Running a JAR file in Pi results in following error "can't find the main class"

Question

I want to make a GUI application in java which can control the gpio pins. I used pi4j lib for control the pins. I wrote a pin toggle code inside the button click event as shown below. while running JAR file in Pi it showing the error as "can't find the main class". I would like to know whether it is possible to write the gpio control code in click event...pls help..

package javatest;

import com.pi4j.io.gpio.GpioController;
import com.pi4j.io.gpio.GpioFactory;
import com.pi4j.io.gpio.GpioPinDigitalOutput;
import com.pi4j.io.gpio.PinState;
import com.pi4j.io.gpio.RaspiPin;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.swing.ImageIcon;
import javax.swing.JButton;
import javax.swing.JFrame;

/**
 *
 * @author R
 */
public class Javatest extends javax.swing.JFrame {



private void jButton4ActionPerformed(java.awt.event.ActionEvent evt) {                                         
            // TODO add your handling code here:
            imageCross = new javax.swing.ImageIcon(cross);
            imageCircle = new javax.swing.ImageIcon(circle);
        if (a==0){
            jButton4.setIcon(imageCross);
        a=1;
        }
        else{
        jButton4.setIcon(imageCircle);
        a=0;
        }
        try {
            System.out.println("PC1<--Pi4J--> GPIO Control Example ... started.");

            // create gpio controller
           final GpioController gpio = GpioFactory.getInstance();
           Thread.sleep(5000);
            System.out.println("PC2<--Pi4J--> GPIO Control Example ... started.");
            // provision gpio pin #01 as an output pin and turn on
            final GpioPinDigitalOutput pin = gpio.provisionDigitalOutputPin(RaspiPin.GPIO_01, "MyLED", PinState.HIGH);
            System.out.println("--> GPIO state should be: ON");

            Thread.sleep(5000);

            // turn off gpio pin #01
            pin.low();
            System.out.println("--> GPIO state should be: OFF");

             Thread.sleep(5000);

            // toggle the current state of gpio pin #01 (should turn on)
            pin.toggle();
            System.out.println("--> GPIO state should be: ON");

            Thread.sleep(5000);

            // toggle the current state of gpio pin #01  (should turn off)
            pin.toggle();
            System.out.println("--> GPIO state should be: OFF");

            Thread.sleep(5000);

            // turn on gpio pin #01 for 1 second and then off
            System.out.println("--> GPIO state should be: ON for only 1 second");
            pin.pulse(1000, true); // set second argument to 'true' use a blocking call

            // stop all GPIO activity/threads by shutting down the GPIO controller
            // (this method will forcefully shutdown all GPIO monitoring threads and scheduled tasks)
            gpio.shutdown();
        } catch (InterruptedException ex) {
            Logger.getLogger(Javatest.class.getName()).log(Level.SEVERE, null, ex);
        }
public static void main(String args[]) {
        /* Set the Nimbus look and feel */
        //<editor-fold defaultstate="collapsed" desc=" Look and feel setting code (optional) ">
        /* If Nimbus (introduced in Java SE 6) is not available, stay with the default look and feel.
         * For details see http://download.oracle.com/javase/tutorial/uiswing/lookandfeel/plaf.html 
         */
        try {
            for (javax.swing.UIManager.LookAndFeelInfo info : javax.swing.UIManager.getInstalledLookAndFeels()) {
                if ("Nimbus".equals(info.getName())) {
                    javax.swing.UIManager.setLookAndFeel(info.getClassName());
                    break;
                }
            }
        } catch (ClassNotFoundException ex) {
            java.util.logging.Logger.getLogger(Javatest.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (InstantiationException ex) {
            java.util.logging.Logger.getLogger(Javatest.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (IllegalAccessException ex) {
            java.util.logging.Logger.getLogger(Javatest.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (javax.swing.UnsupportedLookAndFeelException ex) {
            java.util.logging.Logger.getLogger(Javatest.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        }
        //</editor-fold>
        //</editor-fold>

        /* Create and display the form */
        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                new Javatest().setVisible(true);
            }
        });
    }

    // Variables declaration - do not modify                     
    private javax.swing.JButton jButton2;
    private javax.swing.JButton jButton3;
    private javax.swing.JButton jButton4;
    private javax.swing.JButton jButton5;
    // End of variables declaration                   
}

Answer 1

Use java -cp myjar.jar javatest.Javatest

first argument after '-cp' is the jar file, 
second is the classpath to class containing main(String[]) method

If you dont have a MANIFEST in your jar file, you need to specify which class contains the main(String[] args) method. If you want to avoid it in the future, and be able to run a jar via doubleclick (non console) read How to create a MANIFEST file

Answer 2

You didn't mention what parameters you were passing to Java but I suspect that you are not including PI4J on your classpath.

You need to include the following:

.:/opt/pi4j/lib/'*'

for example

java -cp .:/opt/pi4j/lib/'*' javatest/Javatest