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516

Chapter 18 JDBC and Database Connection Pooling

Using Singleton Classes to Share Connection 

Pools

Rather than using the 

ServletContext

 to share connection pools, you can

use normal static methods. For example, you could write a 

BookPool

 class

with static 

getPool

 and 

setPool

 methods and have each servlet check

BookPool.getPool

 to see if the value is non null, instantiating a new 

Con 

nectionPool

 if necessary. However, each servlet has to repeat similar code,

and a servlet could accidentally overwrite the shared pool that 

Book 

Pool.getPool

 returns. 

A better approach is to use a singleton class to encapsulate the desired

behavior. A singleton class is simply a class for which only a single instance

can be created, enforced through use of a private constructor. The instance is

retrieved through a static method that checks if there is already an object

allocated, returning it if so and allocating and returning a new one if not. For

example, here is the outline of a singleton 

BookPool

 class. Each servlet that

used it would obtain the connection pool by simply calling 

BookPool.getIn 

stance()

.

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