Balking pattern

<h2 id="usage">Usage</h2>
<p>Objects that use this pattern are generally only in a state that is prone to balking temporarily but for an unknown amount of time.  If objects are to remain in a state which is prone to balking for a known, finite period of time, then the <a href="/facts/Guarded_suspension_pattern/d1LuBUso">guarded suspension pattern</a> may be preferred.
</p>
<h2 id="implementation">Implementation</h2>
<p>Below is a general, simple example for an implementation of the balking pattern.<a class="footnote-ref" id="fnref:1" href="#fn:1"><sup>1</sup></a> As demonstrated by the definition above, notice how the "synchronized" line is utilized. If there are multiple calls to the job method, only one will proceed while the other calls will return with nothing. Another thing to note is the jobCompleted() method. The reason it is synchronized is because the only way to guarantee another thread will see a change to a field is to synchronize all access to it. Actually, since it is a boolean variable, it could be left not explicitly synchronized, only declared volatile - to guarantee that the other thread will not read an obsolete cached value.
</p>
public class Example {
    private boolean jobInProgress = false;

public void job() {
        synchronized(this) {
            if (jobInProgress) {
                return;
            }
            jobInProgress = true;
        }
        // Code to execute job goes here
        // ...
        jobCompleted();
    }

void jobCompleted() {
        synchronized(this) {
            jobInProgress = false;
        }
    }
}

<h2 id="see-also">See also</h2>
<ul><li><a href="/facts/Read_and_write_lock_pattern/1zXdg5a7">Read and write lock pattern</a></li>
<li><a href="/facts/Guarded_suspension/d1LuBUso">Guarded suspension pattern</a></li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1"><p>Grand, Mark (2002). Patterns in Java, Volume 1: A Catalog of Reusable Design Patterns Illustrated with UML, Second Edition. Indianapolis, Ind: John Wiley & Sons. <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li>
</ol>

Balking pattern open-in-new

Balking pattern