The main entry point to the Ice run time is represented by the local interface Ice::Communicator. As for the client side, you must initialize the Ice run time by calling
Ice.Util.initialize before you can do anything else in your server.
Ice.Util.initialize returns a reference to an instance of an
Ice.Communicator:
public class Server {
public static void
main(String[] args)
{
int status = 0;
Ice.Communicator ic = null;
try {
ic = Ice.Util.initialize(args);
// ...
} catch (Exception e) {
e.printStackTrace();
status = 1;
}
// ...
}
}
Ice.Util.initialize accepts the argument vector that is passed to
main by the operating system. The function scans the argument vector for any command-line options that are relevant to the Ice run time, but does not remove those options.
1 If anything goes wrong during initialization,
initialize throws an exception.
Before leaving your main function, you
must call
Communicator::destroy. The
destroy operation is responsible for finalizing the Ice run time. In particular,
destroy waits for any operation implementations that are still executing in the server to complete. In addition,
destroy ensures that any outstanding threads are joined with and reclaims a number of operating system resources, such as file descriptors and memory. Never allow your
main function to terminate without calling
destroy first; doing so has undefined behavior.
The general shape of our server-side main function is therefore as follows:
public class Server {
public static void
main(String[] args)
{
int status = 0;
Ice.Communicator ic = null;
try {
ic = Ice.Util.initialize(args);
// ...
} catch (Exception e) {
e.printStackTrace();
status = 1;
}
if (ic != null) {
try {
ic.destroy();
} catch (Exception e) {
e.printStackTrace();
status = 1;
}
}
System.exit(status);
}
}
Note that the code places the call to Ice::initialize into a
try block and takes care to return the correct exit status to the operating system. Also note that an attempt to destroy the communicator is made only if the initialization succeeded.
12.3.1 The Ice.Application Class
The preceding structure for the main function is so common that Ice offers a class,
Ice.Application, that encapsulates all the correct initialization and finalization activities. The synopsis of the class is as follows (with some detail omitted for now):
package Ice;
public enum SignalPolicy { HandleSignals, NoSignalHandling }
public abstract class Application {
public Application()
public Application(SignalPolicy signalPolicy)
public final int main(String appName, String[] args)
public final int main(String appName, String[] args,
String configFile)
public final int main(String appName, String[] args,
InitializationData initData)
public abstract int run(String[] args);
public static String appName()
public static Communicator communicator()
// ...
}
The intent of this class is that you specialize Ice.Application and implement the abstract
run method in your derived class. Whatever code you would normally place in
main goes into the
run method instead. Using
Ice.Application, our program looks as follows:
public class Server extends Ice.Application {
public int
run(String[] args)
{
// Server code here...
return 0;
}
public static void
main(String[] args)
{
Server app = new Server();
int status = app.main("Server", args);
System.exit(status);
}
}
Note that Application.main is overloaded: you can pass an optional file name or an
InitializationData structure (see
Section 32.3 and
Section 30.9).
If you pass a configuration file name to main, the settings in this file are overridden by settings in a file identified by the
ICE_CONFIG environment variable (if defined). Property settings supplied on the command line take precedence over all other settings.
The Application.main function does the following:
1. It installs an exception handler for java.lang.Exception. If your code fails to handle an exception,
Application.main prints the name of an exception and a stack trace on
System.err before returning with a non-zero return value.
2.
It initializes (by calling Ice.Util.initialize) and finalizes (by calling
Communicator.destroy) a communicator. You can get access to the communicator for your server by calling the static
communicator accessor.
4.
It provides the name of your application via the static appName member function. The return value from this call is the first argument in the call to
Application.main, so you can get at this name from anywhere in your code by calling
Ice.Application.appName (which is usually required for error messages). In the example above, the return value from
appName would be
Server.
Using Ice.Application ensures that your program properly finalizes the Ice run time, whether your server terminates normally or in response to an exception. We recommend that all your programs use this class; doing so makes your life easier. In addition
Ice.Application also provides features for signal handling and configuration that you do not have to implement yourself when you use this class.
Using Ice.Application on the Client Side
You can use Ice.Application for your clients as well: simply implement a class that derives from
Ice.Application and place the client code into its
run method. The advantage of this approach is the same as for the server side:
Ice.Application ensures that the communicator is destroyed correctly even in the presence of exceptions.
The simple server we developed in Chapter 3 had no way to shut down cleanly: we simply interrupted the server from the command line to force it to exit. Terminating a server in this fashion is unacceptable for many real-life server applications: typically, the server has to perform some cleanup work before terminating, such as flushing database buffers or closing network connections. This is particularly important on receipt of a signal or keyboard interrupt to prevent possible corruption of database files or other persistent data.
Java does not provide direct support for signals, but it does allow an application to register a
shutdown hook that is invoked when the JVM is shutting down. There are several events that trigger JVM shutdown, such as a call to
System.exit or an interrupt signal from the operating system, but the shutdown hook is not provided with the reason for the shut down.
Ice.Application registers a shutdown hook by default, allowing you to cleanly terminate your application prior to JVM shutdown.
package Ice;
public abstract class Application {
// ...
synchronized public static void destroyOnInterrupt()
synchronized public static void shutdownOnInterrupt()
synchronized public static void setInterruptHook(Thread t)
synchronized public static void defaultInterrupt()
synchronized public static boolean interrupted()
}
By default, Ice.Application behaves as if
destroyOnInterrupt was invoked, therefore our server
main function requires no change to ensure that the program terminates cleanly on JVM shutdown. (You can disable this default shutdown hook by passing the enumerator
NoSignalHandling to the constructor. In that case, shutdown is not intercepted and terminates the VM.) However, we add a diagnostic to report the occurrence, so our
main function now looks like:
public class Server extends Ice.Application {
public int
run(String[] args)
{
// Server code here...
if (interrupted())
System.err.println(appName() + ": terminating");
return 0;
}
public static void
main(String[] args)
{
Server app = new Server();
int status = app.main("Server", args);
System.exit(status);
}
}
During the course of normal execution, the JVM does not terminate until all non-daemon threads have completed. If an interrupt occurs, the JVM ignores the status of active threads and terminates as soon as it has finished invoking all of the installed shutdown hooks.
In a subclass of Ice.Application, the default shutdown hook (as installed by
destroyOnInterrupt) blocks until the application’s main thread completes. As a result, an interrupted application may not terminate successfully if the main thread is blocked. For example, this can occur in an interactive application when the main thread is waiting for console input. To remedy this situation, the application can install an alternate shutdown hook that does not wait for the main thread to finish:
public class Server extends Ice.Application {
class ShutdownHook extends Thread {
public void
run()
{
try
{
communicator().destroy();
}
catch(Ice.LocalException ex)
{
ex.printStackTrace();
}
}
}
public int
run(String[] args)
{
setInterruptHook(new ShutdownHook());
// ...
}
}
After replacing the default shutdown hook using setInterruptHook, the JVM will terminate as soon as the communicator is destroyed.
Apart from the functionality shown in this section, Ice.Application also takes care of initializing the Ice run time with property values. Properties allow you to configure the run time in various ways. For example, you can use properties to control things such as the thread pool size or port number for a server. The
main function of
Ice.Application is overloaded; the second version allows you to specify the name of a configuration file that will be processed during initialization. We discuss Ice properties in more detail in
Chapter 30.
Ice.Application is a singleton class that creates a single communicator. If you are using multiple communicators, you cannot use
Ice.Application. Instead, you must structure your code as we saw in
Chapter 3 (taking care to always destroy the communicator).
