There are several types of resources on the JVM that are rather expensive to initialize. Examples are HTTP connections, execution threads, JDBC connections, and so on. The Java API recognizes such resources and has built-in support for creating a pool of some of those resources, such that the consumer code borrows a resource from a pool when required and at the end of the job simply returns it to the pool. Java's thread pools (discussed in Chapter 5, Concurrency) and JDBC data sources are prominent examples. The idea is to preserve the initialized objects for reuse. Even though Java does not support pooling of a resource type directly, one can always create a pool abstraction around custom expensive resources. Note that the pooling technique is common in I/O activities, but can be equally applicable to non-I/O purposes where initialization cost is high.