Learn

The JVM divides memory into a few regions:

• Heap — all new allocations live here. Subdivided:
  • Young Generation — Eden + Survivor spaces. New objects start here.
  • Old Generation (Tenured) — long-lived objects promoted from Young.
• Metaspace (replaced PermGen in Java 8) — class metadata.
• Stack — one per thread; method frames, local primitives.
• Native heap — JNI allocations, direct ByteBuffers.

Garbage collection lifecycle:

1. New object → Eden
2. Eden fills → minor GC copies live objects to Survivor space
3. Survivors that age out → promoted to Old
4. Old fills → major GC (or full GC) — slower, more thorough

Algorithms (set with -XX:+UseG1GC etc.):

• Serial GC — single-threaded, fine for small heaps
• Parallel GC — multi-threaded, throughput-focused
• G1GC — region-based, predictable pause times. Default in modern Java.
• ZGC, Shenandoah — sub-ms pauses, large heaps

Tuning flags:

-Xms256m       # initial heap
-Xmx2g         # max heap
-XX:+UseG1GC
-XX:MaxGCPauseMillis=200
-Xss512k       # thread stack size

Watching GC:

-Xlog:gc*           # Java 9+ unified logging
-Xlog:gc*:gc.log    # to file

Or at runtime, use jstat, jcmd, or VisualVM/JMC.

Object lifecycle insights:

• finalize() is deprecated — never reliable, never used in modern code
• Use try-with-resources / Cleaner (Java 9+) for cleanup
• Soft / Weak / Phantom references — for caches and reference-aware structures

Memory leaks in Java are usually:

• Static collections that grow unbounded
• Listeners not unregistered
• ThreadLocals that retain large values
• ClassLoader leaks (web apps that hot-reload)

Tools: VisualVM, YourKit, Eclipse MAT for heap dumps. jmap -dump:live,format=b,file=heap.hprof <pid> to capture, then analyze.