Transcript Notes
Proxy Pattern What’s a Proxy? • A remote proxy acts as a local representative of a remote object • Remote Object: instantiated in a different JVM heap (a different address space) • The proxy looks like the remote object, but isn’t • The proxy takes method calls and handles all the details of network communication to the remote object What’s a Proxy? Proxy Remote Object Client Code Local Heap Remote Heap RMI • Luckily, we don’t have to write the code to handle network communications • RMI: Remote Method Invocation • Java provides packages that we can easily use to make our current code ready for network communication with very few changes • The book provides a very detailed guide to this process Proxy, pretends to be the actual service. Packs requests and handles network communications Client Object Client helper Local Heap RMI Service helper Service Object Remote Heap Unpacks requests from client helper; calls requests on the Service Object RMI doBigJob() Client Object Client helper Local Heap Service helper Service Object Remote Heap RMI “client wants to call a method” doBigJob() Client Object Client helper Local Heap Service helper Service Object Remote Heap Client helper packages up the info about the method call (Serializable) & ships it over the network RMI result Client Object Client helper Local Heap Service helper Service Object Remote Heap Service helper packs the result and handles network communications RMI packaged result Client Object Client helper Local Heap Service helper Service Object Remote Heap Client helper unpacks the result and passes it back to the Client Object RMI result Client Object Client helper Local Heap Service helper Service Object Remote Heap The Client Object doesn’t know it isn’t talking to the Service Object in its own heap! RMI RMI STUB Client Object Client helper Local Heap RMI SKELETON Service helper Service Object Remote Heap Newer versions of Java don’t require an explicit skeleton object, but something is still taking care of the skeleton behavior. Making a Remote Service 1. Make a remote Interface Defines the methods that can be called remotely Stub and Service Object will implement it 2. Make a Remote Implementation This is the class that does the “real work” 3. Rmic to generate stubs & skeletons (Server) Rmic comes with the Java SDK Implementation_stub.class Implementation_Skel.class 4. Start the registry (Server): rmiregistry 5. Start the remote service (server) Needs to be in a separate thread from the rmiregistry Back to the Pattern DEFINITION: The Proxy Pattern provides a surrogate or placeholder for another object to control access to it. Class diagram <<interface>> Subject request() RealSubject request() subject Proxy request()