lessons

Lessons:

Lesson 01 - Introduction and Modern VMs, e.g. JVM, Node

23-02-2024 (G0.08)

Bibliography and Lecturing methodology: github and slack.
Tools: javac, javap, kotlinc, JDK 17 and gradle.
Program outline in 3 parts:
1. Java Type System and Reflection;
2. Metaprogramming and Performance;
3. Lazy processing.
Project in 3 parts according to program outline.
Managed Runtime or Execution Environment or informally virtual machine (VM) or runtime.
Execution Environment includes:
- Compiler,
- Programming languages,
- Standard libraries,
- Dependency manager (e.g. gradle, maven)
- Central Repository (e.g. Maven Central Repository, Nuget, NPM, etc)
Examples of languages targeting JVM: Java, kotlin, Scala, Clojure.
Examples of languages targeting Node: JavaScript, TypeScript, Kotlin.
JVM runs .class files with bytecode
JVM translates bytecode to machine code (e.g. IA-32, AMD64, etc depending of the CPU)
In Javascript ecosystem modules are deployed in source code i.e. Javascript.
Distinguish between Programming Language <versus> VM

26-02-2024 (G0.08)

Compiled vs non compiles programming languages
- Compilation -> Transalation in compile time.
  - Translation occurs only once
- Interpretation -> Translation/Interpretation at runtime
  - Translation occurs each time the program runs
The need for Virtual Machines
- Reduce the number of needed compilers
- Code portability

One file .class for each class definition
Metadata:
- data that provides information about other data
- In a .class the metadata provides a description and structure of a Type.
Using javap -c -p AppKt.class to inspect metadata and bytecode definition of class AppKt
CLASSPATH
- e.g. -cp . - local folder
- e.g. -cp '.:~/JetBrains/IntelliJIdea2022.2/plugins/Kotlin/lib/*'
- (for windows use ; rather than :)

01-03-2024 (G0.08)

The fully qualified name of a class includes its package name.
In the JVM the types are always used with its full qualified name.
Distributing Java Programs
- Jar file creation
- Executable jar file creation including the main class fully qualified name in META-INF\MANIFEST.MF

04-03-2024 (G0.08)

Type System - Set of rules and principles that specify how types are defined and behave.
Two kinds of types: Primitive and Reference types.
Classes have Members
Members may be: Fields or Methods.
There are NO properties in Java Type System.
Using javap -p StudentPerson.class to inspect metadata
The fully qualified name of a class includes its package name.
Constructor is a method with the name <init> returning void.
Member access syntax: Receiver.Member.
The Receiver is the target of the member access and it is a Type (for static members) or an Object (for non-static/instance members).
JOL - Java Object Layout:
- java -jar jol-cli.jar internals -cp ./build/classes pt.isel.SavingsAccount
- (linux replace ; by : on -cp (classpath))
Object header = mark word (used for hash, locks, GC, etc) + class word (class-specific metadata).
Fields alignment, reorder and padding gap.
Immutable values:
- constant value calculated at compile time, e.g. Kotlin const
- immutable, yet dynamically initializable

08-03-2024 (G0.08)

Class inheritance and interface implementation
Polymorphism / dynamic method dispatch
At runtime there are always to types to consider in a variable
- The variable type - Can be any, including non-concrete types (e.g abstract classes and interfaces)
- The underlying object type: must be of a compatible type
A type D is compatible with type B if:
- D and B are the same type
- D is a derived type from B
- B is an interface and D is a type that extends or implements B