Writing an Ice Application with Java

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

On this page:

Create Projects for your Client and Server Applications

We will use Gradle to create our application projects. You must install Gradle before continuing with this tutorial.

Open a new Command Prompt and run the following commands to generate a new project:

mkdir printer
cd printer
gradle init

For this demo we're going to use a project with two sub-projects to build the Client and Server applications. The requirements for our sub-projects are the same so we'll do all the setup in the subprojects block of the root project, which applies to all sub-projects. Edit the generated build.gradle file to look like the one below:

// Install the gradle Ice Builder plug-in from the plug-in portal
plugins {
    id 'com.zeroc.gradle.ice-builder.slice' version '1.4.7' apply false
subprojects {
    // Apply Java and Ice Builder plug-ins to all sub-projects
    apply plugin: 'java'
    apply plugin: 'com.zeroc.gradle.ice-builder.slice'

    // Both Client and Server projects share the Printer.ice Slice definitions
    slice {
        java {
            files = [file("../Printer.ice")]

    // Use Ice JAR files from maven central repository
    repositories {

    // Both Client and Server depend only on Ice JAR
    dependencies {
        implementation 'com.zeroc:ice:3.7.2'

    // Create a JAR file with the appropriate Main-Class and Class-Path attributes
    jar {
        manifest {
                "Main-Class": project.name.capitalize(),
                "Class-Path": configurations.runtime.resolve().collect { it.toURI() }.join(' ')

We must also edit the generated settings.gradle to define our sub-projects:

rootProject.name = 'printer'
include 'client'
include 'server'

Finally we need to create the directories for client and server projects:

mkdir client
mkdir server

Compiling a Slice Definition for Java

The next step is to add the Slice file (Printer.ice), and then compile this Slice file. When building the project, the sliceCompile task (added automatically by the Ice Builder plug-in) compiles Printer.ice and places the generated code into build/generated-src using the Slice to Java compiler, slice2java.

Writing and Compiling a Server in Java

To implement our Printer interface, we must create a servant class. By convention, a servant class uses the name of its interface with an I-suffix, so our servant class is called PrinterI and placed into a source file server/src/main/java/PrinterI.java:

public class PrinterI implements Demo.Printer
    public void printString(String s, com.zeroc.Ice.Current current)

The PrinterI class implements the interface Printer, which is generated by the slice2java compiler. The interface defines a printString method that accepts a string for the printer to print and a parameter of type Current. (For now we will ignore the Current parameter.) 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/src/main/java/Server.java, shown in full here:

public class Server
    public static void main(String[] args)
        try(com.zeroc.Ice.Communicator communicator = com.zeroc.Ice.Util.initialize(args))
            com.zeroc.Ice.ObjectAdapter adapter = communicator.createObjectAdapterWithEndpoints("SimplePrinterAdapter", "default -p 10000");
            com.zeroc.Ice.Object object = new PrinterI();
            adapter.add(object, com.zeroc.Ice.Util.stringToIdentity("SimplePrinter"));

The body of main contains a try-with-resources block in which we place all the server code. The Communicator object implements java.lang.AutoCloseable, which allows us to use the try-with-resources statement for the initialization of the Communicator object. This ensures the communicator destroy method is called when the try block goes out of scope. Doing this is essential in order to correctly finalize the Ice run time.

A communicator starts a number of non-background threads. Destroying the communicator terminates all these threads.

The body of our try block contains the actual server code. The code goes through the following steps:

  1. We initialize the Ice run time by calling com.zeroc.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 a Communicator reference, which is the main object in 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 transport 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 Ice object. (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 is shut down (For now, we will simply interrupt the server on the command line when we no longer need it, which terminates the server immediately.)

We can compile the server code as follows:

gradlew :server:build

Writing and Compiling a Client in Java

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

public class Client
    public static void main(String[] args)
        try(com.zeroc.Ice.Communicator communicator = com.zeroc.Ice.Util.initialize(args))
            com.zeroc.Ice.ObjectPrx base = communicator.stringToProxy("SimplePrinter:default -p 10000");
            Demo.PrinterPrx printer = Demo.PrinterPrx.checkedCast(base);
            if(printer == null)
                throw new Error("Invalid proxy");
            printer.printString("Hello World!");

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 com.zeroc.Ice.Util.initialize within the Java try-with-resources statement.
  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 com.zeroc.Ice.ObjectPrx, which is at the root of the inheritance tree for interfaces. 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 PrinterPrx.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:

gradlew :client:build

Running Client and Server in Java

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

java -jar server/build/libs/server.jar

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 -jar client/builds/libs/client.jar

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. 

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:

       error = 0
   	at ...
   	at Client.run(Client.java:65)
Caused by: java.net.ConnectException: Connection refused

See Also