Java，多线程和Raspberry Pi

Question

So I have my multi threading partially working. When I run my program the very first time after compiling it only runs 1 or 2 of the 3 threads. Then running it again right after, it runs perfectly fine with all of the threads activated and running. I can't think of any reason why all of the threads would not run on the first execution of the java class, and then start working on the second execution of the class after the first one has been terminated.

I am using Pi4j on a Raspberry Pi 2. I need the code to run fine the first time because I am trying to run this on bootup of the Pi.

This is where I start the threading.

public class Doors {
public static void main(String[] args) {        
    Thread door1 = new Thread(new DoorSwitch(0));
    door1.start();
    Thread door2 = new Thread(new DoorSwitch(1));
    door2.start();
    Thread door3 = new Thread(new DoorSwitch(2));
    door3.start();
}

This is what the Thread is running. The threading is passing in the value of what I call the door.

public class DoorSwitch implements Runnable{    
int p;
int doorID;
int tempPin;
static DBConnection db = new DBConnection();//create a new object of database connection class
public DoorSwitch(int currPin){
p = currPin;
    doorID = (p + 1);
}
public void run()
{
    try{
        final GpioController gpio = GpioFactory.getInstance();
        final GpioPinDigitalInput[] gpPins = {
                 gpio.provisionDigitalInputPin(RaspiPin.GPIO_28, PinPullResistance.PULL_UP),
                 gpio.provisionDigitalInputPin(RaspiPin.GPIO_29, PinPullResistance.PULL_UP),
                 gpio.provisionDigitalInputPin(RaspiPin.GPIO_27, PinPullResistance.PULL_UP),
         };
        for (int i=1; i<gpPins.length; i++){
            if(p == i){
                tempPin = p;
            }
        }
        final GpioPinDigitalInput door = gpPins[tempPin];
        door.addListener(new GpioPinListenerDigital() {
            @Override
            public void handleGpioPinDigitalStateChangeEvent(GpioPinDigitalStateChangeEvent event) {
                if(event.getState() == PinState.HIGH){
                    try{
                        db.dbConnection();
                        System.out.println(doorID);
                        System.out.println("Door Open! " + p);
                        String update = "UPDATE `door` SET `status`=" + 1 + " WHERE `id`="+ doorID +";";
                        db.getStatement().executeUpdate(update);
                        String log = "INSERT INTO `door_log` (`door_id`, `status`) VALUES ("+ doorID + ",1);";
                        db.getStatement().executeUpdate(log);
                    } catch (SQLException | ClassNotFoundException | InstantiationException | IllegalAccessException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                    }
                }
                if(event.getState() == PinState.LOW){
                    try{
                        db.dbConnection();
                        System.out.println(doorID);
                        System.out.println("                Door Closed! " + p);
                        String update = "UPDATE `door` SET `status`=" + 0 + " WHERE `id`="+ doorID +";";
                        db.getStatement().executeUpdate(update);
                        String log = "INSERT INTO `door_log` (`door_id`, `status`) VALUES ("+ doorID + ",0);";
                        db.getStatement().executeUpdate(log);
                    } catch (SQLException | ClassNotFoundException | InstantiationException | IllegalAccessException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                    }
                }
                //System.out.println(" --> GPIO PIN STATE CHANGE: " + event.getPin() + " = " + event.getState());
            } 
        });
        while(true) {
            Thread.sleep(500);
        }

        // 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();   <--- implement this method call if you wish to terminate the Pi4J GPIO controller
    }catch(Exception e){

    }

}

Any help would be greatly appreciated. Thanks

Answer 1

check db: shared, not protected when used, and created in each thread.

The result can be unpredictable.

I propose:

1 Add this

 static final Object db_sync=new Object();

2 Initialize db only once, and call it in the constructor of DoorSwitch

static DBConnection db=null;

static db_initialize()
{

synchronized(db_sync)
 {
 if (db==null) 
    db = new DBConnection();//create a new object of database connection class
  }

}

3 Surround the use of db

synchronized(db_sync)
 {
  // Do what you want with db
  }