Basics of JNDI

Alessio Bechini

June 2002

Naming and Directory Services: Rationale

A fundamental element in every application is the capability to find and locate components and services.

A client looking for a component/service usually knows its name, but not its physical location.

The component name correspond to the actual name, typically much more difficult to remember and manage.

A well-known naming service is provided on Internet by DNS.

For applications, a naming/directory service is the way to get a reference to a required service (e.g. a JDBC data source, a JMS connection factory, an EJB home interface, etc.)

www.apache.org 64.208.42.41DNS

Naming and Directory Services

A naming service is an application that contains a set of objects, or references to objects, with corresponding names (usually easy to remember).Such correspondances are called bindings.

A directory service allows for the association of attributes to a binding.

Some popular directory implementations:

• Lightweight Directory Access Protocol (LDAP)

• Network Directory Service (NDS)

• Network Information Servis Plus (NIS+)

JNDI Overview

JNDI (Java Naming Directory Interface) provides Java clients with the capability to access naming and directory services.

JNDI is subdivided in the following packages:• javax.naming• javax.naming.directory• javax.naming.event• javax.naming.ldap• javax.naming.spi

JDBC APIs

JDBC Driver

JNDI APIs

SPIService Provider Interface

JNDI Configuration

JNDI configuration could be a quite difficult task.

Whenever we use an EJB server, JNDI is started automatically at the same time of the server itself. Such a service is usually already configured for the specific server.

Also the client applications using JNDI must be configured, and this task is up to the programmer/assempler/deployer.

JNDI Environment Properties

A client (object) uses JNDI to locate remote services; but JNDI might be a remote service as well. Thus, how to locate a naming service without using a naming service?

We can do it using environment properties.

Some standard JNDI properties:

java.naming.factory.initial

java.naming.provider.url

Java.naning.security.authentication

etc.

Context and InitialContext

Context (interface) is used to deal with objects that have been bind to a JNDI name.

javax.naming.Context contains methods to put objects in the naming service, and to retrieve them.

All the naming services have an access point, and in JNDI it is named InitialContext.

To obtain an InitialContext, three steps are required:1. Select the service provider

2. Specify every configuration needed

3. Call the InitialContext constructor, providing the environment properties.

!!! A unique InitialContext is not thread-safe!

Use of InitialContext

InitialContext must be closed after its usage, as it happens for any resource, e.g. a JDBC connection. The best way to do it is to insert the invocation of the close method inside a finally block.

The method getEnvironment of Context returns an Hashtable with all the active properties for the context.

The method lookup(name) returns a reference to the Object corresponding to name. This way, also EJB “home factories” are looked up.

Access to the Environment of an EJB

As an EJB usually must not access files, how can an EJB access its configuration properties?

A smart way to pass a configuration value to a bean is through its deployment descriptor, adding an env-entry tag, e.g.

<env-entry>

<description>what a beautiful entry!</description>

<env-entry-name>theNicestGirl</env-entry-name>

<env-entry-type>java.lang.String</env-entry-type>

<env-entry-value>”Anne”</env-entry-value>

</env-entry>

An EJB accesses environment properties using an InitialContext object, and looking up the desired property by its name.