JVM Internals

The JVM is the runtime that executes all bytecode — managing memory automatically, compiling hot code to native instructions via JIT, and loading classes on demand. Understanding JVM internals helps you diagnose OutOfMemoryError, tune GC pauses, and reason about performance at a level most developers never reach. It also answers interview questions that separate senior from junior engineers.

What You'll Find Here

Notes are being added. Planned topics:

Topic	Description
Class Loading	Bootstrap → Platform → Application classloaders; loading, linking, initialization phases.
JVM Memory Model	Heap regions (Eden, Survivor, Old gen), stack frames, Metaspace.
Garbage Collection	GC roots, reachability, Serial/Parallel/G1/ZGC algorithms, tuning flags.
JIT Compilation	Interpreter → C1 → C2 tiered compilation; inlining, escape analysis.
Bytecode & .class Files	javap disassembler, constant pool, understanding compiled output.

Learning Path

1. JVM Memory Model — understand heap regions and where objects live before studying GC.
2. Garbage Collection — GC roots, minor vs. major GC, and G1 as the default since Java 9.
3. Class Loading — the parent-delegation model; custom classloaders for hot reload and isolation.
4. JIT Compilation — tiered compilation explains "warmup" behavior in benchmarks and production services.
5. Bytecode — javap is a practical tool; reading bytecode demystifies generics, lambdas, and string + concatenation.

Related Domains

• Multithreading & Concurrency — the JVM memory model (happens-before) underpins thread visibility.
• Java Evolution — GC algorithms and JVM features evolve significantly across Java versions.
• Java Type System — type erasure is a consequence of JVM bytecode design.