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Checking For Exceptions

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Checking for Exceptions

Suppose, instead of the Add method, we'd called the Divide method. In that case, we might have had to handle a DivideByZero exception; that is, notice the exception and do something sensible. We do this by establishing a handler for the exception:

        try {
            //now loop over the arguments, adding each in turn
            int i = 1;
            while (i<argv.length) {
                Double v = Double.valueOf(argv[i]); //don't bother...
                i = i+1;
        } catch (Tutorial.DivideByZero e) {
            System.err.println("raised DivideByZero exception: " + e);

And here's an example of what we get when it runs:

% java Tutorial.simple2 12345 6 7 8 9
the sum is 4.08234126984
% java 12345 6 0 8 9
exception signalled:  Tutorial: DivideByZero

Actually, every method may return an exception, as there are a number of standard system exceptions which may be signalled even by methods which have no declared exceptions. So we should check every method to see if it succeeded, even simple ones like GetValue.

Providing the True Module as a Network Service

Now let's see what's involved in providing the calculator functionality as a network service. Basically, there are three things to look at:

  • providing a "factory" to build calculator objects;
  • publishing the name of the factory; and
  • writing a server program.


Using Factories to Build Objects

When one program uses code from another address space, it has to get its hands on an instance of an ILU object, to be able to call methods. In our library application, we simply made a call into the true module, to create an instance of the calculator object. In the networked world, we need to do the same kind of thing, but this time the call into the true module has to be a method on an object type. In short, we need to have some object type which exports a method something like

  CreateCalculator () : Calculator

There are several ways to provide this. The standard way of doing it is to add an object type to our Tutorial interface, which contains this method. This kind of object type is sometimes called a factory, because it exists only in order to build instances of other object types. We'll add the following type definition to our `Tutorial.isl':

    CreateCalculator () : Calculator

Then we need to provide an implementation of the Factory object type, just as we did with the Calculator type:

//we will be lazy and not make an extra file
class FactoryImpl implements Tutorial.Factory  {
    xerox.ilu.IluServer server;
    public FactoryImpl(xerox.ilu.IluServer server) {
        this.server = server;
    public Tutorial.Calculator CreateCalculator() 
        	throws xerox.ilu.IluSystemException
        Tutorial.Calculator calc = new Tutorial.CalculatorImpl();
        return calc;
} //FactoryImpl

Now, to provide other programs a way of creating calculator objects, we'll just create just one instance of Tutorial.Factory, and let programs call the CreateCalculator method on that at will, to obtain new calculator objects.


Publishing a Well-Known Instance

The question then arises, how does a program that wants to use the Factory object get its hands on that one well-known instance? The answer is to use the simple binding system built into ILU. Simple binding allows a program acting as a "server" to publish the location of a well-known object, and allows programs acting as "clients" of that server to look up the location, given the object's name.

The name of an ILU object instance has two parts, which are the instance handle of the object, and the name of its kernel server, called the server ID. (The kernel server is a data structure maintained by the ILU kernel which takes care of all communication between different address spaces.) These two combined must form a universally unique ID for the object. Usually you can simply let the ILU system choose names for your objects automatically, in which case it takes care to choose names which will never conflict with names in use by others. However, for objects which we wish to publish, we need to specify what the name of an object will be, so that users of the well-known object can find it.

When working with the Java programming language, this act of explicitly specifying an object name is divided into two steps. First, we create a kernel server with a specified server ID. Secondly, we create an instance of an object on this new server, with a specified instance handle. Together, the server ID and the instance handle form the name of the instance.

For instance, we might use a server ID of Tutorial.domain, where domain is your Internet domain (typically something like, or This serves to distinguish your server from other servers on the net. Then we can use a simple instance handle, like theFactory. The name, or object ID, of this object would then be theFactory@Tutorial.domain, where domain would vary from place to place. Note that this implies that only one instance of this object is going to exist in the whole domain. If you have many people using different versions of this object in your domain, you should introduce more qualifiers in the server ID so that your kernel server can be distinguished from that run by others.


The Server Program

Given this information, we can now write a complete program that will serve as a provider of calculator objects to other programs. It will create a single Factory instance with a well-known name, publish that instance, then hang out servicing methods invoked on its objects. Here's what it looks like:

package Tutorial;

import Tutorial.CalculatorImpl;
import xerox.ilu.Ilu;
import xerox.ilu.IluException;
import xerox.ilu.IluServer;
import Tutorial.DivideByZero;

//insert the factory code from above here...

public class TutorialServer {
    static FactoryImpl factory;
    static xerox.ilu.IluServer trueServer;
    public static void main(String argv[]) {
        try {
            String serverId;
            if (argv.length < 1) {
                 System.out.println("Must specify a server id");
            //Create a server with appropriate server id (which is
            //taken from the first argument) 
            serverId = argv[0];
            trueServer = xerox.ilu.IluServer.createServer(serverId);
            //Now create an instance of a Factory object on the server
            //with an instance handle "theFactory"
            factory = new FactoryImpl(trueServer);
            //Make the factory well known by publishing it
            //Now we print the string binding handle (the object's name
            //plus its location) of the new Factory instance
            System.out.println("Factory instance published");
            System.out.println("Its SBH is '" + Ilu.sbhOfObject(factory) + "'");
            //the program doesn't terminate because the server is still alive...
        } catch (xerox.ilu.IluException e) {
            System.out.println("IluException: " + e);
} //TutorialServer

When we run this program, we'll see something like:

% java Tutorial.TutorialServer &
Factory instance published.
Its SBH is ''.

This indicates that the object known as is being exported in a particular way, which is encoded in the somegibberish part of the string binding handle. Your specific numbers will vary, but it should look similar.

Using the Network Service

Given that someone has exported a module as a network service, by publishing the location of a well-known instance of an object type, a potential client of that module can then use the module by binding to that well-known instance. It does this by calling the standard ILU routine ilu.LookupObject(), which takes the name and type of an instance, and attempts to find that instance on the net. The name of the object is specified as a pair of strings, the server ID of the object's kernel server, and the instance handle of the object on that kernel server.

So, in our first example, we could replace the call to Create_Tutorial_Calculator with a routine that calls xerox.ilu.IluSimpleBinding.lookup() to find the factory, then creates an instance of a Calculator. The full code of the revised example, `', is available as section, but here's what the new code for obtaining an instance of a Calculator looks like:

    /* We define a new routine, "Get_Tutorial_Calculator", which 
     * finds the tutorial factory, then creates a new Calculator
     * object for us.
    public static Tutorial.Calculator 
    GetTutorialCalculator(String serverId, String factoryId) {
        Tutorial.Factory factory = null;
        Tutorial.Calculator calc = null;
        System.out.println("Looking up factory");
        try {
            /* We have to call lookupObject with the object ID of
             * the factory object, and the "type" of the object we're looking
             * for.
            factory = (Tutorial.Factory) 
        } catch (xerox.ilu.IluSystemException e) {
            System.err.println("Failed to get factory: " + e);
        if (factory==null) {
            System.err.println("Got null factory");
        System.out.println("Got factory " + factory);
        System.out.println("Looking up Calculator");
        try {
            calc = factory.CreateCalculator();
        } catch (xerox.ilu.IluException e) {
            System.err.println("Failed to get Calculator: " + e);
        if (calc==null) {
            System.err.println("Got null Calculator");
        System.out.println("Got Calculator " + calc);
        return calc;
    } //GetTutorialCalculator

We then can use the simple3 program:

% java Tutorial.simple3 theFactory 1 2 3 4 5 6
the sum is 2.10000


Subtyping and Other ISL Types

ILU ISL contains support for a number of types other than object types and REAL. The primitive ISL types include 16, 32, and 64 bit signed and unsigned integers, bytes, 8 and 16 bit characters, a boolean type, and 32, 64, and 128 bit floating point types. A number of type constructors allow specification of arrays, sequences, records, unions, and enumerations, as well as object types. The ISL OPTIONAL type constructor provides an implicit union of some type with NULL, which is useful for building recursive data structures such as linked lists or binary trees.

To illustrate some of these types, we'll extend the Tutorial.Calculator type. Many real-world desktop calculators include a register tape, a printed listing of all the operations that have been performed, with a display of what the value of the calculator was after each operation. We'll add a register tape to Tutorial.Calculator.

We could do it by adding a new method to Tutorial.Calculator, called GetTape. Unfortunately, this would break our existing code, because it would change the Tutorial.Calculator object type, and existing compiled clients wouldn't be able to recognize the new object type. Instead, we'll extend the object type by subtyping; that is, by creating a new object type which uses Tutorial.Calculator as a supertype, but adds new methods of its own. This subtype will actually have two types; both its own new type, and Tutorial.Calculator. We'll also define a subtype of the Tutorial.Factory type, to allow us to create new instances of the new Calculator subtype. Finally, we'll define a new module interface for the new types, so that we don't have to modify the Tutorial interface.

First, let's define the necessary type to represent the operations performed on the calculator:



    SetValue, Add, Subtract, Multiply, Divide END;

TYPE Operation = RECORD
    op : OpType,
    value : REAL,
    accumulator : REAL

TYPE RegisterTape = SEQUENCE OF Operation;

The enumerated type OpType defines an abstract type with five possible values. The type Operation defines a record type (in Java, a dictionary) with 3 fields: the op field, which tells us which of the five possible calculator operations was performed, the value field, which tells us the value of the operand for the operation, and the accumulator field, which tells us what the value of the calculator was after the operation had been performed. Finally, the Operation type is a simple sequence, or list, of Operation. Note that Tutorial2 imports Tutorial; that is, it allows the use of the Tutorial types, exceptions, and constants, in the specifications in Tutorial2.

Now we define the new object types (in the same file):

  SUPERTYPES Tutorial.Calculator END
  DOCUMENTATION "4 function calculator with register tape"
    GetTape () : RegisterTape

TYPE Factory = OBJECT SUPERTYPES Tutorial.Factory END
    CreateTapeCalculator () : TapeCalculator

The SUPERTYPES attribute of an object type may take multiple object type names, so ISL supports multiple inheritance. The Tutorial2.TapeCalculator type will now support the six methods of Tutorial.Calculator, as well as its own method, GetTape.

We then need to provide an implementation for Tutorial2.TapeCalculator (We will be lazy again and put the actual code in the file with the server). We modify each method on the TapeCalculator object to record its invocation, and add a slot to hold the contents of the `tape'. We also provide an implementation for Tutorial2.Factory:

// lazy again... this is in other file

class Factory2Impl implements Tutorial2.Factory {
    public Factory2Impl() {
    public Tutorial.Calculator CreateCalculator() 
        		throws xerox.ilu.IluSystemException {
        System.out.println("Factory2Impl: request for a simple calculator");
        return new Tutorial2.TapeCalculatorImpl();
    } //CreateCalculator
    public Tutorial2.TapeCalculator CreateTapeCalculator() 
		throws xerox.ilu.IluSystemException {
        System.out.println("Factory2Impl: request for a tape calculator");
        return new Tutorial2.TapeCalculatorImpl();
    } //CreateTapeCalculator
} //Factory2Impl

Note that both the Tutorial2.FactoryImpl.CreateCalculator and Tutorial2.FactoryImpl.CreateTapeCalculator methods create and return instances of Tutorial2.TapeCalculator. This is valid, because instances of Tutorial2.TapeCalculator are also instances of Tutorial.Calculator.

Now we modify `' to create an instance of Tutorial2.Factory, instead of Tutorial.Factory, and to initialize the Tutorial2 true-side code

Note that one nice result of this approach to versioning is that old clients, which know nothing about the new TapeCalculator class, or about the whole Tutorial2 interface in general, will continue to function, since every instance of Tutorial2.TapeCalculator is also an instance of Tutorial.Calculator, and every instance of Tutorial2.Factory is also an instance of Tutorial.Factory.

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Keywords: Checking for Exceptions, ILU, ILU, ILU tutorial, ILU tutorial pdf, history of ILU, Custamizing Style Sheet, learn ILU

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