Thread Groups

• With JRadioButtons, we have ButtonGroups to group the JRadioButtons together.  We have a similar tool for threads, called a ThreadGroup.
• This allows us to group multiple related threads together for control purposes.
• To create a ThreadGroup...
• ThreadGroup tg = new ThreadGroup(String groupName);
• And pass this to your Thread, along with a name for the Thread.
• Thread t = new Thread(ThreadGroup tg, String threadName);
• Let's add our previous thread to a ThreadGroup.  Then, we'll click start a number of times, and we'll have the stop button stop the group, instead of one thread at a time.
• Changes (in orange)
• Since we're no longer using the default constructor, we have to specify one in our thread class.
• Make the thread group globally visible.
• We change the call to the constructor for the thread.
• We have a counter variable with class scope because we want each thread to have a unique name.  We use this when constructing instances of the AddRandom thread object.
• Stop the thread group, not the individual thread, in our stop button action listener.

package Threads;

import javax.swing.DefaultListModel;

/**
 *
 * @author  default
 */
public class InterruptThread extends javax.swing.JFrame {

    // These variables are globally visible.
    DefaultListModel model;
    AddRandoms addRandoms;
    ThreadGroup threadGroup;
    int threadCounter;
 
    /** Creates new form RandomNumber */
    public InterruptThread() {
        initComponents();
 
        // Instantiate the model, add it to the list.
        model = new DefaultListModel();
        lstNumbers.setModel(model);
        threadGroup = new ThreadGroup("randoms");
        threadCounter = 0;
 
    }

    // An inner class which will run as a thread.
    class AddRandoms extends Thread {
        public AddRandoms(ThreadGroup t, String s) {
            super(t, s);
        }
 
        public void run() {
            try {
                if ( interrupted() ) { throw new InterruptedException(); }
                for (int j = 0; j < 500; j++) {
                    // Generate a random number between 0 and 1, multiply it by 1000 to make it intersting.
                    int intRandom = (int) (Math.random() * 1000);

                    // add the random number to the model.
                    model.addElement(j + ": " + intRandom);

                    // Sleep for 20 ms.
                    Thread.sleep(20);
                }

            } catch (InterruptedException e) {
                System.out.println("Interrupted, returning");
                return;
            }
        }
    }
 
    /** 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.
     */
    private void initComponents() {
        jScrollPane1 = new javax.swing.JScrollPane();
        lstNumbers = new javax.swing.JList();
        jPanel1 = new javax.swing.JPanel();
        btnStart = new javax.swing.JButton();
        btnStop = new javax.swing.JButton();
        jLabel1 = new javax.swing.JLabel();

        addWindowListener(new java.awt.event.WindowAdapter() {
            public void windowClosing(java.awt.event.WindowEvent evt) {
                exitForm(evt);
            }
        });

        jScrollPane1.setViewportView(lstNumbers);

        getContentPane().add(jScrollPane1, java.awt.BorderLayout.CENTER);

        btnStart.setText("Start");
        btnStart.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                btnStartActionPerformed(evt);
            }
        });

        jPanel1.add(btnStart);

        btnStop.setText("Stop");
        btnStop.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                btnStopActionPerformed(evt);
            }
        });

        jPanel1.add(btnStop);

        getContentPane().add(jPanel1, java.awt.BorderLayout.SOUTH);

        jLabel1.setText("Random Numbers");
        getContentPane().add(jLabel1, java.awt.BorderLayout.NORTH);

        pack();
    }

    private void btnStartActionPerformed(java.awt.event.ActionEvent evt) {
        // Start the AddRandoms thread.
        threadCounter++;
        addRandoms = new AddRandoms(threadGroup, "Thread" + threadCounter);
        addRandoms.start();
 
 
    }

    private void btnStopActionPerformed(java.awt.event.ActionEvent evt) {
        // Stop the group.
        threadGroup.interrupt();
 
    }

    /** Exit the Application */
    private void exitForm(java.awt.event.WindowEvent evt) {
        System.exit(0);
 
    }

    /**
    * @param args the command line arguments
    */
    public static void main(String args[]) {
        new InterruptThread().show();
    }
 

    // Variables declaration - do not modify
    private javax.swing.JButton btnStop;
    private javax.swing.JList lstNumbers;
    private javax.swing.JScrollPane jScrollPane1;
    private javax.swing.JLabel jLabel1;
    private javax.swing.JPanel jPanel1;
    private javax.swing.JButton btnStart;
    // End of variables declaration

}
 

 
• Note what happens when we run it.
• You can see that the counters are not in numerical order.  We have four threads running.
• Further, notice the four messages on the console.  When we click stop, it stops all of them at once.

 Thread Priorities
 

Created by:  Brandan Jones December 17, 2001