Developing URLWriter

To create the URLWriter MIDlet:

  1. Create the URLWriter.java class file.

  2. Import the required classes.

    // Classes for contactless communication
import java.io.IOException;
import javax.microedition.contactless.ContactlessException;
import javax.microedition.contactless.DiscoveryManager;
import javax.microedition.contactless.TargetListener;
import javax.microedition.contactless.TargetProperties;
import javax.microedition.contactless.TargetType;

// Classes for NDEF targets
import javax.microedition.contactless.ndef.NDEFMessage;
import javax.microedition.contactless.ndef.NDEFRecord;
import javax.microedition.contactless.ndef.NDEFRecordType;
import javax.microedition.contactless.ndef.NDEFTagConnection;
import javax.microedition.io.Connector;

// Classes for UI
import javax.microedition.lcdui.Alert;
import javax.microedition.lcdui.AlertType;
import javax.microedition.lcdui.Command;
import javax.microedition.lcdui.CommandListener;
import javax.microedition.lcdui.Display;
import javax.microedition.lcdui.Displayable;
import javax.microedition.lcdui.Form;

import javax.microedition.midlet.*;

  3. Set URLWriter to extend MIDlet and to implement TargetListener and CommandListener.

    public class URLWriter
        extends MIDlet
        implements TargetListener, CommandListener {

  4. Define the global variables used by the MIDlet, and implement the URLWriter method for launching the MIDlet. The URLWriter method sets up the MIDlet UI, and registers the MIDlet to receive notifications when NDEF targets are detected.

        // Global variables for the UI
    private Command exitCommand;
    private Form form;

    public URLWriter() {

        // Create the UI
        exitCommand = new Command("Exit", Command.EXIT, 1);
        form = new Form("NFC Research Lab: URLWriter");

        form.addCommand(exitCommand);
        form.setCommandListener(this);
        form.append("Touch tag to write URL: \"www.nfc-research.at\"");

        // Register TargetListener (this) for NDEF targets
        try {
            DiscoveryManager dm = DiscoveryManager.getInstance();
            dm.addTargetListener(this, TargetType.NDEF_TAG);

        } catch (ContactlessException ce) {
            displayAlert("Unable to register TargetListener: " + ce.toString(), AlertType.ERROR);
        }

    }

  5. Implement the MIDlet lifecycle requirements. The MIDlet uses the startApp method to display the UI.

        public void startApp() {
        Display.getDisplay(this).setCurrent(form);
    }

    public void pauseApp() {
    }

    public void destroyApp(boolean unconditional) {
    }

  6. Implement the targetDetected method mandated by the TargetListener class. The method is called every time the device detects one or more NDEF targets. The MIDlet uses the targetProperties array returned by the method to open an NDEF connection to the target. The custom getNDEFTAGConnection method is used to retrieve the first applicable connection, an NDEF connection, from the set of connections returned in the targetProperties array.

        /**
     * Implements the callback function of TargetListener
     * @param targetProperties: Array of targets found by the device
     */
    public void targetDetected(TargetProperties[] targetProperties) {

        // In case no targets are found, exit the method
        if (targetProperties.length == 0) {
            return;
        }

        // NDEF connection for write operation
        NDEFTagConnection ndconn = getNDEFTAGConnection(targetProperties);
        try {

            // Header byte: open bookmark in browser
            byte[] headerByte = {0x00};

            // Payload text: URL as a byte array
            byte[] urlBytes = "http://www.nfc-research.at/".getBytes();

            // Create NDEF record
            NDEFRecord r = new NDEFRecord(new NDEFRecordType(NDEFRecordType.NFC_FORUM_RTD, "urn:nfc:wkt:U"), null, null);

            // Append payload manually
            r.appendPayload(headerByte);
            r.appendPayload(urlBytes);

            // Create record array and add record
            NDEFRecord[] myRecArray = new NDEFRecord[]{r};

            // Generate a message out of the array
            NDEFMessage myMessage = new NDEFMessage(myRecArray);

            // Write message to tag
            ndconn.writeNDEF(myMessage);

            displayAlert("Information written", AlertType.INFO);

        } catch (IOException io) {
            displayAlert("IOException during open() or Write(): " + io.toString(), AlertType.ERROR);
        } catch (ContactlessException io) {
            displayAlert("ContactlessException during writeNDEF(): " + io.toString(), AlertType.ERROR);
        } catch (Exception e) {
            displayAlert("Exception: " + e.toString(), AlertType.ERROR);
        } // In case of an exception, close connection properly
        finally {
            // Close connection again
            if (ndconn != null) {
                try {
                    ndconn.close();
                } catch (IOException ex) {
                    displayAlert("IOException during close(): " + ex.toString(), AlertType.ERROR);
                }
            }
        }

    }

    private NDEFTagConnection getNDEFTAGConnection(TargetProperties[] tProp) {
        for (int j = 0; j < tProp.length; j++) {
            Class[] connections = tProp[j].getConnectionNames();
            if (connections != null) {
                for (int i = 0; i < connections.length; i++) {
                    if (connections[i].getName().equals(
                            "javax.microedition.contactless.ndef.NDEFTagConnection")) {
                        try {
                            return (NDEFTagConnection) Connector.open(tProp[j].getUrl(connections[i]));
                        } catch (Exception e) {
                            displayAlert("Error writing NDEF: " + e.toString(), AlertType.ERROR);
                        }
                    }
                }
            }
        }
        return null;
    }

  7. Implement command and message display handling for the MIDlet UI. The MIDlet uses the commandAction method to handle the exit command, which involves unregistering TargetListener and closing the MIDlet. The MIDlet uses the displayAlert method to display error and other messages through the MIDlet UI.

        /**
     * Implements the callback function of CommandListener
     * @param command: Command given
     * @param displayable: Associated Displayable object
     */
    public void commandAction(Command command, Displayable displayable) {
        if (command == exitCommand) {
            DiscoveryManager dm = DiscoveryManager.getInstance();
            dm.removeTargetListener(this, TargetType.NDEF_TAG);
            destroyApp(false);
            notifyDestroyed();
        }
    }

    private void displayAlert(String error, AlertType type) {
        Alert err = new Alert(form.getTitle(), error, null, type);
        Display.getDisplay(this).setCurrent(err, form);
    }
}

You can now build and run the MIDlet.

© Nokia 2010.