多线程高效单例模式

// 双检锁方式
public class Singleton {
    
    private static Singleton instance = null;

    private Singleton() {
    }

    public static Singleton getInstance() {
        if (instance == null) {    // 优化 synchronized 同步化方法效率
            synchronized (Singleton.class) {    // 保证下面线程安全
                if (instance == null) {
                    instance = new Singleton();   // 多线程可实例化多次
                }
            }
        }
        return instance;
    }
}

// 静态内部类方式：
public class Singleton {
    
    private static class SingletonHolder {
        public static Singleton instance = new Singleton();
    }

    private Singleton() {
    }

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