Things you need to know about JVM (that matter in Scala)
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Things you need to know about JVM (that matter in Scala)

About the Book

Scala is not Java and Java is not Scala. This may sound obvious, as both languages have different goals and features. They might generate different byte code and even when they do something apparently the same, they might still generate byte code differently. What matters is that at the end of the day you need to understand JVM very well in order to get to the bottom of what is happening, or why something was implemented the way it was. Even in Scala.

This book isn't aiming to be an exhaustive list of what you should know about JVM - that topic is so extensive that you could write a whole book (or books) on that. To meet that requirement you're better of to grab a book on preparing for Java Programmer Certified Professional exam.

The goal of my book is to give you enough knowledge to understand why certain decisions were made by Scala creators and library writers. Why we can do certain things and cannot do other. How it affects functional programming. Give you some general ideas about how things work, so that if you wanted to learn more, you would at least know what to google for.

The book is available as an early access including all the intended content. The author is continuing its editing as he believes that improvements should be driven by the feedback. If you found something that you believe could be done better, feel free to contact the author to help him make the book better for everyone.

The "main" version is PDF while EPUB is experimental, so please consider it a bonus.

About the Author

Mateusz Kubuszok
Mateusz Kubuszok

Mateusz Kubuszok is a software developer. Driven by his long-life curiosity and love for math he specialized in Scala development co-authoring e.g. Chimney library for data transformation and blogging about his findings at his blog, He is an active member of Polish Scala community.

Table of Contents

  1. What is JVM
    1. Primitives and references
    2. Operations
    3. More about memory
      1. Stack
      2. Heap
      3. Boxing and unboxing
      4. JVM types vs Scala type hierarchy
    4. Exceptions
    5. Classpath and ClassLoaders
      1. Classpath exception
  2. Classes
    1. What a normal class is made of
      1. Constructor
      2. Methods
      3. Inheritance
      4. Access control
    2. What are objects (modules)
    3. Parametric classes
      1. Type parameters and type erasure
      2. Issues with type erasure
      3. @specialized
    4. Functions vs methods
    5. case classes and case objects
    6. sealed hierarchies
    7. AnyVals
  3. Threads
    1. Parallel vs concurrent
      1. Concurrency
      2. Parallelism
      3. M:N threads
    2. JVM’s Threads
      1. Interruption and canceling
      2. synchronized and @volatile
      3. sleep, wait and notify
    3. Can we avoid concurrency pitfalls?
  4. Garbage collectors
    1. GC root
    2. How JVM cleans up memory
      1. Mark and sweep
      2. Memory fragmentation
      3. Generations, Serial GC and parallel GC
      4. Concurrent Mark-Sweep
      5. Garbage First (G1)
      6. Zero Garbage Collection (ZGC)
      7. No GC at all?
      8. GCs beyond HotSpot
    3. Value specially treated by memory management
    4. Memory leaks
    5. How does memory matter for Scala?
  5. Reflection
    1. What Class[_] knows?
    2. How to obtain Class[_]?
    3. Best thing since sliced bread?
    4. Reflection in Scala
    5. Why it matters that you understand reflection?
  6. Optimizations
    1. HotSpot
    2. C1 and C2
      1. Architecture Description (.ad files)
    3. Observing JIT at work
      1. Printing assembly
      2. Printing compilation and inlining
      3. JITWatch
    4. Microbenchmarks
      1. Example benchmark
      2. Flight Recorder and Java Mission Control
      3. async -profiler and flame graph
    5. Some of the optimizations done by HotSpot
      1. Inlining
      2. Loop unrooling
      3. Dead code elimination
      4. Escape analysis
      5. Mono-, Bi- and Megamorphism and devirtualization
      6. On Stack Replacement (OSR)
      7. Reflection and optimizations
      8. More about JVM optimizations
    6. Optimizations beyond JVM
      1. Memory alignment
      2. Branch prediction
    7. Why it matters to have some idea about JIT?
  7. Tooling and ecosystem
    1. Official tools
      1. javap and scalap
      2. jdb
      3. jps
      4. jstat and jstatd
      5. jstack
      6. jcmd
      7. Other official tools
    2. Third-party tools
      1. jEnv
      2. Dependency management and building
    3. Printing all JVM flags
  8. Summary
  9. Appendix: JVM flags
    1. GC and memory flags
      1. Generation-based memory layout and flags
      2. GC selection
      3. Memory debugging flags
    2. JIT flags
      1. Compilation mode selection
      2. Compilation options
      3. Compilation debugging

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