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ConnectingROSToAnExternalProgram

Before we configure accessors, we first need to establish a connection between two entities. This page documents how to setup the connection between an external java program and ROS. You will need to install ROS first.
This approach requires two external packages, and the Eclipse IDE:

• rosbridge, a ros package for non-ROS programs to connect with a running ROS system. It provides an API and sets up WebSocket for connection.
• java_rosbridge, a Java library that sets up a connection with a rosbridge WebSocket

The code and steps provided follow closely the code provided in java_rosbridge written by James MacGlashan. To do more, compile the docs and explore the API in java_rosbridge.

• Connecting a ROS Publisher with a Java Subscriber
• First we will set up a ROS publisher

Set up ros with the following terminal commands:
$roscore
$roslaunch rosbridge_server rosbridge_websocket.launch sets up the ROS websocket (assumes you have installed rosbridge)

Then enter $rostopic pub /chatter "std_msgs/String" "hi there". This publishes the string "hi there" to the topic /chatter.

• Now we will write the java listener

Download the java_rosbridge library above from gitHub:
git clone https://github.com/h2r/java_rosbridge

Open Eclipse, select File>New Project>Java Project from Existing Antfile. Select the build.xml file in the toplevel directory of java_rosbridge

We will create the following Listener.java file in the existing src/test folder for this basic tutorial:

package tests;

import ros.Publisher;
import ros.RosBridge;
import ros.RosListenDelegate;

import java.util.HashMap;
import java.util.Map;

public class Listener {

  public static void main(String[] args) {

    if(args.length != 1){
      System.out.println("Need the rosbridge websocket URI provided as argument");
      System.exit(0);
    }

    RosBridge bridge = RosBridge.createConnection(args[0]);
    bridge.waitForConnection();

    bridge.subscribe("/chatter", "std_msgs/String",
      new RosListenDelegate() {
        @Override
        public void receive(Map<String, Object> data, String stringRep) {
          System.out.println("I received: " + stringRep);
        }
      });

  }
}

Eclipse should auto-compile, so supply the WebSocket URI as a command line argument (Right Click on Listener Java, select Run As->Run Configurations, and select the tab Arguments). This is "ws://localhost:9090" by default.

Run the file, and you should see the Eclipse console log: "I received: hi there."

• Connecting a ROS Subscriber with a Java Publisher
• We now make an external java program talk to a ROS subscriber.
  In Eclipse, create this file Talker.java and run it:

package tests;

import ros.Publisher;
import ros.RosBridge;
import ros.RosListenDelegate;

import java.util.HashMap;
import java.util.Map;

public class Talker {

  public static void main(String[] args) {

    if(args.length != 1){
      System.out.println("Need the rosbridge websocket URI provided as argument");
      System.exit(0);
    }

    RosBridge bridge = RosBridge.createConnection(args[0]);
    bridge.waitForConnection();

    Publisher pub = new Publisher("/chatter", "std_msgs/String", bridge);
    final Map<String, String> strData = new HashMap<String, String>();

    for(int i = 0; i < 100; i++) {
      strData.put("data", "hello from java " + i);
      System.out.println("sending...");
      pub.publish(strData);
      try {
        Thread.sleep(500);
      } catch (InterruptedException e) {
        e.printStackTrace();
      }
    }
  }
}

• Now we run a ROS subsciber to the topic /chatter

From a terminal, enter $rostopic echo /chatter
You should see the printout of "hello from java .."

• ROS