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