How to create object per thread

Question

Hi everyone,
This is my problem. I want to have a static method. Inside of it I want to add an element into a List, let's say I want to track something by doing this. But I also want this List to have multiple copies depending on threads, which means for every call to this static method, I want to call the "add" method on different List objects depending which thread it is from.
Does it make sense? Do I need to have a thread pool to keep those thread references or something?
How about I use Thread.currentThread() to determine whether this call is from a different thread so that I create another List?
Thank you very much!
Edited by: feiya200 on Feb 26, 2008 10:31 PM

Answer 1

want to be perfect with singleton as why it is needed and how it works so see this by [email protected], Bijnor
For those who haven't heard of design patterns before, or who are familiar with the term but not its meaning, a design pattern is a template for software development. The purpose of the template is to define a particular behavior or technique that can be used as a building block for the construction of software - to solve universal problems that commonly face developers. Think of design code as a way of passing on some nifty piece of advice, just like your mother used to give. "Never wear your socks for more than one day" might be an old family adage, passed down from generation to generation. It's common sense solutions that are passed on to others. Consider a design pattern as a useful piece of advice for designing software.
Design patterns out of the way, let's look at the singleton. By now, you're probably wondering what a singleton is - isn't jargon terrible? A singleton is an object that cannot be instantiated. At first, that might seem counterintuitive - after all, we need an instance of an object before we can use it. Well yes a singleton can be created, but it can't be instantiated by developers - meaning that the singleton class has control over how it is created. The restriction on the singleton is that there can be only one instance of a singleton created by the Java Virtual Machine (JVM) - by prevent direct instantiation we can ensure that developers don't create a second copy.
So why would this be useful? Often in designing a system, we want to control how an object is used, and prevent others (ourselves included) from making copies of it or creating new instances. For example, a central configuration object that stores setup information should have one and one only instance - a global copy accessible from any part of the application, including any threads that are running. Creating a new configuration object and using it would be fairly useless, as other parts of the application might be looking at the old configuration object, and changes to application settings wouldn't always be acted upon. I'm sure you can think of a other situations where a singleton would be useful - perhaps you've even used one before without giving it a name. It's a common enough design criteria (not used everyday, but you'll come across it from time to time). The singleton pattern can be applied in any language, but since we're all Java programmers here (if you're not, shame!) let's look at how to implement the pattern using Java.
Preventing direct instantiation
We all know how objects are instantiated right? Maybe not everyone? Let's go through a quick refresher.
Objects are instantiated by using the new keyword. The new keyword allows you to create a new instance of an object, and to specify parameters to the class's constructor. You can specify no parameters, in which case the blank constructor (also known as the default constructor) is invoked. Constructors can have access modifiers, like public and private, which allow you to control which classes have access to a constructor. So to prevent direct instantiation, we create a private default constructor, so that other classes can't create a new instance.
We'll start with the class definition, for a SingletonObject class. Next, we provide a default constructor that is marked as private. No actual code needs to be written, but you're free to add some initialization code if you'd like.
public class SingletonObject
     private SingletonObject()
          // no code req'd
So far so good. But unless we add some further code, there'll be absolutely no way to use the class. We want to prevent direct instantiation, but we still need to allow a way to get a reference to an instance of the singleton object.
Getting an instance of the singleton
We need to provide an accessor method, that returns an instance of the SingletonObject class but doesn't allow more than one copy to be accessed. We can manually instantiate an object, but we need to keep a reference to the singleton so that subsequent calls to the accessor method can return the singleton (rather than creating a new one). To do this, provide a public static method called getSingletonObject(), and store a copy of the singleton in a private member variable.
public class SingletonObject
private SingletonObject()
// no code req'd
public static SingletonObject getSingletonObject()
if (ref == null)
// it's ok, we can call this constructor
ref = new SingletonObject();
return ref;
private static SingletonObject ref;
So far, so good. When first called, the getSingletonObject() method creates a singleton instance, assigns it to a member variable, and returns the singleton. Subsequent calls will return the same singleton, and all is well with the world. You could extend the functionality of the singleton object by adding new methods, to perform the types of tasks your singleton needs. So the singleton is done, right? Well almost.....
Preventing thread problems with your singleton
We need to make sure that threads calling the getSingletonObject() method don't cause problems, so it's advisable to mark the method as synchronized. This prevents two threads from calling the getSingletonObject() method at the same time. If one thread entered the method just after the other, you could end up calling the SingletonObject constructor twice and returning different values. To change the method, just add the synchronized keyword as follows to the method declaration :-
public static synchronized
     SingletonObject getSingletonObject()
Are we finished yet?
There, finished. A singleton object that guarantees one instance of the class, and never more than one. Right? Well.... not quite. Where there's a will, there's a way - it is still possible to evade all our defensive programming and create more than one instance of the singleton class defined above. Here's where most articles on singletons fall down, because they forget about cloning. Examine the following code snippet, which clones a singleton object.
public class Clone
     public static void main(String args[])
     throws Exception
     // Get a singleton
     SingletonObject obj =
     SingletonObject.getSingletonObject();
     // Buahahaha. Let's clone the object
     SingletonObject clone =
          (SingletonObject) obj.clone();
Okay, we're cheating a little here. There isn't a clone() method defined in SingletonObject, but there is in the java.lang.Object class which it is inherited from. By default, the clone() method is marked as protected, but if your SingletonObject extends another class that does support cloning, it is possible to violate the design principles of the singleton. So, to be absolutely positively 100% certain that a singleton really is a singleton, we must add a clone() method of our own, and throw a CloneNotSupportedException if anyone dares try!
Here's the final source code for a SingletonObject, which you can use as a template for your own singletons.
public class SingletonObject
private SingletonObject()
// no code req'd
public static SingletonObject getSingletonObject()
if (ref == null)
// it's ok, we can call this constructor
ref = new SingletonObject();          
return ref;
public Object clone()
     throws CloneNotSupportedException
throw new CloneNotSupportedException();
// that'll teach 'em
private static SingletonObject ref;
Summary
A singleton is an class that can be instantiated once, and only once. This is a fairly unique property, but useful in a wide range of object designs. Creating an implementation of the singleton pattern is fairly straightforward - simple block off access to all constructors, provide a static method for getting an instance of the singleton, and prevent cloning.
What is a singleton class?
A: A class whose number of instances that can be instantiated is limited to one is called a singleton class. Thus, at any given time only one instance can exist, no more.
Q: Can you give me an example, where it is used?
A: The singleton design pattern is used whenever the design requires only one instance of a class. Some examples:
�     Application classes. There should only be one application class. (Note: Please bear in mind, MFC class 'CWinApp' is not implemented as a singleton class)
�     Logger classes. For logging purposes of an application there is usually one logger instance required.
Q: How could a singleton class be implemented?
A: There are several ways of creating a singleton class. The most simple approach is shown below:
Code:
class CMySingleton
public:
static CMySingleton& Instance()
static CMySingleton singleton;
return singleton;
// Other non-static member functions
private:
CMySingleton() {}; // Private constructor
CMySingleton(const CMySingleton&); // Prevent copy-construction
CMySingleton& operator=(const CMySingleton&); // Prevent assignment
Can I extend the singleton pattern to allow more than one instance?
A: The general purpose of the singleton design pattern is to limit the number of instances of a class to only one. However, the pattern can be extended by many ways to actually control the number of instances allowed. One way is shown below...
class CMyClass
private:
CMyClass() {} // Private Constructor
static int nCount; // Current number of instances
static int nMaxInstance; // Maximum number of instances
public:
~CMyClass(); // Public Destructor
static CMyClass* CreateInstance(); // Construct Indirectly
//Add whatever members you want
};