public class Singleton {
    public static void main(String[] args) {
        Singleton s1 = Singleton.getInstance();
        Singleton s2 = Singleton.getInstance();
        System.out.println(s1 == s2);
    }
    private static Singleton instance;
    /**将构造器使用private修饰，隐藏该构造器**/
    private Singleton(){
        System.out.println("Singleton被构造！");
    }
    /**线程安全，但性能有待优化，这样写每次调用getInstance方法都需要同步。
        实际上只是第一次加载需要同步**/
    public static synchronized Singleton getInstance() {
        if (instance == null) {
            instance = new Singleton();
        }
        return instance;
    }
}

public class Singleton2 {
    /**注意volatile的修饰，有待改进吧**/
    private static Lock lock=new ReentrantLock();
    private static volatile Singleton2 uniqueInstance;
    private Singleton2() {
    }
    public static void main(String[] args) {
        Singleton2 s1 = Singleton2.getInstance();
        Singleton2 s2 = Singleton2.getInstance();
        System.out.println(s1 == s2);
    }

    public static Singleton2 getInstance() {
        if(uniqueInstance == null) {
            lock.lock();
            try {
                /**double check**/
                if(uniqueInstance == null)
                    uniqueInstance = new Singleton2();
            }catch (Exception e){
            }finally {
                lock.unlock();
            }
        }
        return uniqueInstance;
    }
}

public class SingletonTest {
    /**注意volatile关键字,这个方法还是不错的**/
    private static volatile SingletonTest instance = null;

    private SingletonTest() {
    }

    public static void main(String[] args) {
        SingletonTest s1 = SingletonTest.getInstance();
        SingletonTest s2 = SingletonTest.getInstance();
        System.out.println(s1 == s2);
    }

    private static synchronized void syncInit() {
        if (instance == null) {
            instance = new SingletonTest();
        }
    }

    public static SingletonTest getInstance() {
        if (instance == null) {
            syncInit();
        }
        return instance;
    }
}

public class Singleton_thread {
    /**私有构造方法，防止被实例化**/
    private Singleton_thread() {
    }

    public static void main(String[] args) {
        Singleton_thread s1 = Singleton_thread.getInstance();
        Singleton_thread s2 = Singleton_thread.getInstance();
        System.out.println(s1 == s2);
    }

    /**此处使用一个内部类来维护单例，类的加载线程互斥**/
    private static class SingletonFactory {
        private static Singleton_thread instance = new Singleton_thread();
    }

    /**获取实例**/
    public static Singleton_thread getInstance() {
        return SingletonFactory.instance;
    }

}