Java检查com端口连接

Question

I am looking for a bit of efficient code that can assist me in monitoring if a com port is still open using the RX/TX libraries.

Lets say I have a hardware device that communicates to the PC using a virtual com port and that device can be plugged in and out at any time. I want to show a connection status on the pc.

I have tried this with something like a buffered reader below and it registered that the device gets disconnected but I have to re-open the port from scratch in another method.

I am looking from something short like comPort.isOpen () or something?

// Set the value of is running
Start.isRunning = true;

// Check to see if the device is connected
while (Start.isRunning) {
    // Try to connect to the device
    try {
        // Create a Buffered Reader
        BufferedReader reader = new BufferedReader(
                new InputStreamReader(serialPort.getInputStream()));

        // Read the output
        if (Character.toString((char) reader.read()).equalsIgnoreCase(
            "^")) {
            // Set the connected flag
            Start.CONNECTED_FLAG = true;

            // Set the connected fag
            AddComponents.TFconnected.setText("Connected");
        }

        // Close the reader
        reader.close();

        // Let the thread sleep
        Thread.sleep(500);
    }

    // Catch a error if the device is disconnected
    catch (Exception err) {
        // Set the connected flag
        Start.CONNECTED_FLAG = false;
        // Set the connected fag
        AddComponents.TFconnected.setText("Disconnected");

        // Let the thread sleep
        Thread.sleep(500);
    }
}

Answer 1

Disclaimer: Consider this a partial answer because I do not have intimate knowledge of the workings of serial ports, and my tests could not produce anything useful. Posting here regardless in the hopes any of this is helpful.

Unfortunately, as far as I know, there is no way to receive any kind of "connection / disconnection" event messages. Sadly, as I am not intimately familiar with the workings of serial ports, I cannot give you a full and proper explanation. However, from some research , one of the answers posted in that forum had this to say:

There's no event by the system to inform you of [a disconnection event] because that would require exclusive use of the COM port. If you have a SerialPort object created and have opened a port you should get a CDChanged when a devices is plugged in and unplugged from the serial port. That assumes the device follows the pins standards; not all devices do.

Note that the poster, and the link I've provided, are discussing this within the context of C#. However this seems to be related to how the ports work in general, regardless of language, so I am somewhat confident the same can be applied to RXTX Java.

There are some events you can attempt to listen for. In my tests I was only ever able to receive the DATA_AVAILABLE event, however my setup is a bit different (Raspberry PI) and I can't at the moment physically disconnect the device from the port, I can only attempt to block the device file (which may explain the failure of my test).

If you would like to attempt the event listening yourself, have your class implement SerialPortListener , register for the desired events, check the events in your serialEvent method. Here is an example:

public class YourClass implements SerialPortListener{
    private SerialPort serialPort;

    // ... serial port gets set up at some point ...

    public void registerEvents(){
        serialPort.addEventListener(this);

        // listen to all the events
        serialPort.notifyOnBreakInterrupt(true);
        serialPort.notifyOnCarrierDetect(true);
        serialPort.notifyOnCTS(true);
        serialPort.notifyOnDataAvailable(true);
        serialPort.notifyOnDSR(true);
        serialPort.notifyOnFramingError(true);
        serialPort.notifyOnOutputEmpty(true);
        serialPort.notifyOnOverrunError(true);
        serialPort.notifyOnParityError(true);
        serialPort.notifyOnRingIndicator(true);
    }

    @Override
    public void serialEvent(SerialPortEvent event) {
        System.out.println("Received event. Type: " + event.getEventType() + ", old value: " + event.getOldValue() + ", new value: " + event.getNewValue());
    }
}

If that ultimately fails, I believe the only other alternative is similar to your current solution; attempt to read from the port, and if it fails, consider it disconnected, and set your indicator accordingly. At each iteration, if it is disconnected, attempt to reconnect; if reconnect succeeds, reset your indicator to "connected".

Sorry I cannot be of more assistance. Hopefully some of that may lead to something useful.

Side Note:

If you want to DRY up your code slightly, put the Thread.sleep(500) in a finally block instead, since it appears to be executed regardless.