实现Singleton模式-java

实现Singleton模式

以下写法均为延迟加载！在调用方法时才进行实例化
基础版
要想构造一个单例模式，需要这个类最多智能有一个实例化对象。也就是外部类无法进行new等实例化操作！这样需要让构造方法设置为私有：

 private Singleton(){};

然后在内部进行实例化的控制

public class Singleton {
    
    private Singleton(){};
    private static Singleton singleton = null;
    
    public static Singleton getSingleton() {
            if (singleton == null) {
                singleton = new Singleton();
            }
        return singleton;
    }
}

以上单例模式是非线程安全的！
基础版2.0-线程安全版
线程不安全进行加锁：

public class Singleton {

    private Singleton(){};
    private static Singleton singleton = null;
	private static Object object = new Object();
    public static Singleton getSingleton() {

        synchronized (object){
            if (singleton == null) {
                singleton = new Singleton();
            }
        }
        return singleton;
    }
}

基础版3.0 - 优化版
优化版就是不进行无脑加锁影响效率，在加锁前对已经存在的实例返回

public class Singleton {

    private Singleton(){};
    private static Singleton singleton = null;
	private static Object object = new Object();
	
    public static Singleton getSingleton() {
        if (singleton != null) {
            return singleton;
        }else {
            synchronized (object){
                if (singleton == null) singleton = new Singleton();
            }
            return singleton;
        }
    }
}

基础版4.0-双重加锁
volatile 关键字加锁，会对我们的进行singleton进行弱加锁判断

public class Singleton {

    private static  Object object = new Object();
    private Singleton(){};
    private static volatile  Singleton singleton = null;
    public static Singleton getSingleton() {

        if (singleton != null) {
            return singleton;
        }else {
            synchronized (object){
                if (singleton == null) singleton = new Singleton();
            }
            return singleton;
        }
    }
}

高级版
利用静态内部类 和final 关键字的唯一性创建一个安全的单例！只能说：
牛逼！

public class Singleton {

    private Singleton(){};

    private static class SingletonHelper {
        public static final Singleton instance = new Singleton();
    }
    public static final Singleton getInstance() {
        return SingletonHelper.instance;
    }
}