1A Typical RMI Application• Client and Server run on different machines• Remote Object(s) registered in rmiregistry by Server• Remote Object(s) look’d up by Client• When necessary, code transferred from web server to point of use– Both Client and Server can make code network accessible• Operations on Remote Objects carried out by RMICase Study• This example taken directly from the Java RMI tutorial– http://java.sun.com/docs/books/tutorial/rmi/• Editorial note:– Please do yourself a favor and work through the tutorial yourself– If you get the tutorial to work, you’ll have no problems with project 5 or with the final exam2Compute Server Application• Goal– Execute object methods on a remote machine– Often because local resources aren’t sufficient• Real-life example:– Police car stops a motorist for speeding – Officer wants to check driver for warrants– Places request with a central computer (i.e., the database is not in the police car)Compute Interfacepackage compute;import java.rmi.Remote;import java.rmi.RemoteException;public interface Compute extends Remote {Object executeTask(Task t) throws RemoteException;}• Any class that implements Compute is a remote object– Its remote methods can be called from any JVM– Its implementation does not leave the JVM in which it was created• executeTask() is a remote method – It must throw RemoteException3Task Interfacepackage compute;import java.io.Serializable;public interface Task extends Serializable {Object execute();}• Task implementations implement Serializable, rather than Remote– Why?• execute() method returns an Object– Client must downcast result– This method not required to throw RemoteExceptionImplementing Compute Engine• Our implementation of compute interface will called ComputeEngine• In general, the implementer of a remote interface should at least 1. Declare the remote interfaces being implemented 2. Define the constructor for the remote object 3. Implement each remote method in the remote interfaces4Further Requirements for Servers• The server needs to create and to install the remote objects. – The setup procedure often done in main method of the remote object • but can be done anywhere • The setup procedure should 1. Create and install a security manager 2. Create one or more instances of a remote object 3. Register at least one of the remote objects with the RMI registryDeclare the Remote Interfaces• The ComputeEngine class is declared as – public class ComputeEngine extends UnicastRemoteObject implements Compute • This declaration states that – the class implements the Compute remote interface – extends the class java.rmi.server.UnicastRemoteObject• Much magic happens here!5Define the Constructor• The ComputeEngine class has a single, 0-argument constructor. public ComputeEngine() throws RemoteException { super(); }• The no-argument constructor for Compute-Engine must declare RemoteException– because the UnicastRemoteObject constructor does – A RemoteException can occur during construction if the attempt to export the object fails• for example, if communication resources are unavailable or the appropriate stub class is not found Implement Each Remote Method• The Compute interface contains a single remote method, executeTask, which is implemented as follows: public Object executeTask(Task t) { return t.execute(); } • Client provides the ComputeEngine with a Task object, – which has an implementation of the task's execute method• The ComputeEngine executes the Task and returns the result• Client must downcast the result6Implement the Setup Procedure• Create and install a security manager • Create one or more instances of a remote object • Register at least one of the remote objects with the RMI registryCreate and Install a Security Manager• The security manager determines whether downloaded code has access to the local file system or can perform any other privileged operations. • All programs using RMI must install a security manager, or RMI will not download classes (other than from the local class path) for objects received as parameters, return values, or exceptions in remote method callsif (System.getSecurityManager() == null) { System.setSecurityManager(new RMISecurityManager()); } • We will use a policy file that grants more permissions7Create the Remote Object• The main method creates an instance of the ComputeEngine– Compute engine = new ComputeEngine(); • Note that engine’s type is Compute, not ComputeEngine– The interface is available to clients, not the implementation– At runtime, you’ll pass the stub, not the actual implementation Make the Remote Object Accessible• To invoke a remote object method caller must have a reference to it– Can get it from the program• e.g., as part of the return value of a method or as part of a data structure that contains such a reference, or – You can look it up in an RMI registry• The RMI registry is a simple remote object name service that allows remote clients to get a reference to a remote object by name• Start the registry– From the command line as a separate process, or– From within your Server program (See TIJ)8Add Remote Object to Registry• The java.rmi.Naming interface is used as a front-end API for binding, or registering, and looking up remote objects in the registry• The ComputeEngine class creates a name for the remote object String name = "//host/Compute"; • The ComputeEngine adds remote object to the registryNaming.rebind(name, engine); • Note the following about the arguments to the call to Naming.rebind. – The first parameter is a URL-formatted java.lang.String representing the location and the name of the remote object– If no port number is given in name, defaults to 1099• Note that for security reasons, an application can bind, unbind, or rebind remote object references only with a registry running on the same host• Once the server has registered the remote object, the setup procedure exitsCreating a Client Program• Two separate classes make up the client in our example. – ComputePi– Pi• ComputePi must obtain a reference to a Compute object, create a Task object, and then request that the task be executed• Pi implements the Task interface, calculating Pi to some degree of precision9ComputePi• Begins by installing a security manager• Constructs a name used to look up a Compute remote
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