JAVA设计模式之单例模式详解

public class LazySingleton {

    //定义一个私有的静态对象instance,静态方法和属性属于类,能保障单例对象的唯一性
    private static LazySingleton instance;

    //私有化构造方法,只能在本类中被访问,其他类中不能通过构造方法直接创建对象
    private LazySingleton() {
    }

    //提供一个全局唯一获取实例的方法
    public static synchronized LazySingleton getInstance(){
        if(instance==null){
            instance=new LazySingleton();
        }
        return instance;
    }
}

public class TestLazySingleton {
    public static void main(String[] args) {
        LazySingleton lazySingleton1=LazySingleton.getInstance();
        LazySingleton lazySingleton2=LazySingleton.getInstance();
        LazySingleton lazySingleton3=LazySingleton.getInstance();
        System.out.println(lazySingleton1);
        System.out.println(lazySingleton2);
        System.out.println(lazySingleton3);
    }
}

public class HungrySingleton {
    //类加载完成后该类的实例便已经存在
    private static HungrySingleton instance=new HungrySingleton();
    //私有化构造方法
    private HungrySingleton(){
    }
    //类加载后实例就存在,不会出现线程安全问题,不需要加锁
    public static HungrySingleton getInstance(){
        return  instance;
    }
}

public class TestHungrySingleton {
    public static void main(String[] args) {
       HungrySingleton hungrySingleton1=HungrySingleton.getInstance();
       HungrySingleton hungrySingleton2=HungrySingleton.getInstance();
       HungrySingleton hungrySingleton3=HungrySingleton.getInstance();
        System.out.println(hungrySingleton1);
        System.out.println(hungrySingleton2);
        System.out.println(hungrySingleton3);
    }
}

public class LockSingleton {
    private  volatile static LockSingleton lockSingleton;
    private LockSingleton(){}
    public static LockSingleton getInstance(){
        if(lockSingleton==null){
            synchronized (LockSingleton.class){
                if (lockSingleton==null){
                    lockSingleton=new LockSingleton();
                }
            }
        }
        return lockSingleton;
    }
}

public class LockTest {
    public static void main(String[] args) {
        LockSingleton lockSingleton1=LockSingleton.getInstance();
        LockSingleton lockSingleton2=LockSingleton.getInstance();
        LockSingleton lockSingleton3=LockSingleton.getInstance();
        System.out.println(lockSingleton1);
        System.out.println(lockSingleton2);
        System.out.println(lockSingleton3);
    }
}