Linked EJBs

But in an application designed using EJBs, it would be frequently necessary for one EJB to use the services provided by another.  This case -- the client of an EJB being another EJB -- is handled specially in EJBs, because the EJB technology provides extra features for co-operating EJBs.

If EJB-A is a client of EJB-B, the deployment descriptor of EJB-A (the client EJB) should reference EJB-B.  This is done by using the <ejb-ref> XML node.  In entity beans, this node goes at the same level and after the <reentrant> and also after <cmp-field>s, <primkey-field>s, and/or <env-entry>s, but before any <security-role-ref>s or <resource-ref>s.  (Many of these are optional, so you will not see them in all ejb-jar.xml files.  But if present, the order mentioned above should be preserved.)

In session beans, the <ejb-ref> node goes at the same level and after <transaction-type> and after any <env-entry>s, but before any <security-role-ref>s or <resource-ref>s.

The ejb-ref contains the following nodes, in this order:
 

<description>	This is optional. Example:     <description>Link to stocks EJB</description>
<ejb-ref-name>	The name by which the linked EJB will be referenced.  This should start with "ejb/" e.g.     <ejb-ref-name>ejb/stocks</ejb-ref-name>
<ejb-ref-type>	This must be "Session" or "Entity".
<home>	The fully qualified name of the linked EJB's home interface.
<remote>	The fully qualified name of the linked EJB's remote interface.
<ejb-link>	Optional.  This is container dependent, and is used to point to the name of the linked EJB as known to the container.

A complete ejb-ref node might look as follows:

<ejb-ref>
<description>EJB Reference to stocks EJB</description>
<ejb-ref-name>ejb/stocks</ejb-ref-name>
<ejb-ref-type>Session</ejb-ref-type>
<home>stocks.StockQuotesHome</home>
<remote>stocks.StockQuotes</remote>
<ejb-link>StockQuotes</ejb-link>
</ejb-ref>

Once an ejb-ref has been added, the client ejb can use code like the following:

Context ctx = new InitialContext();
StockQuotesHome = (StockQuotesHome)
    ctx.lookup( "java:comp/env/ejb/stocks" );

This code will return the linked EJB's home object, which can then be used to create EJBs, find them, etc.

The "java:comp/env/" prefix is added to all JNDI lookups which have been made available in the EJB's environment.

Exercise:  Link the stocks EJB to your account's EJB.  Add a function in your account EJB that calls the stock EJB.  Build both EJBs and write a client program to test your new function. (Note:  Make sure to add the stock-ejb's JAR file to the classpath before starting Blazix EJB server.)