懒汉式

public class Singleton{
    private static Singleton singleton;
    private Singleton() {}
    public static Singleton getInstance() {
        if(null == singleton) {
            singleton = new Singleton();
        }
        return singleton;
    }
}

饿汉式

public class Singleton{
    private static Singleton singleton = new Singleton();
    private Singleton() {}
    public static Singleton getInstance() {
        return singleton;
    }
}

双检锁

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

枚举

@Getter
@AllArgsConstructor
public enum Singleton{
   INSTANCE(new Test());
   private Test test;
}