Threads - Java Network Programming - page 74

public void writeEntry ( String message ) throws IOException {
synchronized ( out ) {
Date d = new Date ();
out . write ( d . toString ());
out . write ( '\t' );
out . write ( message );
out . write ( "\r\n" );
}
}
This works because all the threads that use this LogFile object also use the same out
object that's part of that LogFile . It doesn't matter that out is private. Although it is used
by the other threads and objects, it's referenced only within the LogFile class. Further‐
more, although you're synchronizing here on the out object, it's the writeEntry()
method that needs to be protected from interruption. The Writer classes all have their
own internal synchronization, which protects one thread from interfering with a
write() method in another thread. (This is not true of input and output streams, with
the exception of PrintStream . It is possible for a write to an output stream to be inter‐
rupted by another thread.) Each Writer class has a lock field that specifies the object
on which writes to that writer synchronize.
The second possibility is to synchronize on the LogFile object itself. This is simple
enough to arrange with the this keyword. For example:
public void writeEntry ( String message ) throws IOException {
synchronized ( this ) {
Date d = new Date ();
out . write ( d . toString ());
out . write ( '\t' );
out . write ( message );
out . write ( "\r\n" );
}
}
Synchronized Methods
Because synchronizing the entire method body on the object itself is such a common
thing to do, Java provides a shortcut. You can synchronize an entire method on the
current object (the this reference) by adding the synchronized modifier to the method
declaration. For example:
public synchronized void writeEntry ( String message ) throws IOException {
Date d = new Date ();
out . write ( d . toString ());
out . write ( '\t' );
out . write ( message );
out . write ( "\r\n" );
}