JVM Internals & Memory Management

How the JVM loads and runs your code, how memory is organised, how garbage collection works and how to find memory problems.

This section is being written. There are no lessons in it yet: below is the plan for what it will cover. It opens in the syllabus as soon as its first lessons are ready.

What you will be able to do

Explain how a .java file becomes running code: bytecode, class loading and the JIT
Describe where the JVM keeps classes, objects, local variables and threads in memory
Explain how garbage collection finds and removes unused objects
Choose basic heap and garbage collector settings and read a GC log
Find memory leaks and diagnose OutOfMemoryError and StackOverflowError with JDK tools

What it will cover

11.1From Source to Bytecode
Look inside a class file and see the bytecode javac produces.
- What javac does and what a .class file contains
- Reading bytecode with javap
- The stack-based bytecode model
- How the JDK tools fit together
11.2Class Loading
How and when the JVM finds, checks and prepares your classes.
- Loading, linking and initialization
- Bootstrap, platform and application class loaders
- Parent delegation
- When a class is initialized
- ClassNotFoundException versus NoClassDefFoundError
11.3Execution Engine & JIT
How bytecode actually runs and why Java code gets faster after it warms up.
- Interpreter and just-in-time compiler
- Hot methods and tiered compilation
- Inlining and escape analysis in simple terms
- Warm-up and why quick timing tests mislead
11.4JVM Memory Areas
The parts of JVM memory and what lives in each one.
- Heap, stack, metaspace and the program counter
- Stack frames, local variables and references
- Where static fields and the String pool live
- StackOverflowError and OutOfMemoryError: which area ran out
- Each thread's own stack
11.5Objects in Memory
How much space objects really take and what that means for your code.
- Object headers, fields and padding
- Primitive arrays versus arrays of wrappers
- Integer caching and String interning
- Why boxing in big collections costs memory
11.6Garbage Collection Basics
How the JVM decides an object is garbage and cleans it up.
- Reachability and GC roots
- Mark, sweep and compact
- Young and old generations
- Minor and full collections
- Stop-the-world pauses
11.7Garbage Collectors & JVM Options
The main collectors and the few JVM options an entry-level developer should know.
- Serial, Parallel, G1 and ZGC at a glance
- Heap size with -Xms and -Xmx
- Turning on and reading GC logs
- When to change settings and when to fix the code
11.8References & Memory Leaks
How Java programs leak memory even with a garbage collector, and the reference types that help.
- Strong, soft, weak and phantom references
- WeakHashMap
- Leaks from static collections and caches
- Leaks from listeners, unclosed resources and ThreadLocal
- Inner classes that hold the outer object
11.9Diagnosing Memory Problems
Use the JDK's own tools to see what the JVM is doing and find the cause of a problem.
- jcmd, jstat and jps
- Taking and reading a heap dump
- VisualVM and JDK Mission Control
- Reading an OutOfMemoryError message
11.10JVM Practice & Case Studies
Work through broken programs and find out what went wrong inside the JVM.
- Fix a program that runs out of heap
- Fix a deep-recursion StackOverflowError
- Find a leak from a growing cache
- Explain a class loading error

Start with what’s live

These sections are ready to study now.

See the whole roadmap

What you will be able to do

Explain how a .java file becomes running code: bytecode, class loading and the JIT

Describe where the JVM keeps classes, objects, local variables and threads in memory

Explain how garbage collection finds and removes unused objects

Choose basic heap and garbage collector settings and read a GC log

Find memory leaks and diagnose OutOfMemoryError and StackOverflowError with JDK tools

What it will cover

11.1From Source to Bytecode

Look inside a class file and see the bytecode javac produces.

What javac does and what a .class file contains
Reading bytecode with javap
The stack-based bytecode model
How the JDK tools fit together

11.2Class Loading

How and when the JVM finds, checks and prepares your classes.

Loading, linking and initialization
Bootstrap, platform and application class loaders
Parent delegation
When a class is initialized
ClassNotFoundException versus NoClassDefFoundError

11.3Execution Engine & JIT

How bytecode actually runs and why Java code gets faster after it warms up.

Interpreter and just-in-time compiler
Hot methods and tiered compilation
Inlining and escape analysis in simple terms
Warm-up and why quick timing tests mislead

11.4JVM Memory Areas

The parts of JVM memory and what lives in each one.

Heap, stack, metaspace and the program counter
Stack frames, local variables and references
Where static fields and the String pool live
StackOverflowError and OutOfMemoryError: which area ran out
Each thread's own stack

11.5Objects in Memory

How much space objects really take and what that means for your code.

Object headers, fields and padding
Primitive arrays versus arrays of wrappers
Integer caching and String interning
Why boxing in big collections costs memory

11.6Garbage Collection Basics

How the JVM decides an object is garbage and cleans it up.

Reachability and GC roots
Mark, sweep and compact
Young and old generations
Minor and full collections
Stop-the-world pauses

11.7Garbage Collectors & JVM Options

The main collectors and the few JVM options an entry-level developer should know.

Serial, Parallel, G1 and ZGC at a glance
Heap size with -Xms and -Xmx
Turning on and reading GC logs
When to change settings and when to fix the code

11.8References & Memory Leaks

How Java programs leak memory even with a garbage collector, and the reference types that help.

Strong, soft, weak and phantom references
WeakHashMap
Leaks from static collections and caches
Leaks from listeners, unclosed resources and ThreadLocal
Inner classes that hold the outer object

11.9Diagnosing Memory Problems

Use the JDK's own tools to see what the JVM is doing and find the cause of a problem.

jcmd, jstat and jps
Taking and reading a heap dump
VisualVM and JDK Mission Control
Reading an OutOfMemoryError message

11.10JVM Practice & Case Studies

Work through broken programs and find out what went wrong inside the JVM.

Fix a program that runs out of heap
Fix a deep-recursion StackOverflowError
Find a leak from a growing cache
Explain a class loading error