Understanding JVM Garbage Collection
The Java Virtual Machine manages memory automatically through garbage collection (GC). Understanding how GC works is essential for writing high-performance Java applications, especially in latency-sensitive systems.
Generational Hypothesis
Most objects die young. The JVM divides the heap into Young Generation (Eden + Survivor spaces) and Old Generation (Tenured). Minor GC collects Eden frequently; objects that survive multiple collections are promoted to Old Gen.
// Typical JVM flags for tuning
-Xms512m -Xmx512m
-XX:+UseG1GC
-XX:MaxGCPauseMillis=200
-XX:+PrintGCDetailsG1 Garbage Collector
G1 (Garbage-First) is the default collector since Java 9. It divides the heap into equal-sized regions and prioritizes regions with the most reclaimable space. This enables predictable pause times for large heaps.
Key metrics to monitor: GC pause time, allocation rate, and promotion rate. Tools like VisualVM, JFR, and async-profiler help identify allocation hotspots.
Common Pitfalls
- Creating excessive short-lived objects in hot loops
- Holding references longer than necessary (memory leaks)
- Using
System.gc()in production code - Undersized heap causing frequent Full GC
In the next post we'll explore ZGC and Shenandoah for sub-millisecond pause requirements...
