Singleton Instance

Enforcing a Single Instance with the Singleton Property

After covering factories and builders, object creation control moves to a stricter goal: guaranteeing exactly one instance.

This requirement appears frequently in infrastructure code such as configuration loaders, audit loggers, and risk parameter registries.

Traditional Singleton Approach

A common implementation looks like this:

public class Config {
    public static final Config INSTANCE = new Config();
    private Config() { }
}        

The private constructor blocks direct instantiation through new.

This design appears safe but contains two structural weaknesses.

Problem 1: Reflection

Java reflection allows privileged code to bypass access checks. AccessibleObject.setAccessible(true) enables constructor invocation despite private visibility.

Result: multiple instances created inside the same JVM.

Problem 2: Serialization

During serialization and deserialization, Java creates a new object instance unless extra defensive code exists.

Loading the same singleton from disk twice produces two distinct objects.

Java Best Practices Recommendation: Enum Singleton

public enum Config {
    INSTANCE;

    public void refreshConfiguration() { }
}        

This approach enforces the singleton property at the JVM level.

Why Enum Wins

• Reflection safety The JVM forbids reflective creation of enum instances.
• Serialization safety Java guarantees a single enum instance during deserialization without custom logic.
• Simplicity No readResolve method. No defensive checks. Minimal code surface.

Example

Risk engines often rely on a single volatility surface cache shared across pricing threads. A broken singleton leads to inconsistent valuations between desks. Enum based singletons guarantee one authoritative data source across serialization boundaries, restarts, and parallel execution.

Takeaway

When a design requires exactly one instance:

• Class based singletons demand extra defenses.
• Enum based singletons enforce correctness by construction.

For infrastructure code under regulatory and audit pressure, enum singletons reduce failure modes while preserving clarity.