Design Pattern:

Links:
http://www.oodesign.com/factory-pattern.html
Design Pattern :
There are 3 types of design pattenn
1. Creational Pattern.

• Factory Method Design Pattern
• Abstract Factory Design Pattern
• Builder Design Pattern
• Prototype Design Pattern
• Singleton Design Pattern

2. Structural Pattern.

• Adapter Design Pattern
• Decorator Design Pattern
• Facade Design Pattern
• Bridge Design Pattern
• Proxy Design Pattern
• Composite Design Pattern
• Flyweight Design Pattern

3. Behaviour Design Pattern.

Singleton Design Pattern :

What is Singleton class? Have you used Singleton before?
Singleton is a class which has only one instance in whole application and provides a getInstance() method to access the singleton instance. There are many classes in JDK which is implemented using Singleton pattern like java.lang.Runtime which provides getRuntime() method to get access of it and used to get free memory and total memory in Java.

1) Which classes are candidates of Singleton? Which kind of class do you make Singleton in Java?Here they will check whether candidate has enough experience on usage of singleton or not. Does he is familiar of advantage/disadvantage or alternatives available for singleton in Java or not.

Answer: Any class which you want to be available to whole application and whole only one instance is viable is candidate of becoming Singleton. One example of this is Runtime class , since on whole java application only one runtime environment can be possible making Runtime Singleton is right decision. Another example is a utility classes like Popup in GUI application, if you want to show popup with messageyou can have one PopUp class on whole GUI application and anytime just get its instance, and call show() with message.


2) Can you write code for getInstance() method of a Singleton class in Java?Most of the java programmer fail here if they have mugged up the singleton code because you can ask lots of follow-up question based upon the code they have written. I have seen many programmer write Singleton getInstance() method with double checked locking but they are not really familiar with the caveat associated with double checking of singleton prior to Java 5.

Answer: Until asked don’t write code using double checked locking as it is more complex and chances of errors are more but if you have deep knowledge of double checked locking, volatile variable and lazy loading than this is your chance to shine. I have shared code examples of writing singleton classes using enum, using static factory and with double checked locking in my recent post Why Enum Singletons are better in Java, please see there.

3) Is it better to make whole getInstance() method synchronized or just critical section is enough? Which one you will prefer?This is really nice question and I mostly asked to just quickly check whether candidate is aware of performance trade off of unnecessary locking or not. Since locking only make sense when we need to create instance and rest of the time its just read only access so locking of critical section is always better option. read more about synchronization on How Synchronization works in Java
Answer: This is again related to double checked locking pattern, well synchronization is costly and when you apply this on whole method than call to getInstance() will be synchronized and contented. Since synchronization is only needed during initialization on singleton instance, to prevent creating another instance of Singleton,  It’s better to only synchronize critical section and not whole method. Singleton pattern is also closely related to factory design pattern where getInstance() serves as static factory method.

4) What is lazy and early loading of Singleton and how will you implement it?This is another great Singleton interview question in terms of understanding of concept of loading and cost associated with class loading in Java. Many of which I have interviewed not really familiar with this but its good to know concept.

Answer: As there are many ways to implement Singleton like using double checked locking or Singleton class with static final instance initialized during class loading. Former is called lazy loading because Singleton instance is created only when client calls getInstance() method while later is called early loading because Singleton instance is created when class is loaded into memory.


5) Example of Singleton in standard Java Development Kit?This is open question to all, please share which classes are Singleton in JDK. Answer to this question is java.lang.Runtime
Answer: There are many classes in Java Development Kit which is written using singleton pattern, here are few of them:

• Java.lang.Runtime with getRuntime() method
• Java.awt.Toolkit with getDefaultToolkit()
• Java.awt.Desktop with  getDesktop()

6) What is double checked locking in Singleton?One of the most hyped question on Singleton pattern and really demands complete understanding to get it right because of Java Memory model caveat prior to Java 5. If a guy comes up with a solution of using volatile keyword with Singleton instance and explains it then it really shows it has in depth knowledge of Java memory model and he is constantly updating his Java knowledge.

Answer: Double checked locking is a technique to prevent creating another instance of Singleton when call to getInstance() method is made in multi-threading environment. In Double checked locking pattern as shown in below example, singleton instance is checked two times before initialization.

public static Singleton getInstance(){
     if(_INSTANCE == null){
         synchronized(Singleton.class){
         //double checked locking - because second check of Singleton instance with lock
                if(_INSTANCE == null){
                    _INSTANCE = new Singleton();
                }
            }
         }
     return _INSTANCE;
}

Double checked locking should only be used when you have requirement for lazy initialization otherwise use Enum to implement singleton or simple static final variable.

7) How do you prevent for creating another instance of Singleton using clone() method?This type of questions generally comes some time by asking how to break singleton or when Singleton is not Singleton in Java.
Answer: Preferred way is not to implement Clonnable interface as why should one wants to create clone() of Singleton and if you do just throw Exception from clone() method as  “Can not create clone of Singleton class”.

8) How do you prevent for creating another instance of Singleton using reflection?Open to all. In my opinion throwing exception from constructor is an option.
Answer: This is similar to previous interview question. Since constructor of Singleton class is supposed to be private it prevents creating instance of Singleton from outside but Reflection can access private fields and methods, which opens a threat of another instance. This can be avoided by throwing Exception from constructor as “Singleton already initialized”

9) How do you prevent for creating another instance of Singleton during serialization?Another great question which requires knowledge of Serialization in Java and how to use it for persisting Singleton classes. This is open to you all but in my opinion use of readResolve() method can sort this out for you.
Answer: You can prevent this by using readResolve() method, since during serialization readObject() is used to create instance and it return new instance every time but by using readResolve you can replace it with original Singleton instance.  I have shared code on how to do it in my post Enum as Singleton in Java. This is also one of the reason I have said that use Enum to create Singleton because serialization of enum is taken care by JVM and it provides guaranteed of that.

10) When is Singleton not a Singleton in Java?There is a very good article present in Sun's Java site which discusses various scenarios when a Singleton is not really remains Singleton and multiple instance of Singleton is possible. Here is the link of that article http://java.sun.com/developer/technicalArticles/Programming/singletons/

Apart from these questions on Singleton pattern, some of my reader contribute few more questions, which I included here. Thank you guys for your contribution.

11) Why you should avoid the singleton anti-pattern at all and replace it with DI?

Answer: Singleton Dependency Injection: every class that needs access to a singleton gets the object through its constructors or with a DI-container.

Singleton Anti-Pattern: with more and more classes calling getInstance the code gets more and more tightly coupled, monolithic, not testable and hard to change and hard to reuse because of not configurable, hidden dependencies. Also, there would be no need for this clumsy double checked locking if you call getInstance less often (i.e. once).

12) How many ways you can write Singleton Class in Java?
Answer:  I know at least four ways to implement Singleton pattern in Java
1) Singleton by synchronizing getInstance() method
2) Singleton with public static final field initialized during class loading.
3) Singleton generated by static nested class, also referred as Singleton holder pattern.
4) From Java 5 on-wards using Enums

13) How to write thread-safe Singleton in Java?

Answer: Thread safe Singleton usually refers to write thread safe code which creates one and only one instance of Singleton if called by multiple thread at same time. There are many ways to achieve this like by using double checked locking technique as shown above and by using Enum or Singleton initialized by class loader.

At last few more questions for your practice, contributed by Mansi:

14) Singleton vs Static Class?
15) When to choose Singleton over Static Class?
16) Can you replace Singleton with Static Class in Java?
17) Difference between Singleton and Static Class in java?
18) Advantage of Singleton over Static Class?

I have covered answers of couple of these questions in my post, Singleton vs Static Class in Java - Pros and Cons.

If you like to read more Java interview questions you can have a look on some of my favorites below:

How to identify and fix deadlock in Java

Difference between HashMap and HashSet in Java

Top 10 collection interview questions in Java
Difference between Comparator and Comparable in Java
Difference between HashMap and Hashtable?

Read more: http://javarevisited.blogspot.com/2011/03/10-interview-questions-on-singleton.html#ixzz2XNgYLPCa

Singleton Design Pattern

02/05/2011

Singleton design pattern is the first design pattern I learned (many years back). In early days when someone asks me, “do you know any design pattern?” I quickly and promptly answer “I know singleton design pattern” and the question follows, “do you know anything other than singleton” and I stand stumped!

A java beginner will know about singleton design pattern. At least he will think that he knows singleton pattern. The definition is even easier than Newton’s third law. Then what is special about the singleton pattern. Is it so simple and straightforward, does it even deserve an article? Do you believe that you know 100% about singleton design pattern? If you believe so and you are a beginner read through the end, there are surprises for you.
There are only two points in the definition of a singleton design pattern,

1. there should be only one instance allowed for a class and
2. we should allow global point of access to that single instance.

GOF says, “Ensure a class has only one instance, and provide a global point of access to it. [GoF, p127]“.
The key is not the problem and definition. In singleton pattern, trickier part is implementation and management of that single instance. Two points looks very simple, is it so difficult to implement it. Yes it is very difficult to ensure “single instance” rule, given the flexibility of the APIs and many flexible ways available to access an instance. Implementation is very specific to the language you are using. So the security of the single instance is specific to the language used.

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Strategy for Singleton instance creation

We suppress the constructor and don’t allow even a single instance for the class. But we declare an attribute for that same class inside and create instance for that and return it. Factory design pattern can be used to create the singleton instance.

package com.javapapers.sample.designpattern; public class Singleton {   private static Singleton singleInstance;     private Singleton() {}   public static Singleton getSingleInstance() {     if (singleInstance == null) {       synchronized (Singleton.class) {         if (singleInstance == null) {           singleInstance = new Singleton();         }       }     }     return singleInstance;   }

You need to be careful with multiple threads. If you don’t synchronize the method which is going to return the instance then, there is a possibility of allowing multiple instances in a multi-threaded scenario. Do the synchronization at block level considering the performance issues. In the above example for singleton pattern, you can see that it is threadsafe.

Early and lazy instantiation in singleton pattern

The above example code is a sample for lazy instantiation for singleton design pattern. The single instance will be created at the time of first call of the getSingleInstance() method. We can also implement the same singleton design pattern in a simpler way but that would instantiate the single instance early at the time of loading the class. Following example code describes how you can instantiate early. It also takes care of the multithreading scenario.

package com.javapapers.sample.designpattern; public class Singleton {   private static Singleton singleInstance = new Singleton();   private Singleton() {}   public static Singleton getSingleInstance() {     return singleInstance;   } }

Singleton and Serialization

Using serialization, single instance contract of the singleton pattern can be violated. You can serialize and de-serialize and get a new instance of the same singleton class. Using java api, you can implement the below method and override the instance read from the stream. So that you can always ensure that you have single instance.

ANY-ACCESS-MODIFIER Object readResolve() throws ObjectStreamException;

This article is an attempt to explain the basics on singleton design pattern. If you want more insight on singleton refer this technical article “When is a Singleton not a Singleton?” and its references.

The Singleton's purpose is to control object creation, limiting the number to one but allowing the flexibility to create more objects if the situation changes.

Since there is only one Singleton instance, any instance fields of a Singleton will occur only once per class, just like static fields. Singletons often control access to resources such as database connections or sockets.

For example, if you have a license for only one connection for your database or your JDBC driver has trouble with multithreading, the Singleton makes sure that only one connection is made or that only one thread can access the connection at a time.

Implementing Singletons:

Example 1:

The easiest implementation consists of a private constructor and a field to hold its result, and a static accessor method with a name like getInstance().

The private field can be assigned from within a static initializer block or, more simply, using an initializer. The getInstance( ) method (which must be public) then simply returns this instance:

// File Name: Singleton.java
public class Singleton {

   private static Singleton singleton = new Singleton( );
   
   /* A private Constructor prevents any other 
    * class from instantiating.
    */
   private Singleton(){ }
   
   /* Static 'instance' method */
   public static Singleton getInstance( ) {
      return singleton;
   }
   /* Other methods protected by singleton-ness */
   protected static void demoMethod( ) {
      System.out.println("demoMethod for singleton"); 
   }
}

// File Name: SingletonDemo.java
public class SingletonDemo {
   public static void main(String[] args) {
      Singleton tmp = Singleton.getInstance( );
      tmp.demoMethod( );
   }
}

This would produce following result:

demoMethod for singleton

Example 2:

Following implementation shows a classic Singleton design pattern:

public class ClassicSingleton {

   private static ClassicSingleton instance = null;
   protected ClassicSingleton() {
      // Exists only to defeat instantiation.
   }
   public static ClassicSingleton getInstance() {
      if(instance == null) {
         instance = new ClassicSingleton();
      }
      return instance;
   }
}

The ClassicSingleton class maintains a static reference to the lone singleton instance and returns that reference from the static getInstance() method.

Here ClassicSingleton class employs a technique known as lazy instantiation to create the singleton; as a result, the singleton instance is not created until the getInstance() method is called for the first time. This technique ensures that singleton instances are created only when needed.

Singleton Pattern

Motivation

Sometimes it's important to have only one instance for a class. For example, in a system there should be only one window manager (or only a file system or only a print spooler). Usually singletons are used for centralized management of internal or external resources and they provide a global point of access to themselves.

The singleton pattern is one of the simplest design patterns: it involves only one class which is responsible to instantiate itself, to make sure it creates not more than one instance; in the same time it provides a global point of access to that instance. In this case the same instance can be used from everywhere, being impossible to invoke directly the constructor each time.

Intent

• Ensure that only one instance of a class is created.
• Provide a global point of access to the object.

Implementation

The implementation involves a static member in the "Singleton" class, a private constructor and a static public method that returns a reference to the static member.

The Singleton Pattern defines a getInstance operation which exposes the unique instance which is accessed by the clients. getInstance() is is responsible for creating its class unique instance in case it is not created yet and to return that instance.

class Singleton { private static Singleton instance; private Singleton() { ... } public static synchronized Singleton getInstance() { if (instance == null) instance = new Singleton(); return instance; } ... public void doSomething() { ... } }

You can notice in the above code that getInstance method ensures that only one instance of the class is created. The constructor should not be accessible from the outside of the class to ensure the only way of instantiating the class would be only through the getInstance method.

The getInstance method is used also to provide a global point of access to the object and it can be used in the following manner:

Singleton.getInstance().doSomething();

Applicability & Examples

According to the definition the singleton pattern should be used when there must be exactly one instance of a class, and when it must be accessible to clients from a global access point. Here are some real situations where the singleton is used:

Example 1 - Logger Classes

The Singleton pattern is used in the design of logger classes. This classes are ussualy implemented as a singletons, and provides a global logging access point in all the application components without being necessary to create an object each time a logging operations is performed.

Example 2 - Configuration Classes

The Singleton pattern is used to design the classes which provides the configuration settings for an application. By implementing configuration classes as Singleton not only that we provide a global access point, but we also keep the instance we use as a cache object. When the class is instantiated( or when a value is read ) the singleton will keep the values in its internal structure. If the values are read from the database or from files this avoids the reloading the values each time the configuration parameters are used.

Example 3 - Accesing resources in shared mode

It can be used in the design of an application that needs to work with the serial port. Let's say that there are many classes in the application, working in an multi-threading environment, which needs to operate actions on the serial port. In this case a singleton with synchronized methods could be used to be used to manage all the operations on the serial port.

Example 4 - Factories implemented as Singletons

Let's assume that we design an application with a factory to generate new objects(Acount, Customer, Site, Address objects) with their ids, in an multithreading environment. If the factory is instantiated twice in 2 different threads then is possible to have 2 overlapping ids for 2 different objects. If we implement the Factory as a singleton we avoid this problem. Combining Abstract Factory or Factory Method and Singleton design patterns is a common practice.

Specific problems and implementation

Thread-safe implementation for multi-threading use.

A robust singleton implementation should work in any conditions. This is why we need to ensure it works when multiple threads uses it. As seen in the previous examples singletons can be used specifically in multi-threaded application to make sure the reads/writes are synchronized.

Lazy instantiation using double locking mechanism.

The standard implementation shown in the above code is a thread safe implementation, but it's not the best thread-safe implementation beacuse synchronization is very expensive when we are talking about the performance. We can see that the synchronized method getInstance does not need to be checked for syncronization after the object is initialized. If we see that the singleton object is already created we just have to return it without using any syncronized block. This optimization consist in checking in an unsynchronized block if the object is null and if not to check again and create it in an syncronized block. This is called double locking mechanism.

In this case case the singleton instance is created when the getInstance() method is called for the first time. This is called lazy instantiation and it ensures that the singleton instance is created only when it is needed.

//Lazy instantiation using double locking mechanism. class Singleton { private static Singleton instance; private Singleton() { System.out.println("Singleton(): Initializing Instance"); } public static Singleton getInstance() { if (instance == null) { synchronized(Singleton.class) { if (instance == null) { System.out.println("getInstance(): First time getInstance was invoked!"); instance = new Singleton(); } } } return instance; } public void doSomething() { System.out.println("doSomething(): Singleton does something!"); } }

A detialed discussion(double locking mechanism) can be found on http://www-128.ibm.com/developerworks/java/library/j-dcl.html?loc=j

Early instantiation using implementation with static field

In the following implementattion the singleton object is instantiated when the class is loaded and not when it is first used, due to the fact that the instance member is declared static. This is why in we don't need to synchronize any portion of the code in this case. The class is loaded once this guarantee the uniquity of the object.

Singleton - A simple example (java)

//Early instantiation using implementation with static field. class Singleton { private static Singleton instance = new Singleton(); private Singleton() { System.out.println("Singleton(): Initializing Instance"); } public static Singleton getInstance() { return instance; } public void doSomething() { System.out.println("doSomething(): Singleton does something!"); } }

Protected constructor

It is possible to use a protected constructor to in order to permit the subclassing of the singeton. This techique has 2 drawbacks that makes singleton inheritance impractical:

• First of all, if the constructor is protected, it means that the class can be instantiated by calling the constructor from another class in the same package. A possible solution to avoid it is to create a separate package for the singleton.
• Second of all, in order to use the derived class all the getInstance calls should be changed in the existing code from Singleton.getInstance() to NewSingleton.getInstance().

Multiple singleton instances if classes loaded by different classloaders access a singleton.

If a class(same name, same package) is loaded by 2 diferent classloaders they represents 2 different clasess in memory.

Serialization

If the Singleton class implements the java.io.Serializable interface, when a singleton is serialized and then deserialized more than once, there will be multiple instances of Singleton created. In order to avoid this the readResolve method should be implemented. See Serializable () and readResolve Method () in javadocs.

public class Singleton implements Serializable { ... // This method is called immediately after an object of this class is deserialized. // This method returns the singleton instance. protected Object readResolve() { return getInstance(); } }

Abstract Factory and Factory Methods implemented as singletons.

There are certain situations when the a factory should be unique. Having 2 factories might have undesired effects when objects are created. To ensure that a factory is unique it should be implemented as a singleton. By doing so we also avoid to instantiate the class before using it.

Hot Spot:

• Multithreading - A special care should be taken when singleton has to be used in a multithreading application.
• Serialization - When Singletons are implementing Serializable interface they have to implement readResolve method in order to avoid having 2 different objects.
• Classloaders - If the Singleton class is loaded by 2 different class loaders we'll have 2 different classes, one for each class loader.
• Global Access Point represented by the class name - The singleton instance is obtained using the class name. At the first view this is an easy way to access it, but it is not very flexible. If we need to replace the Sigleton class, all the references in the code should be changed accordinglly.