设计模式01 单例模式

单例模式(Singleton)定义：一个类只能有一个实例，且该类能自行创建这个实例的模式。

单例模式有三个特点：

1、单例类只有1个实例对象。

2、该单例对象必须由单例类自己创建。

3、单例类对外提供一个访问该唯一实例的全局访问点。

懒加载实现：

public class LazySingleton {

    private static volatile LazySingleton lazySingleton = null;

    private LazySingleton() {

    }

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

饥饿式实现：

public class HungrySingleton {

    private static final HungrySingleton instance = new HungrySingleton();

    private HungrySingleton() {

    }

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

调用方式：

public class Client {

    public static void main(String[] args) {
        // TODO Auto-generated method stub

        LazySingleton l1 = LazySingleton.getInstance();
        LazySingleton l2 = LazySingleton.getInstance();
        System.out.println(l1 == l2);

        HungrySingleton h1 = HungrySingleton.getInstance();
        HungrySingleton h2 = HungrySingleton.getInstance();
        System.out.println(h1 == h2);
    }

}

执行结果：

多线程环境下的双重检查机制：

public class MultiThreadSingleton {

    private static volatile MultiThreadSingleton multiThreadsSingleton = null;

    private MultiThreadSingleton() {
        System.out.println("MultiThreadSingleton init");
    }

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

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        for (int i = 0; i < 10; i++) {
            new Thread(() -> {
                MultiThreadSingleton.getInstance();
            }).start();
        }

    }
}