Java / Java 17 Garbage Collection Interview Questions

Define a stop-the-world pause?

A stop-the-world (STW) pause is a period during which the JVM suspends every application ("mutator") thread at a safepoint so the garbage collector can safely inspect and move objects without the program changing references out from under it.

STW pauses are necessary because moving or relocating live objects while application threads are simultaneously reading and writing references would produce a corrupted, inconsistent heap.

Collectors differ mainly in how much of their work happens inside an STW pause versus concurrently alongside the running application: Serial and Parallel GC do almost all their work stop-the-world, G1 confines STW pauses to specific phases (initial mark, remark, evacuation), and ZGC/Shenandoah keep STW pauses down to brief root-scanning steps, doing marking and relocation concurrently.

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A stop-the-world pause is necessary because: moving live objects while threads mutate references concurrently would corrupt the heap

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the JVM requires it to compile bytecode to native code

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it is the only way to load new classes

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network sockets must be closed before any GC

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Collectors mainly differ from one another in terms of stop-the-world behavior by: how much GC work happens inside STW pauses versus concurrently with the app

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whether they use GC roots at all

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whether they support the young generation

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whether they run inside the JVM process

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What is garbage collection in Java? What is the purpose of a garbage collector in the JVM? What are the types of garbage collectors available in Java 17? What is the default garbage collector in Java 17? What is the young generation in the JVM heap? What is the old (tenured) generation in the JVM heap? What is Metaspace in Java 17? What is a minor GC? What is a major GC? What is a full GC? Define a stop-the-world pause? What is the Eden space? What are Survivor spaces used for? How do you enable GC logging in Java 17? What is the purpose of the -Xmx and -Xms flags? Why is G1 GC the default collector since Java 9? How does G1 GC divide the heap into regions? What is a humongous object in G1 GC? How does G1 GC decide which regions to collect first? What is a remembered set in G1 GC? What is a card table and how does it help garbage collection? How do you configure G1 GC's pause time goal? What is the difference between G1 GC and Parallel GC? When should you choose ZGC over G1 GC? When should you choose Shenandoah over G1 GC? What happens during a G1 GC mixed collection? Why doesn't Java 17 include the CMS garbage collector? How is Parallel GC different from Serial GC? What is the difference between throughput and low-latency garbage collectors? How do you troubleshoot frequent full GCs in a Java 17 application? What is string deduplication in G1 GC? How does the JVM decide when to trigger a GC cycle? What is promotion in the context of generational garbage collection? Why should you avoid explicit calls to System.gc()? What is the difference between soft, weak, and phantom references? Explain the internal working of G1 GC's concurrent marking cycle? Explain the execution flow of a ZGC collection cycle? How does ZGC achieve sub-millisecond pause times using colored pointers? Explain the lifecycle of an object through the JVM heap under G1 GC? How does Shenandoah's Brooks pointer (forwarding pointer) enable concurrent compaction? What is an evacuation failure in G1 GC and how do you handle it? How does garbage collection behave differently in a containerized environment with cgroup memory limits? How can you optimize G1 GC for large heaps (32GB+)? Explain the tricolor marking algorithm used in concurrent garbage collectors? What are write barriers and load barriers, and how do collectors use them? How do you analyze a GC log to identify a memory leak? What is the difference between G1 GC's young-only phase and space-reclamation phase? How does adaptive sizing (ergonomics) work in the JVM's garbage collectors? Explain how class unloading interacts with garbage collection and Metaspace? Which is better for a low-latency trading application - ZGC or Shenandoah - and why?

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Java / Java 17 Garbage Collection Interview Questions

Define a stop-the-world pause?

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