balancelookup.java

Java Thread Programming (Source

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class BalanceLookup extends JPanel {
	private JTextField acctTF;
	private JTextField pinTF;
	private JButton searchB;
	private JButton cancelB;
	private JLabel balanceL;

	private volatile Thread lookupThread;

	public BalanceLookup() {
		buildGUI();
		hookupEvents();
	}

	private void buildGUI() {
		JLabel acctL = new JLabel("Account Number:");
		JLabel pinL = new JLabel("PIN:");
		acctTF = new JTextField(12);
		pinTF = new JTextField(4);

		JPanel dataEntryP = new JPanel();
		dataEntryP.setLayout(new FlowLayout(FlowLayout.CENTER));
		dataEntryP.add(acctL);
		dataEntryP.add(acctTF);
		dataEntryP.add(pinL);
		dataEntryP.add(pinTF);

		searchB = new JButton("Search");
		cancelB = new JButton("Cancel Search");
		cancelB.setEnabled(false);

		JPanel innerButtonP = new JPanel();
		innerButtonP.setLayout(new GridLayout(1, -1, 5, 5));
		innerButtonP.add(searchB);
		innerButtonP.add(cancelB);

		JPanel buttonP = new JPanel();
		buttonP.setLayout(new FlowLayout(FlowLayout.CENTER));
		buttonP.add(innerButtonP);

		JLabel balancePrefixL = new JLabel("Account Balance:");
		balanceL = new JLabel("BALANCE UNKNOWN");

		JPanel balanceP = new JPanel();
		balanceP.setLayout(new FlowLayout(FlowLayout.CENTER));
		balanceP.add(balancePrefixL);
		balanceP.add(balanceL);

		JPanel northP = new JPanel();
		northP.setLayout(new GridLayout(-1, 1, 5, 5));
		northP.add(dataEntryP);
		northP.add(buttonP);
		northP.add(balanceP);

		setLayout(new BorderLayout());
		add(northP, BorderLayout.NORTH);
	}

	private void hookupEvents() {
		searchB.addActionListener(new ActionListener() {
				public void actionPerformed(ActionEvent e) {
					search();
				}
			});

		cancelB.addActionListener(new ActionListener() {
				public void actionPerformed(ActionEvent e) {
					cancelSearch();
				}
			});
	}

	private void search() {
		// better be called by event thread!
		ensureEventThread();

		searchB.setEnabled(false);
		cancelB.setEnabled(true);
		balanceL.setText("SEARCHING ...");

		// get a snapshot of this info in case it changes
		String acct = acctTF.getText();
		String pin = pinTF.getText();

		lookupAsync(acct, pin);
	}

	private void lookupAsync(String acct, String pin) {
		// Called by event thread, but can be safely 
		// called by any thread.
		final String acctNum = acct;
		final String pinNum = pin;

		Runnable lookupRun = new Runnable() {
				public void run() {
					String bal = lookupBalance(acctNum, pinNum);
					setBalanceSafely(bal);
				}
			};
		
		lookupThread = new Thread(lookupRun, "lookupThread");
		lookupThread.start();
	}
	
	private String lookupBalance(String acct, String pin) {
		// Called by lookupThread, but can be safely 
		// called by any thread.
		try {
			// Simulate a lengthy search that takes 5 seconds
			// to communicate over the network.
			Thread.sleep(5000);

			// result "retrieved", return it
			return "1,234.56";
		} catch ( InterruptedException x ) {
			return "SEARCH CANCELLED";
		}
	}

	private void setBalanceSafely(String newBal) {
		// Called by lookupThread, but can be safely 
		// called by any thread.
		final String newBalance = newBal;

		Runnable r = new Runnable() {
				public void run() {
					try {
						setBalance(newBalance);
					} catch ( Exception x ) {
						x.printStackTrace();
					}
				}
			};
		
		SwingUtilities.invokeLater(r);
	}

	private void setBalance(String newBalance) {
		// better be called by event thread!
		ensureEventThread();

		balanceL.setText(newBalance);
		cancelB.setEnabled(false);
		searchB.setEnabled(true);
	}

	private void cancelSearch() {
		// better be called by event thread!
		ensureEventThread();

		cancelB.setEnabled(false); // prevent additional requests

		if ( lookupThread != null ) {
			lookupThread.interrupt();
		}
	}

	private void ensureEventThread() {
		// throws an exception if not invoked by the 
		// event thread.
		if ( SwingUtilities.isEventDispatchThread() ) {
			return;
		}

		throw new RuntimeException("only the event " +
			"thread should invoke this method");
	}

	public static void main(String[] args) {
		BalanceLookup bl = new BalanceLookup();

		JFrame f = new JFrame("Balance Lookup");
		f.addWindowListener(new WindowAdapter() {
				public void windowClosing(WindowEvent e) {
					System.exit(0);
				}
			});

		f.setContentPane(bl);
		f.setSize(400, 150);
		f.setVisible(true);
	}
}