Java设计模式(一)---单例模式

/**
 *  饿汉式
 * @author illusoryCloud
 */
public class FirstSingleton {
    /**
     * 类变量在类准备阶段就初始化了然后放在<clinit>构造方法中
     * 一旦外部调用了静态方法，那么就会初始化完成。
     * 一个类的<clinit>只会执行一次 保证多线程情况下不会创建多个实例
     */
    private static final FirstSingleton INSTANCE =new FirstSingleton();

    /**
     *
     * 构造函数私有化
     */
    private FirstSingleton(){}

    /**
     *  提供公共方法以获取实例对象
     * @return instance 实例对象
     */
    public static FirstSingleton getInstance(){
        return INSTANCE ;
    }
}

/**
 * 静态内部类方式
 *
 * @author illusoryCloud
 */
public class SecondSingleton {

    private static class SingletonHolder {
        /**
         * 静态变量类加载时才会被创建 且只会创建一次
         */
        private static final SecondSingleton INSTANCE = new SecondSingleton();
    }

    private SecondSingleton() {
    }

    public static SecondSingleton getInstance() {
        return SingletonHolder.INSTANCE;
    }
}

/**
 * 懒汉式
 *
 * @author illusoryCloud
 */
public class ThirdSingleton {

    private static ThirdSingleton instance;

    private ThirdSingleton() {
    }

    /**
     * synchronized 保证线程安全 但效率低
     *
     * @return instance单例对象
     */
    public static synchronized ThirdSingleton getInstance() {
        if (instance == null) {
            instance = new ThirdSingleton();
        }
        return instance;
    }

}

/**
 * 双重校验锁式
 *
 * @author illusoryCloud
 */
public class FourSingleton {
    /**
     * volatile关键字禁止指令重排序
     * 保证多线程下不会获取到未完全初始化的实例
     * 详细请阅读：https://www.lixueduan.com/posts/e7cef119.html
     */
    private static volatile FourSingleton instance;

    private FourSingleton() {
    }

    /**
     * 双重if校验 缩小synchronized代码块范围
     * 若instance不为空 就可直接return
     *
     * @return instance 实例对象
     */
    public static FourSingleton getInstance() {
        if (instance == null) {
            synchronized (FourSingleton.class) {
                if (instance == null) {
                    //非原子操作
                    instance = new FourSingleton();
                }
            }
        }
        return instance;
    }
}

/**
 * 枚举式
 * 序列化及反序列化安全
 * @author illusoryCloud
 */
public enum FiveSingleton {
    //定义一个枚举的元素，它就是 singleton 的一个实例
    INSTANCE;
    public void doSomeThing(FiveSingleton instance) {
        System.out.println("枚举方式实现单例");
    }

}
public class Test {

	public static void main(String[] args) {
		Singleton singleton = Singleton.INSTANCE;
		singleton.doSomeThing();//output:枚举方法实现单例

	}
}

public class Runtime {
    private static Runtime currentRuntime = new Runtime();

    /**
     * Returns the runtime object associated with the current Java application.
     * Most of the methods of class <code>Runtime</code> are instance 
     * methods and must be invoked with respect to the current runtime object. 
     * 
     * @return  the <code>Runtime</code> object associated with the current
     *          Java application.
     */
    public static Runtime getRuntime() { 
    return currentRuntime;
    }

    /** Don't let anyone else instantiate this class */
    private Runtime() {}

    ...
}