Help Reading Network Tables

Technical Java
1

Hi,

I was trying to read a specific value from the robot while following this - https://wpilib.screenstepslive.com/s...client-pc-side

But when I was doing so I noticed a ton of stuff on how the NetworkTables behaved had changed.

We are using Java.

Is there a simple way to get the same result as the screensteps tutorial using the 2018 API?

An example would be amazing.

Regards
Gagan Bhat

2

I’ve been wondering about the same thing, getting (in my case, the limelight) table from NetworkTables… Will definitely be following this thread

3

First, I want to point out that the old API still works. But if you want to switch over to the new API, here’s a simple Java client example:


package networktablesdesktopclient;

import edu.wpi.first.networktables.NetworkTable;
import edu.wpi.first.networktables.NetworkTableEntry;
import edu.wpi.first.networktables.NetworkTableInstance;

public class NetworkTablesDesktopClient {
  public static void main(String] args) {
    new NetworkTablesDesktopClient().run();
  }

  public void run() {
    NetworkTableInstance inst = NetworkTableInstance.getDefault();
    NetworkTable table = inst.getTable("datatable");
    NetworkTableEntry xEntry = table.getEntry("x");
    NetworkTableEntry yEntry = table.getEntry("y");

    inst.startClientTeam(TEAM);  // where TEAM=190, 294, etc, or use inst.startClient("hostname") or similar
    inst.startDSClient();  // recommended if running on DS computer; this gets the robot IP from the DS

    while (true) {
      try {
        Thread.sleep(1000);
      } catch (InterruptedException ex) {
        System.out.println("interrupted");
        return;
      }

      double x = xEntry.getDouble(0.0);
      doubly y = yEntry.getDouble(0.0);
      System.out.println("X: " + x + " Y: " + y);
    }
  }
}
4

If you’re looking to get values just inside of a subsystem etc. read through this. I made this for grabbing values from the limelight/changing values, so I hope it can be of use and help explain some stuff.

5

I created an application using Peter’s code above but when it runs I get the error below. I made two changes to the code to add my team number and fix the “doubly” typo near the end. I referenced the following library which resolves the NetworkTable references but I assume this is my mistake.

C:\Users\kingb\wpilib\java\current\lib
tcore.jar

Don’t I need the client (PC) side version of this library? I believe the one above is for the RoboRIO.

Assuming this is my problem, does anyone know where the correct library/jar is located?

Thanks

Exception in thread “main” java.lang.NoClassDefFoundError: edu/wpi/first/wpiutil/RuntimeDetector
at edu.wpi.first.networktables.NetworkTablesJNI.<clinit>(NetworkTablesJNI.java:27)
at edu.wpi.first.networktables.NetworkTableInstance.getDefault(NetworkTableInstance.java:88)
at networktablesdesktopclient.NetworkTablesDesktopClient.run(NetworkTablesDesktopClient.java:13)
at networktablesdesktopclient.NetworkTablesDesktopClient.main(NetworkTablesDesktopClient.java:9)
Caused by: java.lang.ClassNotFoundException: edu.wpi.first.wpiutil.RuntimeDetector
at java.net.URLClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
… 4 more

6

That’s correct, the jar in the plugins is for the RoboRIO, and you were also missing the wpiutil jar. You’ll need the following 3 jar files for desktop use:

http://first.wpi.edu/FRC/roborio/maven/release/edu/wpi/first/ntcore/ntcore-java/4.0.0/ntcore-java-4.0.0.jar (ntcore java files)

http://first.wpi.edu/FRC/roborio/maven/release/edu/wpi/first/ntcore/ntcore-jni/4.0.0/ntcore-jni-4.0.0-all.jar (ntcore native libs for all desktop platforms)

http://first.wpi.edu/FRC/roborio/maven/release/edu/wpi/first/wpiutil/wpiutil-java/3.0.0/wpiutil-java-3.0.0.jar (wpiutil java files)

7

That fixed it, Thanks