几种常用的单例模式.md

1. 懒汉模式

public class Singleton {
    private Singleton() {
    }

    private volatile static Singleton mInstance;// 第一层锁：保证变量可见性

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

}

2. 恶汉模式

class Singleton {
    private static final Singleton ourInstance = new Singleton();

    static Singleton getInstance() {
        return ourInstance;
    }

    private Singleton() {
    }
}

3. 静态内部类模式

public class Singleton {

    // 私有构造函数
    private Singleton() {
    }
    
    public static Singleton getInstance() {

        return SingletonHolder.instance;
    }

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

}

其中用的最多也是最常用的是第3种，静态内部类单例。