Re: Peterson's Algorithm in java, sequencial instruction execution ?

"Mike Schilling" <mscottschilling@hotmail.com> wrote or quoted in
Message-ID: <Bxoah.15796$Sw1.9105@newssvr13.news.prodigy.com>:


As far as I know, PhantomReference can be used for cleanup.

For example ..

<code_0>

//
// Suppose that Resource R must be cleaned up after used.
//

class Resource_Bank {

    Resource loan() {
        ...
        return R;
    }

    void refund(Resource R) {
        // Clean up R
    }
}

class Customer {

    void X...(..) {
        R = resource_Bank.loan();
        ...
    }

    void Y..(..) {
        ...
        resource_Bank.refund( R );
    }
}
</code_0>

For some reason, if Customer lose the reference of R before
she refunds R, Resource leak will occur.

To prevent this situation, the following code is possible:

<code_1>

class Resource_Wrap {
    Resource R = ....;
    ...
    protected void finalize() ... {
        // Clean up R
    }
}
</code_1>

But ...
If the cleanup process of R take long time, GC may exert
a harmful influence upon the performance of App.

Therefore, the following code will be better:

<code_2>

//
// Data structure, thread-safe etc were ignored to simplify
// this "code_2".
//

class Resource_Wrap {
    Resource R = ....;
    ...
    Resource getR() {
        return R;
    }
}

class Phantom extends PhantomReference<Resource_Wrap> {

    Resource R;

    public Phantom(Resource_Wrap RW, ReferenceQueue<...> rq) {

        super(RW, rq);
        R = RW.getR();
    }

    Resource getR() {
        return R;
    }
}

class Resource_Bank {

    // Resource R0, R1, ... , Rn

    ReferenceQueue<Resource_Wrap> rq = ...;
    final int MAX_NUMBER = ...;
    Resource_Wrap[] rw = ...;
    Phantom[] pm = ...;

    void initialize() {

        for (int i = 0; i < MAX_NUMBER; ..) {
             rw[i] = new Resource_Wrap( ... ); // Ri
             pm[i] = new Phantom( rw[i], rq );
        }
    }

    Resource_Wrap loan() {

       for ( ... ) {
           if ( .. ) {
                Resource_Wrap RW = rw[i];
                rw[i] = null; // if Customer lose RW,
                                 // she will become phontom.
                return RW;
           }
       }
       ....
    }

    void refund(Resource_Wrap RW) {
        // Clean up R
    }

    void cleanupDaemon() {

        Thread tr = new Thread() {
             public void run() {
                 while (true) {
                     Phantom RW = (Phantom) rq.remove();
                     Resource R = RW.getR();
                     ... // Clean up R
                     ... // Clean up RW
                  }
             }
        };
        .....
    }
}
</code_2>

In addition to the above case,
I think that there will be other cases.