This lecture gives a quick introduction to RMI (Remote Method Invocation) and JDBC (Java Database Connectivity).

Agenda

• Introduce RMI
• Talk about stubs and skeletons
• Walk you through an RMI example
• Introduce JDBC
• Walk you through a JDBC example

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RMI

• RMI (Remote Method Invocation) extends Java's object model across JVM boundaries.
• RMI allows you to invoke a method on an object in another JVM instance, located either locally or across the network.
• Can pass data (including objects) as arguments to the remote object.
• Can receive primitive types or an object as a return value.
• RMI is a Java to Java solution to distributed computing.

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Stubs and Skeletons

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RMI Steps

• The interface (Summer) must extend java.rmi.Remote.
• The class that implements the service (SummerServer) must extend java.rmi.server.UnicastRemoteObject.
• You generate the stub and skeleton classes with rmic:

  javac SummerServer
  

  Gives you SummerServer.class

  rmic SummerServer
  

  Gives you SummerServer_Stub.class and SummerServer_Skel.class

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The Summer Server Revisited

// In file rmi/ex1/Summer.java
import java.rmi.*;
public interface Summer extends Remote {
    static class InvalidLongException
        extends Exception {
        private String message;
        InvalidLongException(String s) {
            message = s;
        }
        public String getMessage() {
            return message;
        }
    }
    long sumString(String s)
        throws InvalidLongException,
        RemoteException;
}
// In file rmi/ex1/SummerServer.java
import java.rmi.*;
import java.rmi.server.*;
import java.rmi.registry.*;
import java.net.*;
import java.io.*;
import java.util.StringTokenizer;
public class SummerServer
        extends UnicastRemoteObject
        implements Summer {
    public static final int PORT = 1099;
    public SummerServer() throws RemoteException {
    }
    public static void main(String[] args) {

        System.setSecurityManager(
            new RMISecurityManager());
        try {
            LocateRegistry.createRegistry(PORT);
            SummerServer summer = new SummerServer();
            System.out.println("Ready to rebind");
            Naming.rebind("SUMMER", summer);
            System.out.println("Ready to sum");
        }
        catch(Exception e) {
            e.printStackTrace();
        }
    }
    public long sumString(String s)
        throws Summer.InvalidLongException, RemoteException {
        long sum = 0;
        StringTokenizer st = new StringTokenizer(s);
        String token;
        while (st.hasMoreTokens()) {
            token = st.nextToken();
            try {
                sum += Long.parseLong(token);
            }
            catch (NumberFormatException e) {
                throw new Summer.InvalidLongException(
                    "Invalid number: " + token);
            }
        }

        return sum;
    }
}
// In file rmi/ex1/SummerClient.java
import java.rmi.*;
import java.rmi.registry.*;
import java.net.*;
import java.io.*;
public class SummerClient {
    public static void main(String[] args) {
        System.setSecurityManager(
            new RMISecurityManager());
        try {
            Summer summer = (Summer) Naming.lookup(
                "rmi://localhost:1099/SUMMER");
            LineNumberReader stdinReader =
                new LineNumberReader(new BufferedReader(
                new InputStreamReader(System.in)));
            for (;;) {
                String userLine = stdinReader.readLine();
                if (userLine == null
                    || userLine.length() == 0) {
                    break;
                }
                String outString;
                try {
                    long sum = summer.sumString(userLine);
                    outString = Long.toString(sum);
                }
                catch(Summer.InvalidLongException e) {
                    outString = e.getMessage();
                }
                System.out.println(outString);
            }
        }
        catch (Exception e) {
            e.printStackTrace();
        }
    }
}

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JDBC

• Standardized SQL lets you talk to databases from different vendors in a uniform way. 
• ODBC defined a standard C interface for talking to databases with SQL. 
• The JDBC (Java Database Connectivity) library (java.sql) allows you to use SQL to talk to databases from a Java program. 
• The java.sql calls go through a JDBC driver. 
• JDK comes with a JDBC to ODBC bridge. 
• DB vendors are creating native JDBC drivers; JDBC drivers that talk over a network also exist. 

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JDBC Steps

• On your system or network:
  • First you need a database. 
  • Set up the system so that you can access the database via a JDBC driver. 
• In your Java program: 
  • Load the JDBC driver class, which will register itself.
  • Connect to the datasource through a "database URL" of the form jdbc.subprotocol.name
  • Execute SQL statements.
  • Close the connection.

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The Student Database

• In the upcoming example, I use:  
  • The ODBC text driver that comes with Microsoft Office
  • the JDBC to ODBC bridge that comes with the JDK
• Here's the database, stored in file students.csv: 

  LAST,FIRST,HW1,HW2,HW3,HW4,TOTAL
  Busy,Lizzy,50,50,50,50,200
  Java,Jane,100,100,100,100,400
  Java,Joe,100,100,100,100,400
  Slow,Joe,0,0,0,0,0
  Smart,Bart,90,90,90,90,360
  

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The Student Sorter Application

// In file jdbc/ex1/StudentSorter.java
import java.sql.*;
public class StudentSorter {
    public static void main(String[] args) {
        try {
            Class.forName(
                "sun.jdbc.odbc.JdbcOdbcDriver");
            Connection conn = DriverManager.getConnection(
                "jdbc:odbc:mydb", "", "");
            Statement stmt = conn.createStatement();
            ResultSet rs = stmt.executeQuery(
                "SELECT LAST, FIRST, TOTAL " +
                "FROM students.csv mydb " +
                "ORDER BY TOTAL DESC");
            while (rs.next()) {
                String s = rs.getString("FIRST") + " "
                    + rs.getString("LAST") + ": "
                    + rs.getString("TOTAL");
                System.out.println(s);
            }
            stmt.close();
        }
        catch(Exception e) {
            e.printStackTrace();
        }
    }
}

Here's the the output of StudentSorter:

Joe Java: 400
Jane Java: 400
Bart Smart: 360
Lizzy Busy: 200
Joe Slow: 0