Singleton Pattern says that just"define a class that has only one instance and provides a global point of access to it".

In other words, a class must ensure that only single instance should be created and single object can be used by all other classes.

There are two forms of singleton design pattern

Early Instantiation: creation of instance at load time. Lazy Instantiation: creation of instance when required.

Saves memory because object is not created at each request. Only single instance is reused again and again.

Singleton pattern is mostly used in multi-threaded and database applications. It is used in logging, caching, thread pools, configuration settings etc.

If singleton class is Serializable, you can serialize the singleton instance. Once it is serialized, you can deserialize it but it will not return the singleton object.

To resolve this issue, you need to override the readResolve() method that enforces the singleton. It is called just after the object is deserialized. It returns the singleton object.

public class A implements Serializable { //your code of singleton protected Object readResolve() { return getA(); }

}