How to write a thread-safe Singleton?

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[Method 1]
public class MySingleton 
{
    private static MySingleton instance = new MySingleton();
    private MySingleton() 
    {
        // construct the singleton instance
    }
    
    public static MySingleton getInstance() 
    {
        return instance;
    }
}

[Method 2]
public class MySingleton {
    private static MySingleton instance = null;
    private MySingleton() {
        // construct the singleton instance
    }
    public synchronized static MySingleton getInstance() {
        if (instance == null) {
            instance = new MySingleton();
        }
        return instance;
    }
}

[Method 3]
public class MySingleton {
    private static boolean initialized = false;
    private static Object lock = new Object();
    private static MySingleton instance = null;
    private MySingleton() {
        // construct the singleton instance
    }
    public static MySingleton getInstance() {
        if (!initialized) {
            synchronized(lock) {
                if (instance == null) {
                    instance = new MySingleton();
                    initialized = true;
                }
            }
        }
        return instance;
    }
}