A singleton class is a type of class that has only one instance available to clients. It is a common design pattern that is used to control the creation of objects in a program. It is useful in cases where you need to prevent a single object from being created several times by different client applications or if you want to limit the number of objects that can be created.
How to Create a Singleton Class in Java
The basic idea behind the singleton pattern is that only one instance of a class should ever be created. This is usually accomplished by storing the instance as a private static variable and creating an instance of the class only when it is first needed.
Often, singleton classes are used to control access to resources like database connections or sockets. In this case, the class needs to be thread-safe; it may have to synchronize with the underlying resource to allow only one client to access it at a time.
Another common use of the singleton class is to restrict access to a specific field or method. It can also be used to implement a singleton for an object with a specific interface and only one implementation of that interface.
When creating a singleton class in java, you can use several different methods. These methods include lazy initialization, double check locking, and enum.
This is the most commonly used way to create a singleton class in java. The advantage of using this method is that it delays the instantiation of the object until it is needed.
However, this is not as thread-safe as a synchronized version of the same class. This is because if two threads are accessing the class at the same time, they may run into race conditions that can result in multiple instances of the class being created.
The only way to avoid this problem is to use the synchronized block that creates the instance of the class and prevents other threads from entering it until the first thread exits. This is not very efficient but it solves the issue of multiple instances and is often used.
The second way to implement the singleton class in java is by using the eager initialization technique. The advantage of this approach is that it can be used in a multithreaded environment, as the method only gets called when the class is first loaded.
This method is not as thread-safe as the previous one and it can cause problems when a client attempts to access a singleton class during an operation that uses a synchronized block. The problem is that it can cause race conditions that will destroy the singleton pattern and create multiple instances of the class.
The most efficient way to create a singleton class is by using the enum type. This is because it ensures that only one instance of the enum value is created, which is important for the singleton pattern. The enum type is also thread-safe and does not require synchronization.