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Java / Java 17 Garbage Collection Interview Questions

Why is G1 GC the default collector since Java 9?

G1 replaced Parallel GC as the default in JDK 9 (JEP 248) because it offers a better all-around balance for the majority of modern workloads without requiring heavy manual tuning.

Its region-based heap layout lets it collect only the regions with the most reclaimable garbage first rather than being locked into fixed young/old boundaries, and it exposes a soft, configurable pause-time goal (-XX:MaxGCPauseMillis, 200ms by default) that it actively tries to honor by adjusting how much work it does per pause.

This made it a safer default than Parallel GC, which optimizes purely for throughput and can produce long, unpredictable old-gen pauses on larger heaps, and than the now-removed CMS, which was concurrent but non-compacting and prone to heap fragmentation over time.

G1 replaced which collector as the JVM's default in JDK 9?
A key reason G1 was chosen as a safer general-purpose default than CMS is that G1:

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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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