A singleton class in Java is defined as that class that has only one object, i.e. only one instance of the class. The restricting of the instance to only one per class helps in coding particular programs that require only one instance of the class. The only instance of a singleton class is given a global point of access.

Singleton class is one of the important design patterns in Java programming. Singleton classes help in limiting resources and optimising them; it is used a lot in database connections or sockets. There are implementation syntaxes for singleton class, and a good developer must know about it. Let us see the implementation of the singleton class in Java. 

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Initialisation

The initialisation of a singleton in Java is done through a private constructor. Private constructors help in other classes not being able to create the same instance as of your singleton class. There are five types of initialisation of singleton class in Java which is listed below:

1. Eager Initialisation

 A public method commonly named as getInstance() is created. This method will help in providing only one entry point for creating an instance of the class, thus making it a singleton class. The instance of the class in eager initialisation is created at the time of class loading.

This type of initialisation technique creates the instance of the class automatically, even if the user is not using it. This may cause memory concerns & unnecessary length in code. To counter the memory leak problem, let us see another type of initialisation technique for a singleton class in Java.

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2. Lazy Initialisation

The instance of the class is initialised in the method getInstance () itself in lazy initialisation. The method in lazy initialisation will check whether the instance of the class is created or not and if it is not created, then only it creates an instance, thus solving the memory leak problem.

The method one creates in lazy initialisation is static and has a return type object. The singleton instance in lazy initialisation will not be created until the getInstance() method is called for the first time.

3. Lazy Initialisation with Double Lock Method

This method is used at the time of more than one thread. If we are using two threads in the program, and both of them can be accessed via ‘if’ statement in starting when the instance is null, it will create a situation of conflict & you may find errors.

This problem is solved by the Double Lock Method in which only one thread enters at a time in the synchronised block for the initialisation of the instance while the other thread is blocked. When the first thread exits the synchronised block, then only the second thread enters & creates another instance. By default, the second thread does not check whether the instance is non-null or not.

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4. Lazy Load Method

Nested inner class is created in this method and works on principles of JLS (Java Language Specifications). There would be no static data members in the class; JVM will not create an instance of the class. The instance would be only created on demand when we invoke the getInstance() method.

You do not need to synchronise the method for initialisation & loading as this method helps in automatic synchronisation of all invocations of getting Instance().

5. Enums Method

Multiple instances of a singleton class can be still created by serialisation & reflection in the above methods. In the case of making a strict singleton class, this method can be used. Enums fields are used in this method which is a constant and runs at compile-time only.

An Enum is the instance of the Enum type and will be constructed only when the Enum type is invoked/called for the first time in the code. This method restricts the cloning of instance and makes a perfect singleton class.

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Serialisation & Reflection

Serialisation helps in the transportation of Java objects from one JVM (Java Virtual Machine) to another. Serialisation is one of the methods used for creating multiple instances of a class. Serialisation converts the object into a byte stream, and then the transfer is done.

The byte stream helps in recreating the object in some other JVM using the deserialization process. It helps multiple systems to communicate & share objects among them. It helps in cross JVM synchronisation as the objects will work in different JVMs.

Reflection is also a method of cloning objects in Java. Reflection is an API that helps us in knowing the class of any unknown object & methods of that particular class which can be accessed via the object. Reflection is widely used by programmers to modify the behaviour of classes & methods at run time.

The three major commands used in reflection are getClass() to know the class to which the object belongs, getConstructors() to get the public constructors of the class of the object, getMethods() to know the public methods of the class under observation.

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Normal Class Vs Singleton Class

A standard class in Java uses constructors to initialise itself, whereas we use the getInstance() method to initialise a singleton class. We can also use the same class & method name. We create an object first for normal classes, and then we use its methods & properties.

In contrast, in a singleton class, we create an instance using the static property of the class, and at any point throughout the code, that particular class will return only a single instance.

Besides many advantages of singleton classes, there are some loopholes too like singleton classes tamper unit testing methods if the objects & methods are tightly coupled. You will have to dedicate a fully functional class to a singleton in that case.

Hidden dependencies are also created by singleton class sometimes, as the reference of the singleton class is passed to other methods and it is not fully transparent thus making it challenging for coders/developers to keep track of the methods & classes. One should keep these shortcomings in mind before creating a singleton pattern and should try to minimise it.

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Points to Make a Perfect Singleton Class

1. The points that should be focused to make a perfect singleton class are as follows:

2. If you are creating multiple threads in a singleton class, then make sure that all the threads should not initialise the singleton class at the same time. One should try to make thread-safe singleton classes.

3. Make your singleton class reflection proof, via changing the constructor visibility to the public at the run time & making sure there is only one instance of any particular singleton class.

4. If there are multiple threads, you can use a volatile modifier that prevents a thread from looking at the initialised state of other threads. A volatile modifier works on the happens-before relationship in which the write happens in a thread before any other thread can read it.

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Conclusion

Singleton classes are used to make customised connections to the database or any program in which you want to control the methods & instances of the class. 

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