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Integrate EJBs with CORBA

Access EJBs from non-Java-based applications

By Matjaz B. Juric, JavaWorld.com, 03/29/02

Enterprise JavaBeans (EJBs) are important for developing mission-critical business applications in Java. However, business applications don't exist in isolation; today, companies require integrated applications. In addition, most companies already have existing applications written in programming languages other than Java. Therefore, integrating EJB-based solutions with existing applications is becoming increasingly important.

In this article, I show how to access EJBs from applications written in programming languages other than Java. More specifically, I discuss access to session and entity beans (which use the synchronous IIOP, or Internet Inter-ORB Protocol, communication) from a CORBA C++ client. I won't address message-driven beans, although you could access them too from other programming languages using MOM (message-oriented middleware)-compliant products.

RMI-IIOP

Session and entity beans use synchronous communication with remote method invocation (RMI). Java 2 Platform, Enterprise Edition (J2EE) 1.3 requires that Java clients use RMI-IIOP. RMI-IIOP uses CORBA's IIOP protocol, which makes RMI-IIOP CORBA compatible. In other words, clients not based on Java can use CORBA to communicate with EJBs.

To do this, you must use a J2EE 1.3-compliant application server. Previous EJB specifications did not require you to use RMI-IIOP. Instead, the application server used RMI-JRMP (Java Remote Method Protocol) or some proprietary protocol. You must also use an ORB (Object Request Broker) compliant with CORBA 2.3.1 or higher. Older CORBA versions did not implement the necessary specifications for RMI-IIOP interoperability, particularly the Objects by Value specification later integrated into the CORBA specification (see the Object Management Group's (OMG) CORBA/IIOP Specification 2.6, under the "Value Type Semantics" chapter) and the Java Language Mapping to OMG IDL specification.

Value-type semantics added the concept of transferring objects by value, introduced by RMI, to CORBA. CORBA initially didn't support this functionality; however, that concept is crucial for achieving Java/CORBA interoperability.

The Java Language Mapping to OMG IDL specification defines how Java interfaces map to the CORBA Interface Definition Language (IDL). This definition lets CORBA distributed objects access EJBs (and also RMI-IIOP distributed objects) that originally didn't have CORBA IDL. Specifically, the specification defines a Java RMI subset, called RMI/IDL, which you can map to IDL using IIOP (or, more generally, General Inter-ORB Protocol) as the underlying protocol for communication.

RMI/IDL

Many RMI/IDL data types conform to certain restrictions; we'll look at the most important ones. For more information, please see the Java Language Mapping to OMG IDL specification.

Table 1 shows the mapping of Java primitive data types to IDL.

Table 1. Java-to-IDL mapping

Java OMG IDL
void void
boolean boolean
char wchar
byte octet
short short
int long
long long long
float float
double double


Java packages map to IDL modules. RMI/IDL remote interfaces map to the IDL interfaces with corresponding names. However, methods that use JavaBeans naming for read-write and read-only properties map to IDL attributes. I'll return to this later.

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