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The output from these programs is shown in Listing 13-5. The server and client form a one-to-many relationship. You can start as many client processes as you like; they will all connect and interact with the single Greeter object.
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Listin g 13-4. Remote Method Invocation
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Java: Greeter and Guest Classes public interface Greeter extends Remote { public void sayHello( ) throws java.rmi.RemoteException; public void greetGuest( Guest listener ) throws java.rmi.RemoteException; public String sayGoodbye( ) throws java.rmi.RemoteException; } public class GreeterImpl extends UnicastRemoteObject implements Greeter { private static final long serialVersionUID = 999010092613539924L; public static void main(String[] args) { String greeterServiceURI = makeServiceURI(null,null); try { Greeter greeter = new GreeterImpl(); System.out.println("Starting Greeter service at "+greeterServiceURI); Naming.rebind(greeterServiceURI,greeter); } catch (Exception e) { e.printStackTrace(); } } public static String makeServiceURI( String host, String name ) { if (host==null) host = "localhost"; if (name==null) name = "JavaGreeter"; return "rmi://"+host+"/"+name; } public GreeterImpl() throws RemoteException { super(); } public void sayHello() throws RemoteException { System.out.println("Greeter "+getClass().getName() +" was asked to sayHello()"); }
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public void greetGuest(Guest guest) throws RemoteException { System.out.println("Greeter "+getClass().getName() +" was asked to greetGuest("+guest+")"); guest.listen("I'm pleased to meet you, "+guest+"!"); } public String sayGoodbye() throws RemoteException { System.out.println("Greeter "+getClass().getName() +" was asked to sayGoodbye()"); return "It was a pleasure serving you."; } } public class Guest implements Serializable { private static final long serialVersionUID = -478469725382736366L; public static void main(String[] args) { String greeterServiceURI = GreeterImpl.makeServiceURI(null,null); try { System.out.println("Looking up greeter at "+greeterServiceURI); Greeter greeter = (Greeter)Naming.lookup(greeterServiceURI); Guest guest = new Guest(); greeter.sayHello(); greeter.greetGuest(guest); String lastWord = greeter.sayGoodbye(); System.out.println("Greeter's final response was \""+lastWord+"\""); } catch (Exception e) { e.printStackTrace(); } } public void listen( String message ) { System.out.println(getClass().getName()+" heard \""+message+"\""); } } Objective-C: Greeter Program @class Guest;
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@interface Greeter : NSObject - (void)sayHello; - (void)greetGuest:(Guest*)guest; - (NSString*)sayGoodbye; @end @implementation Greeter - (void)sayHello { NSLog(@"Greeter %@ was asked to sayHello",self); } - (void)greetGuest:(bycopy Guest*)guest { NSLog(@"Greeter %@ was asked to greetGuest:%@",self,guest); [guest listen:[NSString stringWithFormat:@"Pleased to meet you, %@!",guest]]; } - (NSString*)sayGoodbye { NSLog(@"Greeter %@ was asked to sayGoodbye",self); return @"It was a pleasure serving you."; } @end int main (int argc, const char * argv[]) { NSConnection *connection = [NSConnection defaultConnection]; [connection setRootObject:[Greeter new]]; if ([connection registerName:SERVICE_NAME_DEFAULT]) { NSLog(@"Starting Greeter service '%@'",name); [[NSRunLoop currentRunLoop] run]; // never returns } return 0; } Objective-C: Guest Program @interface Guest : NSObject - (void)listen:(NSString*)message; @end @implementation Guest - (void)listen:(NSString*)message { NSLog(@"%@ heard \"%@\"",self,message); }
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@end int main (int argc, const char * argv[]) { NSConnection *connection = nil; NSLog(@"Connecting to greeter '%@' via Mach ports",name); connection = [NSConnection connectionWithRegisteredName:SERVICE_NAME_DEFAULT host:nil]; Greeter *greeter = (Greeter*)[connection rootProxy]; Guest *guest = [Guest new]; [greeter sayHello]; [greeter greetGuest:guest]; NSString *lastWord = [greeter sayGoodbye]; NSLog(@"Greeter's final response was \"%@\"",lastWord); return 0; }
Listin g 13-5. Output of Greeter Demonstration
Java GreeterImpl: $ java com.apress.java.rmi.GreeterImpl Starting Greeter service at rmi://localhost/JavaGreeter Greeter com.apress.java.rmi.GreeterImpl was asked to sayHello() Greeter com.apress.java.rmi.GreeterImpl was asked to talkBackTo(Guest@e9cb75) com.apress.java.rmi.Guest heard "I'm pleased to meet you, Guest@e9cb75!" Greeter com.apress.java.rmi.GreeterImpl was asked to sayGoodbye() Java Guest: $ java com.apress.java.rmi.Guest Looking up greeter at rmi://localhost/JavaGreeter Greeter's final response was "It was a pleasure serving you." Objective-C Greeter: $ ./Greeter --mach Starting Greeter service 'ObjCGreeter' Greeter <Greeter: 0x1011bf0> was asked to sayHello Greeter <Greeter: 0x1011bf0> was asked to greetGuest:<Guest: 0x1014bc0> Greeter <Greeter: 0x1011bf0> was asked to sayGoodbye Objective-C Guest: $ ./Guest --mach Connecting to greeter 'ObjCGreeter' via Mach ports <Guest: 0x1014bc0> heard "I'm pleased to meet you, <Guest: 0x1014bc0>!" Greeter's final response was "It was a pleasure serving you." Java and Objective-C remote method invocations are, conceptually, quite similar: a server process instantiates an object that implements a service. One or more client processes connect to the
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service and obtain a proxy object. The proxy object doesn t implement any of the server s code. Instead, the proxy forwards any method invocations through the connection to the server process, where the desired method is actually executed. Any parameters and return values are similarly encoded and transported through the connection. There are two significant differences between the Java and Objective-C implementations. The first is how proxy objects are created. In Java, you must first design an interface that defines the methods for the server object, along with a class that implements them. The Java rmic compiler then creates a special _Stub class suitable for use as a proxy object. The client process obtains the proxy object and invokes its methods, which are forwarded back to the server for execution. Objective-C provides the NSProxy class. This is a very special class, because it is not a subclass of NSObject. NSObject and NSProxy are both subclasses of the root Object class. Most of what you consider to be base-class methods are defined in NSObject, not Object. Consequently, NSProxy inherits no methods which makes it perfect for what it does. NSProxy overrides the -forwardInvocation: method described in 6. Since it implements almost no methods, virtually any method you send it will end up invoking -forwardInvocation:. NSDistantObject, the useable subclass of NSProxy, connects an NSProxy with an NSConnection. Any message sent to the proxy object is archived and transported through the NSConnection for execution by the remote process. By leveraging Objective-C s unimplemented method handling, a lightweight proxy object can be spontaneously created for any Objective-C object. This means that you can share virtually any object through an NSConnection.
Caution NSProxy objects do not contain any of the instance variables of the objects they stand in for. For
convenience, proxy object pointers are often cast as a pointer to the actual object type. Be careful not to directly access any instance variable of the object using a pointer to its proxy, as in distantObject->value. This will most certainly have disastrous results. Use property accessors instead ( [distantObject value] or distantObject.value) as these translate into messages, or use the object identifier type (id) which inherently prohibits direct variable access. When designing distributed objects, program defensively. Make all instance variables @protected or @private and provide accessor methods for all public properties.
The other significant difference is how parameters and return values are exchanged. In Java, all parameters and return values are passed by copy, and must therefore be serializable. It also means that the classes that implement those values must exist in the server. Find the Guest heard I m pleased to meet you, Guest! message for both Java and Objective-C in the example output in Listing 13-5. In the Java version, the message is emitted by the server process. The client statement greeter.greetGuest(guest) serialized a copy of the Guest object and sent it to the server. When the Greeter executed guest.listen("I'm pleased to meet you, guest!"), the listen( ) method executed on the server s local copy of the guest object. In the Objective-C output, the Guest heard I m pleased to meet you, Guest! message is emitted by the client. That s because Objective-C s default is to pass values by reference. The [greeter greetGuest:guest] statement passed a reference to the guest object to the server. The server s greetGuest: method received a proxy NSDistantObject to the instance of Guest that exists in the client. When the server sends the message [guest listen:@"I'm pleased to meet you, guest!"], the message invocation is intercepted, archived, and sent back to the client for execution, where the output appears. This allows you to pass virtually any object to a remote method. The object doesn t have to be archivable, you just need to be aware that the receiver will get a proxy to the original object, spontaneously created by NSDistantObject. If you need objects to be passed by copy, you can do that by making them conform to NSCoding and implement sequential archiving, as described in 12.
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