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Java / JVM Architecture (Java21) Interview questions

What are the types of garbage collectors available in Java 21?

Java 21 ships five HotSpot garbage collectors that a developer can select with a command-line flag.

CollectorEnable flagFocus
Serial GC-XX:+UseSerialGCSingle-threaded, small heaps
Parallel GC-XX:+UseParallelGCMulti-threaded, throughput
G1 GC-XX:+UseG1GCBalanced pause/throughput (default)
ZGC-XX:+UseZGCVery low pause, large heaps
Shenandoah-XX:+UseShenandoahGCLow pause, concurrent compaction

G1 remains the default collector, while ZGC runs in its new generational mode by default as of Java 21 (JEP 439), replacing the older single-generation ZGC behavior.

The default garbage collector in Java 21 (when no flag is set) is:
Java 21 changed ZGC's default behavior to be:

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