Java.Multithreading.Why enum singletone is it thread safe ?

The thread-safety of Enum singletons comes from how Java handles enums internally.

Here’s the magic:

---

🔧 1. Enum instances are created during class loading

When you declare an enum like this:

public enum Singleton {
    INSTANCE;
}

The INSTANCE is created once when the enum class Singleton is loaded into memory, and class loading is:

✅ Thread-safe by design (guaranteed by the Java Language Specification).

Only one thread can initialize a class at a time, so multiple threads can’t create multiple enum instances.

🧠 JVM Guarantees for Enum

• Enum constants (INSTANCE) are static final fields.
• Static initializers are run by the JVM in a synchronized way.
• So even if multiple threads call Singleton.INSTANCE simultaneously, the JVM ensures:
  • The enum class is loaded only once
  • The INSTANCE is created only once

📦 Behind the Scenes

The enum:

public enum Singleton {
    INSTANCE;
}

Is roughly compiled by the JVM into something like:

public final class Singleton extends Enum<Singleton> {
    public static final Singleton INSTANCE = new Singleton();

    private Singleton() {
        super("INSTANCE", 0);
    }
}

✅ INSTANCE is static final — initialized once, thread-safe.

---

💡 Enum = Built-in Singleton

JVM handles:

• Thread-safe instantiation ✅
• Serialization ✅
• Reflection attack prevention ✅

You don’t need to write synchronized, volatile, or worry about double-checked locking.

🔐 Summary: Why Enum Singleton is Thread-Safe

Reason	Explanation
Class loading is thread-safe	Only one thread loads the class and creates the instance
Enum instances are static final	Created once, safely, by JVM
No custom synchronization needed	JVM takes care of everything