Distributed Applications Programming. Lab 7

Transcription

1 Lebanese University Info Distributed Applications Programming Faculty of Sciences Section I Antoun Yaacoub Info 408 Distributed Applications Programming Lab 7 Chat Server with RMI - Part I (90 minutes) 1. Overview and Installation We will learn to write remote interfaces to existing remotely accessible objects, use the advanced features of RMI and understand the problems of deploying a distributed application. It is strongly recommended to make a diagram showing the relationships between different parts of an application at runtime. (a) Installation We will start from a simple chat application. This application consists of three packages: i. a server package that includes the chat server ii. a client package that includes a client that can connect to the server and send and receive messages iii. a package that contains common remote interfaces A package, which is not directly related to the application, will be used to enable the downloading of code with RMI. Retrieve the chatserver.zip file and unzip it in a directory of your choice. You get a directory that contains in the src directory, the following subdirectories: Directory name chat/client/ chat/server/ chat/remote/ classserver/ Content Client chat application package Server chat application package Remote interfaces of the chat application package The package downloading code that implements an HTTP server capable of responding to GET methods (b) How it works The principle is as follows: The chat server is a well-defined ServerImpl class that implements an interface Server offering a set of services. This class can not be accessed remotely. To make the server available to distributed clients, we define a remote object RemoteServerImpl exposing the RemoteServer interface. RemoteServerImpl instantiates a ServerImpl. Once RemoteServerImpl server runs on a host machine, it registers in the registry. The client is also a remote object because he needs to be contacted by the server. He exposes the RemoteClient interface. The client gets in parameter the server URL to which he should 1

2 connect. Once launched, he does a lookup for the server and obtains a remote reference to the server (a stub) that implements the interface RemoteServer. This interface define a logon method that allows the client to become known by the server. public RemoteConnection logon (String name, RemoteClient c) throws RemoteException; This method takes as parameters the name of the client and a remote reference for the callback. It returns a remote reference (RemoteConnection) on a connection that allows the client to talk to the server. RemoteConnection defines two methods, one to send a message to the server, the other to allow the client to disconnect. (c) Description of available files List of files and their usage File name Role Common Interfaces RemoteClient.java remote interface of the client (for the server callback s). RemoteServer.java remote interface of the server that the clients retrieve from the registry and allow them to logon. RemoteConnection.java Remote interface that the server returns to the client as a result of the logon and that allows the client to communicate with the server. The interface provided is empty. Its writing is the subject of a question below. Server RemoteServerImpl.java RemoteServer implementation based on UnicastRemoteObject. The logon method is empty. Its implementation is the subject of a question below. RemoteConnectionImpl.java RemoteConnection implementation based on UnicastRemoteObject. The provided class has only the constructor. The implementation of the RemoteConnection interface is the subject of a question below. Server.java Interface offering features of a chat server. This interface is not remote and represents a local view of the server. ServerImpl.java Implementation of the Server interface. ServerImpl is the actual server code that handles the clients. It is used by the class RemoteServerImpl.java Client ClientImpl.java An implementation of the RemoteClient interface that reads the messages using System.in and displays using System.out (d) Compilation Try to compile the 3 packages then use rmic to generate the stub for the 3 remote objects. Note that, it is still impossible to test the server before writing the missing code (next question). 2

3 It is important to understand that in order to compile, the package chat.server only requires the common interfaces and server classes. Similarly, the chat.client package requires only the common interfaces and the client class. At execution, the server will need the stub on the client and the client will need the stub on the server and the connection. These classes can be downloaded automatically by the RMI runtime as we shall see later. Chat Client Chat Server Class Server (Note that the server classes will be used later) 3

4 2. Writing a remote interface (a) Writing and implementation of the connection interface As explained above, the connection interface RemoteConnection that allows the client to communicate with the server is empty. The purpose of this part is to write the interface, to write its implementation and modify the client and server code in order to use it. i. In the server package chat.remote, edit the empty interface RemoteConnection by adding two methods. One that sends a message passed as a parameter to the server, and a second one that allows the client to disconnect from the server (logoff). ii. In chat.server package, edit the RemoteConnectionImpl class in order to implement the two methods that you have defined. iii. Also in the package chat.server, edit RemoteServerImpl class to implement the logon method that instantiates a RemoteConnectionImpl and returns it to the client. We can also send a welcome message to the client. iv. In the chat.client package, edit the ClientImpl class and modify the startclient method to use the connection object in order to talk to the server. (b) Test sharing the classpath Recompile the client and server packages to reflect the changes. Do not forget to use rmic on RemoteConnectionImpl because the remote interface it implements has changed. For each command described below, the CLASSPATH must point to the classes of the client and the server. For this you should use: set CLASSPATH=classes Launching the registry Launching the server 4

5 Launching a client 3. Launching the application in a distributed environment (a) To highlight the problems of deploying a distributed application, the server and the client should not now share the same classpath (the most common case in reality). The consequence is that we allow to RMI to download classes dynamically. For this, we must launch two servers of classes that are a very simple HTTP server capable of reading class files from a given directory. Follow the steps below to start the application. i. Observe what happens in each window and explain. In particular notice the two windows of ClassServer and explain who is dowloading and what. ii. Spend some time looking at the server code. Try to understand the chain of responsibility that is established between RemoteServerImpl, RemoteConnectionImpl and ServerImpl classes. How many instances of each class are present in a given time in the server JVM. (b) Separation of class files Create two directories (serverside and clientside). Move in the first the.class files required by the server, and in the second the.class files required by the client. (c) Launching the registry Make sure that the classpath in the command line contains nothing then start the rmiregistry: (d) Launching the Server ClassServer In a second command line, launch the ClassServer by pointing it to the server classes. The 5

6 port number chosen is arbitrary but must match the one passed as a parameter to the Server. (e) Launching the ChatClient ClassServer In a third command line, launch the ClassServer by pointing it to the client classes. The port number chosen is arbitrary but must match the one passed as a parameter to ChatClient. (f) Launching the Server In a fourth command line, launch the Server by specifying the security policy and the codebase, that is to say what is the URL of the HTTP server capable of serving the necessary classes in order to use this server. This is the server we started on port In parameter, you must give the name under which the server will register in the rmiregistry. (g) Launching a Client In a fifth command line, launch the ChatClient by specifying the security policy and the codebase, that is to say what is the URL of the HTTP server capable of serving the necessary classes in order to use the remote part of the client. This is the ClassServer that we have started on port As parameter, you must set the ChatServer URL and the client username. 6

7 (h) Launching another ChatClient Start another client and try to communicate by typing messages. 7