java设计模式-单例模式实现方法详解

class Singleton {
	//私有化构造器
	private Singleton() {	
	}
	//内部创建对象实例
	private final static Singleton instance = new Singleton();
	//对外公有的静态方法
	public static Singleton getInstance() {
		return instance;
	}
}

class Singleton {  //静态代码块
	//私有化构造器
	private Singleton() {}
	//内部创建对象实例
	private  static Singleton instance;
	static { // 在静态代码块中，创建单例对象
		instance = new Singleton();
	}
	//对外公有的静态方法
	public static Singleton getInstance() {
		return instance;
	}
}

public static void main(String[] args) {
		Singleton instance1 = Singleton.getInstance();
		Singleton instance2 = Singleton.getInstance();
		System.out.println(instance1 == instance2);
		System.out.println("instance1.hashCode=" + instance1.hashCode());
		System.out.println("instance2.hashCode=" + instance2.hashCode());
	}

class Singleton { //线程不安全
    private static Singleton instance;
    private Singleton() {}
    public static Singleton getInstance() {  //调用时才实例化对象，懒汉式
        if(instance == null) {
            instance = new Singleton();
        }
        return instance;
    }
}

class Singleton { //线程安全
    private static Singleton instance;
    private Singleton() {}
    //synchronized同步处理
    public static synchronized Singleton getInstance() {
        if(instance == null) {
            instance = new Singleton();
        }
        return instance;
    }
}

class Singleton { //双重检查
	private static volatile Singleton instance;
	private Singleton() {}
	public static synchronized Singleton getInstance() {
		if(instance == null) { //判断是否实例化
			synchronized (Singleton.class) {
				if(instance == null) {
					instance = new Singleton();
				}
			}
		}
		return instance; //否则直接return
	}
}

class Singleton {
	private static volatile Singleton instance;
	private Singleton() {}
	//静态内部类,包含一个静态属性：Singleton
	private static class SingletonInstance {
		private static final Singleton INSTANCE = new Singleton(); 
	}
	//对外公有的静态方法，直接返回SingletonInstance.INSTANCE
	public static synchronized Singleton getInstance() {
		return SingletonInstance.INSTANCE;
	}
}

enum Singleton {
	INSTANCE; //属性
	public void say() {
		System.out.println("记得三连~");
	}
}

public static void main(String[] args) {
		Singleton instance1 = Singleton.INSTANCE;
		Singleton instance2 = Singleton.INSTANCE;
		System.out.println(instance1 == instance2);
		System.out.println(instance1.hashCode());
		System.out.println(instance2.hashCode());
		instance1.say();
	}