Formatted 14 January 1998.
Copyright (C) 1995 Xerox Corporation
All Rights Reserved.
This tutorial will show how to use the ILU system with the programming language Java, both as a way of developing software libraries, and as a way of building distributed systems. In an extended example, we'll build an ILU module that implements a simple four-function calculator, capable of addition, subtraction, multiplication, and division. It will signal an error if the user attempts to divide by zero. The example demonstrates how to specify the interface for the module; how to implement the module in Java; how to use that implementation as a simple library; how to provide the module as a remote service; how to write a client of that remote service; and how to use subtyping to extend an object type and provide different versions of a module. We'll also demonstrate how to use OMG IDL with ILU, and discuss the notion of network garbage collection.
Each of the programs and files referenced in this tutorial is available as a complete program in a separate appendix to this document; parts of programs are quoted in the text of the tutorial.
Our first task is to specify more exactly what it is we're trying to provide. A typical four-function calculator lets a user enter a value, then press an operation key, either +, -, /, or *, then enter another number, then press = to actually have the operation happen. There's usually a CLEAR button to press to reset the state of the calculator. We want to provide something like that.
We'll recast this a bit more formally as the interface of our module; that is, the way the module will appear to clients of its functionality. The interface typically describes a number of function calls which can be made into the module, listing their arguments and return types, and describing their effects. ILU uses object-oriented interfaces, in which the functions in the interface are grouped into sets, each of which applies to an object type. These functions are called methods.
For example, we can think of the calculator as an object type, with several methods: Add, Subtract, Multiply, Divide, Clear, etc. ILU provides a standard notation to write this down with, called ISL (which stands for "Interface Specification Language"). ISL is a declarative language which can be processed by computer programs. It allows you to define object types (with methods), other non-object types, exceptions, and constants.
The interface for our calculator would be written in ISL as:
INTERFACE Tutorial;
EXCEPTION DivideByZero;
TYPE Calculator = OBJECT
METHODS
SetValue (v : REAL),
GetValue () : REAL,
Add (v : REAL),
Subtract (v : REAL),
Multiply (v : REAL),
Divide (v : REAL) RAISES DivideByZero END
END;
This defines an interface Tutorial, an exception DivideByZero,
and an object type Calculator. Let's consider these one by one.
The interface, Tutorial, is a way of grouping a number of type
and exception definitions. This is important to prevent collisions
between names defined by one group and names defined by another group.
For example, suppose two different people had defined two different
object types, with different methods, but both called Calculator!
It would be impossible to tell which calculator was meant. By
defining the Calculator object type within the scope of the
Tutorial interface, this confusion can be avoided.
The exception, DivideByZero, is a formal name for a particular
kind of error, division by zero. Exceptions in ILU can specify
an exception-value type, as well, which means that real errors
of that kind have a value of the exception-value type associated with them.
This allows the error to contain useful information about why it might
have come about. However, DivideByZero is a simple exception,
and has no exception-value type defined. We should note that the full
name of this exception is Tutorial.DivideByZero, but for this
tutorial we'll simply call our exceptions and types by their short name.
The object type, Calculator (again, really Tutorial.Calculator),
is a set of six methods. Two of those methods, SetValue and
GetValue, allow us to enter a number into the calculator object,
and "read" the number. Note that SetValue takes a single
argument, v, of type REAL. REAL is a
built-in ISL type, denoting a 64-bit floating point number.
Built-in ISL types are things like INTEGER (32-bit
signed integer), BYTE (8-bit unsigned byte), and CHARACTER
(16-bit Unicode character). Other more complicated types are
built up from these simple types using ISL type constructors,
such as SEQUENCE OF, RECORD, or ARRAY OF.
Note also that SetValue does not return a value,
and neither do Add, Subtract, Multiply,
or Divide. Rather,
when you want to see what the current value of the calculator
is, you must call GetValue, a method which has no arguments,
but which returns a REAL value, which is the value of the
calculator object. This is an arbitrary decision on our part;
we could have written the interface differently, say as
TYPE NotOurCalculator = OBJECT
METHODS
SetValue () : REAL,
Add (v : REAL) : REAL,
Subtract (v : REAL) : REAL,
Multiply (v : REAL) : REAL,
Divide (v : REAL) : REAL RAISES DivideByZero END
END;
-- but we didn't.
Our list of methods on Calculator is bracketed by the two
keywords METHODS and END, and the elements are separated
from each other by commas. This is pretty standard in ISL:
elements of a list are separated by commas; the keyword END
is used when an explicit list-end marker is needed (but not when it's
not necessary, as in the list of arguments to a method); the list often
begins with some keyword, like METHODS.
The raises clause (the list of exceptions which a method
might raise) of the method Divide provides another example
of a list, this time with only one member, introduced by the keyword
RAISES.
Another standard
feature of ISL is separating a name, like v,
from a type, like REAL, with a colon character. For example,
constants are defined with syntax like
CONSTANT Zero : INTEGER = 0;Definitions, of interface, types, constants, and exceptions, are terminated with a semicolon.
We should expand our interface a bit by adding more documentation on what our methods actually do. We can do this with the docstring feature of ISL, which allows the user to add arbitrary text to object type definitions and method definitions. Using this, we can write
INTERFACE Tutorial;
EXCEPTION DivideByZero
"this error is signalled if the client of the Calculator calls
the Divide method with a value of 0";
TYPE Calculator = OBJECT
COLLECTIBLE
DOCUMENTATION "4-function calculator"
METHODS
SetValue (v : REAL) "Set the value of the calculator to `v'",
GetValue () : REAL "Return the value of the calculator",
Add (v : REAL) "Adds `v' to the calculator's value",
Subtract (v : REAL) "Subtracts `v' from the calculator's value",
Multiply (v : REAL) "Multiplies the calculator's value by `v'",
Divide (v : REAL) RAISES DivideByZero END
"Divides the calculator's value by `v'"
END;
Note that we can use the DOCUMENTATION keyword on object types
to add documentation about the object type, and can simply add documentation
strings to the end of exception and method definitions. These docstrings
would be passed on to the lisp docstring system, so that they are available
at runtime from lisp. Documentation
strings cannot currently be used for non-object types or from Java.
ILU provides a program, islscan, which can be used
to check the syntax of an ISL specification. islscan
parses the specification and summarizes it to standard output:
% islscan Tutorial.isl
Interface "Tutorial", imports "ilu"
{defined on line 1
of file /tmp/tutorial/Tutorial.isl (Fri Jan 27 09:41:12 1995)}
Types:
real {<built-in>, referenced on 10 11 12 13 14 15}
Classes:
Calculator {defined on line 17}
methods:
SetValue (v : real); {defined 10, id 1}
"Set the value of the calculator to `v'"
GetValue () : real; {defined 11, id 2}
"Return the value of the calculator"
Add (v : real); {defined 12, id 3}
"Adds `v' to the calculator's value"
Subtract (v : real); {defined 13, id 4}
"Subtracts `v' from the calculator's value"
Multiply (v : real); {defined 14, id 5}
"Multiplies the calculator's value by `v'"
Divide (v : real) {DivideByZero}; {defined 16, id 6}
"Divides the calculator's value by `v'"
documentation:
"4-function calculator"
unique id: ilu:cigqcW09P1FF98gYVOhf5XxGf15
Exceptions:
DivideByZero {defined on line 5, refs 15}
%
islscan simply lists the types defined in the interface, separating
out object types (which it calls "classes"), the exceptions, and
the constants. Note that for the Calculator object type,
it also lists something called its unique id. This is a 160-bit
number (expressed in base 64) that ILU assigns automatically
to every type, as a way of distinguishing them. While
it might interesting to know that it exists (:-),
the ILU user never has know what it is; islscan
supplies it for the convenience of the ILU implementors, who
sometimes do have to know it.
After we've defined an interface, we then need to supply an implementation of our module. Implementations can be done in any language supported by ILU. Which language you choose often depends on what sort of operations have to be performed in implementing the specific functions of the module. Different languages have specific advantages and disadvantages in different areas. Another consideration is whether you wish to use the implementation mainly as a library, in which case it should probably be done in the same language as the rest of your applications, or mainly as a remote service, in which case the specific implementation language is less important.
We'll demonstrate an implementation of the Calculator
object type in Java, which is one of the most capable
of all the ILU-supported languages. This is just a matter
of defining a Java class, corresponding to the Tutorial.Calculator type. Before we do that,
though, we'll explain how the names and signatures of the Java functions
are arrived at.
For every programming language supported by ILU, there is a standard mapping defined from ISL to that programming language. This mapping defines what ISL type names, exception names, method names, and so on look like in that programming language.
The mapping for Java is simple. For type names,
such as Tutorial.Calculator, the Java name
of the ISL type Interface.Name
is Interface.Name, with any hyphens replaced by underscores. That is, the name of the interface in ISL
becomes the name of a class in Java.
So the name of our Calculator type in Java
would be Tutorial.Calculator, which is really the name of a Java class.
The Java mapping for a method name such as SetValue
is the method name, with any hyphens replaced by underscores.
The return type of this Java method is whatever is specified
in the ISL specification for the method, or void if
no type is specified. The arguments for the Java method are the
same as specified in the ISL; their types are the
Java types corresponding to the ISL types. An instance is simply a value of that
type. Thus the Java method corresponding
to our ISL SetValue would have the prototype signature
void SetValue(float v) throws xerox.ilu.SystemException
Similarly, the signatures for the all methods, in Java, are encapsulated in this generated interface
package Tutorial;
public interface Calculator extends xerox.ilu.IluObject{
public void SetValue(double v)
throws xerox.ilu.IluSystemException;
public double GetValue()
throws xerox.ilu.IluSystemException;
public void Add(double v)
throws xerox.ilu.IluSystemException;
public void Subtract(double v)
throws xerox.ilu.IluSystemException;
public void Multiply(double v)
throws xerox.ilu.IluSystemException;
public void Divide(double v)
throws DivideByZero, xerox.ilu.IluSystemException;
} //Calculator
Note that we don't always want to generate code in a package name derived directly from
the interface name. The stubber allows to specify a prefix package.
Note that even though most methods do not raise an exception,
we still dedclare the system exceptions.
The mapping of exception names is similar to the mapping used for types.
So the exception Tutorial.DivideByZero
would also have the name Tutorial.DivideByZero, in Java.
One way to see what all the Java names for an interface
look like is to run the program java-stubber. This program
reads an ISL file, and generates the necessary Java
code to support that interface in Java. One of the files
generated is `Interface.java', which contains the definitions
of all the Java types for that interface.
% java-stubber Tutorial.isl 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 %
Now we provide an implementation of our object; the CalculatorImpl class implement the
generated Java interface for our Calculator class:
package Tutorial;
/*
* While this class complies to the Tutorial.isl specification
* it is a local implementation. Its instances need to be
* registered explicitely or implicitely with Ilu before they
* are publicly accessible.
*/
public class CalculatorImpl implements
Tutorial.Calculator {
double theValue = 0.0;
public CalculatorImpl(){
theValue = 0.0;
}
public void SetValue(double v) {
theValue = v;
}
public double GetValue() {
return theValue;
}
public void Add(double v) {
theValue = theValue + v;
}
public void Subtract(double v) {
theValue = theValue - v;
}
public void Multiply(double v) {
theValue = theValue * v;
}
public void Divide(double v) throws Tutorial.DivideByZero {
if (v==0.0) throw new Tutorial.DivideByZero();
theValue = theValue / v;
}
} //CalculatorImpl
Each instance of a Tutorial.CalculatorImpl implements a Tutorial.Calculator. Each has an instance
variable called theValue, which maintains a running total
of the `accumulator' for that instance. We can create an instance
of a Tutorial.CalculatorImpl object by simply calling new Tutorial.CalculatorImpl().
So, a very simple program to use the Tutorial module might be
the following:
/*
* A simple client program that demonstrates the use of the
* Calculator module as a library.
*/
/*
* Run this like
* java Tutorial.simple1 number [number...]
*/
package Tutorial;
public class simple1 {
public static void main(String argv[]) {
CalculatorImpl calc;
//create the calculator
calc = new CalculatorImpl();
if (calc==null) {
System.err.println("Got null TapeCalculator");
System.exit(1);
}
//clear the calculator before using it
calc.SetValue(0.0);
//now loop over the arguments, adding each in turn
int i = 0;
while (i<argv.length) {
Double v = Double.valueOf(argv[i]); //don't bother about exceptions
calc.Add(v.doubleValue());
i = i+1;
}
//and print the result
System.out.println("The sum is " + calc.GetValue());
} //main
} //simple1
This program would be compiled and then run as follows:
% java Tutorial.simple1 34.9 45.23111 12 the sum is 92.13111 %
This is a completely self-contained use of the Tutorial
implementation; when a method is called, it is the true method
that is invoked. The use of ILU in this program adds
some overhead in terms of included code, but has almost
the same performance as a version of this program that does not
use ILU.
CalculatorImpl calc;
calc = new CalculatorImpl();
we will write
Tutorial.Calculator calc;
calc = new CalculatorImpl();
This obvously doesn't cause any real change but conceptionally the change is quite large: calc isn't simply a local class anymore but now is the java interface representing the ILU object.
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...
calc.Divide(v.doubleValue());
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 simple2.py 12345 6 0 8 9 exception signalled: Tutorial: DivideByZeroActually, 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.
Now let's see what's involved in providing the calculator functionality as a network service. Basically, there are three things to look at:
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':
TYPE Factory = OBJECT
METHODS
CreateCalculator () : Calculator
END;
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();
Ilu.registerTrueObject(
Ilu.inventID(),
calc,
this.server,
Tutorial.CalculatorStub.iluClass(),
Ilu.unspec
);
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.
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
department.company.com, or
department.univerity.edu). 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.
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");
return;
}
//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);
xerox.ilu.Ilu.registerTrueObject(
"theFactory",
factory,
trueServer,
Tutorial.FactoryStub.iluClass(),
xerox.ilu.IluLifetimeArgs.iluLifetimeRemember
);
//Make the factory well known by publishing it
xerox.ilu.IluSimpleBinding.publish(factory);
//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 Tutorial.dept.company.com & Factory instance published. Its SBH is 'theFactory@Tutorial.dept.company.com@somegibberish'. %This indicates that the object known as
theFactory@Tutorial.dept.company.com 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.
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, `simple3.java',
is available as section simple3.java, 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)
xerox.ilu.IluSimpleBinding.lookup(
serverId,
factoryId,
Tutorial.FactoryStub.iluClass()
);
} catch (xerox.ilu.IluSystemException e) {
System.err.println("Failed to get factory: " + e);
System.exit(1);
}
if (factory==null) {
System.err.println("Got null factory");
System.exit(1);
}
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);
System.exit(1);
}
if (calc==null) {
System.err.println("Got null Calculator");
System.exit(1);
}
System.out.println("Got Calculator " + calc);
return calc;
} //GetTutorialCalculator
We then can use the simple3 program:
% java Tutorial.simple3 Tutorial.dept.company.com theFactory 1 2 3 4 5 6 the sum is 2.10000 %
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:
INTERFACE Tutorial2 IMPORTS Tutorial END;
TYPE OpType = ENUMERATION
SetValue, Add, Subtract, Multiply, Divide END;
TYPE Operation = RECORD
op : OpType,
value : REAL,
accumulator : REAL
END;
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):
TYPE TapeCalculator = OBJECT COLLECTIBLE
SUPERTYPES Tutorial.Calculator END
DOCUMENTATION "4 function calculator with register tape"
METHODS
GetTape () : RegisterTape
END;
TYPE Factory = OBJECT SUPERTYPES Tutorial.Factory END
METHODS
CreateTapeCalculator () : TapeCalculator
END;
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 `TutorialServer.java' to create an instance of Tutorial2.Factory,
instead of Tutorial.Factory, and to initialize the Tutorial2
true-side code (See section Tutorial2Server.java for the actual code).
Finally, see section simple4.java, for an example of a client of the
TapeCalculator type.
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.
OPTIONAL types, so
not every ILU interface can be expressed in OMG IDL, but
many of them can. For example, here is the OMG IDL version
of the Tutorial interface:
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.
The OMG IDL version of Tutorial2 can be found in section Tutorial2.idl.
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.
The 2.0 release of ILU contains support for the programming languages ANSI C, C++, Modula-3, Java, and Common Lisp. It has been installed on many flavors of UNIX, including SPARC machines running SunOS 4.1.3 and Solaris 2, SGI MIPS machines running IRIX 5.2, Intel 486 machines running Linux 1.1.78, DEC Alpha machines with OSF/1, IBM RS/6000 machines running AIX, and HP machines running HP/UX. It runs on Microsoft Windows 3.1, Windows 95, and Windows NT environments. It supports both threaded and non-threaded operation. Since one of the implementation goals of ILU is to maximize compatibility with existing open standards, ILU provides support for use of the OMG CORBA IDL interface description language, and can be thought of as a CORBA ORB system (though with omissions from and extensions to the CORBA spec). As another result, ILU includes a self-contained implementation of ONC RPC.
ILU
is available free from ftp://ftp.parc.xerox.com/pub/ilu/ilu.html.
Make sure the "java" interpreter is on your PATH, and that "$(ILUHOME)/bin" is on your path. If you use the literal expression "$(ILUHOME)/bin", make sure that the environment variable ILUHOME is properly defined.
2. Make sure "./classes" and "$(ILUHOME)/lib/ilu.jar" are on your CLASSPATH.
3. Make sure your LD_LIBRARY_PATH has "$(ILUHOME)/lib" on it.
INTERFACE Tutorial;
EXCEPTION DivideByZero
"this error is signalled if the client of the Calculator calls
the Divide method with a value of 0";
TYPE Calculator = OBJECT COLLECTIBLE
DOCUMENTATION "4-function calculator"
METHODS
SetValue (v : REAL) "Set the value of the calculator to `v'",
GetValue () : REAL "Return the value of the calculator",
Add (v : REAL) "Adds `v' to the calculator's value",
Subtract (v : REAL) "Subtracts `v' from the calculator's value",
Multiply (v : REAL) "Multiplies the calculator's value by `v'",
Divide (v : REAL) RAISES DivideByZero END
"Divides the calculator's value by `v'"
END;
TYPE Factory = OBJECT
METHODS
CreateCalculator () : Calculator
END;
package Tutorial;
/*
* While this class matches the Tutorial.isl specification
* it is a local implementation. Its instances need to be
* registered explicitely or implicitely with Ilu before they
* are publicly accessible.
*/
public class CalculatorImpl implements
Tutorial.Calculator {
double theValue = 0.0;
public CalculatorImpl(){
theValue = 0.0;
}
public void SetValue(double v) {
theValue = v;
}
public double GetValue() {
return theValue;
}
public void Add(double v) {
theValue = theValue + v;
}
public void Subtract(double v) {
theValue = theValue - v;
}
public void Multiply(double v) {
theValue = theValue * v;
}
public void Divide(double v) throws Tutorial.DivideByZero {
if (v==0.0) throw new Tutorial.DivideByZero();
theValue = theValue / v;
}
} //CalculatorImpl
/*
* A simple client program that demonstrates the use of the
* Calculator module as a library.
*/
/*
* Run this like
* java Tutorial.simple1 number [number...]
*/
package Tutorial;
public class simple1 {
public static void main(String argv[]) {
CalculatorImpl calc;
//create the calculator
calc = new CalculatorImpl();
if (calc==null) {
System.err.println("Got null TapeCalculator");
System.exit(1);
}
//clear the calculator before using it
calc.SetValue(0.0);
//now loop over the arguments, adding each in turn
int i = 0;
while (i<argv.length) {
Double v = Double.valueOf(argv[i]); //don't bother about exceptions
calc.Add(v.doubleValue());
i = i+1;
}
//and print the result
System.out.println("The sum is " + calc.GetValue());
} //main
} //simple1
/*
* A simple client program that demonstrates the use of the
* Calculator stub module with a local implementation.
*/
/*
* Run this like
* java Tutorial.simple2 number [number...]
*/
package Tutorial;
public class simple2 {
public static void main(String argv[]) {
Tutorial.Calculator calc; //Interface from stubbing...
try {
//create the calculator
calc = new Tutorial.CalculatorImpl();
if (calc==null) {
System.err.println("Couldn't create calculator");
System.exit(1);
}
//clear the calculator before using it
if (argv.length<1) {
calc.SetValue(0.0);
} else {
Double v = Double.valueOf(argv[0]);
calc.SetValue(v.doubleValue());
}
//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...
calc.Divide(v.doubleValue());
i = i+1;
}
//and print the result
System.out.println("the 'sum' is " + calc.GetValue());
} catch (Tutorial.DivideByZero e) {
System.err.println("raised DivideByZero exception: " + e);
} catch (xerox.ilu.IluSystemException e) {
System.err.println("raised IluSystemException exception: " + e);
}
} //main
} //simple2
/*
* Run this like
* java Tutorial.TutorialServer servername
*/
package Tutorial;
import Tutorial.Calculator;
import Tutorial.Factory;
import Tutorial.DivideByZero;
import Tutorial.CalculatorStub;
import Tutorial.FactoryStub;
import Tutorial.CalculatorImpl;
import xerox.ilu.Ilu;
import xerox.ilu.IluSimpleBinding;
import xerox.ilu.IluSystemException;
import xerox.ilu.IluServer;
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();
Ilu.registerTrueObject(
Ilu.inventID(),
calc,
this.server,
Tutorial.CalculatorStub.iluClass(),
Ilu.unspec
);
return calc;
}
} //FactoryImpl
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");
return;
}
//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);
xerox.ilu.Ilu.registerTrueObject(
"theFactory",
factory,
trueServer,
Tutorial.FactoryStub.iluClass(),
xerox.ilu.IluLifetimeArgs.iluLifetimeRemember
);
//Make the factory well known by publishing it
xerox.ilu.IluSimpleBinding.publish(factory);
//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.IluSystemException e) {
System.out.println("raised IluSystemException: " + e);
}
}
} //TutorialServer
/*
* A simple client program that finds the Calculator-Factory,
* creates a calculator, and adds up its arguments.
*/
/*
* Run this like
* java Tutorial.simple3 servername number [number...]
* after making sure a server is running.
*/
package Tutorial;
public class simple3 {
/* 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 lookup with the object ID of
* the factory object, and the "type" of the object
* we're looking for.
*/
factory = (Tutorial.Factory)
xerox.ilu.IluSimpleBinding.lookup(
serverId,
factoryId,
Tutorial.FactoryStub.iluClass()
);
} catch (xerox.ilu.IluSystemException e) {
System.err.println("Failed to get factory: " + e);
System.exit(1);
}
if (factory==null) {
System.err.println("Got null factory");
System.exit(1);
}
System.out.println("Got factory " + factory);
System.out.println("Looking up Calculator");
try {
calc = factory.CreateCalculator();
} catch (xerox.ilu.IluSystemException e) {
System.err.println("Failed to get Calculator: " + e);
System.exit(1);
}
if (calc==null) {
System.err.println("Got null Calculator");
System.exit(1);
}
System.out.println("Got Calculator " + calc);
return calc;
} //GetTutorialCalculator
public static void main(String argv[]) {
Tutorial.Calculator calc;
if (argv.length < 2) {
System.err.println("usage: java Tutorial.simple3 number*");
System.exit(1);
}
//Find a calculator
String serverId = argv[0];
calc = GetTutorialCalculator(serverId, "theFactory");
if (calc==null) {
System.out.println("Null calculator");
System.exit(1);
}
try {
//clear the calculator before using it
calc.SetValue(0.0);
//now loop over the arguments, adding each in turn
int i = 1;
while (i<argv.length) {
Double v = Double.valueOf(argv[i]);//exceptions possible
calc.Add(v.doubleValue());
i = i+1;
}
//and print the result
System.out.println("The sum is " + calc.GetValue());
/* This is awfull but Java does not let you catch exceptions which
* are not raised. What an idiotic feature!
* } catch (Tutorial.DivideByZero e) {
* System.err.println("Division by zero: " + e);
*/
} catch (xerox.ilu.IluSystemException e) {
System.err.println("Some IluSystemException: " + e);
}
} //main
} //simple3/*
* A simple client program that finds the Calculator-Factory,
* creates a calculator, and adds up its arguments.
*/
/*
* Run this like
* java Tutorial.simple3 servername number [number...]
* after making sure a server is running.
*/
package Tutorial;
import xerox.ilu.Ilu;
import xerox.ilu.IluException;
import xerox.ilu.IluServer;
import Tutorial.DivideByZero;
public class simple3 {
/* 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)
xerox.ilu.Ilu.lookupObject(
serverId,
factoryId,
Tutorial.FactoryStub.iluClass()
);
} catch (xerox.ilu.IluException e) {
System.err.println("Failed to get factory: " + e);
System.exit(1);
}
if (factory==null) {
System.err.println("Got null factory");
System.exit(1);
}
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);
System.exit(1);
}
if (calc==null) {
System.err.println("Got null Calculator");
System.exit(1);
}
System.out.println("Got Calculator " + calc);
return calc;
} //GetTutorialCalculator
public static void main(String argv[]) {
Tutorial.Calculator calc;
if (argv.length < 2) {
System.err.println("usage: java Tutorial.simple3 number*");
System.exit(1);
}
//Find a calculator
String serverId = argv[0];
calc = GetTutorialCalculator(serverId, "theFactory");
if (calc==null) {
System.out.println("Null calculator");
System.exit(1);
}
try {
//clear the calculator before using it
calc.SetValue(0.0);
//now loop over the arguments, adding each in turn
int i = 1;
while (i<argv.length) {
Double v = Double.valueOf(argv[i]);//exceptions possible
calc.Add(v.doubleValue());
i = i+1;
}
//and print the result
System.out.println("The sum is " + calc.GetValue());
} catch (xerox.ilu.IluException e) {
System.err.println("Some IluException: " + e);
}
} //main
} //simple3
INTERFACE Tutorial2 IMPORTS Tutorial END;
TYPE OpType = ENUMERATION
SetValue, Add, Subtract, Multiply, Divide END;
TYPE Operation = RECORD
op : OpType,
value : REAL,
accumulator : REAL
END;
TYPE RegisterTape = SEQUENCE OF Operation;
TYPE TapeCalculator = OBJECT COLLECTIBLE
SUPERTYPES Tutorial.Calculator END
DOCUMENTATION "4 function calculator with register tape"
METHODS
GetTape () : RegisterTape
END;
TYPE Factory = OBJECT SUPERTYPES Tutorial.Factory END
METHODS
CreateTapeCalculator () : TapeCalculator
END;
package Tutorial2;
import java.util.Vector;
import Tutorial2.OpType;
import Tutorial2.Operation;
/*
* While this class complies to the Tutorial2.isl specification
* it is a local implementation. Its instances need to be
* registered with Ilu before they are publicly accessible.
*/
public class TapeCalculatorImpl implements
Tutorial2.TapeCalculator {
double value;
java.util.Vector tape;
public TapeCalculatorImpl() {
value = 0.0;
tape = new java.util.Vector();
}
public void SetValue(double v) {
value = v;
Operation op = new Operation(OpType.SetValue, v, value);
tape.addElement(op);
}
public double GetValue() {
return value;
}
public void Add(double v) {
value = value + v;
Operation op = new Operation(OpType.Add, v, value);
tape.addElement(op);
}
public void Subtract(double v) {
value = value - v;
Operation op = new Operation(OpType.Subtract, v, value);
tape.addElement(op);
}
public void Multiply(double v) {
value = value * v;
Operation op = new Operation(OpType.Multiply, v, value);
tape.addElement(op);
}
public void Divide(double v) throws Tutorial.DivideByZero {
if (v==0.0) throw new Tutorial.DivideByZero();
if (v==1.0) tape = null; //raise an unexpected exception for debugging
value = value / v;
Operation op = new Operation(OpType.Divide, v, value);
tape.addElement(op);
}
public Operation[] GetTape() {
Operation retVal[];
//We protect structural integrity even if we don't care
//about the value in case of a conflict.
synchronized (tape) {
retVal = new Operation[tape.size()];
tape.copyInto(retVal);
}
return retVal;
}
} //TapeCalculatorImpl
/*
* Run this like
* java Tutorial2.Tutorial2Server servername
*/
package Tutorial2;
import xerox.ilu.Ilu;
import xerox.ilu.IluSystemException;
import xerox.ilu.IluServer;
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
public class Tutorial2Server {
static Factory2Impl factory;
static xerox.ilu.IluServer trueServer;
public static void main(String argv[]) {
String serverId;
if (argv.length != 1) {
System.err.println("usage: java Tutorial2.Tutorial2Server servername");
System.exit(1);
}
System.out.println("Create the server");
try {
//Create a server with appropriate server id (which is
//taken from the first argument)
serverId = argv[0];
trueServer = xerox.ilu.IluServer.createServer(serverId);
} catch (xerox.ilu.IluSystemException e) {
System.err.println("Failed creating server: " + e);
System.exit(1);
}
System.out.println("Create the factory");
try {
//Now create an instance of a Factory object on the server
//with an instance handle "theFactory"
factory = new Factory2Impl();
Tutorial2.FactoryStub.registerTrueObject(
"theFactory",
factory,
trueServer
);
//Make the factory well known by publishing it
xerox.ilu.IluSimpleBinding.publish(factory);
} catch (xerox.ilu.IluSystemException e) {
System.err.println("Failed creating Factory: " + e);
System.exit(1);
}
//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...
} //main
} //Tutorial2Server
/*
* A simple client program that finds the TapeCalculator-Factory,
* creates a calculator and executes the users input.
*/
/*
* Run this like
* java Tutorial2.simple4 serverId
*/
package Tutorial2;
public class simple4 {
/* We define a new routine, "Get_Tutorial_Calculator", which
* finds the tutorial factory, then creates a new TapeCalculator
* object for us.
*/
public static Tutorial2.TapeCalculator
GetTutorialTapeCalculator(String serverId, String factoryId) {
Tutorial2.Factory factory = null;
Tutorial2.TapeCalculator calc = null;
System.out.println("Looking up factory");
try {
/* We have to call lookup with the object ID of
* the factory object, and the "type" of the object we're looking
* for.
*/
factory = (Tutorial2.Factory)
xerox.ilu.IluSimpleBinding.lookup(
serverId,
factoryId,
Tutorial2.FactoryStub.iluClass()
);
} catch (xerox.ilu.IluSystemException e) {
System.err.println("Failed to get factory: " + e);
System.exit(1);
}
if (factory==null) {
System.err.println("Got null factory");
System.exit(1);
}
System.out.println("Got factory " + factory);
System.out.println("Looking up TapeCalculator");
try {
calc = factory.CreateTapeCalculator();
} catch (xerox.ilu.IluSystemException e) {
System.err.println("Failed to get TapeCalculator: " + e);
System.exit(1);
}
if (calc==null) {
System.err.println("Got null TapeCalculator");
System.exit(1);
}
System.out.println("Got TapeCalculator " + calc);
return calc;
} //GetTutorialTapeCalculator
static String opNames[]
= {"SetValue", "Add", "Subtract", "Multiply", "Divide"};
static double argToDouble(String inputLine) {
return Double.valueOf(inputLine.substring(1)).doubleValue();
} //argToDouble
public static void main(String argv[]) {
Tutorial2.TapeCalculator calc;
Tutorial2.Operation[] tape;
boolean quitFlag = false;
double value = 0.0;
String line;
java.io.DataInputStream sysIn =
new java.io.DataInputStream(System.in);
if (argv.length < 1) {
System.err.println("USAGE: java Tutorial2.simple4 serverid");
System.exit(1);
}
//Find a calculator
String serverId = argv[0];
calc = GetTutorialTapeCalculator(serverId, "theFactory");
if (calc==null) {
System.err.println("Didn't get a calculator");
System.exit(1);
}
System.out.println("Got tape calculator");
try {
//Clear the calculator before using it
calc.SetValue(0.0);
//Loop over user inputs and perform the requested operation
while (!quitFlag) {
value = calc.GetValue();
System.out.print(value + "\n> "); System.out.flush();
line = sysIn.readLine();
if (line == null) line = "q";
switch (line.charAt(0)) {
case '\n':
break;
case '+':
value = argToDouble(line);
calc.Add(value);
break;
case '-':
value = argToDouble(line);
calc.Subtract(value);
break;
case '*':
value = argToDouble(line);
calc.Multiply(value);
break;
case '/':
try {
value = argToDouble(line);
calc.Divide(value);
} catch (Tutorial.DivideByZero e) {
System.out.println("** division by zero " + e);
}
break;
case 'q':
quitFlag = true;
break;
case 't':
tape = calc.GetTape();
for (int i = 0; i < tape.length; i++) {
System.out.println(" "
+ opNames[tape[i].op.value()]
+ "(" + tape[i].value + ") => "
+ tape[i].accumulator
);
}
break;
case 'c':
calc.SetValue(0.0);
break;
default:
System.out.println("Invalid Operation <" + line + ">");
System.out.println("Valid ops are +, -, *, /, "
+ "t (for tape),"
+ "c (for clear),"
+ "q (for quit),"
);
}
}
} catch (java.io.IOException e) {
System.err.println("Example raised IOException: " + e);
} catch (xerox.ilu.IluSystemException e) {
System.err.println("Example raised IluSystemException: " + e);
}
} //main
} //simple4
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();
};
};
#include "Tutorial.idl"
module Tutorial2 {
enum OpType { SetValue, Add, Subtract, Multiply, Divide };
struct Operation {
OpType op;
double value;
double accumulator;
};
typedef sequence<Operation> RegisterTape;
// A four function calculator with a register tape
interface TapeCalculator : Tutorial::Calculator {
RegisterTape GetTape ();
};
// A factory that produces TapeCalculators
interface Factory : Tutorial::Factory {
TapeCalculator CreateTapeCalculator ();
};
};