Programming Concurrency on the JVM

Isolating Mutability Using Actors

Java turned OOP into mutability-driven development,^[20] while functional programming emphasizes immutability; both extremes are problematic. If everything is mutable, we have to tackle visibility and race conditions. In a realistic application, everything can’t be immutable. Even pure functional languages provide restricted areas of code that allow side effects and ways to sequence them. Whichever programming model we favor, it’s clear we must avoid shared mutability.

Shared mutability—the root of concurrency problems—is where multiple threads can modify a variable. Isolated mutability—a nice compromise that removes most concurrency concerns—is where only one thread (or actor) can access a mutable variable, ...