Singleton design pattern in Java ensures that a class has only one instance and provides a global access point through private constructors and static methods, which is suitable for controlling access to shared resources. Implementation methods include: 1. Lazy loading, that is, creating an instance only when the first request is requested, which is suitable for situations where resource consumption is high and not necessarily required; 2. Thread-safe processing, ensuring that only one instance is created in a multi-threaded environment through synchronization methods or double check locking, and reducing performance impact; 3. Hungry loading, which directly initializes the instance during class loading, is suitable for lightweight objects or scenarios that can be initialized in advance; 4. Enumeration implementation, using Java enumeration to naturally support serialization, thread safety and prevent reflective attacks, is a recommended concise and reliable method. Different implementation methods can be selected according to specific needs to meet performance and security requirements.

A singleton design pattern in Java ensures that a class has only one instance and provides a global point of access to it. This is useful when you want to control access to shared resources, like configuration settings or connection pools.

Here's how it works and how you can implement it effectively.

Lazy Initialization – Create the Instance When Needed

Lazy initialization means the singleton instance is created only when it's first requested, not when the class loads. This can be helpful if creating the instance is resource-intensive and might not always be needed.

Here's a basic example:

 public class Singleton {
    private static Singleton instance;

    private Singleton() {}

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

• The constructor is private so no other class can instantiate it.
• The getInstance() method controls access to the single instance.
• This version isn't thread-safe — more on that next.

Use this approach when startup performance matters and the instance isn't always needed.

Thread-Safety – Make It Work Across Multiple Threads

If multiple threads call getInstance() at the same time, the basic lazy version might create multiple instances. To fix that, you can make the method synchronized:

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

But synchronizing the whole method can hurt performance. A better alternative is double-checked locking :

 private static volatile Singleton instance;

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

• Using volatile ensures visibility across threads.
• Double-checking avoids unnecessary synchronization after the instance is created.

This version gives you thread safety with minimal performance impact.

Eager Initialization – Create the Instance Upfront

If you know the instance will always be needed, you can create it when the class loads:

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

    private Singleton() {}

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

• Simpler and thread-safe without needing synchronization.
• Best for lightweight objects or when early creation is acceptable.

This is often the easiest way to implement a singleton if lazy loading isn't required.

Enum-Based Singleton – Cleanest and Safe Approach

Java enums provide a simple and effective way to implement singletons:

 public enum Singleton {
    INSTANCE;

    public void doSomething() {
        // method implementation
    }
}

• Enums are inherently serializable and thread-safe.
• Prevents reflection-based attacks that could create multiple instances.
• Simple syntax and safe by default.

If you're using Java 1.5 or later, this is generally the most robust option.

That's the core of how singletons work in Java — different methods suit different needs, but they all aim to enforce a single instance across your app.

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