Understanding JVM Basics for Effective Tuning

1. JVM Components and Workflow

JVM Components

Class loader, runtime data area (memory model), bytecode execution engine.

General Workflow

Java source is compiled to .class files. When execution starts, the JVM loads classes via the class loader into the runtime data area, then the bytecode execution engine runs the code.

Consider which component is tuned: the runtime data area.

Inspecting the underlying binary with javap can reveal assembly‑like output; consult the official documentation for each line's meaning.

A diagram shows the main method’s data in the runtime data area.

Garbage Collection

GC starts from "GC Roots" (thread stack locals, static variables, native method stack variables, etc.) and marks reachable objects; unmarked objects are garbage.

GC Roots include thread stack locals, static variables, native method stack variables, and similar references.

STW (Stop-The-World) pauses all Java threads while GC runs.

During STW, all Java threads stop (GC threads excluded).

The main optimization goal is to reduce STW time; JVM tuning focuses on STW operations.

Internal heap flow diagram:

Tuning Tools

1. java VisualVM (built‑in tool)

2. Alibaba Arthas (open‑source tuning tool)

That concludes the basic concepts.