Writing an Ice Application with JavaScript

This page shows how to create an Ice client application with JavaScript.

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Compiling a Slice Definition for JavaScript

The first step in creating our JavaScript application is to compile our Slice definition to generate JavaScript proxies. You can compile the definition as follows:

$ slice2js Printer.ice

The slice2js compiler produces a single source file, Printer.js, from this definition. The exact contents of the source file do not concern us for now — it contains the generated code that corresponds to the Printer interface we defined in Printer.ice.

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Writing a Client in JavaScript

The client code, in Client.js, is shown below in full:

JavaScript
var Ice = require("ice").Ice;
var Demo = require("./Printer").Demo;

var ic;

Ice.Promise.try(
    function()
    {
        ic = Ice.initialize();
        var base = ic.stringToProxy("SimplePrinter:default -p 10000");
        return Demo.PrinterPrx.checkedCast(base).then(
            function(printer)
            {
                return printer.printString("Hello World!");
            });
    }
).finally(
    function()
    {
        if(ic)
        {
            return ic.destroy();
        }
    }
).exception(
    function(ex)
    {
        console.log(ex.toString());
        process.exit(1);
    });

The program begins with require statements that assign modules from the Ice run time and the generated code to convenient local variables. (These statements are necessary for use with NodeJS. Browser applications would omit these statements and load the modules a different way.)

The program begins with a call to Ice.Promise.try to launch a chain of promises (or futures) that handles the asynchronous nature of Ice invocations with a structure that resembles synchronous code.

  1. The function passed to try is executed immediately. The body of this function begins by calling Ice.initialize to initialize the Ice run time. The call to initialize returns an Ice.Communicator reference, which is the main object in the Ice run time.
  2. The next step is to obtain a proxy for the remote printer. We create a proxy by calling stringToProxy on the communicator, with the string "SimplePrinter:default -p 10000". Note that the string contains the object identity and the port number that were used by the server. (Obviously, hard-coding object identities and port numbers into our applications is a bad idea, but it will do for now; we will see more architecturally sound ways of doing this when we discuss IceGrid.)
  3. The proxy returned by stringToProxy is of type Ice.ObjectPrx, which is at the root of the inheritance tree for interfaces and classes. But to actually talk to our printer, we need a proxy for a Demo::Printer interface, not an Object interface. To do this, we need to do a down-cast by calling Demo.PrinterPrx.checkedCast. A checked cast sends a message to the server, effectively asking "is this a proxy for a Demo::Printer interface?" If so, the call returns a proxy of type Demo::PrinterPrx; otherwise, if the proxy denotes an interface of some other type, the call returns null.
  4. The checkedCast function involves a remote invocation to the server, which means this function has asynchronous semantics and therefore it returns a new promise object.
  5. We call then on the promise returned by checkedCast and supply a "success" function, meaning the code that's executed when checkedCast succeeds. This inner function accepts one argument, printer, representing a proxy to the newly-downcasted object, or null if the remote object doesn't support the Printer interface.
  6. Inside the success function, we now have a live proxy in our address space and can call the printString method, passing it the time-honored "Hello World!" string. The server prints that string on its terminal. Again, printString is a remote invocation, and it returns a promise that the success function passes along as its own return value.
  7. The then function also returns a new promise which our outer function passes back to try. This outer promise is chained to the promise associated with the printString invocation; the outer promise completes successfully if and when the printString invocation completes successfully.
  8. The function passed to finally is executed after the try block has completed, whether or not it completes successfully. If we created a communicator in the try block, we destroy it here. Doing this is essential in order to correctly finalize the Ice run time: the program must call destroy on any communicator it has created; otherwise, undefined behavior results. The destroy function has asynchronous semantics, so we return its promise to ensure no subsequent code is executed until destroy completes.
  9. Lastly, the function passed to exception is the default exception handler for this entire promise chain.

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Running the Client in JavaScript

The server must be started before the client. Since Ice for JavaScript does not currently include a complete server-side implementation, we need to use a server from another language mapping. In this case, we will use the C++ server:

$ server

At this point, we won't see anything because the server simply waits for a client to connect to it. We run the client in a different window:

$ node Client.js
$

The client runs and exits without producing any output; however, in the server window, we see the "Hello World!" that is produced by the printer. To get rid of the server, we interrupt it on the command line.

If anything goes wrong, the client will print an error message. For example, if we run the client without having first started the server, we get something like the following:

Ice::ConnectionRefusedException
    ice_cause: "Error: connect ECONNREFUSED"
    error: "ECONNREFUSED"

Note that, to successfully run the client, NodeJS must be able to locate the Ice for JavaScript modules. See the Ice for JavaScript installation instructions for more information.

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See Also