This page shows how to create an Ice application with Java.
On this page:
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. You can compile the definition as follows:
--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
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
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.java, shown in full here:
main method is declared to throw
Exception. If the code throws an exception, it will be handled by the JVM which typically prints out the exception and then returns failure to the operating system. The body of
main contains a
try-with-resources block in which we place all the server code.
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.
Failure to call
destroy on the communicator before the program exits results in undefined behavior.
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
com.zeroc.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.)
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 provides such a helper class, called
Application.) 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 three lines to instantiate a
PrinterI object and register it with the object adapter.
We can compile the server code as follows:
This compiles both our application code and the code that was generated by the Slice compiler. We assume that the
ICE_HOME environment variable is set to the top-level directory containing the Ice run time. (For example, if you have installed Ice in
ICE_HOME to that path.) Note that Ice for Java uses Gradle to control building of source code. (Gradle 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 in Java
The client code, in
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
com.zeroc.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
com.zeroc.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
PrinterPrx.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. (We will see cleaner ways to terminate a server in our discussion of the
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:
Note that, to successfully run client and server, your
CLASSPATH must include the Ice library and the classes directory, for example:
Please have a look at the demo applications that ship with Ice for the details for your platform.