Re: Two more multithreading questions

"Daniel Pitts" <>
30 Jan 2007 13:54:11 -0800
On Jan 30, 10:55 am, Knute Johnson <>

Daniel Pitts wrote:

On Jan 30, 9:44 am, Knute Johnson <>

Patricia Shanahan wrote:

Knute Johnson wrote:

I've got two specific scenarios I want to ask about:
1) I have a class with an instance variable that is a reference to a
JDialog. In one thread I create new instances of JDialog and make
them visible. They might get closed in this thread as well. In
another thread I close the JDialog using the class instance variable.
To ensure that my dialog closing thread always has a reference to the
current dialog I created the instance variable with volatile. Is this
adequate to guarantee that my closing thread always has a reference to
the latest dialog?

I believe most javax.swing component access is supposed to be done in
the event handling thread anyway. Swing was not designed to be

Sorry, bad example. Say it is an Integer that is being created in one
thread and in the other you are using the intValue() method.



Knute Johnson
email s/nospam/knute/

Integer is a bad example too, since it is immutable, which means onces
its created, its value doesn't change.
A good example might be a File object.

Thread W can alter the file object, and Thread R can query it.
The safest way to insure that your Thread R only sees what its
supposed to is to wrap bother the object modifying and object querying
code in synchronize blocks that sync on the same object O. That
object O can be ANY object.

public class ThreadSafeFileAccessor {
   private final Object sync = new Object();
   private File file = new File();
   public void modifyFile() {
      synchronize(sync) {
         // do modification of file

  public String queryFile() {
    synchronize(sync) {
      return file.toString();

Also be aware that Java 1.5 includes a new locking mechanism and other
concurrency utilities which gives you more fine grained control over
thread synchronization.

Good luck :-)


Thank you very much for your response but it doesn't really answer my
question. As to the immutable, I'm not muting. So let me set the
scenario again.

Class with instance variable that is reference to Integer. One thread
makes new Integers and assigns them to the instance variable. The other
thread calls some method on the Integer. I want to know if making the
variable volatile will guarantee that the second thread always sees the
latest integer created by the first thread.

I know that I can wrap both pieces of code in a synchronized block but I
want to understand my question.

Thanks very much,


Knute Johnson
email s/nospam/knute/

Ah, you didn't ask about volatile before. Yes, volatile is supposed
to make it so that one thread can see the primative data written by
another in a thread safe manor. A reference to an object is a
primative for this argument.

So, if all you care about is the reference, then you don't need
synchronization. Although, you still might be better off looking into
java.util.concurrent.AtomicInteger or

Hope this answers your question a little better.

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