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Remote Method Invocation, Advanced programming
- 1. Advanced Programming Remote MethodInvocation Gera 2020 1
- 2. Remote Method Invocation(RMI) •RMI stands for Remote Method Invocation. •It is a mechanism that allows an object residing in one system (JVM) to access/invoke an object running on another JVM. •RMI is used to build distributed applications; it provides remote communication between Java programs. •It is provided in the package java.rmi. 2
- 3. Architecture of anRMI Application •In an RMI application, we write two programs, a server program (resides on the server) and a client program (resides on the client). • Inside the server program, a remote object is created and reference of that object is made available for the client (using the registry). • The client program requests the remote objects on the server and tries to invoke its methods 3
- 4. Architecture of anRMI Application… • The following diagram shows the architecture of an RMI application. 4
- 5. Architecture of anRMI Application… Let us now discuss the components of this architecture. • Transport Layer ─ This layer connects the client and the server. It manages the existing connection and also sets up new connections. • Stub ─ A stub is a representation (proxy) of the remote object at client. It resides in the client system; it acts as a gateway for the client program. • Skeleton ─ This is the object which resides on the server side. stub communicates with this skeleton to pass request to the remote object. • RRL (Remote Reference Layer) ─ It is the layer which manages the references made by the client to the remote object. 5
- 6. Working of anRMI Application • When the client makes a call to the remote object, it is received by the stub which eventually passes this request to the RRL. • When the client-side RRL receives the request, it invokes a method called invoke() of the object remoteRef. It passes the request to the RRL on the server side. • The RRL on the server side passes the request to the Skeleton (proxy on the server) which finally invokes the required object on the server. • The result is passed all the way back to the client. 6
- 7. RMI Registry • RMIregistryis a namespace on which all server objects are placed. • Each time the server creates an object, it registers this object with the RMIregistry (using bind() or reBind() methods). These are registered using a unique name known as bind name. • To invoke a remote object, the client needs a reference of that object. • At that time, the client fetches the object from the registry using its bind name (using lookup() method). 7
- 8. RMI Registry… • Thefollowing illustration explains the entire process: 8
- 9. Goals of RMI Followingare the goals of RMI: • To minimize the complexity of the application • To preserve type safety • Distributed garbage collection • Minimize the difference between working with local and remote objects 9
- 10. The Remote Interface •A remote interface provides the description of all the methods of a particular remote object. • The client communicates with this remote interface. To create a remote interface – • Create an interface that extends the predefined interface Remote which belongs to the package. • Declare all the business methods that can be invoked by the client in this interface. • Since there is a chance of network issues during remote calls, an exception named RemoteException may occur; 10
- 11. Developing the ImplementationClass (Remote Object) •We need to implement the remote interface created in the earlier step. (We can write an implementation class separately or we can directly make the server program implement this interface.) To develop an implementation class – • Implement the interface created in the previous step. • Provide implementation to all the abstract methods of the remote interface. 11
- 12. Developing the ServerProgram • An RMI server program should implement the remote interface or extend the implementation class. Here, we should create a remote object and bind it to the RMIregistry. To develop a server program – • Create a class that extends the implementation class implemented in the previous step. (or implement the remote interface) • Create a remote object by instantiating the implementation class as shown below. • Export the remote object using the method exportObject() of the class named UnicastRemoteObject which belongs to the package java.rmi.server. • Get the RMI registry using the getRegistry() method of the LocateRegistry class which belongs to the package java.rmi.registry. • Bind the remote object created to the registry using the bind() method of the class named Registry. To this method, pass a string representing the bind name and the object exported, as parameters. 12
- 13. Developing the ClientProgram • Write a client program in it, fetch the remote object and invoke the required method using this object. To develop a client program – • Create a client class from where you want invoke the remote object. • Get the RMI registry using the getRegistry() method of the LocateRegistry class which belongs to the package java.rmi.registry. • Fetch the object from the registry using the method lookup() of the class Registry which belongs to the package java.rmi.registry. To this method you need to pass a string value representing the bind name as a parameter. This will return you the remote object down cast it. • The lookup() returns an object of type remote, down cast it to the type Hello. • Finally invoke the required method using the obtained remote object. 13
- 14. Developing the ClientProgram • Write a client program in it, fetch the remote object and invoke the required method using this object. To develop a client program – • Create a client class from where you want invoke the remote object. • Get the RMI registry using the getRegistry() method of the LocateRegistry class which belongs to the package java.rmi.registry. • Fetch the object from the registry using the method lookup() of the class Registry which belongs to the package java.rmi.registry. To this method you need to pass a string value representing the bind name as a parameter. This will return you the remote object down cast it. • The lookup() returns an object of type remote, down cast it to the type Hello. • Finally invoke the required method using the obtained remote object. 14
- 15. RMI Implementation For implementingRMI system The steps are as follows: • Step–1: Create an Remote interface • A remote interface specifies the methods that can be invoked remotely by a client. • Such interfaces includes the types of objects that will be used as the parameters and return values for these methods. • This interface extends java.rmi.Remote interface, It is part of java.rmi package. • Its purpose is simply indicate that an interface uses remote methods and must specify the exception RemoteException in throws statement. 15
- 16. RMI Implementation… Step–2: Createa Class that implements the interface • This class must extends java.rmi.UnicastRemoteObject and must implements an interface defined in step–1. • This class provides the functionality needed to make object available from remote machines. • The Constructor must call super(), and it should call Naming.rebind(name, this):- It makes RMI registry which informs that an object having “name” is available. 16
- 17. RMI Implementation… Step–3: CreatesServer that creates an instance of this class. • Create an instance of the Class which implements Remote Interface, and gives it a name. • With the RMI registry, the RMI Object is available to Remote clients. • This can be done by rebind() method of Naming class. • It required 2 arguments: • First argument is String that names the Server as “AddServer”. • Second argument is reference to an instanse of AddServerImpl class. 17
- 18. RMI Implementation… Step–4: CreatesClient that connects to Server object • Clients uses a method Naming.lookup() to get the reference of Remote objects. • The client using the interface to hold the reference and for method call. So that, We should create interface before client request it. • The Client required 3 command line arguments: • first argument is IP address or name of Server machine, • second and third arguments are two numbers required for addition. • The client forms a string that follows the URL syntax. • It includes IP address or name of the server and String “AddServer”. 18
- 19. RMI Implementation… Step–5: Compileall these source classes • Includes Remote Interfaces, their implementations, any other Server and Client classes. Step–6: Run the RMI Interface compiler • RMI Interface compiler (i.e. rmic) can be run on the class that implements Remote Interface. • rmic compiler runs on the .class file and not on .java files. Step–7: Start RMI Registry • The program rmiregistry comes with JDK in bin directory. • For Windows the command is start rmiregistry on command prompt. • Before starting you RMI Server, RMI Registry must be started. 19
- 20. RMI Implementation… Step–8: StartServer Class • For windows, the command for staring server is start java MyRMIServer on command prompt. • After executing this command, Server starts running and creates an instance of RMI classes which implements Remote Interface. Step–9: Run Client Program • For widows, the command for starting client is java MyRMIClient on command prompt. • The client program will ask for RMI Registry for reference. • After getting the reference, it can invoke any Remote method. 20
- 21. Marshalling • Marshalling isthe process of transforming the memory representation of an object to a data format suitable for storage or transmission. i.e. It is the process of converting a data structure or object into a sequence of bits so that it can be stored in a file, a memory buffer, or transmitted across a network. • It is typically used when data must be moved between different parts of a computer program or from one program to another. Unmarshalling • The reverse process of marshalling is called unmarshalling. • The process of, extracting a data structure from a series of bytes, is called unmarshalling. 21
- 22. References ➢S. Horstmann andGary Cornell, Core Java 2 – Volume II- Advanced Features, Sun Microsystems Press ➢Harvey M. Deitel and Paul J. Deitel, Java How to Program, Deitel & Associates ➢Tutorials Point “Java RMI,” tutorialspoint.com ➢Prof. Shailesh T. Gahane, Dr. D Y Patil School of MCA, Pune, “Java Programming Remote Method Invocation ” 22 Gerabirhan Paulos ToCourseInfo@gmail.com