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Table of Contents
2. Static Inner Class (Bill Pugh Solution)
3. Eager Initialization (Simple but Not Lazy)
4. Using Enum (Best for Serialization Safety)
Common Pitfalls to Avoid
Final Recommendations
Home Java javaTutorial Implementing a Thread-Safe Singleton in Java

Implementing a Thread-Safe Singleton in Java

Jul 24, 2025 am 03:35 AM

When using double check lock to implement lazy loading singletons, the volatile keyword is required to ensure thread visibility and prevent instruction re-arrangement; 2. It is recommended to use static internal classes (Bill Pugh scheme) to implement thread-safe lazy loading singletons, because the JVM ensures thread safety and no synchronization overhead; 3. If lazy loading is not required, a static constant can be used to implement simple and efficient singletons; 4. When serialization is involved, enumeration should be used because it can naturally prevent multiple instance problems caused by reflection and serialization; in summary, static internal classes are preferred in general scenarios, and enumeration is selected for serialization scenarios. Both have the advantages of thread safety, high performance and concise code.

Implementing a Thread-Safe Singleton in Java

Implementing a thread-safe singleton in Java is a common requirement in multi-threaded applications where you want to ensure that only one instance of a class exists and that it's safely accessible across multiple threads. Here's how to do it properly using different approaches, each with its own trade-offs.

Implementing a Thread-Safe Singleton in Java

1. Lazy Initialization with Double-Checked Locking

This approach creates the instance only when needed (lazy loading) and ensures thread safety with minimal performance overhead after the first creation.

 public class Singleton {
    // Volatile ensures visibility across threads
    private static volatile Singleton instance;

    private Singleton() {
        // Prevent instantiation via reflection
        if (instance != null) {
            throw new RuntimeException("Use getInstance() method to create instance.");
        }
    }

    public static Singleton getInstance() {
        if (instance == null) { // First check (no lock)
            synchronized (Singleton.class) {
                if (instance == null) { // Second check (with lock)
                    instance = new Singleton();
                }
            }
        }
        return instance;
    }
}

Key Points:

Implementing a Thread-Safe Singleton in Java
  • volatile keyword prevents instructions reordering and ensures that changes to instance are immediately visible to all threads.
  • The first null check avoids synchronization after the instance is created.
  • Double-checked locking reduces the overhead of acquiring a lock every time getInstance() is called.

?? Without volatile , this pattern can fail due to memory model issues — one thread might see a partially constructed object.


2. Static Inner Class (Bill Pugh Solution)

This is the most widely recommended approach for lazy, thread-safe singletons without synchronization overhead.

Implementing a Thread-Safe Singleton in Java
 public class Singleton {
    private Singleton() {
        if (InstanceHolder.INSTANCE != null) {
            throw new RuntimeException("Use getInstance() method.");
        }
    }

    public static Singleton getInstance() {
        return InstanceHolder.INSTANCE;
    }

    // Inner static class is not loaded until referenced
    private static class InstanceHolder {
        private static final Singleton INSTANCE = new Singleton();
    }
}

Why it works:

  • The inner class InstanceHolder is loaded only when getInstance() is called.
  • JVM guaranteees that class initialization is thread-safe.
  • No need for synchronized or volatile .
  • Lazy loading is achieved naturally.

? This is clean, efficient, and recommended for most use cases.


3. Eager Initialization (Simple but Not Lazy)

If you don't need lazy loading, this is the simplest and thread-safe way.

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

    private Singleton() {}

    public static Singleton getInstance() {
        return INSTANCE;
    }
}

Pros:

  • Simple and inherently thread-safe.
  • No synchronization needed.

Cons:

  • Instance is created at class loading time, even if never used.

Use this only if the cost of instantiation is low or guaranteed to be used.


4. Using Enum (Best for Serialization Safety)

Joshua Bloch in Effective Java recommends this approach, especially if serialization is involved.

 public enum Singleton {
    INSTANCE;

    public void doSomething() {
        // Your method logic
    }
}

Advantages:

  • Automatically thread-safe.
  • Prevents multiple instantiation via reflection, serialization, or deserialization.
  • Very concise.

Use case: When you want to be 100% safe from multiple instances in complex environments (eg, with serialization or reflection).


Common Pitfalls to Avoid

  • ? Basic lazy initialization without synchronization :

     public static Singleton getInstance() {
        if (instance == null) {
            instance = new Singleton(); // Not thread-safe!
        }
        return instance;
    }

    This can result in multiple instances if two threads enter the block simultaneously.

  • ? Synchronizing the whole method (inefficient):

     public static synchronized Singleton getInstance() {
        if (instance == null) {
            instance = new Singleton();
        }
        return instance;
    }

    This works but causes performance bottlenecks — synchronization is needed only once.


    Final Recommendations

    Use Case Recommended Approach
    General-purpose lazy singleton Static inner class
    Serialization involved Enum singleton
    Simple, early initialization Eager initialization
    Need fine-grained control (rare) Double-checked locking

    Basically, use the static inner class approach unless you're dealing with serialization — then go with enum . Both are thread-safe, clean, and perform.

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