Jonathan Lalou's Blog

Lecturer

Per Minborg is a Java Core Library Developer at Oracle, specializing in language features and performance improvements. He has contributed to projects enhancing Java’s concurrency and immutability models, drawing from his background in database acceleration tools like Speedment.

Abstract

This investigation explores Java’s ‘final’ keyword as a mechanism for immutability, examining its semantic guarantees from compilation to runtime execution. It contextualizes the balance between mutable flexibility and immutable predictability, delving into JVM trust dynamics and recent proposals like strict finals. Through code demonstrations and JVM internals, the narrative assesses approaches for stronger immutability, including lazy constants, and their effects on threading, optimization, and application speed. Forward implications for Java’s roadmap are considered, emphasizing enhanced reliability and efficiency.

Balancing Mutability and Immutability in Java

Java’s design philosophy accommodates both mutability for dynamic state changes and immutability for stability, yet tensions arise in performance and safety. Immutability, praised in resources like Effective Java, ensures objects maintain a single state, enabling safe concurrent access without locks. Immutable instances can be cached or reused, functioning reliably as keys in collections—unlike mutables that may cause hash inconsistencies after alterations.

Mutability, however, provides adaptability for evolving data, crucial in interactive systems. The ‘final’ keyword aims to enforce constancy, but runtime behaviors complicate this. Declaring ‘final int value = 42;’ implies permanence, yet advanced techniques like reflection with Unsafe can modify it, breaching expectations.

This duality impacts optimizations: trusted finals allow constant propagation, minimizing memory accesses. Yet, Java permits post-constructor updates for deserialization, eroding trust. Demonstrations show reflection altering finals, highlighting vulnerabilities where JVMs must hedge against changes, forgoing inlining.

Historically, Java prioritized flexibility, allowing such mutations for practicality, but contemporary demands favor integrity. Initiatives like “final truly final” seek to ban post-construction changes, bolstering trust for aggressive enhancements.

JVM Trust Mechanisms and Final Field Handling

JVM trust refers to assuming ‘final’ fields remain unchanged post-initialization, enabling efficiencies like constant folding. Current semantics, however, permit reflective or deserialization modifications, limiting optimizations.

Examples illustrate:

class Coordinate {
    final int coord;
    Coordinate(int coord) { this.coord = coord; }
}

Post-constructor, ‘coord’ is trusted, supporting optimizations. Reflection via Field.setAccessible(true) overrides this, as Unsafe manipulations demonstrate. Performance tests reveal trusted finals outperform volatiles in accesses, but untrusted ones underperform.

Java’s model historically allowed mutations for deserialization, but stricter enforcement proposals aim to restrict this. Implications include safer multi-threading, as finals ensure visibility without volatiles.

Advancements in Strict Finals and Leak Prevention

Strict finals strengthen guarantees by prohibiting constructor leaks—publishing ‘this’ before all finals are set. This prevents partial states in concurrent environments, where threads might observe defaults like zero before assignments.

Problematic code:

class LeakyClass {
    final int val = 42;
    LeakyClass() { Registry.register(this); } // Leak
}

Leaks risk threads seeing val=0. Strict finals reject this at compile time, enforcing safe initialization.

Methodologically, this requires refactoring to avoid early publications, but yields threading reliability and optimization opportunities. Benchmarks quantify benefits: trusted paths execute quicker, with fewer barriers.

Lazy Constants for Deferred Initialization

Previewed in Java 25, lazy constants merge mutable deferral with final optimizations. Declared ‘lazy final int val;’, they compute once on access via suppliers:

lazy final int heavy = () -> heavyComputation();

JVM views them as constants after initialization, supporting folding. Use cases include infrequent or expensive values, avoiding startup costs.

Unlike volatiles, lazy constants ensure at-most-once execution, with threads blocking on contention. This fits singletons or caches, surpassing synchronized options in efficiency.

Roadmap and Performance Consequences

Strict finals and lazy constants fortify Java’s immutability, complementing concurrent trends. Consequences include accelerated, secure code, with JVMs exploiting trust for vector operations.

Developers balance: finals for absolutes, lazies for postponements. Roadmaps indicate stabilization in Java 26, expanding usage.

In overview, these evolutions make ‘final’ definitively final, boosting Java’s sturdiness and proficiency.

Links:

• Lecture video: https://www.youtube.com/watch?v=J754RsoUd00
• Per Minborg on Twitter/X: https://twitter.com/PMinborg
• Oracle website: https://www.oracle.com/