How to use PropertyChangeSupport and PropertyChangeListener?

Question

@Controller
@RequestMapping("/scrape")
public class ScrapeController {
    private final PropertyChangeSupport pcs = new PropertyChangeSupport(this);

    public void addPropertyChangeListener(PropertyChangeListener listener) {
        this.pcs.addPropertyChangeListener(listener);
    }

    public void removePropertyChangeListener(PropertyChangeListener listener) {
        this.pcs.removePropertyChangeListener(listener);
    }

    @GetMapping({"/", ""})
    public String scrape(){

        this.pcs.firePropertyChange("value", "wait", "scrape"); // Breakpoint.
        return "done";
    }
}


public class ScrapeService implements PropertyChangeListener {

    private final ScrapeController sc;

    public ScrapeService(ScrapeController sc) {
        this.sc = sc;
        sc.addPropertyChangeListener(this);
    }

    public void propertyChange(PropertyChangeEvent evt) {
        System.out.println("Name      = " + evt.getPropertyName()); // Breakpoint.
        System.out.println("Old Value = " + evt.getOldValue());
        System.out.println("New Value = " + evt.getNewValue());
    }
}

This project is based on Spring. But I don't believe that the problem is Spring related.

There are two breakpoints here. The debugger stops at the first breakpoint in the scrape method where I fire the property change. But the debugger fails to stop at the breakpoint in the propertyChange method. So the debug messages are not being printed.

Answer 1

My guess is that the ScrapeService is never actually instantiated and added to the ScrapeController 's addPropertyChangeListener method.