JNDI is a Java API that defines an interface to Naming and Directory Services for Java programs. JNDI is just an API and not, in itself, a Naming and Directory Service. To use JNDI, an implementation of a Naming and Directory service must be available. JNDI provides a service-independent interface to the underlying Service Provider implementation.
Figure 3.1 shows how the JNDI layer interfaces between the Java program and the underlying naming services. Additional naming services can be plugged into the JNDI layer by implementing the Service Provider Interface (SPI) for JNDI.