I have created three threads in a java program. One is the main program, the others are two classes that extend Thread. The main thread represent a controller for a machine. Another thread is the actuators and the third is the sensors. The controller sets variables in its own class which is read by the actuator thread. The actuator performs certain actions according to the instructions and update its own internal variables. These are in turn read by the sensor thread which reads the actuator variables (representing real world actions) and sets its own internal variables which in turn is read by the controller and we have come full circle. The controller then sets variables according to the new sensed world etc.
The actuators are in a eternal loop sleeping 100 ms in each loop.
The sensors are also in an eternal loop sleeping 20ms per loop.
The system almost works. The main loop will miss the updates from the sensor unless I add a sleep to it as well. My question is now why that is? Even sleep(0) makes the system work. I’ve placed the statement inside the performJob(Job input) while loop. How does the java main thread without a sleep call act differently than the same thread with?
Concurrency is a fairly new subject to me.
This is the code I am using:
Main:
public class Main {
public static void main(String[] args) {
Controller Press = new Controller();
Press.processUnits(1); // no reset code at the moment, only use 1 at a time
Press.shutdownThreads();
}
}
Controller:
import java.util.LinkedList;
public class Controller extends Thread {
// Constants
static final int STATE_WAITING = 0;
static final int STATE_MOVE_ARMS = 1;
static final int STATE_MOVE_PRESS = 2;
static final int LOADINGARM = 2;
static final int UNLOADINGARM = 1;
static final int NOARM = 0;
static final boolean EXTEND = true;
static final boolean RETRACT = false;
private enum Jobs {
EXTENDRETRACT, ROTATE, MAGNETONOFF, PRESSLOWERRAISE
}
// Class variables
private int currentState;
// Component instructions
private int armChoice = 0;
private boolean bool = false; // on, up, extend / off, down, retract
private boolean[] rotInstr = {false, false}; // {rotate?, left true/right false}
private boolean errorHasOccurred = false;
private boolean pressDir = false;
private Sensors sense = null;
private Actuators act = null;
private LinkedList<Job> queue = null;
// Constructor
public Controller() {
act = new Actuators(0.0f, this);
sense = new Sensors();
act.start();
sense.start();
currentState = STATE_WAITING;
queue = new LinkedList<Job>();
}
// Methods
int[] getArmInstructions() {
int extret = (bool) ? 1 : 0;
int[] ret = {armChoice, extret};
return ret;
}
boolean[] getRotation() {
return rotInstr;
}
int getControllerState() {
return currentState;
}
boolean getPressDirection() {
return pressDir;
}
public boolean processUnits(int nAmount) {
if (run(nAmount)) {
System.out.println("Controller: All units have been processed successfully.");
return true;
} else { // procUnits returning false means something went wrong
System.out.println("Controller: An error has occured. The process cannot complete.");
return false;
}
}
/*
* This is the main run routine. Here the controller analyses its internal state and its sensors
* to determine what is happening. To control the arms and press, it sets variables, these symbolize
* the instructions that are sent to the actuators. The actuators run in a separate thread which constantly
* reads instructions from the controller and act accordingly. The sensors and actuators are dumb, they
* will only do what they are told, and if they malfunction it is up to the controller to detect dangers or
* malfunctions and either abort or correct.
*/
private boolean run(int nUnits) {
/*
Here depending on internal state, different actions will take place. The process uses a queue of jobs
to keep track of what to do, it reads a job, sets the variables and then waits until that job has finished
according to the sensors, it will listen to all the sensors to make sure the correct sequence of events is
taking place. The controller reads the sensor information from the sensor thread which runs on its own
similar to the controller.
In state waiting the controller is waiting for input, when given the thread cues up the appropriate sequence of jobs
and changes its internal state to state move arms
In Move arms, the controller is actively updating its variables as its jobs are performed,
In this state the jobs are, extend, retract and rotate, pickup and drop.
From this state it is possible to go to move press state once certain conditions apply
In Move Press state, the controller through its variables control the press, the arms must be out of the way!
In this state the jobs are press goes up or down. Pressing is taking place at the topmost position, middle position is for drop off
and lower is for pickup. When the press is considered done, the state reverts to move arms,
*/
//PSEUDO CODE:
//This routine being called means that there are units to be processed
//Make sure the controller is not in a blocking state, that is, it shut down previously due to errors
//dangers or malfunctions
//If all ok - ASSUMED SO AS COMPONENTS ARE FAULT FREE IN THIS VERSION
if (!errorHasOccurred) {
//retract arms
currentState = STATE_MOVE_ARMS;
queue.add(new Job(Jobs.EXTENDRETRACT, LOADINGARM, RETRACT));
queue.add(new Job(Jobs.EXTENDRETRACT, UNLOADINGARM, RETRACT));
performWork();
//System.out.println("Jobs added");
//while there are still units to process
for (;nUnits != 0; nUnits--) {
//move the press to lower position, for unloading
currentState = STATE_MOVE_PRESS;
//rotate to pickup area and pickup the metal, also pickup processed
currentState = STATE_MOVE_ARMS;
queue.add(new Job(Jobs.EXTENDRETRACT, LOADINGARM, EXTEND));
queue.add(new Job(Jobs.EXTENDRETRACT, UNLOADINGARM, EXTEND));
performWork();
//retract and rotate
queue.add(new Job(Jobs.EXTENDRETRACT, LOADINGARM, RETRACT));
queue.add(new Job(Jobs.EXTENDRETRACT, UNLOADINGARM, RETRACT));
performWork();
//state change, press moves to middle position
currentState = STATE_MOVE_PRESS;
//state change back, put the metal on the press, drop processed and pull arms back
currentState = STATE_MOVE_ARMS;
queue.add(new Job(Jobs.EXTENDRETRACT, LOADINGARM, EXTEND));
queue.add(new Job(Jobs.EXTENDRETRACT, UNLOADINGARM, EXTEND));
queue.add(new Job(Jobs.EXTENDRETRACT, LOADINGARM, RETRACT));
queue.add(new Job(Jobs.EXTENDRETRACT, UNLOADINGARM, RETRACT));
performWork();
//state change, press the metal in upper position
currentState = STATE_MOVE_PRESS;
//repeat until done
}
//unload final piece
//move the press to lower position for unload
//rotate and pickup processed piece
//drop it off at unloading and wait for more orders
currentState = STATE_WAITING;
}
return true;
}
private boolean performWork() {
while (!queue.isEmpty()) {
performJob(queue.removeFirst());
}
return true;
}
private boolean performJob(Job input) {
//The purpose of this function is to update the variables and wait until they are completed
// read in the job and check appropriate sensors
boolean[] data = sense.getSensorData(); // LExt, LRet, UlExt, UlRet
printBools(data);
int Instruction = input.Instruction;
boolean skipVars = false;
if (input.Job == Jobs.EXTENDRETRACT) {
if (currentState != STATE_MOVE_ARMS){
System.out.println("Wrong state in performJob. State is "+currentState+" expected "+STATE_MOVE_ARMS);
return false;
}
if ((Instruction == LOADINGARM) && (input.Bool)) skipVars = data[0];
if ((Instruction == LOADINGARM) && (!input.Bool)) skipVars = data[1];
if ((Instruction == UNLOADINGARM) && (input.Bool)) skipVars = data[2];
if ((Instruction == UNLOADINGARM) && (!input.Bool)) skipVars = data[3];
}
if (!skipVars) {
// if sensors not at intended values, update correct variables
System.out.println("Controller: Did not skip vars");
switch (input.Job) {
case EXTENDRETRACT:
armChoice = (Instruction == LOADINGARM) ? LOADINGARM : UNLOADINGARM;
bool = input.Bool;
break;
case ROTATE:
break;
case MAGNETONOFF:
break;
case PRESSLOWERRAISE:
break;
default:
System.out.println("Default called in performJob()");
break;
}
}
// listen to sensors until completed
boolean done = false;
System.out.println("Waiting for sensor data.");
//System.out.print("Instruction is "+Instruction+" and bool is "); if (input.Bool) System.out.print("true\n"); else System.out.print("false\n");
while (!done) {
data = sense.getSensorData();
// REMOVING THIS TRY STATEMENT BREAKS THE PROGRAM
try {
Thread.currentThread().sleep(10);
} catch (InterruptedException e) {
System.out.println("Main thread couldn't sleep.");
}
// Check appropriate sensors
if (input.Job == Jobs.EXTENDRETRACT) {
if ((Instruction == LOADINGARM) && (input.Bool)) done = data[0];
if ((Instruction == LOADINGARM) && (!input.Bool)) done = data[1];
if ((Instruction == UNLOADINGARM) && (input.Bool)) done = data[2];
if ((Instruction == UNLOADINGARM) && (!input.Bool)) done = data[3];
}
}
// reset all variables
armChoice = 0;
bool = false;
// when done return
System.out.println("Finished "+input.Job);
return true;
}
public void shutdownThreads() {
sense.shutDown();
act.shutDown();
}
private class Job {
// Class variables
Jobs Job;
int Instruction;
boolean Bool; // used for directions, up/down, left/right, extend/retract
// Constructor
Job(Jobs newJob, int newInstruction, boolean dir) {
Job = newJob;
Instruction = newInstruction;
Bool = dir;
}
}
private void printBools(boolean[] input) {
System.out.println();
for (int i = 0; i < input.length; i++) {
if (input[i]) System.out.print("true "); else System.out.print("false ");
}
System.out.println();
}
}
Actuators:
public class Actuators extends Thread {
// Constants
private final int ARM = 0, ROTATE = 0; // array indexes - which arm, rotate yes/no?
private final int DIR = 1, ROTDIR = 1; // array indexes - which direction ext/ret, rotate direction
private final int EXT = 1;//, RET = 0;
private final double ARM_SPEED = 5.0;
// Class variables
private Controller Owner = null;
private boolean run = true;
// Constructor
Actuators(float nPos, Controller Owner) {
Reality.changeAngle(nPos);
this.Owner = Owner;
}
// Methods
private void rotate(boolean dir) {
float nAngle = dir ? 0.1f : -0.1f;
Reality.changeAngle(nAngle);
}
public void run() {
while (run) {
try {
sleep(100);
} catch (InterruptedException e) {
System.out.println("Actuators couldn't sleep");
}
// read variables in controller
int nState = Owner.getControllerState();
if (nState == Controller.STATE_MOVE_ARMS) {
boolean[] rot = Owner.getRotation();
if (rot[ROTATE]) { // Rotation?
rotate(rot[ROTDIR]);
} else { // or arm extensions
int[] instr = Owner.getArmInstructions();
if (instr[ARM] != Controller.NOARM) { // 0 = no arm movement
//System.out.println("Actuator arm is "+instr[ARM]);
double dir = (instr[DIR] == EXT) ? ARM_SPEED : -ARM_SPEED; // 1 = extend, 0 = retract
Reality.changeArmLength(instr[ARM], dir);
}
}
}
}
}
void shutDown() {
run = false;
}
}
Reality is a class composed of static fields and methods, written to by the actuators and read by sensors.
public class Reality {
// Constants
static private final double EXTEND_LIMIT = 100.0;
static private final double RETRACT_LIMIT = 0.0;
// Variables
private static float ArmsAngle = 0.0f;
// Read by Sensor
static double LoadingArmPos = 0.0;
static double UnloadingArmPos = 0.0;
// Methods
static void changeAngle(float newAngle) {
ArmsAngle = ArmsAngle + newAngle;
if ((ArmsAngle < 0.0f) || (ArmsAngle > 90.0f))
System.out.println("Reality: Unallowed Angle");
}
static void changeArmLength(int nArm, double dPos) { // true = extend, false = retract
switch (nArm) {
case Controller.LOADINGARM:
LoadingArmPos += dPos;
checkArmPos(LoadingArmPos);
break;
case Controller.UNLOADINGARM:
UnloadingArmPos += dPos;
checkArmPos(UnloadingArmPos);
break;
default:
System.out.println("Arm other than 2 (load) or 1 (unload) in changeArmLength in Reality");
break;
}
}
static float senseAngle() {
return ArmsAngle;
}
static private boolean checkArmPos(double dPos) {
// Allowed positions are 100.0 to 0.0
if ((dPos > EXTEND_LIMIT) || (dPos < RETRACT_LIMIT)) {
System.out.println("Arm position impossible in reality. Is "+dPos);
return true;
} else {
return false;
}
}
}
Finally the sensors:
public class Sensors extends Thread {
// Constants
private final double EXTENDED = 100.0;
private final double RETRACTED = 0.0;
private final double MARGIN = 0.1;
// Class Variables
private boolean run = true;
// Read by Controller
private boolean LoadingExtended = true;
private boolean LoadingRetracted = true;
private boolean UnloadingExtended = true;
private boolean UnloadingRetracted = true;
// Constructor
Sensors() {
LoadingExtended = false;
LoadingRetracted = true;
UnloadingExtended = false;
UnloadingRetracted = true;
}
// Methods
boolean senseLoadingExtended() {
return (Math.abs(Reality.LoadingArmPos - EXTENDED) < MARGIN);
}
boolean senseLoadingRetracted() {
return (Math.abs(Reality.LoadingArmPos - RETRACTED) < MARGIN);
}
boolean senseUnloadingExtended() {
return (Math.abs(Reality.UnloadingArmPos - EXTENDED) < MARGIN);
}
boolean senseUnloadingRetracted() {
return (Math.abs(Reality.UnloadingArmPos - RETRACTED) < MARGIN);
}
// called by Controller
boolean[] getSensorData() {
boolean[] ret = {LoadingExtended, LoadingRetracted, UnloadingExtended, UnloadingRetracted};
return ret;
}
// Sensor primary loop
public void run() {
while (run) {
try {
sleep(20);
}
catch (InterruptedException e) {
System.out.println("Sensors couldn't sleep");
}
LoadingExtended = senseLoadingExtended();
LoadingRetracted = senseLoadingRetracted();
UnloadingExtended = senseUnloadingExtended();
UnloadingRetracted = senseUnloadingRetracted();
}
}
void shutDown() {
run = false;
}
}
Not all fields and functions are read in this version. The program is a reworking of a previous single thread application mostly using function calls. I’ve cleaned up the code a bit for readability. Constructive design remarks are welcome even though it was not the original question. There is something really fishy going on. I am usually not a superstitious coder but I can for example replace the sleep call with a System.out.println() call and the program will work.
What occurred here was most probably a Memory Consistency Error. When the controller class set the internal control variables and then entered the loop waiting for sensors it most likely prevented the Actuators and Sensors classes from properly updating the readings seen to the controller and as such prevented the controller from seeing the correct values. By adding the synchronize statement to all functions which read from another class the problem was solved. I can only speculate that the sleep call had the controller thread enter a synchronized block of some kind which let the other threads’ changes to the variables become visible.