懒汉模式

public class Singleton {
    private static Singleton instance;
    private Singleton(){}

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

饿汉模式

public class Singleton {
    private static Singleton instance = new Singleton();
    private Singleton(){}

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

双重检查锁

public class Singleton {
    private volatile static Singleton instance;
    private Singleton(){}

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