Java Technologies. Lecture VII. Valdas Rapševičius. Vilnius University Faculty of Mathematics and Informatics

Transcription

1 Preparation of the material was supported by the project Increasing Internationality in Study Programs of the Department of Computer Science II, project number VP1 2.2 ŠMM-07-K , funded by The European Social Fund Agency and the Government of Lithuania. Java Technologies Lecture VII Valdas Rapševičius Vilnius University Faculty of Mathematics and Informatics

2 Session Outline You will learn networking basics in Java, namely TCP Sockets Datagram Sockets Network Interfaces URL RMI Java Policy for Networking 2

3 Protocols Computers running on the Internet communicate to each other using either the Transmission Control Protocol (TCP) or the User Datagram Protocol (UDP) TCP (Transmission Control Protocol) is a connection-based protocol that provides a reliable flow of data between two computers. UDP (User Datagram Protocol) is a protocol that sends independent packets of data, called datagrams, from one computer to another with no guarantees about arrival. UDP is not connection-based like TCP. The TCP and UDP protocols use ports to map incoming data to a particular process running on a computer. 3

4 Basic networking classes in JDK java.net package TCP protocol classes URL URLConnection Socket ServerSocket SocketChannel UDP protocol classes DatagramPacket DatagramSocket MulticastSocket java.net.networkinterface class 4

5 Sockets A socket is one endpoint of a two-way communication link between two programs running on the network. A socket is bound to a port number so that the TCP layer can identify the application that data is destined to be sent. 5

6 Socket class try (Socket socket = new Socket(HOSTNAME, PORT)) { try (PrintWriter out = new PrintWriter(socket.getOutputStream(), true) { try (Reader isr = new InputStreamReader(socket.getInputStream())) { try (BufferedReader in = new BufferedReader(isr)) { String somedata = "this is a message!"; out.println(somedata); System.out.println("return: " + in.readline()); catch (UnknownHostException e) { System.err.format("Don't know about host: %s.%n, HOSTNAME); catch (IOException e) { System.err.format("I/O problem for the connection to: %s.%n", HOSTNAME); 6

7 ServerSocket class try (ServerSocket ssocket = new ServerSocket(PORT)) { Socket socket = ssocket.accept(); OutputStream os = socket.getoutputstream(); try (PrintWriter out = new PrintWriter(os, true)) { InputStream is = csocket.getinputstream(); try (BufferedReader in = new BufferedReader(new InputStreamReader(is))) { String line; while ((line = in.readline())!= null) { out.println("thanks for: " + line); catch (IOException e) { System.err.format("Could not listen on port: %d.%n", PORT); 7

8 DatagramSocket class A datagram is an independent, self-contained message sent over the network whose arrival, arrival time, and content are not guaranteed. byte[] buf = new byte[256]; // Binds Datagram socket to the certain port (aka Server) DatagramSocket socket = new DatagramSocket(4445); DatagramPacket packet = new DatagramPacket(buf, buf.length); socket.receive(packet); // Get client socket info from the packet InetAddress address = packet.getaddress(); int port = packet.getport(); // And send something back packet = new DatagramPacket(buf, buf.length, address, port); socket.send(packet); // Create Datagram socket on any available local port (aka Client) DatagramSocket socket = new DatagramSocket(); InetAddress serveraddr = InetAddress.getByName("localhost"); DatagramPacket packet = new DatagramPacket(buf, buf.length, serveraddr, 4445); socket.send(packet); 8

9 MulticastSocket class MulticastSocket is used on the client-side to listen for packets that the server broadcasts to multiple clients. byte[] buf = new byte[256]; // Server sends a message to the group DatagramSocket socket = new DatagramSocket(); InetAddress group = InetAddress.getByName(" "); DatagramPacket packet = new DatagramPacket(buf, buf.length, group, 4446); socket.send(packet); // Client to listen to message broadcast MulticastSocket socket = new MulticastSocket(4446); InetAddress group = InetAddress.getByName(" "); socket.joingroup(group); DatagramPacket packet = new DatagramPacket(buf, buf.length); socket.receive(packet); String received = new String(packet.getData()); System.out.println(received); socket.leavegroup(group); socket.close(); 9

10 Network Interface A network interface is the point of interconnection between a computer and a private or public network. A network interface is generally a network interface card (NIC), but does not have to have a physical form. Instead, the network interface can be implemented in software. For example, the loopback interface ( for IPv4 and ::1 for IPv6) is not a physical device but a piece of software simulating a network interface. The loopback interface is commonly used in test environments. // Create socket and connect to InetSocketAddress Socket soc = new java.net.socket(); soc.connect(new InetSocketAddress(IP, PORT)); // Get NIC addresses NetworkInterface nif = NetworkInterface.getByName("lo"); Enumeration<InetAddress> addrs = nif.getinetaddresses(); InetAddress addr = addrs.nextelement(); // Bind to existing NIC address InetSocketAddress sockaddr = new InetSocketAddress(addr, PORT); soc.bind(sockaddr); soc.connect(sockaddr); 10

11 NetworkInterface class Enumeration<NetworkInterface> nets = NetworkInterface.getNetworkInterfaces(); for (NetworkInterface net : Collections.list(nets)) { out.printf("display name: %s\n", net.getdisplayname()); out.printf("name: %s\n", net.getname()); Enumeration<NetworkInterface> subnets = net.getsubinterfaces(); for (NetworkInterface subnet : Collections.list(subnets)) { out.printf("\tsub Interface Display name: %s\n", subnet.getdisplayname()); out.printf("\tsub Interface Name: %s\n", subnet.getname()); Enumeration<InetAddress> inetaddresses = net.getinetaddresses(); for (InetAddress inetaddress : Collections.list(inetAddresses)) { out.printf("inetaddress: %s\n", inetaddress); out.printf("up? %s\n", net.isup()); out.printf("loopback? %s\n", net.isloopback()); out.printf("pointtopoint? %s\n", net.ispointtopoint()); out.printf("supports multicast? %s\n", net.supportsmulticast()); out.printf("virtual? %s\n", net.isvirtual()); out.printf("hardware address: %s\n", Arrays.toString(net.getHardwareAddress())); out.printf("mtu: %s\n", net.getmtu()); 11

12 URI Uniform Resource Identifiers (URI) provide a simple and extensible means for identifying a resource. URIs: RFC1630, issued in June 1994, was called "Universal Resource Identifiers in WWW: A Unifying Syntax for the Expression of Names and Addresses of Objects on the Network as used in the World-Wide Web". URI consists of 2 parts: URLs: RFC1738, issued in December 1994, was called "Uniform Resource Locators. mailto:valdas.rapsevicius@mif.vu.lt jar: URNs: RFC1737, issued December 1994, was called "Functional Requirements for Uniform Resource Names. urn:isan: e o urn:uuid:6e8bc430-9c3a-11d c9a66 urn:nbn:de:bvb: urn:isbn:

13 URL class import java.net.*; import java.io.*; public class ParseURL { public static void main(string[] args) throws Exception { URL url = new URL(" + "/index.html?name=networking#downloading"); System.out.println("protocol = " + url.getprotocol()); System.out.println("authority = " + url.getauthority()); System.out.println("host = " + url.gethost()); System.out.println("port = " + url.getport()); System.out.println("path = " + url.getpath()); System.out.println("query = " + url.getquery()); System.out.println("filename = " + url.getfile()); System.out.println("ref = " + url.getref()); 13

14 URL: direct InputStream import java.net.*; import java.io.*; public class URLReader { public static void main(string[] args) throws Exception { URL mif = new URL(" BufferedReader in = new BufferedReader(new InputStreamReader(mif.openStream())); String inputline; while ((inputline = in.readline())!= null) { System.out.println(inputLine); in.close(); 14

15 URLConnection class // Create connection URL url = new URL(myUrl); // HttpURLConnection, JarURLConnection concrete classes // based on URL type URLConnection con = url.openconnection(); // Write some data connection.setdooutput(true); OutputStreamWriter out = new OutputStreamWriter(con.getOutputStream()); out.write(mydata); out.close(); // Read data Reader reader = new InputStreamReader(con.getInputStream()); BufferedReader in = new BufferedReader(reader); String data; while ((data = in.readline())!= null) { System.out.println(data); in.close(); 15

16 The network is the computer Consider the following program organization: method call SomeClass AnotherClass returned object computer 1 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 16

17 Roadmap to Remote Method Invocation 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 17

18 RMI overview 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 Do this by looking it up in a registry The client object then has to marshal the parameters (prepare them for transmission) 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 18

19 RMI components Steps for RMI application implementation: Defining the remote interfaces. A remote interface specifies the methods that can be invoked remotely by a client. Clients program to remote interfaces, not to the implementation classes of those interfaces. The design of such interfaces includes the determination of the types of objects that will be used as the parameters and return values for these methods. If any of these interfaces or classes do not yet exist, you need to define them as well. Implementing the remote objects. Remote objects must implement one or more remote interfaces. The remote object class may include implementations of other interfaces and methods that are available only locally. If any local classes are to be used for parameters or return values of any of these methods, they must be implemented as well. Implementing the clients. Clients that use remote objects can be implemented at any time after the remote interfaces are defined, including after the remote objects have been deployed. 19

20 RMI: Remote interface An object becomes remote by implementing a remote interface, which has the following characteristics: A remote interface extends the interface java.rmi.remote. Each method of the interface declares java.rmi.remoteexception in its throws clause, in addition to any application-specific exceptions. package lt.vu.mif.javatech.rmi.remote; import java.rmi.remote; import java.rmi.remoteexception; public interface Calc extends Remote { public int add(int i,int j) throws RemoteException; 20

21 RMI: server implementation package lt.vu.mif.javatech.rmi.server; import java.rmi.*; import lt.vu.mif.javatech.rmi.remote.calc; public class CalcImpl implements Calc { public int add(int i,int j)throws RemoteException { return i + j; public static void main(string[] args) { if (System.getSecurityManager() == null) { System.setSecurityManager(new SecurityManager()); try { CalcImpl c = new CalcImpl(); CalcImpl stub = (CalcImpl) UnicastRemoteObject.exportObject(c, 0); Registry registry = LocateRegistry.getRegistry(); registry.rebind("calc", stub); catch (Exception e) { e.printstacktrace(); 21

22 RMI: client implementation package lt.vu.mif.javatech.rmi.client; import java.rmi.registry.*; import lt.vu.mif.javatech.rmi.remote.calc; public class CalcClient { public static void main(string args[]) { if (System.getSecurityManager() == null) { System.setSecurityManager(new SecurityManager()); try { Registry registry = LocateRegistry.getRegistry(args[0]); Calc calc = (Calc) registry.lookup("calc"); System.out.println(calc.add(5, 9)); catch (Exception e) { e.printstacktrace(); 22

23 RMI: running example Policy files Server and client applications have to have policy files defined to grant permissions to execute the code on/from remote machine. I.e. grant codebase "file:/path/to/class/files/" { permission java.security.allpermission; ; Start RMI registry The RMI registry is a simple server-side bootstrap naming facility that enables remote clients to obtain a reference to an initial remote object. By default, the registry runs on port rmiregistry & Start server application java -cp rmi_server.jar -Djava.rmi.server.hostname= Djava.security.policy=server.policy lt.vu.mif.javatech.rmi.server.calcimpl Execute client application java -cp rmi_client.jar -Djava.security.policy=client.policy lt.vu.mif.javatech.rmi.client.calcclient

24 Java Policy File In Java 2, the java application must first obtain information regarding its privileges. It can obtain the security policy through a policy file. In above example, we allow Java code to have all permissions: grant { permission java.security.allpermission; ; Now, we given an example for assigning resource permissions: grant { permission java.io.filepermission "/tmp/*", "read", "write"; permission java.net.socketpermission "somehost.somedomain.com:999", "connect"; permission java.net.socketpermission "*: ", "connect,request"; permission java.net.socketpermission "*:80", "connect"; ; allow the Java code to read/write any files only under the /tmp directory, includes any subdirectories allow all java classes to establish a network connection with the host somehost.somedomain.com on port 999 allows classes to connection to or accept connections on unprivileged ports greater than 1024, on any host allows all classes to connect to the HTTP port 80 on any host. 24

25 Session Conclusions You must be able to program application layer on top of Java provided transport layer tools You now understand the Java RMI protocol and basic usage. Start from there! 25