JVM Internals: Advanced

TLAB is a per-thread private buffer in Eden space. Object allocation is a lock-free pointer bump, making new extremely fast.

Escape analysis checks if an object escapes its creating method. If not, the JIT eliminates heap allocation (scalar replacement) or removes locks (lock elision).

GC roots are the starting points of reachability analysis. Objects reachable from roots are live. Everything else is garbage to be collected.

Compressed Oops use 4 bytes for object references on 64-bit JVMs with <32GB heap, saving ~20-30% memory by encoding 35-bit addresses in 32 bits via 8-byte alignment.

Every Java object has a 12-byte header (mark word + class pointer). Project Lilliput aims to reduce this to 8 bytes, saving memory for applications with many small objects.

A safepoint is a pause where all application threads stop so the JVM can safely perform GC, deoptimization, or thread dumps. TTSP measures how long it takes all threads to reach one.

G1 String Deduplication finds Strings with the same content and makes them share one byte[]. Saves memory without changing code. Enable with -XX:+UseStringDeduplication.

finalize() is deprecated because it is unpredictable, delays GC, and causes security issues. Use try-with-resources for deterministic cleanup and Cleaner for native resources.

Strong (never GC'd while reachable), Soft (GC'd when memory is low), Weak (GC'd at next GC), Phantom (post-finalization cleanup). Each is weaker than the previous.

WeakHashMap uses weak reference keys. When no strong reference to a key exists outside the map, the entry is automatically removed after GC.