This page shows how to create an Ice application with Java Compat.
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:
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:
We must also edit the generated
settings.gradle to define our sub-projects:
Finally we need to create the directories for client and server projects:
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 s
liceCompile 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,
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:
PrinterI class extends the class
_PrinterDisp, which is generated by the
slice2java compiler. The class 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:
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:
- We initialize the Ice run time by calling
Ice.Util.initialize. (We pass
argsto 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
Communicatorreference, which is the main object in the Ice run time.
- We create an object adapter by calling
Communicatorinstance. 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.
- At this point, the server-side run time is initialized and we create a servant for our
Printerinterface by instantiating a
- We inform the object adapter of the presence of a new servant by calling
addon the adapter; the arguments to
addare 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.)
- Next, we activate the adapter by calling its
activatemethod. (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.)
- 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.)
We can compile the server code as follows:
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:
Note that the overall code layout is the same as for the server: we use the same
catch blocks to deal with errors. The code in the
try block does the following:
- As for the server, we initialize the Ice run time by calling
Ice.Util.initializewithin the Java
- The next step is to obtain a proxy for the remote printer. We create a proxy by calling
stringToProxyon 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.)
- The proxy returned by
stringToProxyis 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
Printerinterface, not an
Objectinterface. 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
Printerinterface?" 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.
- We test that the down-cast succeeded and, if not, throw an error message that terminates the client.
- We now have a live proxy in our address space and can call the
printStringmethod, 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:
Running Client and Server in Java
To run client and server, we first start the server in a separate window:
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:
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: