Remote Method Invocation (RMI)

Remote Method Invocation (RMI)

Client-Server Communication

• Sockets• Remote Procedure Calls• Remote Method Invocation (Java)

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.

Socket Communication

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)

Execution of RPC

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.

Marshalling Parameters

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.

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

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

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



• 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

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

Defining the Remote Interface

• Interface TemperatureServer– Extends Remote– Describes method getWeatherInfo

1. import

1.1 extends Remote

2. getWeatherInfo

2.1 throws RemoteException

1 // Fig. 20.1: TemperatureServer.java

2 // TemperatureServer interface definition

33 import java.rmi.*;

4

5 public interface TemperatureServer extends Remote {

6 public WeatherInfo[] getWeatherInfo()

77 throws RemoteException;

8 }

Interface Remote in java.rmi

Methods in Remote interface (is a relationship) must be able to throw a RemoteException.

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


– 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 RemoteExceptions

– URL object• Contains URL for Traveler's Forecast web page

• Throws MalformedURLException

18 public 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" );


– 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() ) );


– 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 = "</PRE><HR> <BR><PRE>";

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


– 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


– 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";


– 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();


1. Interface------------------

1. extends UnicastRemote Object, implements TemperatureServer

1.1 Constructor

1 // Fig. 20.1: TemperatureServer.java

2 // TemperatureServer interface definition


4

5 public interface TemperatureServer extends Remote {


7 throws RemoteException;

8 }

9 TemperatureServer interface.

10 // Fig. 20.2: TemperatureServerImpl.java

11 // TemperatureServerImpl definition


13 import java.rmi.server.*;

14 import java.util.*;

15 import java.io.*;

16 import java.net.*;

17

1818 public class TemperatureServerImpl extends UnicastRemoteObject

19 implements TemperatureServer {

20 private WeatherInfo weatherInformation[];

21

2222 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.

2. updateWeather Conditions

2.1 URL

2.2 BufferedReader

2.3 readLine

34 "Updating weather information..." );

35

36 // Traveler's Forecast Web Page

3737 URL url = new URL(

38 "http://iwin.nws.noaa.gov/iwin/us/traveler.html" );

39

4040 BufferedReader in =

41 new BufferedReader(

42 new InputStreamReader( url.openStream() ) );

43

44 String separator = "</PRE><HR> <BR><PRE>";

45

46 // locate first horizontal line on Web page

4747 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.

2.4 Locate header

2.5 Loop

2.5.1 WeatherInfo

2.5.2 readLine

2.6 WeatherInfo array

67

6868 while ( !inputLine.equals( "" ) ) {

69 // create WeatherInfo object for city70 WeatherInfo w = new WeatherInfo(

71 inputLine.substring( 0, 16 ),

72 inputLine.substring( 16, 22 ),73 inputLine.substring( 23, 29 ) );

7475 cityVector.addElement( w ); // add to Vector

76 inputLine = in.readLine(); // get next city's info

77 }

78

79 // create array to return to client

8080 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

6263

64 Vector cityVector = new Vector();6566 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.

2.7 close

3. getWeatherInfo

4. main

4.1 temp

100 // implementation for TemperatureServer interface method


102 {

103103 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();


97 }

98 }

99

Return the WeatherInfo array.

4.2 serverObjectName

4.3 rebind

115 // bind TemperatureServerImpl object to the rmiregistry


117117 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.

1. Class WeatherInfo implements Serializable

1. Instance variables

1.1 Constructor

2. Get methods

1 / Fig. 20.3: WeatherInfo.java

2 // WeatherInfo class definition


4 import java.io.Serializable;

5

66 public 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.

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)

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();

Define the client

– Add WeatherItems• 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 else77 gt = new TemperatureClient( args[ 0 ] );

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 }

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 )

1. import

1.1 Constructor

2. getRemoteTemp

2.1 serverObjectName

2.2 Naming.lookup

1 // Fig. 20.4: TemperatureClient.java 2 // TemperatureClient definition3 import java.awt.*;4 import java.awt.event.*;5 import javax.swing.*;6 import java.rmi.*; 78 public class TemperatureClient extends JFrame9 {10 public TemperatureClient( String ip ) 11 {12 super( "RMI TemperatureClient..." ); 13 getRemoteTemp( ip );14 15 setSize( 625, 567 );16 setResizable( false );17 show();18 }1920 // obtain weather information from TemperatureServerImpl21 // remote object22 private void getRemoteTemp( String ip )23 { 24 try {25 // name of remote server object bound to rmi registry2626 String serverObjectName = "//" + ip + "/TempServer"; 2728 // lookup TemperatureServerImpl remote object29 // in rmiregistry3030 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.

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" );3940 JPanel p = new JPanel();4142 // determine number of rows for the GridLayout;43 // add 3 to accommodate the two header JLabels44 // and balance the columns45 p.setLayout(46 new GridLayout( ( w.length + 3 ) / 2, 2 ) );47 p.add( new JLabel( headerImage ) ); // header 148 p.add( new JLabel( headerImage ) ); // header 24950 for ( int i = 0; i < w.length; i++ ) { 51 w[ i ] = new WeatherItem( weatherInfo[ i ] );52 p.add( w[ i ] ); 53 }5455 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.

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

7777 gt = new TemperatureClient( args[ 0 ] );

78

79 gt.addWindowListener(

80 new WindowAdapter() {

81 public void windowClosing( WindowEvent e )

82 {


84 }

85 }

86 );

87 }

88 }

62 catch ( Exception e ) {

63 e.printStackTrace();


65 }

66 }

args[ 0 ] is the first argument, which should be the IP address.

1. Class WeatherItem

1.1 static variables

1.2 Initializer block

1.3 Load ImageIcons

1 // Fig. 20.5: WeatherItem.java2 // WeatherItem definition3 import java.awt.*;4 import javax.swing.*;56 public 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" };1617 // static initializer block to load weather images18 static {19 backgroundImage = new ImageIcon( "images/back.jpg" );20 weatherImages =21 new ImageIcon[ weatherImageNames.length ];2223 for ( int i = 0; i < weatherImageNames.length; ++i )24 weatherImages[ i ] = new ImageIcon(

2525 "images/" + weatherImageNames[ i ] + ".jpg" );26 }27 28 // instance variables29 private ImageIcon weather;30 private WeatherInfo weatherInfo;

Use names in weatherImageNames array to load ImageIcons.

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(

4040 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 );

5858 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.

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 );

6363 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.

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


• Start rmiregistry– Type rmiregistry at command window

• No text in response


• 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


• Execute TemperatureClient– java TemperatureClient

– If server on different machine, specify IP on command line

java TemperatureClient 192.168.150.4– Result on next slide

Program Output

Documents

Remote Method Invocation (RMI)