饿汉单例

public class Singleton {

    private static Singleton singleton = new Singleton();

    private Singleton() {}

    public static Singleton createInstance() {
        return singleton;
    }
}

饱汉单例

public class Singleton {

    private static volatile Singleton baoHan;

    private Singleton() {
    }

    // double check 双重判断检查锁
    public static Singleton createInstance() {
        if (singleton == null) {
            synchronized (Singleton.class) {
                if (singleton == null) {
                    singleton = new Singleton();
                }
            }
        }
        return singleton;
    }
}