How to instantiate a Singleton multiple times?

I need a singleton in my code. I implemented it in Java and it works well. The re开发者_开发百科ason I did it, is to ensure that in a mulitple environment, there is only one instance of this class.

But now I want to test my Singleton object locally with a Unit test. For this reason I need to simulate another instance of this Singleton (the object that would be from another device). So is there a possiblity to instantiate a Singleton a second time for testing purpose or do I have to mock it?

I'm not sure, but I think it could be possible by using a different class loader?

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Traditionally, a Singleton creates its own instance, and it creates it only once. In this case it is not possible to create a second instance.

If you use Dependency Injection, you can let the framework create the singleton for you. The singleton does not guard against other instances (i.e. it has a public constructor), but the dependency injection framework instantiates only one instance. In this case, you can create more instances for testing, and your object is not cluttered with singleton code.

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The point of a Singleton is that you can only instantiate it once.

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You can invoke the private constructor of your singleton class using reflection to create a new instance of the class.

class MySingleton {
    private MySingleton() {
    }
}

class Test {
    public void test() throws Exception {
        Constructor<MySingleton> constructor = MySingleton.class.getConstructor();
        constructor.setAccessible(true);
        MySingleton otherSingleton = constructor.newInstance();
    }
}

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A singleton, by definition, can only be instantiated once. However, the fact that your unit test requires two singletons is a strong indication that your object shouldn't really be a singleton, and that you should rethink the singleton design.

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I feel compelled to post this series of articles about how Singletons destroy testability and are poor design choices:

• Singletons are Pathological Liars
• Where Have All the Singletons Gone?
• Root Cause of Singletons

Short summary: combining logic for what a class does with how it is instantiated makes code that is ugly to test, and should be avoided.

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First, why do you need to create a new singleton to run a unit test? A unit test should not be running concurrently with the normal application, so you should be able to access the original singleton without fear of modifying it..

Is there a particular reason you need an explicit second singleton?

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you could just make another static method getInstance2 that looks like this:

class MySingleton
{
    private MySingleton(){}
    private static MySingleton instance1 = new MySingleton();
    private static MySingleton instance2 = new MySingleton();

    public static MySingleton getInstance(){ return instance1; }
    public static MySingleton getInstance2(){ return instance2; }
}

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Singleton ston=Singleton.getInstance(); will return singleton object. By making use of "ston" object, if we call the method createNewSingleTonInstance() which is written in Singleton class will give new instance.

public class Singleton {

private String userName;
private String Password;
private static Singleton firstInstance=null;
private Singleton(){}


public static synchronized Singleton getInstance(){
    if(firstInstance==null){
        firstInstance=new Singleton();
        firstInstance.setUserName("Prathap");
        firstInstance.setPassword("Mandya");
    }
    return firstInstance;
}


public void setUserName(String userName) {
    this.userName = userName;
}
public String getUserName() {
    return userName;
}
public void setPassword(String password) {
    Password = password;
}
public String getPassword() {
    return Password;
}

public Singleton createNewSingleTonInstance(){
    Singleton s=new Singleton();
    s.setUserName("ASDF");
    s.setPassword("QWER");
    return s;
}
}

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You may keep a key on a map and populate instance with key

public class MultiSingleton {
/**** Non-static Global Variables ***/
String status = "";
private BaseSmartCard bsc;
/***********************************/
private static Object lockObject = new Object();
private String serialNo;

private static Map<String, MultiSingleton> mappedObjects = new TreeMap<String, MultiSingleton>();

protected MultiSingleton() {

}


public static MultiSingleton getInstance(String serialNo,long slotNo){
    if (mappedObjects.isEmpty() || !mappedObjects.containsKey(serialNo)) {
        MultiSingleton instance = new MultiSingleton();
        instance.setSerialNo(serialNo);
        mappedObjects.put(serialNo, instance);
        return instance;
    } else if (mappedObjects.containsKey(serialNo)) {
        return mappedObjects.get(serialNo);
    }else {
        return null;
    }
}