Singleton design java - Best INTERVIEW Material
Java Singleton Pattern comes in the Creational Design Pattern. Singleton restricts the instantiation of a class & ensures that only 1 instance of the class exists in the JVM. Singleton pattern is used for logging, drivers objects, caching and thread pool.
Several approaches but the basic concepts are:
Private constructor: This will prevent anyone to instantiate the class
Private static variable: This is the only instance of the class
Static public method : Global access point for the outer world to get instance of this class
Eager Initialization - Created at time of class loading even though client application might not be using it
Static block initialization - Similar to eager initialization except for instance getting created in static block which has option for exception handling
Lazy initialization- Creates the instance in the global acess method
Drawbacks of lazy initialization - Works fine in single threaded environment
In multi-threaded environment it can destroy the singleton pattern if two threads get different instances of the class
So we will be creating thread-safe singleton class
Thread-safe Singleton Class - Make the global access method synchronized so that only 1 thread can execute this at a time
Drawback of Threadsafe Singleton - By using synchronized we prevent Thread 2 or Thread 3 to access the method while Thread 1 is inside the method. So every time the method is called there is additional overhead.
Double Checked Locking - To overcome the drawback of simple thread safe singleton, we use double checked locking to limit the expensive operation
Drawback of Double Checked Locking- To understand this, 1st we need to understand that object creation can be broadly classified into 3 steps:
Allocating memory to an object
Executing code inside the constructor
Assigning memory reference to the field
Problem is some of the JIT compilers reorder last 2 steps.
In that case instance variable will be assigned non-null reference & then constructor code will be executed
Half-baked object : If an object is assigned non-null reference & code inside the constructor is still being executed, then the object is not fully constructed/baked & is called half-baked object
Solution to this - We need to declare the instance variable as volatile
Because a volatile instance gurantees completion of all the steps of object creation before another thread reads the reference from the instance variable
En esta página del sitio puede ver el video en línea Singleton design java - Best INTERVIEW Material de Duración hora minuto segunda en buena calidad , que subió el usuario Sudipto Roy 10 marzo 2018, comparta el enlace con amigos y conocidos, en youtube este video ya ha sido visto 203 veces y le gustó 5 a los espectadores. Disfruta viendo!
