interact?

Stacks hold references to heap objects; method area stores class metadata referenced by those objects; the program counter links to the current execution point.

Parent‑delegation mechanism

Ensures core Java classes are loaded by the bootstrap class loader, preventing tampering; certain frameworks (JDBC, Tomcat, OSGi) may break this mechanism.

Memory allocation strategies

Free‑list, pointer‑bumping, and CAS‑based allocation are discussed.

Why Metaspace?

Replaces the Permanent Generation to overcome its size limits; however, Metaspace can still suffer fragmentation.

5. Ant Financial Interview: G1 collector overview

G1 divides the heap into equal regions, allowing independent collection and more predictable pause times.

6. Bytedance Interview: CMS pause behavior

CMS performs two short STW pauses (initial mark and remark) to minimize impact while still achieving concurrent collection.

7. JD Interview: JVM tuning methodology

Three aspects: identify whether performance issues stem from code or JVM parameters, monitor and diagnose using tools (jstat, jmap, jvisualvm, MAT), and adjust parameters such as -Xms/-Xmx, -Xmn, and -XX:+DisableExplicitGC.

-XX:+PrintGCTimeStamps -XX:+PrintGCDetails -Xloggc:gc.log

-XX:+PrintGCDetails -XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=D:\jvm.dump

jmap -dump:format=b,file=D:/demo.hprof pid

8. Alibaba Interview: When does GC trigger?

Minor GC occurs when Eden is full; Full GC occurs when promotion fails, old‑gen usage exceeds a threshold, Metaspace is exhausted, or System.gc() is invoked.

9. Meituan Interview: Using jstat, jmap, MAT

Commands to locate process IDs, evaluate memory usage, generate heap dumps, and analyze them with MAT are provided.

10. Additional Q&A: Effect of enlarging Eden on Minor GC

Increasing Eden reduces Minor GC frequency but does not necessarily increase each GC’s duration; the impact depends on the proportion of long‑lived objects.

— End —