Threaded Java - Java - page 879

To demonstrate ReadWriteLock in action, I wrote the business logic portion of a web-based
voting application. It could be used in voting for candidates or for the more common web
poll. Presuming that you display the results on the home page and change the data only when
somebody takes the time to click a response to vote, this application fits one of the intended
criteria for ReadWriteLock —i.e., that you have more readers than writers. The main class,
ReadersWritersDemo , is shown in Example 22-11 . The helper class BallotBox is online; it
simply keeps track of the votes and returns a read-only Iterator upon request. Note that in
the run( ) method of the reading threads, you could obtain the iterator while holding the
lock but release the lock before printing it; this allows greater concurrency and better per-
formance, but could (depending on your application) require additional locking against con-
current update.

Example 22-11. ReadersWriterDemo.java
public public class class ReadersWriterDemo ReadersWriterDemo {

private private static static final final int int NUM_READER_THREADS = 3 ;

public public static static void void main ( String [] args ) {

new new ReadersWriterDemo (). demo ();

}

/** Set this to true to end the program */
private

private volatile volatile boolean boolean done = false false ;

/** The data being protected. */
private private BallotBox theData ;

/** The read lock / write lock combination */
private private ReadWriteLock lock = new new ReentrantReadWriteLock ();

/**
* Constructor: set up some quasi-random initial data
*/
public

public ReadersWriterDemo () {
List < String > questionsList = new new ArrayList <>();

questionsList . add ( "Agree" );
questionsList . add ( "Disagree" );
questionsList . add ( "No opinion" );
theData = new

new BallotBox ( questionsList );

}

/**
* Run a demo with more readers than writers
*/