RMI Remote Method Invocation Jan 13 2019 The network is the computer Consider the following program organization method call SomeClass computer 1 AnotherClass returned object computer 2 If the network is the computer we ought to be able to put the two classes on different computers RMI is one technology that makes this possible 2 RMI and other technologies CORBA Common Object Request Broker Architecture was used for a long time CORBA supports object transmission between virtually any languages Objects have to be described in IDL Interface Definition Language which looks a lot like C data definitions CORBA is complex and flaky CORBA has fallen out of favor Microsoft supported CORBA then COM now NET RMI is purely Java specific Java to Java communications only As a result RMI is much simpler than CORBA 3 What is needed for RMI Java makes RMI Remote Method Invocation fairly easy but there are some extra steps To send a message to a remote server object The client object has to find the object The client object then has to marshal the parameters prepare them for transmission Do this by looking it up in a registry Java requires Serializable parameters The server object has to unmarshal its parameters do its computation and marshal its response The client object has to unmarshal the response Much of this is done for you by special software 4 Terminology A remote object is an object on another computer The client object is the object making the request sending a message to the other object The server object is the object receiving the request As usual client and server can easily trade roles each can make requests of the other The rmiregistry is a special server that looks up objects by name Hopefully the name is unique rmic is a special compiler for creating stub client and skeleton server classes 5 Processes For RMI you need to be running three processes The Client The Server The Object Registry rmiregistry which is like a DNS service for objects You also need TCP IP active 6 Interfaces Interfaces define behavior Classes define implementation Therefore In order to use a remote object the client must know its behavior interface but does not need to know its implementation class In order to provide an object the server must know both its interface behavior and its class implementation In short The interface must be available to both client and server The class of any transmitted object must be on both client and server The class whose method is being used should only be on the server 7 Classes A Remote class is one whose instances can be accessed remotely A Serializable class is one whose instances can be marshaled turned into a linear sequence of bits On the computer where it is defined instances of this class can be accessed just like any other object On other computers the remote object can be accessed via object handles Serializable objects can be transmitted from one computer to another It probably isn t a good idea for an object to be both remote and serializable 8 Conditions for serializability If an object is to be serialized The class must be declared as public The class must implement Serializable However Serializable does not declare any methods The class must have a no argument constructor All fields of the class must be serializable either primitive types or Serializable objects Exception Fields marked transient will be ignored during serialization 9 Remote interfaces and class A Remote class has two parts The interface used by both client and server Must be public Must extend the interface java rmi Remote Every method in the interface must declare that it throws java rmi RemoteException other exceptions may also be thrown The class itself used only by the server Must implement the Remote interface Should extend java rmi server UnicastRemoteObject May have locally accessible methods that are not in its Remote interface 10 Remote vs Serializable A Remote object lives on another computer such as the Server You can send messages to a Remote object and get responses back from the object All you need to know about the Remote object is its interface Remote objects don t pose much of a security issue You can transmit a copy of a Serializable object between computers The receiving object needs to know how the object is implemented it needs the class as well as the interface There is a way to transmit the class definition Accepting classes does pose a security issue 11 Security It isn t safe for the client to use somebody else s code on some random server System setSecurityManager new RMISecurityManager The security policy of RMISecurityManager is the same as that of the default SecurityManager Your client program should use a more conservative security manager than the default Most discussions of RMI assume you should do this on both the client and the server Unless your server also acts as a client it isn t really necessary on the server 12 The server class The class that defines the server object should extend UnicastRemoteObject This makes a connection with exactly one other computer If you must extend some other class you can use exportObject instead Sun does not provide a MulticastRemoteObject class The server class needs to register its server object String url rmi host port objectName The default port is 1099 Naming rebind url object Every remotely available method must throw a RemoteException because connections can fail Every remotely available method should be synchronized 13 Hello world server interface import java rmi public interface HelloInterface extends Remote public String say throws RemoteException 14 Hello world server class import java rmi import java rmi server public class Hello extends UnicastRemoteObject implements HelloInterface private String message Strings are serializable public Hello String msg throws RemoteException message msg public String say throws RemoteException return message 15 Registering the hello world server class HelloServer public static void main String argv try Naming rebind rmi localhost HelloServer new Hello Hello world System out println Hello Server is ready catch Exception e System out println Hello Server failed e 16 The hello world client program class HelloClient public static void main String args HelloInterface hello String name rmi localhost HelloServer try hello HelloInterface Naming lookup name System out println hello say catch Exception e System out println HelloClient exception e 17 rmic The class that implements the
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