Using OMG IDL With ILU

Using OMG IDL with ILU

ILU also supports the use of the interface definition language OMG IDL, defined by the Object Management Group (OMG) for their Common Object Request Broker Architecture (CORBA).

module Tutorial {

  exception DivideByZero {};

  interface Calculator {
    // Set the value of the calculator to `v'
    void SetValue (in double v);
    // Return the value of the calculator
    double GetValue ();
    // Adds `v' to the calculator's value
    void Add (in double v);
    // Subtracts `v' from the calculator's value
    void Subtract (in double v);
    // Multiplies the calculator's value by `v'
    void Multiply (in double v);
    // Divides the calculator's value by `v'
    void Divide (in double v) raises (DivideByZero);
  };

  interface Factory {
    // Create and return an instance of a Calculator object
    Calculator CreateCalculator();
  };
};

This can be used with the java-stubber:

% java-stubber Tutorial.idl
writing file javastubs/Tutorial/DivideByZero.java
writing file javastubs/Tutorial/Factory.java
writing file javastubs/Tutorial/FactoryStub.java
writing file javastubs/Tutorial/Calculator.java
writing file javastubs/Tutorial/CalculatorStub.java
% 

This will be a bit slower than running the java-stubber on the equivalent ISL file, as the program works by converting the OMG IDL into ISL, then compiling from the ISL description. OMG IDL interfaces can be checked by running the OMG IDL-to-ILU ISL converter, idl2isl, directly:

% idl2isl Tutorial.idl
INTERFACE Tutorial;

EXCEPTION DivideByZero;
TYPE Calculator = OBJECT OPTIONAL
        METHODS
                SetValue (v : REAL),
                GetValue () : REAL,
                Add (v : REAL),
                Subtract (v : REAL),
                Multiply (v : REAL),
                Divide (v : REAL)
                        RAISES DivideByZero END
        END;
TYPE Factory = OBJECT OPTIONAL
        METHODS
                CreateCalculator () : Calculator
        END;
% 

You will notice that the ISL interface generated by idl2isl is a bit different, in that the object type modifier OPTIONAL is used in the description of the Calculator and Factory types. This is because CORBA has the notion that any object type instance passed as a parameter or return value (or field in an array, or element of a sequence, etc.) may be null, instead of being a valid instance pointer. Thus, when working with OMG IDL descriptions of your interfaces, it is necessary to check the return type of methods like Tutorial.Factory.CreateCalculator to see that a valid object reference has been returned, before using the object. ISL allows you to have these CORBA-style objects, by using the OPTIONAL modifier in the declaration of an object type, but it also allows object pointers which can't be null. By default ILU object instances may not be null.

Transient and Permanent Objects

Keen-eyed readers will have seen that the Calculator type in `Tutorial.isl' is marked with the modifier COLLECTIBLE, while the Factory type is not. ILU differentiates between two kinds of objects, which we call transient and permanent.

Transient objects are typically created for a specific use, and typically have a fixed lifetime. They often have state associated with them. They are frequently used to represent some ongoing computation or operation.

Permanent objects are usually expected to be around forever (or until some activity not captured by a programming concept terminates). Permanent objects are often gateways to some sort of service, or represent some external piece of data, such as a cell in a spreadsheet, or a file on a disk somewhere. They are often stateless; when they do have state, it is typically backed by some form of stable storage, as they have to be available across crashes of their server program.

ILU provides an automatic system for distributed garbage collection of transient objects; that is, of removing them from everyone's address space when everyone is done with them. This uses the underlying garbage collection techniques of the particular programming language being used. With Java, the ILU garbage collector works in conjunction with the Java collector; with ANSI C, we use the extremely primitive collector provided for C; that is, when the process terminates, the objects are collected. To indicate to ILU that a particular object type is to be treated as transient, you mark the object type specification, in the ISL, with the keyword COLLECTIBLE.

For permanent objects, like the Tutorial.Factory object type, the ILU Java support will maintain a reference to the instance, preventing the Java collector from collecting it.