Integrate EJBs with CORBA
Tutorial Details:
Integrate EJBs with CORBA
Integrate EJBs with CORBA
By: By Matjaz B. Juric
Access EJBs from non-Java-based applications
nterprise 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.
Java serializable objects map to CORBA value types. Value types provide pass-by-value semantics to CORBA. Value types are local and cannot be called remotely. They aren't registered with the ORB and don't require an identity, as their value is their identity. For more information, please refer to Professional J2EE EAI and the CORBA/IIOP Specification 2.6 .
As I mentioned, all Java serializable objects, built-in and user-defined, map to value types. However, there are certain exceptions to the rule -- you'll encounter one when attempting to map java.lang.String , for example. If defined as a constant ( final static ), the object maps to IDL wstring . In all other cases, including method parameters and return values, the object maps to the CORBA::WStringValue value type. This value type is defined as a part of the CORBA module; the IDL definition is as follows:
valuetype WStringValue wstring;
This is equivalent to the following IDL definition:
valuetype WStringValue {
public wstring data;
};
Keep in mind, however, that the first definition maps more cleanly to Java. Table 2 shows other special case mappings.
Table 2. Other important special case mappings
Java
OMG IDL
java.lang.Object
::java::lang::_Object
java.lang.String
::CORBA::WStringValue or wstring
java.lang.Class
::javax::rmi::CORBA::ClassDesc
java.io.Serializable
::java::io::Serializable
java.io.Externalizable
::java::io::Externalizable
java.rmi.Remote
::java::rmi::Remote
org.omg.CORBA.Object
Object
Achieve integration
I'll return to value types later for user-defined classes and then for built-in classes, such as Vectors , Collections , and Enumerations . For now, let's look at the basic approach for CORBA and EJB integration. First, we'll need an EJB. In this first example, we use a simple session bean that only uses primitive data types as method parameters and return values. That way, we aren't forced to use value types. ( Note: Accessing entity beans from CORBA clients mirrors the process of accessing session beans.)
This approach is the simplest; however, you cannot use it for more complex interfaces. Its benefit: you can use CORBA ORBs, which don't support value types. This is the case for some CORBA products, particularly for those based on languages other than C++.
For the examples, I'll use Inprise/Borland's VisiBroker for C++ 4.5 as the CORBA ORB and Microsoft's Visual C++ 5.0 (or higher) C++ compiler to compile the C++ client code. To deploy the example EJBs, I'll use BEA's WebLogic 6.1. You can download free trial versions of VisiBroker for C++ and WebLogic 6.1 from Resources .
You can use any CORBA 2.3.1 (or higher)-compliant ORB product that provides C++ mapping, a corresponding C++ compiler, and a J2EE 1.3-compliant application server. Theoretically, no code modifications are needed; however, minor modifications might be necessary if you use other products.
EJB session bean
Let's look briefly at a simple session bean, called CorbaEai . For our first example, the simple remote component interface contains a single method that sums two integer numbers:
package eai;
import java.rmi.RemoteException;
import javax.ejb.EJBObject;
public interface CorbaEai extends EJBObject {
public int sum(int num1, int num2) throws RemoteException;
}
After you become familiar with the integration process, I'll show you how to extend the interface to include methods with user-defined and built-in objects.
To run these examples, you must deploy the session bean to an application server. This requires that you define the home interface, implement the bean implementation class, define the deployment descriptor, build the jar file, and finally deploy the EJB. I won't show these steps; however, you can download the file containing the full code example for WebLogic 6.1, jw-0329-corba.zip , from Resources below.
Develop the CORBA client
To develop the CORBA client, we'll perform the following steps:
Generate the IDL from session bean home and component remote interfaces
Simplify the generated IDL
Compile the IDL interfaces to the corresponding programming language -- C++ in our case -- to generate stubs and other necessary mappings
Find out how to use JNDI (Java Naming and Directory Interface) as a CORBA naming service
Develop the C++ client and the support for value types
Build the client
Generate the IDL
To generate the IDL from Java interfaces, you can use any tool that supports the Java Language Mapping to OMG IDL specification. Examples include:
rmic compiler, using the -idl option, bundled with Java SDK 1.3 and higher
java2idl , bundled with VisiBroker
rmic and ejbc (using -idl ) compilers, bundled with BEA WebLogic
Other application servers provide similar tools. Be aware that generating the IDL interfaces from EJB interfaces is more complicated than generating them from RMI-IIOP interfaces. First, the EJB interfaces inherit from the EJBObject and EJBHome interfaces that define some base methods. Because the IDL interfaces also have to inherit from those interfaces, the tool must generate the IDL interfaces for the EJB interfaces as well. Second, the methods in EJB home interfaces throw at least CreateException , so the tool must map this exception, as well as any other user-defined exceptions, to the IDL.
You can use WebLogic's ejbc compiler, which provides all necessary IDL definitions for the enterprise bean. Suppose we built our CorbaEai session bean and generated the eai.jar file. To use ejbc to generate IDL files, we must use an additional -idl switch, which we use to tell the ejbc to generate the IDL interfaces. We also use the -idlDirectory switch to specify the directory in which to store the generated IDL files. First, we have to set WebLogic's environment variables. Then, we can use the following command:
java weblogic.ejbc eai.jar eaiC.jar -idl -idlDirectory ./idl
The ejbc compiler creates IDL files in the ./idl directory, generating 17 IDL files.
Simplify the generated IDL
Compiling and implementing all the generated interfaces and value types in C++ is time consuming. However, we won't need all the generated interfaces because we won't use certain methods. Therefore, we can save a lot of work by manually simplifying the generated IDLs.
For this first example, suppose we don't invoke methods on the EJBObject and EJBHome interfaces, and we don't need the EJBMetaData . We won't need a specialized RemoveException , so we can leave out the RemoteException and RemoveEx IDL interfaces. We can also eliminate all the IDL interfaces: java.io ,
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