Send String from Java to Arduino (simple example)

Question

I'm new to Arduino and Java, but I made a program so simple that I don't get why it isn't working.

I send a String to the serial port that correspond to Arduino (COM5):

import java.io.*;
import java.util.*;
import gnu.io.*;

public class SimpleWrite {

static Enumeration portList;
static CommPortIdentifier portId;
static String messageString = "color FF00FFEND";
static SerialPort serialPort;
static OutputStream outputStream;

public static void main(String[] args) {
    portList = CommPortIdentifier.getPortIdentifiers();


    while (portList.hasMoreElements()) {

        portId = (CommPortIdentifier) portList.nextElement();
        if (portId.getPortType() == CommPortIdentifier.PORT_SERIAL) {

             if (portId.getName().equals("COM5")) {

                try {
                    serialPort = (SerialPort)
                    portId.open("SimpleWriteApp", 2000);

                    outputStream = serialPort.getOutputStream();

                    serialPort.setSerialPortParams(9600,
                        SerialPort.DATABITS_8,
                        SerialPort.STOPBITS_1,
                        SerialPort.PARITY_NONE);

                    outputStream.write(messageString.getBytes());
                    System.out.println(messageString);

                    outputStream.close();
                    serialPort.close();
                } 
                catch (IOException e) {System.out.println("err3");}
                catch (PortInUseException e) {System.out.println("err");}
                catch (IOException e) {System.out.println("err1");}
                catch (UnsupportedCommOperationException e) {System.out.println("err2");}
            }
        }
    }
}
}

and the code in Arduino to get that string:

char inputBuffer[10];   

void setup() {                
  Serial.begin(9600);  
}

void loop() {
    while (true) 
    {
      if (Serial.available() > 0) {
          Serial.readBytes(inputBuffer, Serial.available());
          delay(5000);
          Serial.print("I got this ->");
          Serial.print(inputBuffer);
          Serial.println("<-");
      }
    }
}

The while(true) is for testing purposes. I get nothing printed.

I'm using RXTXcomm.jar. Version: RXTX-2.2-20081207

Can you tell us which Lib (and Version of it) you are using for Serial I/O? Are you sure you open COM5? Are you sure you can change serialPort Params after getting the outputstream? — Fildor
– Fildor, Commented Aug 26, 2014 at 9:50
Im using RXTXcomm.jar. Version: RXTX-2.2-20081207. I put traces, and im sure it opens the com5, and that it changes serialport params — takluiper
– takluiper, Commented Aug 26, 2014 at 9:56
@takluiper Post your solution you edited in your question as an answer and accept it so this question doesn't appear as unanswered. Thanks and cheers! — Michal
– Michal, Commented Jul 19, 2015 at 17:21
why not contact arduino directly and wait for arduino to send a "i'm ready!" flag? — Andrea Bori
– Andrea Bori, Commented Jun 3, 2020 at 22:06

Jeff Loughlin · Accepted Answer · 2016-06-22 14:29:29Z

4

It is solved. I put a Thread.sleep(4000) after opening the port in the java code and now it works. The problem was that the Arduino is reset everytime the port is opened, so I was sending the data when the Arduino wasn't ready to listen.

char inputBuffer[10];   

void setup()
{                
    Serial.begin(9600);  
}

void loop()
{
     while (true) 
     {
          if (Serial.available() > 0)
          {
              Serial.readBytes(inputBuffer, 10);
              delay(5000);
              Serial.print("I got this ->");
              Serial.print(inputBuffer);
              Serial.println("<-");
          }
     }
}

edited Jun 22, 2016 at 14:29

Jeff Loughlin

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answered Feb 15, 2016 at 10:26

takluiper

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1 Comment

Georgi Peev Over a year ago

For ~1500ms are enough. Another way is to send a message from the Arduino in the setup function and have a polling for the data from the Java side waiting for that message and when received to unblock your sending of the data from the Java side.

Hirday Gupta · Accepted Answer · 2016-08-23 17:46:53Z

You could use the Java-Arduino Communication Library. It can be found here: https://sourceforge.net/projects/javaarduinolibrary/. (be sure to download both jars) Then your code would look something like this:

import arduino.*;
class JavaArduinoComm {
        public static void main(String[] args) {
        Arduino obj = new Arduino('PortDescription', BAUD_RATE);
        obj.openConnection();
    }
}

Then you can use any of the following methods:

String serialRead(int limit) - returns a string containing as many readings as the value of limit. recommended for reading
void serialWrite(String s) - writes the contents of the entire string to the serial at once. written as string to serial.
void serialWrite(String s, int noOfChars, int delay) - writes the contents of the strings to the serial gradually. It writes the string in incremental steps with 'noOfChars' charaacters each time, with a pause of 'delay' milliseconds between each write. written as string to serial. recommended to write String
void serialWrite(char c) - writes the individual char to the serial in datatype char.
void serialWrite(char c, int delay) - writes the individual char to the serial in datatype char and pauses the thread for delay milliseconds after. recommended to write char

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