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Yatin
The author is graduated in Electronics & Telecommunication. During his studies, he has been involved with a significant number of projects ranging from programming and software engineering to telecommunications analysis. He works as a technical lead in the information technology sector where he is primarily involved with projects based on Java/J2EE technologies platform and novel UI technologies.

Java Singleton Design Pattern – Best Practices with Examples

In this post, we feature a comprehensive Tutorial on Java Singleton Design Pattern. Design Patterns in Java are incredibly popular among the software developers. One of the most common interview questions is the Singleton Design Pattern. So in this tutorial, I’ll summarize the best practices which will help developers dodge general issues and develop better applications.

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1. What are Design Patterns?

A design pattern is known as a well-proven solution to a commonly occurring problem in the software design. They can speed up the development process by providing the tested and proven development paradigms. Using design patterns offer few advantages such as:

  • Reusable in multiple projects
  • Provide solutions for an effective system architecture
  • Provide clarity and transparency to the application design
  • Provide robust and highly maintainable code for easy understanding and debug

Java has several design patterns and the Singleton Pattern is the most commonly used.

1.1 What is Java Singleton Design Pattern?

  • The Singleton pattern is a part of Creational design patterns
  • In Java, the new keyword creates an instance of a class whenever required. But there are some cases where developers need to have an isolated instance of a class such as:
    • A single database connection object
    • A single shared resource
  • Therefore, this design pattern ensures that only one instance of a given class is created and a global access to that instance is provided
Java Singleton Design Pattern - Classic Singleton Design Pattern

Fig. 1: Classic Singleton Design Pattern

1.2 Pre-requisites

To implement the Singleton design pattern in the Java programming language, developers need to have the following:

  • Static Member: It will create a single instance in the JVM memory as static are class level variables.
  • Private Constructor: It will restrict the instantiation of the Singleton class from the outside world (i.e. Initialization of this class using the new keyword is prevented)
  • Static factory method: This provides the global point of access to the Singleton object and returns the instance to the caller

1.3 Structure

In the Java programming language, there are different implementations of the singleton pattern. But before we start, the singleton design pattern should be considered only if all the three criteria’s are satisfied i.e.

  • Ownership of the single instance cannot be changed
  • Lazy initialization is recommended
  • Global access to the singleton instance is not otherwise provided

Over here is the classic implementation of the Singleton Design Pattern.

SingletonClassDemo.java

package com.java.design.pattern.singleton;

// Lazy Initialization
public class SingletonClassDemo {

	// Static attribute.
	private static SingletonClassDemo instance = null;

	// Private constructor.
	private SingletonClassDemo() {

	}

	// Static function.
	public static SingletonClassDemo getInstance() {
		// If there is no instance available, create new one (i.e. lazy initialization).
		if (instance == null) {
			instance = new SingletonClassDemo();
		}
		return instance;
	}
}

In the preceding example, we wrote a class with a method that creates a new instance of the class if one does not exist. Do note:

  • The instance attribute in the class is defined private and static
  • The constructor of the class is made private so that there is no other way to instantiate the class
  • The accessor function for obtaining the reference to the singleton object is defined public and static

This example is known as Lazy Initialization – which means that it restricts the instance creation until it is requested for the first time.

1.4 Real-time examples

Here represent some significant scenarios where the singleton design pattern is used.

  • The singleton pattern is used in the logger classes. These classes provide a global logging access point in all the application components without being necessary to produce an object each time a logging operation is performed
  • The singleton pattern is used to design the classes which provide the configuration settings for an application. This approach provides a global access point and keeps the instance as a cache object
  • The singleton pattern is used to design the application that needs to work with the serial port
  • The singleton pattern can be used with Abstract Factory, Builder, and Prototype design patterns to have a unique object

Note: If a Singleton class is loaded by two classloaders, two instances of the Singleton class will be created (i.e. one for each classloader).

2. Java Singleton Design Pattern Best Practices

In this section, we will explore the strategies that can be adopted to improve the Singleton design pattern.

2.1 Eager Initialization

In eager initialization, the instance of the singleton class is created at the time of class loading. This approach offers the easiest execution and helps improve the runtime performance of the application. Here is an example of Eager Initialization singleton class.

SingletonClassDemo2.java

package com.java.design.pattern.singleton;

// Eager Initialization
public class SingletonClassDemo2 {

	// Static attribute.
	// Making the "instance" attribute as "final" ensures that only one instance of the class exists.
	private static final SingletonClassDemo2 instance = new SingletonClassDemo2();

	// Private constructor.
	private SingletonClassDemo2() {

	}

	// Static function.
	public static SingletonClassDemo2 getInstance() {
		return instance;
	}
}

This approach is similar to lazy initialization, but it has a drawback i.e. the instance is always created even though the application is not utilizing it. This is considered a destructive practice for creating the database connections or sockets as it may lead to memory leak problems.

2.1.1 Static Initialization

The implementation of static block initialization is similar to the eager initialization, except that instance of the class is created in a static block that provides an option for the exception handling.

// Static block initialization for exception handling.
static {
	try {
		instance = new StaticSingletonClassDemo();
	} catch(Exception ex) {
		throw new RuntimeException("Exception occurred in creating the singleton instance ...!");
	}
}

2.2 Bill Pugh Singleton

Prior to Java 5, Java memory model had many issues and the developers had to use the Bill Pugh Solution for implementing the Single design pattern in their applications. This approach is based on initialization on demand idiom and uses the Inner classes’ concept. Here is an example of Bill Pugh Solution singleton class.

SingletonBillPughDemo.java

package com.java.design.pattern.singleton;

public class SingletonBillPughDemo {

	// Private constructor.
	private SingletonBillPughDemo() {

	}

	// Static inner class.
	private static class Lazyholder {
		// Static attribute.
		private static final SingletonBillPughDemo INSTANCE = new SingletonBillPughDemo();
	}

	// Static function.
	public static SingletonBillPughDemo getInstance() {
		return Lazyholder.INSTANCE;
	}
}

To a degree, Bill Pugh Singleton has been the good approach, but it is easily destroyed through the Java Reflection API. Hence, this approached was not heartily recommended by the Java developers.

2.3 Using Enum

Enum was introduced in Java 5 and provides a thread-safe implementation. The objects returned by Enum are Singleton in nature and therefore can be effectively used for implementing the Singleton design pattern in the multi-threaded environment.

SingletonEnum.java

package com.java.design.pattern.singleton;

public enum SingletonEnum {
	INSTANCE;
}

// The singleton instance can be accessed via "SingletonEnum.INSTANCE".

This approach is easy but it has 2 drawbacks i.e.

  • Enum does not support the lazy initialization
  • Changing the Singleton class to Multi-ton is not possible with Enum

2.4 Thread-Safe Singleton

Consider a scenario if two threads try to create an instance of a singleton class at the same time. In a multi-threaded environment, there is a possibility that separate objects get created, due to different times of accessing the (instance == null) check. This will break the singleton principle. The simplest way of achieving the thread safety in the singleton design pattern is to make the getInstance() method synchronized.

Here is an example of the Thread-Safe singleton class.

SingletonClassDemo3.java

package com.java.design.pattern.singleton;

// Thread-Safe Implementation
public class SingletonClassDemo3 {

	// Static attribute.
	private static SingletonClassDemo3 instance = null;

	// Private constructor.
	private SingletonClassDemo3() {

	}

	// Static function.
	// Only one thread can execute this at a time.
	public static synchronized SingletonClassDemo3 getInstance() {
		// If there is no instance available, create new one (i.e. lazy initialization).
		if (instance == null) {
			instance = new SingletonClassDemo3();
		}
		return instance;
	}
}

At this location using the synchronized keyword will ensure thread-safety but the application performance will be degraded. So at one side, we are resolving the problem on another side we are creating one more. To solve this, Double Check Lock principle is used.

2.5 Double Check Locking Principle

The locking mechanism in the singleton design pattern causes the thread to get a lock on the getInstance method only when the instance is null. This prevents the unnecessary synchronization once the instance variable is initialized. Here is an example of a Double Check Locking singleton class.

SingletonClassDemo4.java

package com.java.design.pattern.singleton;

// Double Check Locking Principle
public class SingletonClassDemo4 {

	// Static attribute.
	private static SingletonClassDemo4 instance = null;

	// Private constructor.
	private SingletonClassDemo4() {

	}

	// Static function.
	public static SingletonClassDemo4 getInstance() {
		// Double check locking principle.
		// If there is no instance available, create new one (i.e. lazy initialization).
		if (instance == null) {

			// To provide thread-safe implementation.
			synchronized(SingletonClassDemo4.class) {

				// Check again as multiple threads can reach above step.
				if (instance == null) {
					instance = new SingletonClassDemo4();
				}
			}
		}
		return instance;
	}
}

2.6 Using Volatile Keyword

At this point, the singleton implementation looks perfect. But it will still be incomplete without the use of the volatile keyword. This keyword guarantees the happens-before relationship i.e. all the write will happen in the volatile instance before any read of the instance.

SingletonClassDemo5.java

package com.java.design.pattern.singleton;

// Double Check Locking Principle
public class SingletonClassDemo5 {

	// Static and Volatile attribute.
	private static volatile SingletonClassDemo5 instance = null;

	// Private constructor.
	private SingletonClassDemo5() {

	}

	// Static function.
	public static SingletonClassDemo5 getInstance() {
		// Double check locking principle.
		// If there is no instance available, create new one (i.e. lazy initialization).
		if (instance == null) {

			// To provide thread-safe implementation.
			synchronized(SingletonClassDemo5.class) {

				// Check again as multiple threads can reach above step.
				if (instance == null) {
					instance = new SingletonClassDemo5();
				}
			}
		}
		return instance;
	}
}

Up till now, these have been the most widely used approaches for the Singleton design pattern. I am using 5th and the 6th approach in many of my projects as it is easy to understand and implement in the multi-threaded environment.

3. Ways to Kill Singleton

In this section, we will discuss three essential concepts which can break the singleton property of a class and how to prevent them. Let’s discuss them one by one.

3.1 Reflection

Reflection can easily destroy the Singleton design of a class by calling the private constructor and setting the access level to true. Let’s understand this with the help of a code snippet:

try {
	Constructor[] constructors = Singleton.class.getDeclaredConstructors();
	for (Constructor constructor : constructors) {
				
		// Below Code Will Destroy the Singleton Pattern
		constructor.setAccessible(true);
		instanceTwo = (Singleton) constructor.newInstance();
		break;
	}
} catch (Exception ex) {
	ex.printStackTrace();
}

To overcome this, Enum is used because JVM ensures that the Enum value is instantiated only once and the objects returned by Enum are Singleton in nature. The purpose of using Enum is that its default constructor is private in nature and developers cannot invoke them through the program. Its only drawback is it does not allow the Lazy initialization for the Singleton design pattern.

3.2 Serialization

In distributed systems, the Singleton design can be destroyed during the deserialization process as it’ll create a new instance of the Singleton class. To overcome this issue, developers have to implement the readResolve() method in the Singleton class implementing the Serializable interface. Let’s understand this with the help of a code snippet:

// Implementing the 'readResolve()' method.
protected Object readResolve() {
	return getInstance();
}

3.3 Cloning

Cloning is a concept where one can produce the copy of an instance and therefore destroying the Singleton design of a class. To overcome this issue, developers have to override the clone() method and throw the CloneNotSupportedException exception from that method. Let’s understand this with the help of a code snippet:

@Override
protected Object clone() throws CloneNotSupportedException {
	throw new CloneNotSupportedException();
}

4. Conclusion

These tips and examples on the Singleton Design Pattern are based on my experience and how I use this design pattern in the Java programming language.

4.1 Thumb Rules

  • Abstract Factory, Builder, Façade, and Prototype design patterns can use the Singleton in their implementation
  • Implement the double locking principle in the singleton design pattern to ensure thread-safety in the application
  • Beware of the object cloning as it can break your singleton. Remember restricting the object’s clone() method
  • Beware of the Java Reflection API as it can again break your singleton. Throw the runtime exception in the constructor if the (instance != null) check is true
  • Make the singleton class safe from Serialization
  • java.lang.Runtime and java.awt.Desktop are the two Singleton classes provided by the JVM

That’s all for this tutorial and I hope the article served you whatever you were looking for. Happy Learning and don’t forget to share!

5. Download the Eclipse Project

This was an example of following the best practices in the Java Singleton Design Pattern.

Download
You can download the full source code of this example here: SingletonDesignPattern
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Jarrod Roberson
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Jarrod Roberson

There is no such thing as a true “Singleton” in Java because of the ability to have multiple classloaders no single instance of any give class can be guaranteed. “Singleton” is a proven “anti-pattern” and if your design relies on it, it is fundamentally flawed.

ricardogeek
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I can tell you are not a spring framework fan :D