JFrame观察Controller，但从未收到通知

Question

从另一个问题开始，我提出了一个Singleton Observable。为什么不执行ArticleSelect.update（）？

package net.bounceme.dur.usenet.swing;
import java.util.Observable;
import java.util.Observer;
import java.util.logging.Logger;
import javax.swing.ListModel;
import net.bounceme.dur.usenet.controller.ArticleDefaultListModel;
import net.bounceme.dur.usenet.controller.Controller;
public class ArticleSelect extends javax.swing.JPanel implements Observer {
    private static final Logger LOG = Logger.getLogger(ArticleSelect.class.getName());
    private Controller controller = Controller.getInstance();
    private ListModel defaultListModel = new ArticleDefaultListModel();
    public ArticleSelect() {
        controller.addObserver(this);
        initComponents();
    }
    /**
     * This method is called from within the constructor to initialize the form.
     * WARNING: Do NOT modify this code. The content of this method is always
     * regenerated by the Form Editor.
     */
    @SuppressWarnings("unchecked")
    // <editor-fold defaultstate="collapsed" desc="Generated Code">
    private void initComponents() {
        jScrollPane1 = new javax.swing.JScrollPane();
        jList1 = new javax.swing.JList();
        jScrollPane2 = new javax.swing.JScrollPane();
        jTextPane1 = new javax.swing.JTextPane();
        setLayout(new java.awt.BorderLayout());
        jList1.setModel(defaultListModel);
        jScrollPane1.setViewportView(jList1);
        add(jScrollPane1, java.awt.BorderLayout.WEST);
        jScrollPane2.setViewportView(jTextPane1);
        add(jScrollPane2, java.awt.BorderLayout.CENTER);
    }// </editor-fold>
    // Variables declaration - do not modify
    private javax.swing.JList jList1;
    private javax.swing.JScrollPane jScrollPane1;
    private javax.swing.JScrollPane jScrollPane2;
    private javax.swing.JTextPane jTextPane1;
    // End of variables declaration
    @Override
    public void update(Observable o, Object arg) {
        LOG.info("trying to observe.." + arg);
        LOG.info(controller.getGroup());
        //update defaultListModel
    }
}

扩展Observe的对象从Controller.setGroup（）调用notifyObserver：

package net.bounceme.dur.usenet.controller;
import java.util.Observable;
import java.util.logging.Logger;
public class Controller extends Observable {
    private static final Logger LOG = Logger.getLogger(Controller.class.getName());
    private String group;
    private static Controller instance;
    protected Controller() {
    }
    public static Controller getInstance() {
        if (instance == null) {
            instance = new Controller();
        }
        return instance;
    }

    public void setGroup(String selectedValue) {
        group = selectedValue;
        LOG.fine(group);
        notifyObservers();
    }
    public String getGroup() {
        return group;
    }
}

也许ArticleSelect没有正确注册以观察Controller？

Answer 1

java.util.Observable#notifyObservers()

只有当对象注册为已用setChanged()更改时，才会调用update()方法。