What Are Java Singleton Classes and When Should You Use One?

What is the purpose of Java Singleton classes, and under what circumstances should you implement one?

public class Singleton {
    // private static instance of Singleton
    private static Singleton instance;

    // private constructor to restrict instantiation
    private Singleton() {}

    // public method to provide access to the instance
    public static Singleton getInstance() {
        if (instance == null) {
            instance = new Singleton();
        }
        return instance;
    }
}

A Singleton class in Java ensures that a class has only one instance and provides a global point of access to it. This design pattern is particularly useful in situations where exactly one object is needed to coordinate actions across the system.

public class ConfigurationManager {
    private static ConfigurationManager instance;
    private Properties config;

    private ConfigurationManager() {
        config = new Properties();
        // load properties
    }

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

    public String getProperty(String key) {
        return config.getProperty(key);
    }
}

Causes

• When your application needs to manage global access to a particular resource.
• To control concurrent access to a shared resource, like a configuration manager or connection pool.
• To provide a centralized point of control or a utility across different parts of an application.

Solutions

• Use the Singleton pattern for resource management.
• Implement Singleton to maintain consistency for configuration settings.
• Utilize Singleton for logging mechanisms.

Mistake: Not synchronizing the getInstance method in a multi-threaded environment.

Solution: Use synchronized methods or implement double-checked locking.

Mistake: Failing to make the constructor private, allowing multiple instances to be created.

Solution: Ensure the constructor is private to prevent instantiation from other classes.

Mistake: Using lazy initialization without considering thread safety.

Solution: Consider using an eager initialization or synchronized block.