The Singleton design pattern is simple and easy to understand. This pattern will ensure that only one object of a particular class is created in virtual machine.
In terms of practical usage
01).Logging
02).Caches
03).Thread pool,
04).Configuration Settings
05).Device driver objects
lets have a look on the very basic lazy initialization methodology of Singleton pattern.
We can test the Singleton class using following SingletonTest Class
When you run the SingltonTest class the result would display as following
Instance 1 ID :2018699554
Instance 2 ID :2018699554
BUILD SUCCESSFUL (total time: 1 second)
It is very important to see the multi threading behavior on the Singleton design pattern.So lets modify the code, Create two separate classes for use the singleton instance within a thread and test the implementation within the multiple thread environment.
Instance 1 ID :2068989547
Instance 1 ID :1793542001
BUILD SUCCESSFUL (total time: 1 second)
It is clear that the above implementation is not thread safe. There are two different Singleton instances were created in the heap. We need to have a way to overcome this situation.
It is not that hard, Simple this is where the synchronized key word is used. Lets modify the getInstance method in Singleton class. So, what would be the output, Lets see...
Instance 1 ID :1612655617
Instance 1 ID :1612655617
BUILD SUCCESSFUL (total time: 1 second)
Now we are safe. Same instance is used within the multi-threaded environment.
It is really worth to use the Singleton design pattern with the thread safe way prevent multiple instantiations.
