Java设计模式 create Singleton

单态定义:
Singleton模式主要做用是保证在Java应用程序中，一个类Class只有一个实例存在。

在不少操做中，好比创建目录 数据库链接都须要这样的单线程操做。

还有, singleton可以被状态化;这样，多个单态类在一块儿就能够做为一个状态仓库同样向外提供服务，好比，你要论坛中的帖子计数器，每次浏览一次须要计数，单态类可否保持住这个计数，而且能synchronize的安全自动加1，若是你要把这个数字永久保存到数据库，你能够在不修改单态接口的状况下方便的作到。

另外方面，Singleton也可以被无状态化。提供工具性质的功能，Singleton模式就为咱们提供了这样实现的可能。使用Singleton的好处还在于能够节省内存，由于它限制了实例的个数，有利于Java垃圾回收（garbage collection）。

咱们经常看到工厂模式中类装入器(class loader)中也用Singleton模式实现的,由于被装入的类实际也属于资源。

public class SingletonA  {
    public static boolean instanceFlag = false; //true if 1 instance
    //private SingletonA instance;
    private int i = 0;
    
    // Set constructor private and do nothing
    // Can not new a instance outside class
    private SingletonA() {
    }

    public int getNum() {
        return i;
    }
    
    public void setNum() {
        i++;
    }
    
    public static SingletonA getInstance() {
        if(! instanceFlag) {
            instanceFlag = true;
            return new SingletonA();
        }
        return null;
    }

    public void finalize() {
        instanceFlag = false;
    }
}

测试A

public class TestA  {
    public static void main(String[] args) {
        // Can not create a instance !
        //SingletonA instance1 = new SingletonA();  
        SingletonA instance2 = SingletonA.getInstance();
        if(instance2 != null) {
            instance2.setNum();
            System.out.println("i is:" + instance2.getNum());
        }
        SingletonA instance3 = SingletonA.getInstance();
        if(instance3 == null) {
            System.out.println("Can not get instance twice !");   
        }
    }
}

实例2

public class SingletonB  {
    static private Hashtable registry = new Hashtable();
    //static private SingletonB instance;
    
    public static void Register(String name, SingletonB aInstance) {
        registry.put(name, aInstance);
    }
    public static SingletonB GetInstance(String name) {
        return LookUp(name);
    }
    
    protected static SingletonB LookUp(String name) {
        return (SingletonB)registry.get(name);
    }
}

public class SubSingletonB extends SingletonB {
    public static boolean instanceFlag = false; //true if 1 instance
    //private int i = 0;
    
    public SubSingletonB() throws SingletonException {
        if(instanceFlag) {
            throw new SingletonException("Only can create a instance !");
        } else {
            instanceFlag = true;
            super.Register("Sub1", this);
        }
    }
    
    public void finalize() {
        instanceFlag = false;
    }
}

public class SingletonException extends RuntimeException {
    public SingletonException() {
        super();
    }
    
    public SingletonException(String s) {
        super(s);
    }
    
}

测试2.。

public class TestB  {
    public static void main(String[] args) {
        // First we get a instance from SingletonB
        SingletonB instance1 = SingletonB.GetInstance("Sub1");
        if(instance1 == null) {
            System.out.println("There is no such instance in registry !");
        } else {
            System.out.println(instance1.getClass());
        }
        
        // Then we register a new instance
        try {
            SingletonB instance2 = new SubSingletonB();
            System.out.println("We had created a new instance named \"Sub1\" now");
        } catch (SingletonException e) {
            System.out.println(e.getMessage());
        }
        
        // To get instance again
        instance1 = SingletonB.GetInstance("Sub1");
        if(instance1 == null) {
            System.out.println("There is no such instance in registry !");
        } else {
            System.out.println(instance1.getClass());
        }

        // Finally we create a new instance again
        try {
            SingletonB instance3 = new SubSingletonB();
            System.out.println("We had created a new instance named \"Sub1\" now");
        } catch (SingletonException e) {
            System.out.println(e.getMessage());
        }
        
    }
}

结果

There is no such instance in registry !

We had created a new instance named "Sub1" now

class corner.dp.create.Singleton.SubSingletonB

Only can create a instance !