单例模式

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1. 类图

Singleton：负责创建Singleton类自己的唯一实例，并提供一个getInstance的方法，让外部来访问这个类的唯一实例

2. 示例

2.1 懒汉式单例

/**
 * 饿汉式单例：初始化时直接创建
 */
public class HungrySingleton {
    private static HungrySingleton hungrySingleton = new HungrySingleton();

    private HungrySingleton(){}

    public static HungrySingleton getInstance(){
        return hungrySingleton;
    }
}

2.2 饿汉式单例

/**
 * 懒汉式单例：调用时创建
 */
public class LazySingleton {
    private static LazySingleton lazySingleton = null;

    private LazySingleton(){}

    public static LazySingleton getInstance(){
        if (lazySingleton == null) {
            lazySingleton = new LazySingleton();
        }
        return lazySingleton;
    }
}

2.3 双检锁

/**
 * 双检锁单例：volatile可能会屏蔽掉虚拟机中一些必要的代码优化，运行效率不是很高
 */
public class DoubleCheckSingleton {
    private volatile static DoubleCheckSingleton doubleCheckSingleton = null;

    private DoubleCheckSingleton(){}

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

2.4 静态内部类

/**
 * 静态内部类
 */
public class StaticInnerSingleton {
    private StaticInnerSingleton(){}

     // 类级的内部类，该内部类的实例与外部类的实例无绑定关系，且只有在被调用时才会装载，从而实现延迟加载
    private static class SingletonHolder{
        // 静态初始化器，由JVM来保证线程安全
        private static StaticInnerSingleton staticInnerSingleton = new StaticInnerSingleton();
    }

    public static StaticInnerSingleton getInstance(){
        return SingletonHolder.staticInnerSingleton;
    }
}

2.5 枚举

/**
 * 枚举单例：无偿提供了序列化机制，防止多次实例化
 */
public enum EnumSingelton {
    uniqueInstance;

    public void singletonOperation(){
        // 功能处理
    }
}