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Virtual Machines

  • Managed runtime or execution environment;
  • System virtual machines: full virtualization;
  • Process virtual machines: executes programs.
Virtual Machine IR Language
JVM bytecode Java, Kotlin, Scala
.NET IL C#, F#, VB
Node.js JavaScript JS, TS, Kotlin

IR: Intermediate Representation;

Managed Runtime

  • Portability: Compile once and run everywhere;
  • Safety: No invalid memory accesses;
  • Garbage collection;
  • Jitter: Just-in-time compiler; compiles IR to native code (e.g. x86, amd64, ppc) at runtime;
  • Classloader: lazy file loader (only loaded when needed) and dynamic (loaded at runtime);
  • Interoperability between the languages of the same VM (ex.: Java <=> Kotlin);
  • Type system: Set of rules and principles that specify how types are defined and behave.

Software components = bytecodes + metadata

Metadata

  • Describes the software;
  • Data with information about other data;
  • Equivalent to the header file.

In Kotlin, each class has an associated .class file, containing bytecode and metadata:

Kotlin Compilation

A .class file is generated for each type (class or interface). Files .class are also generated for global functions, with $$ in the file name.

Kotlin/Java Type System

  • A type system is a set of principles and rules that specify how types are defined and behave;
  • Types are defined by classes or interfaces;
  • Classes have members:
JVM Kotlin
Fields Properties
Methods Functions

The concept of property does not exist in JVM.

A property in Kotlin can generate:

  • Field: slot in memory where the property value is stored; accessed through the getfield instruction in bytecode;
  • Getter: method that returns the property value;
  • Setter: method that defines the property value.

The base type of the Kotlin type hierarchy is Any, while in Java it is Object:

Kotlin Java
Any Object
val final
new implicit new explicit

Software Component

  • Reusable software unit;
  • Contains IR and metadata;
  • Ready to use: no need for static compilation;
  • Indivisible;
  • Provided by the provider and used by the client;

The software developed by components promotes the reuse of code and reduces the complexity of development.

Notes

Compile: Translate/convert from one language to another;

  • Dynamic: at runtime;
  • Static: at build time;

Transpile: compilation from a high-level language to another.

Verbose: something expressed by more words than necessary (Java).

Implicit: something that is not formally expressed (in the source code) but is generated by the compiler.

Abstraction: separate the essential from the accessory, to focus attention on more important details.

WARNING: never give more permissions than necessary:

  • Immutability (val > var);
  • Privacy (private > protected > public).

Unmanaged Runtime vs. Managed Runtime

Unmanaged Runtime vs. Managed Runtime

Unmanaged Runtime

  • Component is not a unit, but header + object files:
    • header <=> metadata;
    • object <=> IR;
  • Static linking;
  • Structural modification: compilation + link;
  • Behavioral modification: link.

Unmanaged Runtime

When the link is static, the object files can be deleted and a.out runs the same.

Managed Runtime

  • Component is a unit: .class file;
  • Dynamic linking;
  • Jitter;
  • Lazy loading.

Managed Runtime