1. Remote Method Invocation (RMI)

2. Client-Server Communication Sockets Remote Procedure Calls Remote Method Invocation (Java)

3. Sockets A socket is defined as an endpoint for communication. Concatenation of IP address and port The socket 161.25.19.8:1625 refers to port 1625 on host 161.25.19.8 Communication consists between a pair of sockets. Considered a low-level form of communication between distributed processes. –Sockets allow only an unstructured stream of bytes to be exchanged. It is the responsibility of the client or server application to impose a structure on the data.

4. Socket Communication

5. Remote Procedure Calls Remote procedure call (RPC) abstracts procedure calls between processes on networked systems. Stub – client-side proxy for the actual procedure on the server. Server has a similar stub as well. The client-side stub locates the server and marshals the parameters. The server-side stub receives this message, unpacks the marshaled parameters, and performs the procedure on the server. External data representation (XDR)  I.e most- significant (big-endian), least-significant(little- endian)

6. Execution of RPC

7. Remote Method Invocation Remote Method Invocation (RMI) is a Java mechanism similar to RPCs. RMI allows a Java program on one machine to invoke a method on a remote object.

8. Marshalling Parameters

9. Remote Method Invocation RMI and RPC differs in two ways: 1.RPCs support procedural programming whereby only remote procedures or functions may be called. RMI is object based: It supports invocation of methods on remote objects. 2.The parameters to remote procedures are ordinary data structures in RPC; with RMI it is possible to pass objects as parameters to remote methods. If the marshaled parameters are local (non remote) objects, they are passed by copy using a technique known as object serialization. –Object serialization allowed the state of an object to be written toa byte stream.

10. Introduction to RMI Remote Method Invocation (RMI) –Allows remote method calls Objects in different programs can communicate Method calls appear same as those in same program –Based on Remote Procedure Calls (RPC) Developed in 1980's Allows procedural program (like C) to call function on another computer Performs networking and marshalling of data (packaging arguments and return values) Not compatible with objects Interface Definition Language required - describe functions –RMI is Java's implementation of RPC

11. Introduction to RMI RMI –Register method as remotely accessible Client can look up method and receive a reference Use reference to call method Syntax same as a normal method call –Marshalling of data Can transfer objects as well Class ObjectOutputStream converts Serializable object into stream of bytes –Transmit across network Class ObjectInputStream reconstructs object –No Interface Definition Language needed Use Java's own interface

12. Case Study: Creating a Distributed System with RMI RMI example –Downloads weather information from National Weather Service website http://iwin.nws.noaa.gov/iwin/us/traveler.html Note: Format of website changed several times, if example does not work do the appropriate modifications. –Store information on a server Request information through remote method calls

13. Case Study: Creating a Distributed System with RMI

14. Four major steps –Define remote interface Describes client/server communication –Define server application to implement remote interface Same name as remote interface, ends with Impl –Define client application that uses remote interface reference Interacts with server implementation –Compile and execute server and client

15. Defining the Remote Interface First step –Define remote interface that describes remote methods Client calls remote methods, server implements them To create a remote interface –Define interface that extends interface Remote ( java.rmi ) Tagging interface - no methods to define An object of a class that implements interface Remote directly or indirectly is a remote object and can be accesses from any JVM. –Each method in Remote interface must throw RemoteException Potential network errors

16. Defining the Remote Interface Interface TemperatureServer –Extends Remote –Describes method getWeatherInfo

17. 1. import 1.1 extends Remote 2. getWeatherInfo 2.1 throws RemoteException 1// Fig. 20.1: TemperatureServer.java 2// TemperatureServer interface definition 3 3import java.rmi.*; 4 5public interface TemperatureServer extends Remote { 6 public WeatherInfo[] getWeatherInfo() 7 7 throws RemoteException; 8}8} Interface Remote in java.rmi Methods in Remote interface (is a relationship) must be able to throw a RemoteException.

18. Implementing the Remote Interface Define TemperatureServerImpl –Implements Remote interface TemperatureServer –Client interacts with TemperatureServerImpl object –Uses array of WeatherInfo objects to store data Copy sent to client when calls getWeatherInfo

19. Implementing the Remote Interface –UnicastRemoteObject Provides functionality for remote objects Constructor exports object so it can receive remote calls –Wait for client on anonymous port number Subclass constructors must throw RemoteException s –URL object Contains URL for Traveler's Forecast web page Throws MalformedURLException 18public class TemperatureServerImpl extends UnicastRemoteObject 19 implements TemperatureServer { 22 public TemperatureServerImpl() throws RemoteException 37 URL url = new URL( 38 "http://iwin.nws.noaa.gov/iwin/us/traveler.html" );

20. Implementing the Remote Interface –Open connection to file specified by URL –Method openStream (class URL ) Opens network connection using Http protocol If successful, InputStream object returned (else IOException ) –InputStreamReader Translates bytes to Unicode characters –BufferedReader Buffers characters Method readLine –Returns one line as a String 40 BufferedReader in = 41 new BufferedReader( 42 new InputStreamReader( url.openStream() ) );

21. Implementing the Remote Interface –Sentinel String to find relevant part of HTML code readLine until sentinel found –A string used as column head Second " WEA HI/LO" is for next day, we do not use –Locate column head and get first city's info 44 String separator = " "; 47 while ( !in.readLine().startsWith( separator ) ) 48 ; // do nothing 51 String s1 = 52 "CITY WEA HI/LO WEA HI/LO"; 66 inputLine = in.readLine(); // get first city's info

22. Implementing the Remote Interface –WeatherInfo objects City name, temperature, description of weather –Method substring to extract data from line Store all WeatherInfo objects in a Vector –Store data in WeatherInfo array elementAt returns Object (must be cast) –Close connection 70 WeatherInfo w = new WeatherInfo( 71 inputLine.substring( 0, 16 ), 72 inputLine.substring( 16, 22 ), 73 inputLine.substring( 23, 29 ) ); 75 cityVector.addElement( w ); // add to Vector 84 weatherInformation[ i ] = 85 ( WeatherInfo ) cityVector.elementAt( i ); 88 in.close(); // close connection to NWS server

23. Implementing the Remote Interface –Name of server object Used by clients to connect // host : port / remoteObjectName –host - computer running registry for remote objects »Where remote object executes –port - port number of registry on host (1099 default) –remoteObjectName - client uses to locate object –Registry managed by rmiregistry (located at host and port) Remote objects register with it, clients use it to locate service localhost (same computer) –Same as IP 127.0.0.1 116 String serverObjectName = "//localhost/TempServer";

24. Implementing the Remote Interface –static method rebind (class Naming ) Binds object to rmiregistry Named //localhost/TempServer –Name used by client rebind replaces any previous objects with same name –Method bind does not 117 Naming.rebind( serverObjectName, temp ); 112 TemperatureServerImpl temp = 113 new TemperatureServerImpl(); 116 String serverObjectName = "//localhost/TempServer";

25. 1. Interface ------------------ 1. extends UnicastRemote Object, implements TemperatureServer 1.1 Constructor 1// Fig. 20.1: TemperatureServer.java 2// TemperatureServer interface definition 3import java.rmi.*; 4 5public interface TemperatureServer extends Remote { 6 public WeatherInfo[] getWeatherInfo() 7 throws RemoteException; 8}8} 9TemperatureServer interface. 10// Fig. 20.2: TemperatureServerImpl.java 11// TemperatureServerImpl definition 12import java.rmi.*; 13import java.rmi.server.*; 14import java.util.*; 15import java.io.*; 16import java.net.*; 17 18 18public class TemperatureServerImpl extends UnicastRemoteObject 19 implements TemperatureServer { 20 private WeatherInfo weatherInformation[]; 21 22 22 public TemperatureServerImpl() throws RemoteException 23 { 24 super(); 25 updateWeatherConditions(); 26 } 27 Allows objects to be exported. Superclass constructor exports objects, and this constructor must be able to throw RemoteException.

26. 2. updateWeather Conditions 2.1 URL 2.2 BufferedReader 2.3 readLine 34 "Updating weather information..." ); 35 36 // Traveler's Forecast Web Page 37 37 URL url = new URL( 38 "http://iwin.nws.noaa.gov/iwin/us/traveler.html" ); 39 40 40 BufferedReader in = 41 new BufferedReader( 42 new InputStreamReader( url.openStream() ) ); 43 44 String separator = " "; 45 46 // locate first horizontal line on Web page 47 47 while ( !in.readLine().startsWith( separator ) ) 48 ; // do nothing 49 50 // s1 is the day format and s2 is the night format 51 String s1 = 52 "CITY WEA HI/LO WEA HI/LO"; 53 String s2 = 54 "CITY WEA LO/HI WEA LO/HI"; 55 String inputLine = ""; 56 28 // get weather information from NWS 29 private void updateWeatherConditions() 30 throws RemoteException 31 { 32 try { 33 System.err.println( URL of web site ( URL object). Open connection to file. InputStreamReader formats it to Unicode characters, and BufferedReader buffers the characters. readLine until separator found.

27. 2.4 Locate header 2.5 Loop 2.5.1 WeatherInfo 2.5.2 readLine 2.6 WeatherInfo array 67 68 68 while ( !inputLine.equals( "" ) ) { 69 // create WeatherInfo object for city 70 WeatherInfo w = new WeatherInfo( 71 inputLine.substring( 0, 16 ), 72 inputLine.substring( 16, 22 ), 73 inputLine.substring( 23, 29 ) ); 74 75 cityVector.addElement( w ); // add to Vector 76 inputLine = in.readLine(); // get next city's info 77 } 78 79 // create array to return to client 80 80 weatherInformation = 81 new WeatherInfo[ cityVector.size() ]; 82 83 for ( int i = 0; i < weatherInformation.length; i++ ) 84 weatherInformation[ i ] = 85 ( WeatherInfo ) cityVector.elementAt( i ); 86 62 63 64 Vector cityVector = new Vector(); 65 66 inputLine = in.readLine(); // get first city's info 57 // locate header that begins weather information 58 do { 59 inputLine = in.readLine(); 60 } while ( !inputLine.equals( s1 ) && 61 !inputLine.equals( s2 ) ); Create WeatherInfo object, add data ( substring ), add to Vector. Loop until blank line reached. Create WeatherInfo array, cast Vector elements.

28. 2.7 close 3. getWeatherInfo 4. main 4.1 temp 100 // implementation for TemperatureServer interface method 101 public WeatherInfo[] getWeatherInfo() 102 { 103 103 return weatherInformation; 104 } 105 106 public static void main( String args[] ) throws Exception 107 { 108 System.err.println( 109 "Initializing server: please wait." ); 110 111 // create server object 112 TemperatureServerImpl temp = 113 new TemperatureServerImpl(); 114 87 System.err.println( "Finished Processing Data." ); 88 in.close(); // close connection to NWS server 89 } 90 catch( java.net.ConnectException ce ) { 91 System.err.println( "Connection failed." ); 92 System.exit( 1 ); 93 } 94 catch( Exception e ) { 95 e.printStackTrace(); 96 System.exit( 1 ); 97 } 98 } 99 Return the WeatherInfo array.

29. 4.2 serverObjectName 4.3 rebind 115 // bind TemperatureServerImpl object to the rmiregistry 116 116 String serverObjectName = "//localhost/TempServer"; 117 117 Naming.rebind( serverObjectName, temp ); 118 System.err.println( 119 "The Temperature Server is up and running." ); 120 } 121} Name of server object. rebind binds object to rmiregistry.

30. 1. Class WeatherInfo implements Serializable 1. Instance variables 1.1 Constructor 2. Get methods 1/ Fig. 20.3: WeatherInfo.java 2// WeatherInfo class definition 3import java.rmi.*; 4import java.io.Serializable; 5 6 6public class WeatherInfo implements Serializable { 7 private String cityName; 8 private String temperature; 9 private String description; 10 11 public WeatherInfo( String city, String desc, String temp ) 12 { 13 cityName = city; 14 temperature = temp; 15 description = desc; 16 } 17 18 public String getCityName() { return cityName; } 19 20 public String getTemperature() { return temperature; } 21 22 public String getDescription() { return description; } 23} This allows objects to be passed as a stream of bytes.

31. Define the client Next step –Client code to get weather info from TemperatureServerImpl –Calls getWeatherInfo through RMI –Graphically display weather info Class WeatherItem (extends JLabel ) stores info about each city Display name, High/low, and image (depending on conditions)

32. Define the client –Can specify IP address at command line (more later) –static method lookup (class Naming ) –Returns reference to Remote object Cast to TemperatureServer –Reference may be used as normal Only difference that copy of array returned 22 private void getRemoteTemp( String ip ) 26 String serverObjectName = "//" + ip + "/TempServer"; 30 TemperatureServer mytemp = ( TemperatureServer ) 31 Naming.lookup( serverObjectName ); 34 WeatherInfo weatherInfo[] = mytemp.getWeatherInfo();

33. Define the client –Add WeatherItem s Initialize with WeatherInfo –main Passes command line argument (ip) to constructor localhost default 40 JPanel p = new JPanel(); 50 for ( int i = 0; i < w.length; i++ ) { 51 w[ i ] = new WeatherItem( weatherInfo[ i ] ); 52 p.add( w[ i ] ); 53 } 68 public static void main( String args[] ) 69 { 70 TemperatureClient gt = null; 74 if ( args.length == 0 ) 75 gt = new TemperatureClient( "localhost" ); 76 else 77 gt = new TemperatureClient( args[ 0 ] );

34. Define the client Class WeatherItem –extends JLabel –static initializer block For complex initialization of static variables backgroundImage - ImageIcon, has background weatherImages - ImageIcon array, holds weather images 18 static { 19 backgroundImage = new ImageIcon( "images/back.jpg" ); 20 weatherImages = 21 new ImageIcon[ weatherImageNames.length ]; 22 23 for ( int i = 0; i < weatherImageNames.length; ++i ) 24 weatherImages[ i ] = new ImageIcon( 25 "images/" + weatherImageNames[ i ] + ".jpg" ); 26 }

35. Define the client –Array of descriptions and matching array of images weatherConditions and weatherImages –Tests WeatherInfo object, loads proper image 35 weatherInfo = w; 38 for ( int i = 0; i < weatherConditions.length; ++i ) 39 if ( weatherConditions[ i ].equals( 40 weatherInfo.getDescription().trim() ) ) { 41 weather = weatherImages[ i ]; 32 public WeatherItem( WeatherInfo w )

36. 1. import 1.1 Constructor 2. getRemoteTemp 2.1 serverObjectName 2.2 Naming.lookup 1// Fig. 20.4: TemperatureClient.java 2// TemperatureClient definition 3import java.awt.*; 4import java.awt.event.*; 5import javax.swing.*; 6import java.rmi.*; 7 8public class TemperatureClient extends JFrame 9{9{ 10 public TemperatureClient( String ip ) 11 { 12 super( "RMI TemperatureClient..." ); 13 getRemoteTemp( ip ); 14 15 setSize( 625, 567 ); 16 setResizable( false ); 17 show(); 18 } 19 20 // obtain weather information from TemperatureServerImpl 21 // remote object 22 private void getRemoteTemp( String ip ) 23 { 24 try { 25 // name of remote server object bound to rmi registry 26 26 String serverObjectName = "//" + ip + "/TempServer"; 27 28 // lookup TemperatureServerImpl remote object 29 // in rmiregistry 30 30 TemperatureServer mytemp = ( TemperatureServer ) 31 Naming.lookup( serverObjectName ); Use ip specified at command line.Lookup remote object in registry. Returns Remote reference, cast to proper type.

37. 2.3 getWeatherInfo 2.4 GUI 2.4.1 WeatherItem 34 WeatherInfo weatherInfo[] = mytemp.getWeatherInfo(); 35 WeatherItem w[] = 36 new WeatherItem[ weatherInfo.length ]; 37 ImageIcon headerImage = 38 new ImageIcon( "images/header.jpg" ); 39 40 JPanel p = new JPanel(); 41 42 // determine number of rows for the GridLayout; 43 // add 3 to accommodate the two header JLabels 44 // and balance the columns 45 p.setLayout( 46 new GridLayout( ( w.length + 3 ) / 2, 2 ) ); 47 p.add( new JLabel( headerImage ) ); // header 1 48 p.add( new JLabel( headerImage ) ); // header 2 49 50 for ( int i = 0; i < w.length; i++ ) { 51 w[ i ] = new WeatherItem( weatherInfo[ i ] ); 52 p.add( w[ i ] ); 53 } 54 55 getContentPane().add( new JScrollPane( p ), 56 BorderLayout.CENTER ); 57 } 58 catch ( java.rmi.ConnectException ce ) { 59 System.err.println( "Connection to server failed. " + 60 "Server may be temporarily unavailable." ); 61 } 32 33 // get weather information from server Call like regular method.

38. 3. main 3.1 args[ 0 ] 67 68 public static void main( String args[] ) 69 { 70 TemperatureClient gt = null; 71 72 // if no sever IP address or host name specified, 73 // use "localhost"; otherwise use specified host 74 if ( args.length == 0 ) 75 gt = new TemperatureClient( "localhost" ); 76 else 77 77 gt = new TemperatureClient( args[ 0 ] ); 78 79 gt.addWindowListener( 80 new WindowAdapter() { 81 public void windowClosing( WindowEvent e ) 82 { 83 System.exit( 0 ); 84 } 85 } 86 ); 87 } 88} 62 catch ( Exception e ) { 63 e.printStackTrace(); 64 System.exit( 1 ); 65 } 66 } args[ 0 ] is the first argument, which should be the IP address.

39. 1. Class WeatherItem 1.1 static variables 1.2 Initializer block 1.3 Load ImageIcon s 1// Fig. 20.5: WeatherItem.java 2// WeatherItem definition 3import java.awt.*; 4import javax.swing.*; 5 6public class WeatherItem extends JLabel { 7 private static ImageIcon weatherImages[], backgroundImage; 8 private final static String weatherConditions[] = 9 { "SUNNY", "PTCLDY", "CLOUDY", "MOCLDY", "TSTRMS", 10 "RAIN", "SNOW", "VRYHOT", "FAIR", "RNSNOW", 11 "SHWRS", "WINDY", "NOINFO", "MISG" }; 12 private final static String weatherImageNames[] = 13 { "sunny", "pcloudy", "mcloudy", "mcloudy", "rain", 14 "rain", "snow", "vryhot", "fair", "rnsnow", 15 "showers", "windy", "noinfo", "noinfo" }; 16 17 // static initializer block to load weather images 18 static { 19 backgroundImage = new ImageIcon( "images/back.jpg" ); 20 weatherImages = 21 new ImageIcon[ weatherImageNames.length ]; 22 23 for ( int i = 0; i < weatherImageNames.length; ++i ) 24 weatherImages[ i ] = new ImageIcon( 25 25 "images/" + weatherImageNames[ i ] + ".jpg" ); 26 } 27 28 // instance variables 29 private ImageIcon weather; 30 private WeatherInfo weatherInfo; Use names in weatherImageNames array to load ImageIcon s.

40. 2. Constructor 2.1 Compare conditions 3. paintComponent 3.1 paintIcon 34 weather = null; 35 weatherInfo = w; 36 37 // locate image for city's weather condition 38 for ( int i = 0; i < weatherConditions.length; ++i ) 39 if ( weatherConditions[ i ].equals( 40 40 weatherInfo.getDescription().trim() ) ) { 41 weather = weatherImages[ i ]; 42 break; 43 } 44 45 // pick the "no info" image if either there is no 46 // weather info or no image for the current 47 // weather condition 48 if ( weather == null ) { 49 weather = weatherImages[ weatherImages.length - 1 ]; 50 System.err.println( "No info for: " + 51 weatherInfo.getDescription() ); 52 } 53 } 54 55 public void paintComponent( Graphics g ) 56 { 57 super.paintComponent( g ); 58 58 backgroundImage.paintIcon( this, g, 0, 0 ); 59 31 32 public WeatherItem( WeatherInfo w ) 33 { Loop though weatherConditions and compare to getDescription. Attach background to WeatherItem.

41. 3.2 drawString 3.3 paintIcon 67 } 68 69 // make WeatherItem's preferred size the width and height of 70 // the background image 71 public Dimension getPreferredSize() 72 { 73 return new Dimension( backgroundImage.getIconWidth(), 74 backgroundImage.getIconHeight() ); 75 } 76} 60 Font f = new Font( "SansSerif", Font.BOLD, 12 ); 61 g.setFont( f ); 62 g.setColor( Color.white ); 63 63 g.drawString( weatherInfo.getCityName(), 10, 19 ); 64 g.drawString( weatherInfo.getTemperature(), 130, 19 ); 65 66 weather.paintIcon( this, g, 253, 1 ); Draw city name, high/low, and attach weather image to WeatherItem.

42. Compile and Execute the Server and the Client Build and execute application –All pieces in place –Compile classes with javac –Remote server class ( TemperatureServerImpl ) compiled with rmic compiler Makes a stub class - allows client to access remote methods and server to provide its services Gets remote method calls, passes to RMI system, which performs networking rmic TemperatureServerImpl

43. Compile and Execute the Server and the Client Start rmiregistry –Type rmiregistry at command window No text in response

44. Compile and Execute the Server and the Client Must bind remote server object –Run TemperatureServerImpl application java TemperatureServerImpl –Superclass UnicastRemoteObject Constructor exports remote object main binds object to rmiregistry rmiregistry provides host and port number to clients

45. Compile and Execute the Server and the Client Execute TemperatureClient –java TemperatureClient –If server on different machine, specify IP on command line java TemperatureClient 192.168.150.4 –Result on next slide

46. Program Output