Thread Safety in Java Singleton Classes

Singleton is one of the most widely used creational design pattern to restrict the object created by applications. If you are using it in a multi-threaded environment, then the thread-safety of the singleton class is very important. In real-world applications, resources like Database connections or Enterprise Information Systems (EIS) are limited and should be used wisely to avoid any resource crunch. To achieve this, we can implement a Singleton design pattern. We can create a wrapper class for the resource and limit the number of objects created at runtime to one.

Thread Safe Singleton in Java

In general, we follow the below steps to create a singleton class:

1. Create the private constructor to avoid any new object creation with new operator.
2. Declare a private static instance of the same class.
3. Provide a public static method that will return the singleton class instance variable. If the variable is not initialized then initialize it or else simply return the instance variable.

Using the above steps I have created a singleton class that looks like below. ASingleton.java

package com.journaldev.designpatterns;

public class ASingleton {

	private static ASingleton instance = null;

	private ASingleton() {
	}

	public static ASingleton getInstance() {
		if (instance == null) {
			instance = new ASingleton();
		}
		return instance;
	}

}

In the above code, the getInstance() method is not thread-safe. Multiple threads can access it at the same time. For the first few threads when the instance variable is not initialized, multiple threads can enter the if loop and create multiple instances. It will break our singleton implementation.

How to achieve thread-safety in Singleton Class?

There are three ways through which we can achieve thread safety.

1. Create the instance variable at the time of class loading. Pros:

• Thread safety without synchronization
• Easy to implement

Cons:

• Early creation of resource that might not be used in the application.
• The client application can’t pass any argument, so we can’t reuse it. For example, having a generic singleton class for database connection where client application supplies database server properties.

6. Synchronize the getInstance() method. Pros:

• Thread safety is guaranteed.
• Client application can pass parameters
• Lazy initialization achieved

Cons:

• Slow performance because of locking overhead.
• Unnecessary synchronization that is not required once the instance variable is initialized.

11. Use synchronized block inside the if loop and volatile variable Pros:

• Thread safety is guaranteed
• Client application can pass arguments
• Lazy initialization achieved
• Synchronization overhead is minimal and applicable only for first few threads when the variable is null.

Cons:

• Extra if condition

Looking at all the three ways to achieve thread-safety, I think the third one is the best option. In that case, the modified class will look like this:

package com.journaldev.designpatterns;

public class ASingleton {

	private static volatile ASingleton instance;
	private static Object mutex = new Object();

	private ASingleton() {
	}

	public static ASingleton getInstance() {
		ASingleton result = instance;
		if (result == null) {
			synchronized (mutex) {
				result = instance;
				if (result == null)
					instance = result = new ASingleton();
			}
		}
		return result;
	}

}

The local variable result seems unnecessary. But, it’s there to improve the performance of our code. In cases where the instance is already initialized (most of the time), the volatile field is only accessed once (due to “return result;” instead of “return instance;”). This can improve the method’s overall performance by as much as 25 percent. If you think there are better ways to achieve this or if the thread-safety is compromised in the above implementation, please comment and share it with all of us.

Bonus Tip

String is not a very good candidate to be used with synchronized keyword. It’s because they are stored in a string pool and we don’t want to lock a string that might be getting used by another piece of code. So I am using an Object variable. Learn more about synchronization and thread safety in java.

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