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Singleton Pattern

Lazy Initialization:-The instantiation of an object can be delayed until it is actually needed.

Usage: This especially beneficial when the constructor is doing a costly job like, accessing a remote database.

Example:

This code demonstrates how the Singleton pattern can be used to create a counter to provide unique sequential numbers, such as might be required for use as primary keys in a

Database:


 

Sequence.java


 

public class Sequence {

private static Sequence instance;

private static int counter;

private Sequence()

{

counter = 0; // May be necessary to obtain

// starting value elsewhere...

}

public static synchronized Sequence getInstance()

{

if(instance==null) // Lazy instantiation

{

instance = new Sequence();

}

return instance;

}

public static synchronized int getNext()

{

return ++counter;

}

}


 

Some things to note about this implementation:

  • Synchronized methods are used to ensure that the class is thread-safe.
  • This class cannot be subclassed because the constructor is private. This may or may not be a good thing depending on the resource being protected. To allow subclassing, the visibility of the constructor should be changed to protected.
  • Object serialization can cause problems; if a Singleton is serialized and then deserialized more than once, there will be multiple objects and not a singleton
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