When do you need to explicity specify initial context factory?

public class WsnInitialContextFactory
extends java.lang.Object
implements javax.naming.spi.InitialContextFactory

This class is the initial JNDI context factory for WebSphere Application Server. To use this class as the initial context factory for initial JNDI contexts, set the property java.naming.factory.initial to a value of com.ibm.websphere.naming.WsnInitialContextFactory. JNDI clients running in the WebSphere runtime environment should not have to set this property because it is already set in a copy of jndi.properties contained in a WebSphere runtime jar file. To set this property from a program, you can use the Java constant, javax.naming.Context.INITIAL_CONTEXT_FACTORY, for the property name, and the constant, com.ibm.websphere.naming.PROPS.INITIAL_CONTEXT_FACTORY, for the property value.

This class is only a facade of sorts and does not contain any real implementation. It delegates to an instance of the class specified by an internal property set in the properties file, jndiprovider.properties, which is part of the WebSphere installation. Users of WebSphere do not need to set this property. However, they do need jndiprovider.properties to be in their runtime environment so that it can be found by the class loader in effect.