3.4 Writing an Ice Application with Java

This section shows how to create an Ice application with Java.

Compiling a Slice Definition for Java

The first step in creating our Java application is to compile our Slice definition to generate Java proxies and skeletons. Under Unix, you can compile the definition as follows:¹

$ mkdir generated
$ slice2java ‑‑output‑dir generated Printer.ice

The ‑‑output‑dir option instructs the compiler to place the generated files into the generated directory. This avoids cluttering the working directory with the generated files. The slice2java compiler produces a number of Java source files from this definition. The exact contents of these files do not concern us for now—they contain the generated code that corresponds to the Printer interface we defined in Printer.ice.

Writing and Compiling a Server

To implement our Printer interface, we must create a servant class. By conven­tion, servant classes use the name of their interface with an I‑suffix, so our servant class is called PrinterI and placed into a source file PrinterI.java:

public class PrinterI extends Demo._PrinterDisp {
    public void
    printString(String s, Ice.Current current)
    {
        System.out.println(s);
    }
}

The PrinterI class inherits from a base class called _PrinterDisp, which is generated by the slice2java compiler. The base class is abstract and contains a printString method that accepts a string for the printer to print and a parameter of type Ice.Current. (For now we will ignore the Ice.Current parameter. We will see its purpose in detail in Section 32.6.) Our implementation of the printString method simply writes its argument to the terminal.

The remainder of the server code is in a source file called Server.java, shown in full here:

public class Server {
    public static void
    main(String[] args)
    {
        int status = 0;
        Ice.Communicator ic = null;
        try {
            ic = Ice.Util.initialize(args);
            Ice.ObjectAdapter adapter
                = ic.createObjectAdapterWithEndpoints(
                    "SimplePrinterAdapter", "default ‑p 10000");
            Ice.Object object = new PrinterI();
            adapter.add(
                    object,
                    ic.stringToIdentity("SimplePrinter"));
            adapter.activate();
            ic.waitForShutdown();
        } catch (Ice.LocalException e) {
            e.printStackTrace();
            status = 1;
        } catch (Exception e) {
            System.err.println(e.getMessage());
            status = 1;
        }
        if (ic != null) {
            // Clean up
            //
            try {
                ic.destroy();
            } catch (Exception e) {
                System.err.println(e.getMessage());
                status = 1;
            }
        }
        System.exit(status);
    }
}

Note the general structure of the code:

public class Server {
    public static void
    main(String[] args) {
        int status = 0;
        Ice.Communicator ic = null;
        try {

            // Server implementation here...

        } catch (Ice.LocalException e) {
            e.printStackTrace();
            status = 1;
        } catch (Exception e) {
            System.err.println(e.getMessage());
            status = 1;
        }
        if (ic != null) {
            // Clean up
            //
            try {
                ic.destroy();
            } catch (Exception e) {
                System.err.println(e.getMessage());
                status = 1;
            }
        }
        System.exit(status);
    }
}

The body of main contains a try block in which we place all the server code, followed by two catch blocks. The first block catches all exceptions that may be thrown by the Ice run time; the intent is that, if the code encounters an unexpected Ice run-time exception anywhere, the stack is unwound all the way back to main, which prints the exception and then returns failure to the operating system. The second block catches Exception exceptions; the intent is that, if we encounter a fatal error condition somewhere in our code, we can simply throw an exception with an error message. Again, this unwinds the stack all the way back to main, which prints the error message and then returns failure to the operating system.

Before the code exits, it destroys the communicator (if one was created successfully). 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 body of our try block contains the actual server code:

            ic = Ice.Util.initialize(args);
            Ice.ObjectAdapter adapter
                = ic.createObjectAdapterWithEndpoints(
                    "SimplePrinterAdapter", "default ‑p 10000");
            Ice.Object object = new PrinterI();
            adapter.add(
                    object,
                    ic.stringToIdentity("SimplePrinter"));
            adapter.activate();
            ic.waitForShutdown();

The code goes through the following steps:

1. We initialize the Ice run time by calling Ice.Util.initialize. (We pass args to this call because the server may have command-line arguments that are of interest to the run time; for this example, the server does not require any command-line arguments.) The call to initialize returns an Ice::Communicator reference, which is the main handle to the Ice run time.

2. We create an object adapter by calling createObjectAdapterWith­Endpoints on the Communicator instance. The arguments we pass are "SimplePrinterAdapter" (which is the name of the adapter) and "default ‑p 10000", which instructs the adapter to listen for incoming requests using the default protocol (TCP/IP) at port number 10000.

3. At this point, the server-side run time is initialized and we create a servant for our Printer interface by instantiating a PrinterI object.

4. We inform the object adapter of the presence of a new servant by calling add on the adapter; the arguments to add are the servant we have just instantiated, plus an identifier. In this case, the string "SimplePrinter" is the name of the servant. (If we had multiple printers, each would have a different name or, more correctly, a different object identity.)

5. Next, we activate the adapter by calling its activate method. (The adapter is initially created in a holding state; this is useful if we have many servants that share the same adapter and do not want requests to be processed until after all the servants have been instantiated.)

6. Finally, we call waitForShutdown. This call suspends the calling thread until the server implementation terminates, either by making a call to shut down the run time, or in response to a signal. (For now, we will simply inter­rupt the server on the command line when we no longer need it.)

Note that, even though there is quite a bit of code here, that code is essentially the same for all servers. You can put that code into a helper class and, thereafter, will not have to bother with it again. (Ice ships with such a helper class, called Ice.Application—see Section 12.3.1.) As far as actual application code is concerned, the server contains only a few lines: seven lines for the definition of the PrinterI class, plus four² lines to instantiate a PrinterI object and register it with the object adapter.

We can compile the server code as follows:

$ mkdir classes
$ javac ‑d classes ‑classpath classes:$ICEJ_HOME/lib/Ice.jar\
‑source 1.4 Server.java PrinterI.java generated/Demo/*.java

This compiles both our application code and the code that was generated by the Slice compiler. We assume that the ICEJ_HOME environment variable is set to the top-level directory containing the Ice run time. (For example, if you have installed Ice in /opt/Icej, set ICEJ_HOME to that path.) Note that Ice for Java uses the ant build environment to control building of source code. (ant is similar to make, but more flexible for Java applications.) You can have a look at the demo code that ships with Ice to see how to use this tool.

Writing and Compiling a Client

The client code, in Client.java, looks very similar to the server. Here it is in full:

public class Client {
    public static void
    main(String[] args)
    {
        int status = 0;
        Ice.Communicator ic = null;
        try {
            ic = Ice.Util.initialize(args);
            Ice.ObjectPrx base = ic.stringToProxy(
                    "SimplePrinter:default ‑p 10000");
            Demo.PrinterPrx printer
                = Demo.PrinterPrxHelper.checkedCast(base);
            if (printer == null)
                throw new Error("Invalid proxy");

            printer.printString("Hello World!");
        } catch (Ice.LocalException e) {
            e.printStackTrace();
            status = 1;
        } catch (Exception e) {
            System.err.println(e.getMessage());
            status = 1;
        }
        if (ic != null) {
            // Clean up
            //
            try {
                ic.destroy();
            } catch (Exception e) {
                System.err.println(e.getMessage());
                status = 1;
            }
        }
        System.exit(status);
    }
}

Note that the overall code layout is the same as for the server: we use the same try and catch blocks to deal with errors. The code in the try block does the following:

1. As for the server, we initialize the Ice run time by calling Ice.Util.initialize.

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 in Chapter 38.)

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 Printer interface, not an Object interface. To do this, we need to do a down-cast by calling PrinterPrxHelper.checkedCast. A checked cast sends a message to the server, effectively asking “is this a proxy for a Printer interface?” If so, the call returns a proxy of type Demo::Printer; otherwise, if the proxy denotes an interface of some other type, the call returns null.

4. We test that the down-cast succeeded and, if not, throw an error message that terminates the client.

5. 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.

Compiling the client looks much the same as for the server:

$ javac ‑d classes ‑classpath classes:$ICEJ_HOME/lib/Ice.jar\
‑source 1.4 Client.java PrinterI.java generated/Demo/*.java

Running Client and Server

To run client and server, we first start the server in a separate window:

$ java 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:

$ java Client
$

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 for now. (We will see cleaner ways to terminate a server in Chapter 12.)

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.ConnectFailedException
        at IceInternal.Network.doConnect(Network.java:201)
        at IceInternal.TcpConnector.connect(TcpConnector.java:26)
        at
IceInternal.OutgoingConnectionFactory.create(OutgoingConnectionFac
tory.java:80)
        at Ice._ObjectDelM.setup(_ObjectDelM.java:251)
        at Ice.ObjectPrxHelper.__getDelegate(ObjectPrxHelper.java:
642)
        at Ice.ObjectPrxHelper.ice_isA(ObjectPrxHelper.java:41)
        at Ice.ObjectPrxHelper.ice_isA(ObjectPrxHelper.java:30)
        at Demo.PrinterPrxHelper.checkedCast(Unknown Source)
        at Client.main(Unknown Source)
Caused by: java.net.ConnectException: Connection refused
        at sun.nio.ch.SocketChannelImpl.checkConnect(Native Method
)
        at
sun.nio.ch.SocketChannelImpl.finishConnect(SocketChannelImpl.java:
518)
        at IceInternal.Network.doConnect(Network.java:173)
        ... 8 more

Note that, to successfully run client and server, your CLASSPATH must include the Ice library and the classes directory, for example:

$ export CLASSPATH=$CLASSPATH:./classes:$ICEJ_HOME/lib/Ice.jar

Please have a look at the demo applications that ship with Ice for the details for your platform.

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1

Whenever we show Unix commands in this book, we assume a Bourne or Bash shell. The commands for Windows are essentially identical and therefore not shown.

2

Well, two lines, really: printing space limitations force us to break source lines more often than you would in your actual source files.