Threads - Java Network Programming - page 73

was that several threads had access to the same PrintStream object, System.out . In this
case, it was a static class variable that led to the conflict. However, instance variables can
also have problems.
For example, suppose your web server keeps a logfile. The logfile may be represented
by a class like the one shown in Example 3-11 . This class itself doesn't use multiple
threads. However, if the web server uses multiple threads to handle incoming connec‐
tions, then each of those threads will need access to the same logfile and consequently
to the same LogFile object.
Example 3-11. LogFile
import java.io.* ;
import java.util.* ;
public class LogFile {
private Writer out ;
public LogFile ( File f ) throws IOException {
FileWriter fw = new FileWriter ( f );
this . out = new BufferedWriter ( fw );
}
public void writeEntry ( String message ) throws IOException {
Date d = new Date ();
out . write ( d . toString ());
out . write ( '\t' );
out . write ( message );
out . write ( "\r\n" );
}
public void close () throws IOException {
out . flush ();
out . close ();
}
}
In this class, the writeEntry() method finds the current date and time, then writes into
the underlying file using four separate invocations of out.write() . A problem occurs
if two or more threads each have a reference to the same LogFile object and one of
those threads interrupts another in the process of writing the data. One thread may
write the date and a tab, then the next thread might write three complete entries; then,
the first thread could write the message, a carriage return, and a linefeed. The solution,
once again, is synchronization. However, here there are two good choices for which
object to synchronize on. The first choice is to synchronize on the Writer object out .
For example: