jrosclient keeps all standard ROS message definitions in jrosmessages Java module. This module provides definition for some basic ROS messages (from std_msgs and others) which are identical for all versions of ROS (ROS1, ROS2). Messages which are different between ROS versions available in separate modules: jros2messages, jros1messages.
In case some of the standard ROS messages are missing or you want to use your own messages, it is highly recommended to use msgmonster application. Additionally jrosclient allows you to define them manually as described in this document.
jrosclient supports following mapping of ROS built-in types to Java types:
Complete list of ROS built-in types:
In Java size of char is 2 bytes whereas in ROS2 it is the same as in C++ and equals to 1 byte. To prevent possible data loss when converting Java char (2 bytes) to ROS char (1 byte), jros2client maps both ROS built-in types "byte" and "char" into single Java primitive type "byte" (see CharMessageSubscriberApp for example). In ROS1 char is deprecated.
Every message class should:
For example this is wrong, since stamp field contains null:
public Time stamp;
Correct, field stamp is non-null and Time class will be serialized as part of the message class where it is declared:
public Time stamp = new Time();
public Point32Message[] points = new Point32Message[0];
# ROS shape_msgs/Plane definition float64[4] coef // Java definition @Array(size = 4) public double[] coef = new double[0];
As an example lets define message Java class for geometry_msgs/PolygonStamped (it is already part of jrosclient).
Its ROS definition can be found in ROS_INSTALL_DIR/share/geometry_msgs/msg/PolygonStamped.msg and it looks like:
# This represents a Polygon with reference coordinate frame and timestamp Header header Polygon polygon
As we see it consist from two other messages:
Because Polygon type is not available, before proceeding with PolygonStamped we need to define Polygon first.
Polygon definition can be found in ROS_INSTALL_DIR/geometry_msgs/msg/Polygon.msg and it consist from:
#A specification of a polygon where the first and last points are assumed to be connected Point32[] points
Point32 type is available in jrosclient so we can define Polygon message type right away.
Before writing a message class make sure you have:
To calculate md5 sum for geometry_msgs/Polygon (required for ROS1 only) we can use following ROS1 command:
cd60a26494a087f577976f0329fa120e
Now, since we have all the information let's define a Java message class for geometry_msgs/Polygon:
/** Example of custom message definition */
@MessageMetadata(
name = PolygonMessage.NAME,
/*fields = {"points"} /* This parameter is optional because message has only one field */
md5sum = "cd60a26494a087f577976f0329fa120e")
public static class PolygonMessage implements Message {
static final String NAME = "geometry_msgs/Polygon";
public Point32Message[] points = new Point32Message[0];
public PolygonMessage withPoints(Point32Message[] points) {
this.points = points;
return this;
}
}
Since this message has same format between ROS1 and ROS2 we would like to use it for both and therefore we specify md5sum. For messages which target only ROS2 md5sum is not required.
As we saw before geometry_msgs/PolygonStamped message consist from:
Because now we have all these types available we are ready to define geometry_msgs/PolygonStamped itself. But due to difference of Header type format in ROS2 and ROS1 we will have to create separate PolygonStamped for each of them.
Java message class definition for geometry_msgs/PolygonStamped:
import id.jros2messages.std_msgs.HeaderMessage;
/** Example of custom message definition */
@MessageMetadata(
name = PolygonStampedMessage.NAME,
fields = {"header", "polygon"})
public static class PolygonStampedMessage implements Message {
static final String NAME = "geometry_msgs/PolygonStamped";
public HeaderMessage header = new HeaderMessage();
public PolygonMessage polygon = new PolygonMessage();
public PolygonStampedMessage withPolygon(PolygonMessage polygon) {
this.polygon = polygon;
return this;
}
public PolygonStampedMessage withHeader(HeaderMessage header) {
this.header = header;
return this;
}
}
As always first step is to calculate the md5 sum or the ROS1 message definition:
c6be8f7dc3bee7fe9e8d296070f53340
Now Java message class definition for geometry_msgs/PolygonStamped:
import id.jros1messages.std_msgs.HeaderMessage;
/** Example of custom message definition */
@MessageMetadata(
name = PolygonStampedMessage.NAME,
fields = {"header", "polygon"},
md5sum = "c6be8f7dc3bee7fe9e8d296070f53340")
public static class PolygonStampedMessage implements Message {
static final String NAME = "geometry_msgs/PolygonStamped";
public HeaderMessage header = new HeaderMessage();
public PolygonMessage polygon = new PolygonMessage();
public PolygonStampedMessage withPolygon(PolygonMessage polygon) {
this.polygon = polygon;
return this;
}
public PolygonStampedMessage withHeader(HeaderMessage header) {
this.header = header;
return this;
}
}
Since we just defined geometry_msgs/PolygonStamped message, lets create a topic for it and start to publish messages from Java using jrosclient. Complete examples:
Once you start the application you should see its output like:
Published Published Published Published Published Published Published Published Published ...
Now start RViz and go to Displays -> Add -> By Topic -> /PolygonExample. This should subscribe you to the topic and you should see the polygon: