lessons

Lessons:

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

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

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 :)

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 Person.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 -cp .;jol-cli-0.17-full.jar org.openjdk.jol.Main estimates <classqualifiedname>
- (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.

Static initializer: initializes static fields.
Immutable values:
- constant value calculated at compile time, e.g. Kotlin const
- immutable, yet dynamically initializable
Boxing and Unboxing.
Boxing : <Wrapper>.valueOf(value)
1. Heap allocation
2. Object header initialization
3. Copy value
Unboxing: <Wrapper Instance>.<primitive>Value(), e.g. nr.intValue()

Homework 2 - virtual and non-virtual methods.
Kotlin Class Members
Analyzing Kotin properties in JVM.
There are NO properties in Java Type System.
A Kotlin property may generate:
- Backing field
- Getter, i.e. function get... -- called with invoke... bytecode.
- Setter, i.e. function set... (if defined with var).
Top-level declarations
Extension functions

Singleton design pattern
object keyword:
- private constructor
- Singleton instance in static INSTANCE field;
companion object - specific type of object declaration associated with its owner class.

Function Types
Kotlin compiler generates an _anonymous class: that implements the interface aligned with the respective function type.

Reflection object oriented API for metadata
Reflection ---> metadata ---> Type System
Type System: types have members
Kotlin Reflection API: KClass ----->* KCallable
- An instance of KClass may represent a type in Kotlin.
- An instance of KCallable may represent a member in Kotlin.
KCallable base type of KFunction and KProperty
KProperty and KMutableProperty
KCallable ----->* KParameter
KFunction properties: name, type, parameters and instanceParameter.
KParameter property kind: INSTANCE versus EXTENSION_RECEIVER
KParameter property isOptional
KClass::createInstance()
KFunction::call()

To directly reference a KType, we use the typeOf function:
- e.g. func.returnType != typeOf<Unit>()
KType holds information about nullability and type arguments.
KType properties: isMarkedNullable, arguments, and classifier.
- arguments provide information about the type arguments (i.e. List<KType>)
- classifier provides a reference to the associated class (i.e. KClassifier).
  - KClassifier is the base type of KClass

Implement an utility extension Appendable.log(obj:Any)
isAccessible - Provides a way to suppress JVM access checks for a callable.
Test with Kotlin domain classes and Java domain classes
Appendable.logGetters(obj:Any) in Kotlin to inspect Java getters:
- methods with prefix get
- a single argument corresponding to the instance parameter
- return type different from Unit:
- e.g. m.returnType.classifier != Unit::class

NaiveMapper, takes inspiration from libraries like AutoMapper or MapStruct:
- Simplify the process of mapping data between objects of different types by copying values from properties of one object to corresponding properties of another object.
- Offers an extension function like Any.mapTo(dest: KClass<*>): Any
1st version - through mutable properties (i.e. KMutableProperty):
- The destination type must have a parameterless constructor
- The source and destination properties share the same name and type
- The destination properties are mutable
2nd version - through constructor parameters:
- Call constructor via: fun call(vararg args: Any?): R