Singleton Class in Java: A Comprehensive Guide to Design Patterns

All You Need to Know About Singleton Class in Java

Edited By Team Careers360 | Updated on Feb 09, 2024 02:26 PM IST | #Java

In Java programming, the singleton class stands as a cornerstone design pattern, wielding profound influence over code structure and application dynamics. This article aims to demystify the complexities of the singleton class in Java, unravelling the singleton design pattern Java nuances. The Java singleton pattern proves invaluable for scenarios requiring a solitary point of control.

All You Need to Know About Singleton Class in Java

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What is a Singleton Class in Java?

Singleton Class in Java is a design pattern that restricts the instantiation of a class to a single object, ensuring that there is only one instance of the class in the Java Virtual Machine (JVM). This pattern is widely used when exactly one object is needed to coordinate actions across the system, like in scenarios where a single point of control is desirable.

A singleton class in Java is a class that can have only one instance, and it provides a method to get that instance. They are usually implemented at places where one needs to control access to resources, such as a configuration file. The classic implementation involves making the constructor of the class private, ensuring that no other class can instantiate it. Additionally, a static method is often provided to access the single instance.

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Singleton Design Pattern

The singleton design pattern is a blueprint for ensuring that a class has only one instance and provides a global point of access to that instance. It falls under the creational design patterns category and is particularly useful when precisely one object is required to coordinate actions, such as logging, driver objects, caching, thread pools, or database connections.

How to Create a Singleton Class in Java

Creating a singleton class in Java involves following a set of well-defined steps to ensure that only one instance of the class exists. The classic implementation includes declaring a private constructor, a private static instance variable, and a public static method to retrieve the instance. Here is a singleton pattern Java example of how to create singleton class in Java:

public class CustomSingleton {

private static CustomSingleton instance;

// Private constructor to restrict instantiation

private CustomSingleton() {

}

// Static method to get the single instance

public static CustomSingleton getInstance() {

if (instance == null) {

instance = new CustomSingleton();

}

return instance;

}

In this singleton design pattern example in Java, the private constructor prevents external instantiation, and the getInstance method ensures that only one instance is created and returned, making it a robust implementation of a singleton class in Java.

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Java Singleton Class Example

Let us delve into a simple Java singleton example to understand the implementation of a Java Singleton Class.

public class Singleton {

private static Singleton instance;

// Private constructor to restrict instantiation

private Singleton() {

}

// Static method to get the single instance

public static Singleton getInstance() {

if (instance == null) {

instance = new Singleton();

}

return instance;

}

In this example, the private constructor prevents the class from being instantiated by other classes. The ‘getInstance’ method ensures that only one instance of the class is created and returned.

Thread Safe Singleton Class in Java

One challenge with the basic implementation is that it is not thread-safe. In a multithreaded environment, multiple threads could potentially create separate instances of the singleton class. To address this, we can introduce synchronisation in the ‘getInstance’ method.

public class ThreadSafeSingleton {

private static ThreadSafeSingleton instance;

private ThreadSafeSingleton() {

}

// Synchronized method to ensure thread safety

public static synchronised ThreadSafeSingleton getInstance() {

if (instance == null) {

instance = new ThreadSafeSingleton();

}

return instance;

}

By adding the ‘synchronised’ keyword to the ‘getInstance’ method, we ensure that only one thread can execute this method at a time, preventing the creation of multiple instances in a multithreaded environment. Alternatively, you can use the "Bill Pugh Singleton" or other manipulative measures, to achieve thread safety without the overhead of synchronisation in every method.

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How to Break Singleton Pattern in Java

While the singleton pattern is designed to enforce a single instance of a class, there are ways it can be inadvertently broken. Common scenarios include:

Reflection: Using reflection, it is possible to access the private constructor and instantiate multiple objects.
Serialisation: If a singleton class implements Serialisable, multiple instances can be created during deserialization.
Multithreading Issues: In certain multithreading scenarios, without proper synchronisation, multiple instances might be created.

Use of Singleton Class in Java

The use of a singleton class in Java is prevalent in scenarios where having a single point of control is crucial. Some common use cases include:

Database Connection: Ensuring a single connection instance to a database to avoid resource wastage.
Logger Classes: Maintaining a single logging instance for consistent logging across the application.
Caching: Using a single cache manager instance to efficiently manage application caching.

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Conclusion

Understanding the intricacies of the singleton class in Java is essential for creating robust, efficient, and maintainable code. The design pattern ensures a single instance of a class, providing a global point of access to that instance. From basic implementations to addressing thread safety concerns, the singleton pattern is a valuable tool in the Java programmer's arsenal. By grasping its principles and implementations, developers can enhance the efficiency and reliability of their Java applications.

Frequently Asked Questions (FAQs)

1. How do you create a Singleton Class in Java?

To create a singleton class, declare a private constructor, a private static instance variable, and provide a public static method (usually named getInstance) to retrieve the instance. Ensure the constructor is private to prevent external instantiation.

2. What is a Singleton Class in Java?

A singleton class in Java is a design pattern ensuring that only one instance of the class is created and providing a global point of access to that instance. It is widely used when precisely one object is needed to coordinate actions across the system.

3. How do you create a Singleton Class in Java?

4. What is the significance of the Singleton Design Pattern?

The singleton design pattern ensures that a class has only one instance, providing a global point of access to that instance. It is valuable for scenarios requiring a single point of control, such as logging, driver objects, caching, thread pools, or database connections.

5. How do you achieve thread safety in a Singleton Class?

Thread safety in a singleton class can be achieved by introducing synchronisation in the getInstance method. Using the synchronised keyword ensures that only one thread can execute the method at a time, preventing the creation of multiple instances in a multithreaded environment.

6. Can the Singleton Pattern be broken in Java, and how can it be avoided?

Yes, the Singleton Pattern can be broken through techniques like reflection or serialisation. To mitigate this, developers can implement additional measures, such as using an enum or employing the "Bill Pugh Singleton Implementation," to ensure the pattern's integrity in various scenarios.