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UMD CMSC 433 - Java RMI

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Java RMI

Java RMI

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Java RMI

Java RMI

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1CMSC 433 – Programming Language Technologies and ParadigmsSpring 2007Java RMIMay 3, 20072Distributed Computing• Programs that cooperate and communicate over a network–E-mail– Web server and web client–SETI @Home3Key Features of Distrib. Comp.• Machines are not all the same– But all adhere to same communication protocol• Network is “slow”– Sending a message takes a lot of time• Network is unreliable– Machines may join and leave with no warning– Part of the network may fail4Different Approaches to Distributed Computation• Connecting via sockets– Custom protocols for each application• RPC/DCOM/CORBA/RMI– Make what looks like a normal function call– Function actually invoked on another machine– Arguments are marshalled for transport–Value is unmarshalled on return25Remote Method Invocation• Easy way to get distributed computation• Have stub for remote object– Calls to stub get translated into network call– Implemented on top of sockets• Arguments and return values are passed over network– Java takes care of the details6A Simple Exampleclass ChatServerImpl ... { // runs on one mach.public void say(String s) {System.out.println(s);}...}class Chatter { // runs on another mach.public static void main(String args[]) {ChatServer c = // get remote object;BufferedReader br = new BufferedReader(newInputStreamReader(System.in));while (true) {System.out.print(“> “);c.say(br.readLine());}}}7Remote Objects• Object should– Extend java.rmi.server.UnicastRemoteObject• Constructor declared to throw RemoteException– Fine Print: actually, don’t NEED to extend UnicastRemoteObject, but it’s much easier– Implement a remote interface• A remote interface extends java.rmi.Remote• All methods in a remote interface throw RemoteException– “Something bad happened on the network”8Remote Interfaces39Stubs• Client only sees the RemoteInterface– ConcreteObject can have other methods • Remote objects represented using stub– Stub sends arguments over network– Stub receives result back from network10Compiling Stubs with rmic• Generates stub code for a class– Generates position-independent code• Generates stubs for all methods declared in the class’ Remote interface– Other methods don’t get a stub11Stub Code• Objects contain both data and code– When you receive a remote object, you need the stub for that remote object• Solution #1: All clients have stub code on their classpath– Or stub code for another class with same remote interface12Downloading Code• Solution #2: Provide a code base where stub code for objects can be downloadedjava -Djava.rmi.server.codebase=<url> ... – Specifies location of classes originating from this server– url can be, e.g., http:// or file:/413Security Manager• Downloading code (even stub code) from the internet is potentially risky– Need to limit what downloaded code could do– Must install a Security Manager before you download any code from RMI code bases• Can useSystem.setSecurityManager(new RMISecurityManager());14Policy Files• In addition to security manager, need to specify a security policygrant {permission java.net.SocketPermission“*:1024-65535”, “connect,accept”;permission java.net.SocketPermission “*:80”,“connect”;};• Set security policy when JVM started– java -Djava.security.policy=<file name>15Getting the First Remote Object• Can make objects available in RMI registry– Each object has a name (that you specify)– Registry listens on a port (1099 default)• Naming.lookup(url) gets object from reg.– E.g., Naming.lookup(“rmi://localhost/Chat”);– Use to get first reference to remote object16Starting an RMI Registry• Method 1: Separate RMI registry process– Command rmiregistry• Run with stubs in classpath, or specify codebase– Listens on port 1099 by default• Method 2: Start in same JVM– LocateRegistry.createRegistry(int port)– Advantage: dies when your program dies• No registries lying around on machine517Advertising Remote Objects• Call Naming.{bind/unbind/rebind} to place objects in registry– E.g., Naming.bind(“rmi://localhost/Chat”);• Can bind/unbind/rebind name on localhost• Can lookup name on any host18Example: RMI Chat Server•Server – Runs the chat room• Client– Participant in chat room– Receives messages from others in room• Connection– Uniquely identifies a client– Used to speak in chat room19Serverinterface Server extends Remote {Connection logon(String name, Client c)throws RemoteException;}20Connectioninterface Connection extends Remote {/** Say to everyone */void say(String msg)throws RemoteException;/ ** Say to one person */void say(String who, String msg)throws RemoteException;String [] who()throws RemoteException;void logoff()throws RemoteException;}621Clientinterface Client extends Remote {void said(String who, String msg)throws RemoteException;void whoChanged(String [] who)throws RemoteException;}22Server’s Remote Object creationServerImplServer s = new ServerImpl();HostedRemote ObjectssServerObject added to table because it implements extension of Remoteinterface23Remote Object registryServerImplNaming.rebind(“ChatServer”, s);HostedRemote ObjectsChatServerServerImplStubsServer RMI Registry24Client’s Remote Object creationClientImplClient c = new ClientImpl();HostedRemote ObjectscClientClient object also implements extension of Remote interface725Client looks up ServerServer s = (Server) Naming.lookup(“//host/ChatServer”);sServerImplStubHostedRemote ObjectsServerImplClient ServerRMI RegistryChatServerServerImplStublookupreturnsstub26After lookup finishedClientImplHostedRemote ObjectscsServerImplStubHostedRemote ObjectsServerImplClient Server27Invokes remote Server methodConnection conn = s.logon(“Bill”, c);sServerImplStubClientStub code for remote logon callString “Bill”Stub for cMethod: logon… to server processlogonClientImplc28Receives remote callServer(Skeleton) code for remote logon callString “Bill”Stub for cMethod: logon… from client processHostedRemote ObjectsServerImpl“Bill”ClientImplStub cunmarshalled arguments829Executes the callServerHostedRemote ObjectsServerImpl“Bill”ClientImplStub ccall logon …ConnectionImpl… create new Connection object30Returns the resultServerHostedRemote ObjectsServerImplConnectionImpl… return this as the result(Skeleton) code for remote logon callStub for connReturn value:… to


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