⟩ Explain A Time Server?
Sending binary, nontext data is not significantly harder. Example demonstrates with a time server. This follows the time protocol outlined in RFC 868. When a client connects, the server sends a 4-byte, big-endian, unsigned integer specifying the number of seconds that have passed since 12:00 A.M., January 1, 1900 GMT (the epoch). The current time can be retrieved simply by creating a new Date object. However, since the Date class counts milliseconds since 12:00 A.M., January 1, 1970 GMT rather than seconds since 12:00 A.M., January 1, 1900 GMT, some conversion is necessary.
Example: A Time Server
import java.net.*;
import java.io.*;
import java.util.Date;
public class TimeServer {
public final static int DEFAULT_PORT = 37;
public static void main(String[] args) {
int port = DEFAULT_PORT;
if (args.length > 0) {
try {
port = Integer.parseInt(args[0]);
if (port < 0 || port >= 65536) {
System.out.println
("Port must between 0 and 65535");
return;
}
}
catch (NumberFormatException e) {}
}
// The time protocol sets the epoch at 1900,
// the java Date class at 1970. This number
// converts between them.
long differenceBetweenEpochs = 2208988800L;
try {
ServerSocket server = new ServerSocket(port);
while (true) {
Socket connection = null;
try {
OutputStream out = connection.getOutputStream( );
Date now = new Date( );
long msSince1970 = now.getTime( );
long secondsSince1970 = msSince1970/1000;
long secondsSince1900 = secondsSince1970
+ differenceBetweenEpochs;
byte[] time = new byte[4];
time[0]
= (byte) ((secondsSince1900 & 0x00000000FF000000L)
>> 24); time[1]
= (byte) ((secondsSince1900 & 0x0000000000FF0000L)
>> 16); time[2]
= (byte) ((secondsSince1900 & 0x000000000000FF00L)
>> 8); time[3]
= (byte) (secondsSince1900 & 0x00000000000000FFL);
out.write(time);
out.flush( );
} // end try
catch (IOException e) {
} // end catch
finally {
if (connection != null) connection.close( );
}
} // end while
} // end try
catch (IOException e) {
System.err.println(e);
} // end catch
} // end main
} // end TimeServer
As with the TimeClient of the previous chapter, most of the effort here goes into working with a data format (32-bit unsigned integers) that Java doesn't natively support.
public void close( ) throws IOException
