what is java:comp/env?
java:comp/env
is the node in the JNDI tree where you can find properties for the current Java EE component (a webapp, or an EJB).
Context envContext = (Context)initContext.lookup("java:comp/env");
allows defining a variable pointing directly to this node. It allows doing
SomeBean s = (SomeBean) envContext.lookup("ejb/someBean");
DataSource ds = (DataSource) envContext.lookup("jdbc/dataSource");
rather than
SomeBean s = (SomeBean) initContext.lookup("java:comp/env/ejb/someBean");
DataSource ds = (DataSource) initContext.lookup("java:comp/env/jdbc/dataSource");
Relative paths instead of absolute paths. That's what it's used for.
It's an in-memory global hashtable where you can store global variables by name.
The "java:" url scheme causes JNDI to look for a javaURLContextFactory
class, which is usually provided by your app container, e.g. here is Tomcat's implementation javadoc
See also NamingManager.getURLContext