CON33-J. When using lazy initialization in Singleton, synchronize the getInstance() method

The Java Singleton pattern is a design pattern that governs the instantiation process. According to this design pattern, there can only be one instance of your class per JVM at any time. "Since there is only one Singleton instance, any instance fields of a Singleton will occur only once per class, just like static fields. Singletons often control access to resources such as database connections or sockets." [When is a Singleton not a Singleton?]

A typical implementation of a Singleton in Java is the creation of a single instance of the class as a static field.

The instance can be created using lazy initialization, which means that the instance is not created when the class loads but when it is first used.

Noncompliant Code Example

When the getter method is called by two (or more) threads or classes simultaneously, multiple instances of the Singleton class might result if one neglects to synchronize access.

class MySingleton {
  private static MySingleton _instance;

  private MySingleton() {
    // construct object . . .
    // private constructor prevents instantiation by outside callers
  }

  // lazy initialization
  // error, no synchronization on method access

  public static MySingleton getInstance() {
    if (_instance==null) {
     _instance = new MySingleton();
    }
    return _instance;
  }

  // Remainder of class definition . . .
}

Noncompliant Code Example

Multiple instances can be created even if you add a synchronized(this) block to the constructor call.

// Also an error, synchronization does not prevent
// two calls of constructor.
public static MySingleton getInstance() {
 if (_instance==null) {
   synchronized (MySingleton.class) {
      _instance = new MySingleton();
   }
 }
 return _instance;
}

Compliant Solution

To avoid these issues, make getInstance() a synchronized method.

class MySingleton {

  private static MySingleton _instance;

  private MySingleton() {
    // construct object . . .
    // private constructor prevents instantiation by outside callers
  }

   // lazy initialization
   public static synchronized MySingleton getInstance() {
     if (_instance==null) {
       _instance = new MySingleton();
     }
     return _instance;
   }
   // Remainder of class definition . . .
}

Applying a static modifier to the getInstance() method which returns the Singleton allows the method to be accessed subsequently (after the initial call) without creating a new object.

Noncompliant Code Example

Another solution for Singletons to be thread-safe is double-checked locking. Unfortunately, it is not guaranteed to work because compiler optimizations can force the assignment of the new Singleton object before all its fields are initialized.

// double-checked locking
public static MySingleton getInstance() {
 if (_instance==null) {
   synchronized (MySingleton.class) {
     if (_instance==null) {
        _instance = new MySingleton();
     }
   }
 }
}

Compliant Solution

It is still possible to create a copy of the Singleton object by cloning it using the object's clone method, which violates the Singleton Design Pattern's objective. To prevent this, one needs to override the object's clone method and throw a CloneNotSupportedException exception from within it. [[Daconta 03]]

class MySingleton {
  private static MySingleton _instance;

  private MySingleton() {
    // construct object . .
    //  private constructor prevents instantiation by outside callers
  }

  // lazy initialization
  public static synchronized MySingleton getInstance() {
    if (_instance==null) {
     _instance = new MySingleton();
    }
    return _instance;
  }

  public Object clone() throws CloneNotSupportedException {
    throw new CloneNotSupportedException();
  }

  // Remainder of class definition . . .
}

See MSC05-J. Make your classes noncloneable unless required for more details about restricting the clone() method.

To be fully compliant, it must be ensured that the class obeys the Singleton pattern's design contract. It is unreasonable to use the class for anything else, for example, as a method to share global state. [[Daconta 03]]

Risk Assessment

Using lazy initialization in a Singleton without synchronizing the getInstance() method may lead to multiple instances and can thus violate the expected contract.