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Lesson 6 Mapping of method signatures
Objective Describe mapping of operations in IDL interfaces

Mapping of Method Signatures in Corba

To use an interface, you need to be able to use the operations defined in that interface. As stated previously, an IDL interface will map to a Java interface. Any operations in the interface will map to methods in the Java interface. The return type and all parameters will map natively to Java types in accordance with the primitive type mappings.
By value
In CORBA the in parameters are all passed one way over the wire[1], so they are all sent by value[2].
In fact, because all communication is across the wire rather than in the same address space, all elements passed back and forth must be copied, and as such pass by value[3]. In Java, all parameters are always by value, so this works well. All values are copied and push across to the server. When the information about a parameter reaches the server process, it is reconstructed into a new object of the appropriate type, generated from the IDL in the language of choice. Any variable passed in as a parameter cannot have its local value changed during the processing of a remote method. Even if the parameter is an object reference (nonremote), that object cannot be side-effected[4] like in a normal local method call. The interface-method below illustrates the mapping of individual features of method signatures:

Simple Mapped Operations

1) All elements defined in a module are the same package
MyIdl.idl** 
module MyMod {
    interface MyInterface {
        void doit();
        long anotherMethod();
        string yetAnotherMethod(in string arg1, in long arg2);
        object evenAnotherMethod(in long long number);
    };
};

MyInterface.java** 
package MyMod;

public interface MyInterface extends org.omg.CORBA.Object {
    public void doit();
    public int anotherMethod();
    public String yetAnotherMethod(String arg1, int arg2);
    public org.omg.CORBA.Object evenAnotherMethod(long number);
}

This IDL (Interface Definition Language) file describes the interface, and the corresponding Java file represents its implementation in a CORBA (Common Object Request Broker Architecture) system. All elements defined in a module are the same package

2) The interface definition results in a class for the interface containing all the elements defined in the IDL interface. 
MyIdl.idl:
  module MyMod{
The interface definition results in a class for the interface containing all the elements defined in the IDL interface. The Java interface extends from the common object reference interface org.omg.CORBA.Object.

3) Each operation maps to a single mehod in the Java interface generated for the IDL interface 
MyInterface.java:
Each operation maps to a single method in the Java interface generated for the IDL interface. The method name and operation name are the same.


4) The return type of the operation maps to the return type of the method following the rules for primitive type mapping. 
public interface MyInterface extends
The return type of the operation maps to the return type of the method following the rules for primitive type mapping.

5) The in parameters of the operation map, in order, to parameters of the same names in the Java method signature. 
public interface MyInterface extends
The in parameters of the operation map, in order, to parameters of the same names in the Java method signature.
The parameter types follow the mapping rules for primitive types.

6) The type object in IDL is an object reference, and so maps to the object reference base type org.omg.CORBA.Object 
MyIdl.idl;
The type object in IDL is an object reference, and so maps to the object reference base type org.omg.CORBA.Object. The type MyInterface in IDL refers to the IDL interface, so parameters and return values map to the Java interface generated for the IDL interface. The Java interface derives from org.omg.CORBA.Object.

  1. All elements defined in a module are the same package
  2. The interface definition results in a class for the interface containing all the elements defined in the IDL interface.
  3. Each operation maps to a single method in the Java interface generated for the IDL interface.
  4. The return type of the operation maps to the return type of the method following the rules for primitive type mapping.
  5. The in parameters of the operation map, in order, to parameters of the sames names in the Java method signature.
  6. The type object in IDL is an object reference, and so maps to the object reference base type org.omg.CORBA.Object.

Return value

The return value can be thought of as passed by value from the server instead of to the server. Any value the server passes back as a return value is copied and reconstructed in the local space of the client. Any operation defined as oneway maps to the Java interface exactly as if it was not declared oneway. The difference between a oneway and a regular operation is how the stub handles the communication, not in the programming interface.
In the next lesson, you will learn how to describe the mapping of IDL operations with out and inout parameters.

Java Mapping - Quiz

Click the Quiz link below to take another quiz on IDL-to-Java mapping.
Java Mapping - Quiz

Mapping Method Signatures - Exercise

Click the Exercise link below to implement calling remote methods by using the Java mapping for the IDL in the course project.
Mapping Method Signatures - Exercise

[1]Wire: The communication socket between the servers where all communication is reduced to a byte stream.
[2]By value: The value of the parameter is copied, and so any changes to the value are made on the copy, not the original reference.
[3] Pass by value: The value of the parameter is copied, and so any changes to the value are made on the copy, not the original reference.
[4]Side-effected: In pass by value, the value of the parameter cannot change, but if it is an object reference, methods on that reference can still be called, even accessors and mutators. So the internal state of that object can be changed, even if the parameter cannot be made to reference a different object .

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