Singleton pattern is a software design pattern that restricts the instantiation of a class to one "single" instance. This is useful when exactly one object is needed to coordinate actions across the system. The term comes from the mathematical concept of a singleton.
The singleton design pattern solves problems like:
- How can it be ensured that a class has only one instance?
- How can the sole instance of a class be accessed easily?
- How can a class control its instantiation?
- How can the number of instances of a class be restricted?
To create the singleton class, we need to have static member of class, private constructor and static factory method.
- Static member: It gets memory only once because of static, itcontains the instance of the Singleton class.
- Private constructor: It will prevent to instantiate the Singleton class from outside the class.
- Static factory method: This provides the global point of access to the Singleton object and returns the instance to the caller.
public final class ClassSingleton {
private static ClassSingleton INSTANCE;
private String info = "Initial info class";
private ClassSingleton() {
}
public static ClassSingleton getInstance() {
if(INSTANCE == null) {
INSTANCE = new ClassSingleton();
}
return INSTANCE;
}
// getters and setters
}
Common uses:
- The abstract factory, factory method, builder, and prototype patterns can use singletons in their implementation.
- Facade objects are often singletons because only one facade object is required.
- State objects are often singletons.
https://en.wikipedia.org/wiki/Singleton_pattern
https://www.journaldev.com/1377/java-singleton-design-pattern-best-practices-examples
https://www.geeksforgeeks.org/singleton-class-java/
https://www.javatpoint.com/singleton-design-pattern-in-java
https://www.baeldung.com/java-singleton
https://sourcemaking.com/design_patterns/singleton