Java ProgrammingThe Language of Now & the Future

Clients and Serversincluding Internet connectivity,

I/O and JDBC

John Morris and Peter JonesDepartment of Electrical and Electronic Engineering,

University of Western Australia

JohnMorris

PeterJones

Sockets

• Sockets are the “endpoints” of Internet connections

• Socket constructors require Internet addresses• Socket( String host, int port ) throws UnknownHostException, IOException;

• Socket(InetAddress address, int port) throws IOException;

... and a host of other alternatives, see the API documentation!

• Note: these constructors throw exceptions• Use them in a try { } catch .. block

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Internet

Sockets

• Note: these constructors throw exceptions• Use them in a try { } catch .. Block

• Note: UnknownHostException is a subclass of IOException

catch( IOException e ) alone would suffice

String name = new String(“ciips.ee.uwa.edu.au”);try { Socket t = new Socket( name, 80 ); ... }catch( UnknownHostException e ) { System.out.println(“Unknown host:” + name ); }catch( IOException e ) { ...

}

Aside: Datagrams, ...• Use a DatagramSocket

• Constructor DatagramSocket( int port ) throws IOException;

creates a datagram socket on port • Methods

void receive( DatagramPacket p ) throws IOException;

void send( DatagramPacket p ) throws IOException;• DatagramPacket

• Constructor DatagramPacket( byte[] buf, int length, InetAddress add, int port );

creates a packet to be sent to port at add• Methods

• byte[] getData();• void setData( byte[] buf );• InetAddress getAddress();

Datagrams, ...• Use a DatagramSocket

• Constructor DatagramSocket( int port ) throws IOException;

creates a datagram socket on port • Methods

void receive( DatagramPacket p ) throws IOException;

void send( DatagramPacket p ) throws IOException;• DatagramPacket

• Constructor DatagramPacket( byte[] buf, int length, InetAddress add, int port );

creates a packet to be sent to port at add• Methods

• byte[] getData();• void setData( byte[] buf );• InetAddress getAddress();

Note: byte[]not char[]... more later!

Multicast sockets ...

• Messages to multiple hosts• Use a MulticastSocket• ConstructorMulticastSocket( int port ) throws IOException;creates a datagram socket bound to port

• Methods• void receive( DatagramPacket p ) throws IOException;• void send( DatagramPacket p ) throws IOException;• void joinGroup( InetAddress mcastaddr ) throws IOException;

• void leaveGroup( InetAddress mcastaddr ) throws IOException;

• Hosts can join and leave multicast groups• Security feature:

• An applet is not allowed to use a multicast socket

Inherited fromDatagramSocket

static final int lut_port = 3062;String name = new String(“ciips.ee.uwa.edu.au”);static final String query = “?”;try { Socket server = new Socket( name, lut_port ); DataInputStream input = new DataInputStream( server.getInputStream() ); PrintStream output = new PrintStream( server.getOutputStream() ); do { output.println( query ); String resp = input.readLine(); System.out.println( name + “ response: “ + resp ); } while ( !( resp.equal( quit_string ) ) ); server.close(); }catch( UnknownHostException e ) { System.out.println(“Unknown host:” + name ); }catch( IOException e ) { ...

}

Talking to a server

static final int lut_port = 3062;String name = new String(“ciips.ee.uwa.edu.au”);static final String query = “?”;try { Socket server = new Socket( name, lut_port ); DataInputStream input = new DataInputStream( server.getInputStream() ); PrintStream output = new PrintStream( server.getOutputStream() ); do { output.println( query ); String resp = input.readLine(); System.out.println( name + “ response: “ + resp ); } while ( !( resp.equal( quit_string ) ) ); server.close(); }catch( UnknownHostException e ) { System.out.println(“Unknown host:” + name ); }catch( IOException e ) { ...

}

Talking to a server Socket (and almost all the rest) throws an exception, so put it all in a try { } block

Once the Socket has been created,create input and outputstreams on it

DataInputStreamDataOutputStreamPrintStream

Just part of the streamzoo - there are 55 more!

static final int lut_port = 3062;String name = new String(“ciips.ee.uwa.edu.au”);static final String query = “?”;try { Socket server = new Socket( name, lut_port ); DataInputStream input = new DataInputStream( server.getInputStream() ); PrintStream output = new PrintStream( server.getOutputStream() ); do { output.println( query ); String resp = input.readLine(); System.out.println( name + “ response: “ + resp ); } while ( !( resp.equal( quit_string ) ) ); server.close(); }catch( UnknownHostException e ) { System.out.println(“Unknown host:” + name ); }catch( IOException e ) { ...

}

Talking to a server

Now use methods on thestreams just like any otherI/O device println on PrintStreamreadLine on DataInputStream

static final int lut_port = 3062;String name = new String(“ciips.ee.uwa.edu.au”);static final String query = “?”;try { Socket server = new Socket( name, lut_port ); DataInputStream input = new DataInputStream( server.getInputStream() ); PrintStream output = new PrintStream( server.getOutputStream() ); do { output.println( query ); String resp = input.readLine(); System.out.println( name + “ response: “ + resp ); } while ( !( resp.equal( quit_string ) ) ); server.close(); }catch( UnknownHostException e ) { System.out.println(“Unknown host:” + name ); }catch( IOException e ) { ...

}

Talking to a serverFinally add some code to handle exceptions!Choices (Java usually gives you plenty ) Separate catch blocks for different types One “catch all” (IOException) try { } catch { } for each method call Throw the exception from here

static final int lut_port = 3062;String name = new String(“ciips.ee.uwa.edu.au”);static final String query = “?”;try { Socket server = new Socket( name, lut_port ); }catch( UnknownHostException e ) { System.out.println(“Unknown host:” + name ); return error_flag; }catch( IOException e ) { /* process other errors .... */ return error_flag; }

try { DataInputStream input = new DataInputStream( server.getInputStream() ); PrintStream output = new PrintStream( server.getOutputStream() ); }catch( IOException e ) { System.out.println(“Cant form streams”); return error_flag;

} do { /* do some real work ... */

Talking to a server - variant 1Exception Choices (Java usually gives you plenty ) try { } catch { } for each method call Throw the exception from here

try { } catch { } around each exception throwing call

public String ServerResp( String query ) throws IOException { static final int lut_port = 3062; String name = new String(“ciips.ee.uwa.edu.au”); Socket server = new Socket( name, lut_port ); DataInputStream input = new DataInputStream( server.getInputStream() ); PrintStream output = new PrintStream( server.getOutputStream() ); output.println( query ); String resp = input.readLine(); server.close(); return resp; }

Talking to a server - variant 2

Exception Choices (Java usually gives you plenty ) try { } catch { } for each method call Throw the exception from here This method

throws any exceptions up a level

static final int lut_port = 3062;static final int queue_len = 10;try { Socket server = new ServerSocket( lut_port, queue_len ); }catch( IOException e ) { System.out.println(“Can’t establish server on port “ + lut_port ); }while( true ) { // The server never dies! // Wait for a connection attempt try { Socket s = server.accept(); PrintStream ps = new PrintStream( s.getOutputStream() ); DataInputStream in = new DataInputStream( s.getInputStream() ); while( true ) { String q = in.readLine(); if ( q.equal( “?” ) ) ps.println( “Hello” ); else if ( q.equal( “quit” ) ) break; else ps.println( “??” ); } s.close(); // Close this connection } catch ( IOException e ) { /* Do something about it! */ } }

Simple Server

static final int lut_port = 3062;static final int queue_len = 10;try { Socket server = new ServerSocket( lut_port, queue_len ); }catch( IOException e ) { System.out.println(“Can’t establish server on port “ + lut_port ); }while( true ) { // The server never dies! // Wait for a connection attemtp try { Socket s = server.accept(); PrintStream ps = new PrintStream( s.getOutputStream() ); DataInputStream in = new DataInputStream( s.getInputStream() ); while( true ) { String q = in.readLine(); if ( q.equal( “?” ) ) ps.println( “Hello” ); else if ( q.equal( “quit” ) ) break; else ps.println( “??” ); } s.close(); // Close this connection } catch ( IOException e ) { /* Do something about it! */ } }

Simple ServerSet up a ServerSocket- basically just a socketwith an accept method

Servers usually liveforever!!

The code will blockhere until a

client attempts a connection

static final int lut_port = 3062;static final int queue_len = 10;try { Socket server = new ServerSocket( lut_port, queue_len ); }catch( IOException e ) { System.out.println(“Can’t establish server on port “ + lut_port ); }while( true ) { // The server never dies! // Wait for a connection attemtp try { Socket s = server.accept(); PrintStream ps = new PrintStream( s.getOutputStream() ); DataInputStream in = new DataInputStream( s.getInputStream() ); while( true ) { String q = in.readLine(); if ( q.equal( “?” ) ) ps.println( “Hello” ); else if ( q.equal( “quit” ) ) break; else ps.println( “??” ); } s.close(); // Close this connection } catch ( IOException e ) { /* Do something about it! */ } }

Simple Server

Now we just create the input & output streams read and write to them close the connection when we’re finished

static final int lut_port = 3062;static final int queue_len = 10;try { Socket server = new ServerSocket( lut_port, queue_len ); }catch( IOException e ) { System.out.println(“Can’t establish server on port “ + lut_port ); }while( true ) { // The server never dies! // Wait for a connection attemtp try { Socket s = server.accept(); PrintStream ps = new PrintStream( s.getOutputStream() ); DataInputStream in = new DataInputStream( s.getInputStream() ); while( true ) { String q = in.readLine(); if ( q.equal( “?” ) ) ps.println( “Hello” ); else if ( q.equal( “quit” ) ) break; else ps.println( “??” ); } s.close(); // Close this connection } catch ( IOException e ) { /* Do something about it! */ } }

Simple Server

But ...This server blocks whilst one user is using it!A second client can’t connect ‘til the first one finishes!

Solution ...A thread for each client

static final int lut_port = 3062;static final int queue_len = 10;try { Socket server = new ServerSocket( lut_port, queue_len ); }catch( IOException e ) { System.out.println(“Can’t establish server on port “ + lut_port ); }while( true ) { // The server never dies! // Wait for a connection attempt try { Socket s = server.accept(); // This thread will do the talking ClientHandler ch = new ClientHandler( s ); ch.start(); } catch ( IOException e ) { /* Do something about it! */ } }

Multi-threaded Server

Make this parameter relevant!Number of queued requests

As soon as we get a connection,

spawn a handler thread to talk to the client

Start the thread

class ClientHandler extends Thread { Socket s; public ClientHandler( Socket s ) { this.s = s; } public void run() { PrintStream ps = new PrintStream( s.getOutputStream() ); DataInputStream in = new DataInputStream( s.getInputStream() ); while( true ) { String q = in.readLine(); if ( q.equal( “?” ) ) ps.println( “Hello” ); else if ( q.equal( “quit” ) ) break; else ps.println( “??” ); } s.close(); // Close this connection } }

Multi-threaded Server - The Handler

Same operational code ..Just embedded in the

thread’s run method

Constructor - just save a reference to the

socket

This class is a Thread

I/O Streams - Too much choice?• There’s a bewildering array of choices ... “Stream Zoo”

• Considering input ..• InputStream

ByteArrayInputStream FileInputStream FilterInputStream

BufferedInputStream DataInputStream

DataInput

InflaterInputStream GZIPInputStream ZipInputStream

... 3 more!

PipedInputStream ... 3 more!

Matching Output classes

also!

I/O Streams - Too much choice?• ... and the Reader classes!• Characters (16 bits!) rather than bytes• Reader

BufferedReader LineNumberReader

CharArrayReader FilterReader

PushbackReader InputStreamReader

FileReader PipedReader StringReader

Matching Writer classes

also!

Writing Objects to Streams• ObjectOutputStream• Enables writing of serialized objects• Serializable objects implement the java.io.Serializable interface

• private void writeObject( ObjectOutputStream out ) throws IOException;

• private void readObject( ObjectInputStream in ) throws IOException, ClassNotFoundException;

• ClassNotFoundException?• A remote program could send you an object that you don’t know about ..• .. and therefore don’t know how to de-serialize!

Bytes and characters in Java• Java has

• Primitive types• byte• char

• Classes• Byte• Character

• Class Byte has a single attribute of type byte• It’s just a class “wrapper” for a byte

• Class Character ...• Characters are Unicode characters

• For the full tables, see• ftp://ftp.unicode.org/Public

• Character methods

• isDigit, isLetter, isLowerCase, ...• Full story: Gosling, Joy & Steele, “The Java Language Specification”

Bytes and characters in Java

• Class Character ...• Characters are Unicode characters

• Main trap ...• “char” files created by other programs, eg C, C++• Files created by text editors

• ... but Windows “NotePad” often creates Unicode files for you!

• Suggestion• Use byte / Byte if the file, stream, ... may be

generated/read by any other language

URLs• Not surprisingly, Java provides good support for URLs• URL class

• Constructors• URL( String spec );• URL( String protocol, String host, int port, String file );

• URL( String protocol, String host, String file );

• Methods• Object getContent();

• Retrieve the whole file• URLConnection openConnection();

• Used when URLConnection subclasses, eg HttpURLConnection, JarURLConnection exist

• InputStream openStream();

SQL databases

• JDBC • provides a bridge to an Open Database Connectivity

(ODBC) interface to a specific database

• Getting started• import java.sql.*;• Load the bridge ..

• Class.forName( “sun.jdbc.odbc.JdbcOdbcDriver” );

• Connect to a database• Connection c = DriverManager.getConnection( String URL, String username, String passwd );

• Create an SQL statement• Statement s = c.createStatement();

• Execute a statement• s.execute( String sql_statement );

SQL databases

• JDBC • Results from a query

• ResultSet res = s.executeQuery( String s );

• Goto the first row ...• res.next();

• Extract data• String name = res.getString( String col_name );• int value = res.getInt( String col_name );• ... and, of course, several pages more!

• Examing the table returned• ResultSetMetaData m = res.getMetaData();

• Information about the table• int columns = m.getColumnCount();• String label = m.getColumnLabel( int col_no );

• Columns number from 1!

• String ctype = m.getColumnType( int col_no ); • ... and, of course, 19 more!