Ask any Java developer to name a design pattern that they have used at work and I guarantee you that one of their first choices will be the Singleton Design Pattern. This design pattern is used in cases where it is desirable to have one and only one instance of a class where other classes access the required functionality. It is one of the easiest design patterns to implement, yet it presents many pitfalls especially in a multithreaded environment where multiple threads may be trying to access the singleton class. Our goal in a multithreaded environment is to create a Thread Safe Singleton Class that creates an instance of a class if one has not yet been created, if one has been created then it should simply return an instance of the already created class. In this tutorial, “Creating a Thread Safe Singleton Class with Examples“, we will show you many examples of how to create a singleton class and discuss some of the pitfalls associated with each.

Advantages of a Singleton Class

Ensures only one instance of class is created
Provides a global single point of access
One of the simplest design patterns to implement

In the following example, I will illustrate creating the default or classic example of a Java Singleton Class. I will point out a few details to help you understand the finer points in a ClassicSingleton. First, you will notice that I have created a private static Singleton variable called instance. Second, you will note that the constructor is made private so that outside so that it cannot be instantiated from the outside. Third, you will notice that in the getInstance method I check to see if instance is equal to null and if it is I will create an instance of ClassicSingleton with the new operator in Java. Otherwise, an instance must have been created already and I simply return that previous created instance to the caller.

Singleton Class Example (Single Threaded)

package com.avaldes.tutorials;

public class ClassicSingleton{
  private static ClassicSingleton instance= null;
  
  private ClassicSingleton() {
  }
  
  public static ClassicSingleton getInstance() {
      if (instance == null) {
        instance = new ClassicSingleton();
      }
      return instance;
  }
}

In the previous example, the following code is not thread-safe:

if (instance == null) {
  instance = new ClassicSingleton();
}

You call this Singleton with: ClassicSingleton.getInstance();

In this example, we risk the possibility of the code being interrupted after line 1 (if statement) and before line 2, calling new ClassicSingleton(). The possibility, albeit small exists that if another thread would get into same block scope it is possible for more than one Singleton instances to be created thus violating the singleton design pattern we have tried to create. This lazy loading example of a singleton class works fine in a single threaded environment, but in a multithreaded environment more needs to be done. Let’s see some more examples.

Pros/Cons

Working Singleton for Single-Threaded Model
Not Thread-Safe (cannot be used in concurrent environment)

Singleton Class Example (Lazy Initialization)

package com.avaldes.tutorials;

public class LazyInitSingleton {
  private static LazyInitSingleton instance;
  
  private LazyInitSingleton() {
  }
  
  public synchronized static LazyInitSingleton getInstance() {
      if (instance == null) {
        instance = new LazyInitSingleton();
      }
      return instance;
  }
}

You call this Singleton with: LazyInitSingleton.getInstance();

In this example, we synchronize the getInstance() method using the synchronized keyword. All threads that need to get access to the instance in the singleton class will go through this public synchronized method. The first time the call is made, the instance will be null and a new instance will be created using the new operator on the private constructor. After that, all subsequent calls will simply return the instance that was previously created. However, the problem with this example, is that every call is incurring the synchronization overhead and performance penalty because getInstance is synchronized. Even after the instance has been created, every call to getInstance incurs the performance hit. A better approach is needed…

Pros/Cons

Synchronization works
Thread Safe
Lazy Initialization
Slow Synchronization
Wasteful synchronization after instance is created (not needed)

Singleton Class Example (Double-Checked Locking Idiom)

package com.avaldes.tutorials;

public class DoubleCheckedSingleton {
  private static DoubleCheckedSingleton instance;
  private static Object lock = new Object();

  private DoubleCheckedSingleton() {
  }
  
  // This is a broken multithreaded version
  // using the double-checked idiom 
  public static DoubleCheckedSingleton getInstance() {
      if (instance == null) {
        synchronized (lock) {
          if (instance == null) {
            instance = new DoubleCheckedSingleton();
          }
        }
      }
      return instance;
  }
  
}

You call this Singleton with: DoubleCheckedSingleton.getInstance();

In Java versions 1.4 and earlier this mechanism appears to work fine as it is extremely difficult to distinguish between an correctly working implementation and one that may have some subtle problems. With incorrect implementations, the failures happen intermittently. Incorrect implementations of double-checked locking may make failures difficult to reproduce.

Pros/Cons

Attempts to fix problem with synchronized method
Lazy Initialization
Broken synchronization
Very subtle errors, difficult to distinguish from correct working version
Reproduction of failures hard to pinpoint as they do not occur all the time

Singleton Class Example (Double-Checked Locking Idiom with Volatile)

package com.avaldes.tutorials;

public class DoubleCheckedSingletonFixed {
  private volatile static DoubleCheckedSingletonFixed instance;
  private static Object lock = new Object();

  private DoubleCheckedSingletonFixed() {
  }
  
  // This is a fixed multithreaded version
  // using the double-checked idiom 
  // fixed for Java 1.5 and above
  public static  DoubleCheckedSingletonFixed getInstance() {
      if (instance == null) {
        synchronized (lock) {
          if (instance == null) {
            instance = new DoubleCheckedSingletonFixed();
          }
        }
      }
      return instance;
  }
  
}

You call this Singleton with: DoubleCheckedSingletonFixed.getInstance();

As of Java versions 1.5 and above, the problems with the previous implementation of fixed. This version uses volatile keyword which ensures that all threads see the same variable as soon as it is changed.

Pros/Cons

Attempts to fix problem with synchronized method
Works under JDK 1.5
Lazy Initialization
Broken under JDK 1.4 and lower because of volatile usage and semantics

Singleton Class Example (Pre-initialized Class Loading)

package com.avaldes.tutorials;

public class PreInitializedSingleton {
  private static final PreInitializedSingleton instance 
                              = new PreInitializedSingleton();

  private PreInitializedSingleton() {}

  public static PreInitializedSingleton getInstance() {
    return instance;
  }
}

You call this Singleton with: PreInitializedSingleton.getInstance();

In this example, the static variable is created and initialized as soon as the class is loaded. It is thread-safe, however, this example’s main disadvantage is that it uses up memory and CPU resources prior to actually needing it. If the class is never uses than this may have been a waste of time and resources. Let’s look at a better approach.

Pros/Cons

Thread-Safe as initialization is done during class loading
No need for any synchronization
Does not make use of Lazy Initialization
May use up memory and CPU resources even if never used

Singleton Class Example (Initialization On-Demand Holder)

package com.avaldes.tutorials;

public class InitializationOnDemandSingleton {

  // Inner class for correct implementation of lazy loading
  private static class Holder {
    private static final InitializationOnDemandSingleton instance 
                             = new InitializationOnDemandSingleton();
  }
  
  private InitializationOnDemandSingleton() {}

  public static InitializationOnDemandSingleton getInstance() {
    return Holder.instance;
  }
}

You call this Singleton with: InitializationOnDemandSingleton.getInstance();

This example works well because it provides a thread-safe, fast and very efficient lazy loading initialization mechanism. This singleton example completes quickly because during the initialization there are no static variables to initialize early on. The static inner class Holder is not initialized by the JVM until it used in the getInstance() method. Once this happens the JVM will load and initialize the Holder class. This initialization is guaranteed to be sequential and non-concurrent by the Java Language Specification. This removes any need to add any type of synchronization over from our singleton class.

Pros/Cons

Thread-Safe as initialization is done on-demand
No need for any synchronization
Lazy Initialization
Uses static inner class to ensure on-demand loading

Singleton Class Example (Using enum)

package com.avaldes.tutorials;

public enum EnumSingleton {
  INSTANCE;
	
  public void doSomething() {
    // do something specific to your class.....
  }	
}

You call this Singleton with: EnumSingleton.INSTANCE;

In the book, “Effective Java“, Joshua Bloch suggests that this is best best approach to implement a singleton for any JVM that supports enums.

Pros/Cons

Thread-Safe as JVM ensures only one instantiation of enum
Enums are very easy to write
No need for any synchronization
No drawbacks with respect to serialization and reflection
Enum is somewhat inflexible as superclass is always enum
Enum contains some additional public methods which may muddy the waters…

Final Thoughts

There you have it. There are quite a few different approaches to creating a thread-safe singleton class each with some advantages and disadvantages. I still believe the best one for me is the Initialization On-Demand Singleton which performs the lazy loading and does not add the extra methods (valueOf, compareTo, values(), etc) found with enum class implementation.

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