Now I'm struggling with the task from the title. I create X threads, each of them prints Y equal digits (getting from constructor, for example "11111", "222222" etc) for Z times in cycle. So the result looks like:
111111111
222222222
333333333
111111111
222222222
333333333
for X = 3, Y = 9 and Z = 2.
Firstly I've solved this issue using sleep, interrupt and passing "next" thread to the constructor of previous one. One interrupts another etc. Next step is to get the same output using wait/notify instead sleep and interrupt. As far as I can see, it's neccesary to create the shared monitor object, to invoke wait after every printing and in a some moment " I should invoke notifyAll.
Current code is:
public class PrinterController {
private static final int THREADS_NUMBER = 5;
public static void main(String[] args) {
Printer[] printers = new Printer[THREADS_NUMBER];
for (int i = 0; i < THREADS_NUMBER; i++) {
printers[i] = new Printer(i);
printers[i].start();
}
}
}
public class Printer extends Thread {
private static int portion = 10;
private static int totalNumber = 100;
private int digit;
private static final Object monitor = new Object();
public Printer(int digit) {
this.digit = digit;
}
#Override
public void run() {
synchronized (monitor) {
int portionsNumber = totalNumber / portion;
for (int i = 0; i < portionsNumber; i++) {
printLine();
try {
monitor.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
private void printLine() {
for (int j = 0; j < portion; j++) {
System.out.print(digit);
}
System.out.println();
}
}
Could you help to improve it? I found similar tasks but they don't contain appropriate answers. Thanks.
Final solution based on the Nadir's answer:
public class Printer extends Thread {
private static int portion = 10;
private static int totalNumber = 100;
private int digit;
static Object monitor = new Object();
static Integer counter = 0;
public Printer(int digit) {
this.digit = digit;
}
#Override
public void run() {
int portionsNumber = totalNumber / portion;
for (int i = 0; i < portionsNumber; i++) {
synchronized (monitor) {
while (digit != counter) {
try {
monitor.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
printLine();
monitor.notifyAll();
}
}
}
private void printLine() {
for (int j = 0; j < portion; j++) {
System.out.print(digit);
}
System.out.println();
counter = (counter + 1) % PrinterController.THREADS_NUMBER;
}
}
It can be accomplished with a class used to synchronize the threads (and even make sure they are orderer). All threads would share the same instance.
public class Synchronizer
{
private int nextThread;
private int maxNumThreads;
public Synchronizer(int numThreads)
{
maxNumThreads = numThreads;
nextThread = 0;
}
public void doSync(int threadId) throws Exception
{
synchronized(this)
{
while(nextThread != threadId)
{
wait();
}
}
}
public void threadDone(int threadId) throws Exception
{
synchronized(this)
{
nextThread = (threadId + 1) % maxNumThreads;
notifyAll();
}
}
}
On your thread's run(), you would call doSync() before printing anything. Then you would put the code for printing, and afterwards, you would call threadDone(), allowing the next thread to be released. The id is used to enforce an order.
Related
I need to initialize an Array of n Threads. Each Thread represents a number which randomly initialized in the Thread's Object creation.
The program should run m iterations. On each iteration, the Threads number needs to be updated. If both left and right neighbors in the Array are bigger than this Thread then the Thread's value will get +1 in the next iteration. If both neighbors values are lower then the Thread's value then the Thread's value will get -1. Otherwise, the Thread's value will not change.
The code starts from a code line in a GUI class. The line is:
Controller cont = new Controller(n,m)
The code enters updated method in class Controller just once and then all the Threads are going to sleep without implementing updated method.
Why is that?
This is my code:
public class Threads extends Thread
{
private int num;
private Controller cont;
private int toUpdate;
private int index;
public Threads(Controller c, int i)
{
num = (int)(Math.random() * 100 + 1); // random number between 1-100
cont = c;
index = i;
}
public void run()
{
for(int j = 1; j <= cont.getIterations(); j++)
{
toUpdate = cont.checkValue(this, this.getIndex());
cont.finished();
cont.threadWait();
num += toUpdate;
cont.updated();
cont.threadWait();
}
}
public int getValue()
{
return this.num;
}
public int getIndex()
{
return this.index;
}
}
public class Controller
{
private Threads[] threadsArray;
private int iterations;
private boolean finished = false;
private static int numOfThreads;
public Controller(int n, int m)
{
threadsArray = new Threads[n];
for(int i=0; i < threadsArray.length; i++)
threadsArray[i] = new Threads(this, i);
iterations = m;
numOfThreads = n;
printResults();
for(int i=0; i < threadsArray.length; i++)
threadsArray[i].start();
}
public synchronized void threadWait()
{
while(!finished)
{
try{ wait(); }
catch(InterruptedException e) {}
}
numOfThreads++;
waitForAll();
if(numOfThreads == threadsArray.length)
{
finished = false;
notifyAll();
}
}
public int checkValue(Threads t, int ind) //returns the thread's value to be updated. 1,-1 or 0.
public int getIterations()
{
return this.iterations;
}
public synchronized void finished()
{
numOfThreads--;
notifyAll();
if(numOfThreads == 0)
finished = true;
}
public void iteration()
{
for(int i=0; i < threadsArray.length; i++)
threadsArray[i].start();
}
public synchronized void updated()
{
numOfThreads--;
if(numOfThreads == 0)
{
printResults();
finished = true;
notifyAll();
}
}
public synchronized void waitForAll()
{
while(numOfThreads != threadsArray.length)
{
try{ wait(); }
catch(InterruptedException e) {}
}
}
public void printResults() // prints the result on each iteration.
}
This problem has puzzled me for a long time, please help me,thanks.
This is my java code.
package com.concurrent.example;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;
/**
* P683
*/
class CircularSet {
private int[] array;
private int len;
private int index = 0;
public CircularSet (int size) {
array = new int[size];
len = size;
for (int i = 0; i < size; i++) {
array[i] = -1;
}
}
public synchronized void add(int i ) {
array[index] = i;
index = ++index % len;
}
public synchronized boolean contains(int val) {
for (int i = 0; i < len; i++) {
if(array[i] == val) {
return true;
}
}
return false;
}
}
public class SerialNumberChecker {
private static final int SIZE = 10;
private static CircularSet serials = new CircularSet(1000);
private static ExecutorService exec = Executors.newCachedThreadPool();
private static int serial;
static class SerialChecker implements Runnable {
#Override
public void run() {
while(true) {
//int serial;
synchronized (serials) {
serial = SerialNumberGenerator.nextSerialNumber();
}
if (serials.contains(serial)) {
System.out.println("Duplicate: " + serial);
System.exit(0);
}
System.out.println(serial);
serials.add(serial);
}
}
}
public static void main(String[] args) throws InterruptedException {
for (int i = 0; i < SIZE; i++) {
exec.execute(new SerialChecker());
if (args.length > 0) {
TimeUnit.SECONDS.sleep(new Integer(args[0]));
System.out.println("No duplicates detected");
System.exit(0);
}
}
}
}
It can stop, but when i uncomment //int serial;The result is different,it can't stop.Why does this temporary variable have a different result than the static variable of the external class. Is this the reason of using a thread?
The code of SerialNumberGenerator:
public class SerialNumberGenerator {
private static volatile int serialNumber = 0;
public static int nextSerialNumber() {
return serialNumber ++; //Not thread-safe
}
}
With private static int serial, all SerialNumberCheckers share the same serial. For example:
Thread1 set serial = 1
Thread2 set serial = 2
Thread1 put 2 into CircularSet.
Thread2 found it duplicate and exit.
However, if you declare another int serial in the run method, It will shadow the private static int serial, which means all threads has its own serial and they will assign & check it. Since the generation of serial is in the synchronized block, there will be no duplicates.
I am trying to create a working progress bar program in Java so it can support both console and GUI applications.
The idea is use the thread to provide the current progress information, but it seems the thread code is not working well. Concurrency is so new to me.
I want it to advance the bar every time it is stepped up by one rather than completing the loop prematurely before the progress bar catches up. I guess the problem is timing?
[=====> ] 10% 1
2
3
4
5
6
7
8
9
10
[==================================================] 100%
Can someone tell me what I have gone wrong?
Main code
package console;
import java.util.ArrayList;
import console.ProgressThread;
public class ConsoleProgressBar
{
private static final long REFRESH_DELAY = 50;
private ProgressValue progress;
private ProgressThread target;
private Thread thread;
protected static class ProgressValue
{
protected long total = 0;
protected long current = 0;
protected ProgressValue(long n)
{
total = n;
}
protected synchronized void setMaxTotal(long n)
{
total = n;
}
protected synchronized void stepBy(long n)
{
current = current + n;
if (current > total) total = current;
}
protected synchronized void stepTo(long n)
{
current = n;
if (current > total) total = current;
}
protected synchronized long getCurrent()
{
return current;
}
protected synchronized long getTotal()
{
return total;
}
}
public ConsoleProgressBar(long totalItem)
{
this(totalItem, REFRESH_DELAY);
}
public ConsoleProgressBar(long totalItem, long refreshDelay)
{
progress = new ProgressValue(totalItem);
target = new ProgressThread(progress, refreshDelay);
}
public void start()
{
thread = new Thread(target);
thread.start();
}
public void stepBy(long n)
{
progress.stepBy(n);
}
public void stepTo(long n)
{
progress.stepTo(n);
}
public void step()
{
progress.stepBy(1);
}
public void setMaxTotal(long n)
{
progress.setMaxTotal(n);
}
public void stop()
{
target.terminate();
try
{
thread.join();
}
catch (InterruptedException ex)
{
}
}
public long getCurrent()
{
return progress.getCurrent();
}
public long getTotal()
{
return progress.getTotal();
}
public static void main(String[] args)
{
ArrayList<Integer> test = new ArrayList<>();
ConsoleProgressBar bar = new ConsoleProgressBar(10, 50);
bar.start();
for (int i = 0; i < 10; i++)
{
int sum = i + 5;
test.add(sum);
bar.step();
System.out.format("%s%n", bar.getCurrent());
}
bar.stop();
}
}
Thread code
package console;
import console.ConsoleProgressBar.ProgressValue;
public class ProgressThread implements Runnable
{
private static final int WIDTH = 50;
private volatile boolean terminated;
private ProgressValue progressRef;
private long timeMS;
public ProgressThread(ProgressValue ref, long refreshDelay)
{
progressRef = ref;
timeMS = refreshDelay;
terminated = false;
}
private void refreshProgressBar()
{
StringBuilder sb = new StringBuilder("\r[");
int percent = (int) Math.floor(100.0 * progressRef.current / progressRef.total);
for (int i = 0; i < WIDTH; i++)
{
if (i < (percent / 2)) sb.append("=");
else if (i == (percent / 2)) sb.append(">");
else sb.append(" ");
}
sb.append("] %s ");
if (percent >= 100) sb.append("%n");
System.out.printf(sb.toString(), percent + "%");
}
void terminate()
{
terminated = true;
}
public void run()
{
try
{
while (terminated == false)
{
refreshProgressBar();
Thread.sleep(timeMS);
}
refreshProgressBar();
}
catch (InterruptedException exc)
{
}
}
}
Why do you need a multithreaded application when it is just one task you are trying to achieve?
Nonetheless, to achieve what you want I suggest moving your execution entirely into either the thread class or into the main class.
If the main application is going to run something else, then ideally you'd put the execution in the thread class. However here I've put the execution into the main class. It could also just as easily go in the thread class.
As an example, I've edited run() in ProgressThread to just be this,
public void run()
{
while( terminated )
{
}
}
And I edited main in ConsoleProgressBar to this,
public static void main(String[] args)
{
ArrayList<Integer> test = new ArrayList<>();
ConsoleProgressBar bar = new ConsoleProgressBar(10, 50);
bar.start();
for (int i = 0; i <= 10; i++)
{
int sum = i + 5;
test.add(sum);
bar.refreshProgressBar();
System.out.format( "%s", bar.getCurrent() );
bar.step();
bar.sleep( 1000 );
}
bar.stop();
}
Note that I added the methods sleep( int n ) and refreshProgressBar() to bar so I can call the thread methods, similar to what you did with bar.start() and bar.stop().
To be clear, in ProgressThread I changed refreshProgressBar to public just for the sake of the example and added the following,
void sleep( int n )
{
try
{
Thread.sleep( n );
}
catch( InterruptedException ie )
{
ie.printStackTrace();
}
}
and the following to ConsoleProgressBar,
private void sleep( int n )
{
target.sleep( n );
}
private void refreshProgressBar()
{
target.refreshProgressBar();
}
The output (each line printing at one second intervals) is,
[> ] 0% 0
[=====> ] 10% 1
[==========> ] 20% 2
[===============> ] 30% 3
[====================> ] 40% 4
[=========================> ] 50% 5
[==============================> ] 60% 6
[===================================> ] 70% 7
[========================================> ] 80% 8
[=============================================> ] 90% 9
[==================================================] 100% 10
Not sure if this is what you are looking for but I suggest putting the execution into one place.
Im trying to make 2 threads that read/write to a counter using thread safe methods.
I have written some code to try test this but the read thread just reads the counter at its max (1000)
Main:
public static void main(String[] args) {
Counter c = new Counter();
Thread inc = new Increment(c);
Thread read = new Read(c);
inc.start();
read.start();
}
Counter:
public class Counter {
private int count;
public Counter() {
count = 0;
}
public synchronized void increment() {
count++;
}
public synchronized int getVal() {
return count;
}
}
Increment:
public class Increment extends Thread {
private static final int MAX = 1000;
private Counter myCounter;
public Increment(Counter c) {
myCounter = c;
}
public void run() {
for (int i = 0; i < MAX; i++) {
myCounter.increment();
}
}
}
Read:
public class Read extends Thread {
private static final int MAX = 1000;
private Counter myCounter;
public Read(Counter c) {
myCounter = c;
}
public void run() {
for (int i = 0; i < MAX; i++) {
System.out.println(myCounter.getVal());
}
}
}
Would I be better off using Atomic Integer to hold the value of the counter to allow me to safely increment it and get the value?
Your code is perfectly fine as is. It just so happened that your increment thread finished all its increments before the read thread got a chance to read. 1,000 increments takes almost no time at all.
If you want interleave execution of Read thread and Increment thread much more often then the natural operating system thread pre-emption, just make each thread give up their lock (by calling <lockedObject>.wait() followed by <lockedObject>.notify() or notifyAll() in the respective run() methods:
[In Reader]:
public void run() {
for (int i = 0; i < MAX; i++) {
synchronized (myCounter) {
System.out.println(myCounter.getVal());
try {
myCounter.wait(0L, 1);
myCounter.notifyAll();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
[In Increment]:
public void run() {
for (int i = 0; i < MAX; i++) {
synchronized (myCounter) {
myCounter.increment();
try {
myCounter.wait(0L, 1);
myCounter.notifyAll();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
Upping the MAX constant to 1_000_000_000 (1 billion) made the treads interleave as well every now and then (on my machine interleave happened just by gazing at few printouts between 150 and 400_000 iterations).
It's a little program written with a purpose of studying multithreading. I expected to get in main method different random numbers after run. About 4 numbers per second. But I got many thousands of zeros. Where is an error?
Main Class:
public class Main {
public static void main(String[] args) {
ExternalWorld externalWorld = new ExternalWorld();
externalWorld.start();
int x = 0;
while (true) {
while(!externalWorld.signal){
System.out.println("qqq");}
System.out.println(++x + ") " + externalWorld.getAnInt());
}
}
}
ExternalWorld Class:
import java.util.Random;
public class ExternalWorld extends Thread {
private int anInt = 0;
public boolean signal = false;
#Override
public void run() {
Random random = new Random(100);
while(true) {
anInt = random.nextInt(100);
signal = true;
try {
Thread.sleep(200);
signal = false;
Thread.sleep(50);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
public int getAnInt() {
if (!signal) {
int p = 1 / 0;
}
int result = anInt;
anInt = 0;
return result;
}
}
problem:
private int anInt = 0;
public boolean signal = false;
You are access those variables from one thread to another thus giving you 0 and false on the main thread
solution:
use volatile keyword to access those variables from multiple threads
sample:
private volatile int anInt = 0;
public volatile boolean signal = false;