I'm new to programming and been studying threads for some time now.
So, the following code should give an output of:
one 98098
two 98099
and it does sometimes.
When I try to run it for a couple of times, it gives different outputs. I can understand that the JVM controls the threads and I can't directly affect it, but some of the outputs are less than 98,000 even though the for loop is adding 1000 for 98 times. How is this happening? Can a thread leave lines behind? Or did I do something wrong (note: the expected output sometimes shows on the screen, but not always)
public class TestThreads {
public static void main(String [] args) {
ThreadOne t1 = new ThreadOne();
Thread one = new Thread(t1);
ThreadTwo t2 = new ThreadTwo();
Thread two = new Thread(t2);
one.start();
two.start();
}
}
class Accum {
private int counter = 0;
private static Accum a = new Accum();
private Accum() {
}
public static Accum getAccum() {
return a;
}
public int getCount() {
return counter;
}
public void updateCounter(int add) {
counter += add;
}
}
class ThreadOne implements Runnable {
Accum a = Accum.getAccum();
public void run() {
for(int x=0; x < 98; x++) {
a.updateCounter(1000);
try {
Thread.sleep(50);
} catch(InterruptedException ex) { }
}
System.out.println("one "+a.getCount());
}
}
class ThreadTwo implements Runnable {
Accum a = Accum.getAccum();
public void run() {
for(int x=0; x < 99; x++) {
a.updateCounter(1);
try {
Thread.sleep(50);
} catch(InterruptedException ex) { }
}
System.out.println("two "+a.getCount());
}
}
Basically, your updateCounter method isn't thread-safe. If it's called from two threads at the same time, you can lose information.
Let's rewrite it to make it more obvious why that's the case:
public void updateCounter(int add) {
// Fetch
int originalValue = counter;
// Compute
int newValue = originalValue + add;
// Store
counter = newValue;
}
Imagine what happens if two threads come into the method at the same time. We'll pretend that there's some "total ordering" of what happens - the reality is more complex than that, but even the simplified form shows the problem. Suppose counter has a value of 5 to start with, and on thread x we're calling updateCounter(3) and on thread y we're calling updateCounter(4). We could imagine this sequence of events:
Thread x executes the "fetch" operation: originalValue = 5 (local variable, unaffected by thread y)
Thread y executes the "fetch" operation: originalValue = 5
Thread x executes the "compute" operation: newValue = 8
Thread y executes the "compute" operation: newValue = 9
Thread x executes the "store" operation: counter = 8 (note that newValue in thread x is separate to the one in thread y)
Thread y executes the "store" operation: counter = 9
So we end up with the value of counter being 9... as if the updateCounter(3) call had never taken place. If the last two operations happened in the reverse order, then counter would be 8 instead.
The way to fix this is to use the AtomicInteger class which is designed specifically to make operations like this atomic:
class Accum {
private final AtomicInteger counter = new AtomicInteger(0);
private static Accum a = new Accum();
private Accum() {
}
public static Accum getAccum() {
return a;
}
public int getCount() {
return counter.get();
}
public void updateCounter(int add) {
counter.addAndGet(add);
}
}
Related
I am trying to learn Multi-threading and I am trying to print odd & even number using two thread but i am not sure how to synchronized the for loop and make it print from 1 to 10 in order.
public class Counter implements Runnable {
public static void main(String[] args) {
Thread t1 = new Thread(new Counter(1, " ODD")); // Thread 1 runs the Odd number
Thread t2 = new Thread(new Counter(0, " EVEN")); // Thread 2 runs the Even number
t1.start();
t2.start();
}
constructor:
int num; // gets the number
String name; // gets the name
public Counter(int i, String name) {
this.num = i;
this.name = name;
}
This is the Loop im using to create Odd and Even number and i dont know how to synchronized this loop.
public void printNum() {
synchronized (this) {
for (int j = this.num; j <= 10; j += 2) {
System.out.println(name + "-->" + j);
}
}
}
#Override
public void run() {
//this will run the printNum to the Threads
printNum();
}
Mb something like this
public class Counter implements Runnable{
#Override
public void run() {
try {
printNum();
} catch (InterruptedException e) {
throw new RuntimeException(e);
}
}
public static void main(String[] args){
Thread t1 = new Thread(new Counter(1, " ODD")); // Thread 1 runs the Odd number
Thread t2 = new Thread(new Counter(0, " EVEN")); // Thread 2 runs the Even number
t2.start();
t1.start();
}
int num; // gets the number
String name; // gets the name
public Counter(int i, String name) {
this.num = i;
this.name = name;
}
public void printNum() throws InterruptedException {
synchronized (this) {
for (int j = this.num; j <= 10; j += 2) {
System.out.println(name + "-->" + j);
Thread.sleep(100);
}
}
}
}
Result:
public class HelloWorld {
public static void main(String[] args) {
counter e = new counter();
counter o = new counter();
e.neighbor = o;
o.neighbor = e;
e.wait = false;
o.wait = true;
e.count = 0;
o.count = 1;
Thread te = new Thread(e);
Thread to = new Thread(o);
te.start();
to.start();
}
static class counter implements Runnable{
public counter neighbor = null;
public boolean wait = false;
public int count = -1;
#Override
public void run(){
while (count <= 10){
if (!wait){
System.out.print("count = " + count + "\n");
count += 2;
wait = true;
neighbor.wait = false;
}
}
wait = true;
neighbor.wait = false;
}
}
}
Often when you have two threads interdependent on each other, like in these case where odd needs to wait until even has finished and vice versa, we need to establish some kind of relation in order for them to communicate with each other, the reason why your code wasn't working was because synchronize makes the thread wait until the other one has finished, in the loop however, the entire loop is considered one task and one thread will wait for the other to finish their loop first.
Basically I have a method that I want to synchronize (only let one thread in at a time), however I'm not allowed to use the synchronized keyword for this particular practice. Instead, I decided to create a semaphore and just set the value to 1, therefore acting like a mutually exclusive lock (which is what I believe the synchronized keyword does right?).
So basically, my Semaphore class looks like this:
import java.util.*;
public class Semaphore
{
private int count = 0;
public Semaphore (int init_val) {
count = init_val;
}
public synchronized void P() {
count = count - 1;
while(count < 0) {
try {
wait();
} catch(InterruptedException e) {
}
}
}
public synchronized void V() {
count = count + 1;
notifyAll();
}
}
And I'm using it inside the method I want to synchronize like this (just an example):
Semaphore s = new semaphore(1);
int x = 0;
public void add() {
s.P()
int x = x + 1;
System.out.println(x);
s.V()
}
I have 100 threads calling the add method, but for some reason the value of x is concurrent, but not going up in order (race condition). I'm not sure what I'm doing wrong, any advice?
I am wondering why the result is not 400 000. There are two threads why does it gets blocked?
class IntCell {
private int n = 0;
public int getN() {return n;}
public void setN(int n) {this.n = n;}
}
class Count extends Thread {
private static IntCell n = new IntCell();
#Override public void run() {
int temp;
for (int i = 0; i < 200000; i++) {
temp = n.getN();
n.setN(temp + 1);
}
}
public static void main(String[] args) {
Count p = new Count();
Count q = new Count();
p.start();
q.start();
try { p.join(); q.join(); }
catch (InterruptedException e) { }
System.out.println("The value of n is " + n.getN());
}
}
Why there is so problem with that?
Because the way you increment your variable is not an atomic operation indeed to increment it you:
Get the previous value
Add one to this value
Set a new value
They are 3 operations not done atomically you should either us a synchronized block or use an AtomicInteger instead.
With a synchronized block it would be something like:
synchronized (n) {
temp = n.getN();
n.setN(temp + 1);
}
With an AtomicInteger you will need to rewrite your code as next:
class IntCell {
private final AtomicInteger n = new AtomicInteger();
public int getN() {return n.get();}
public void incrementN(int n) {this.n.addAndGet(n);}
}
for (int i = 0; i < 200000; i++) {
n.incrementN(1);
}
The approach with an AtomicInteger is non blocking so it will be faster
When two threads access one object at the same time, they interfere with each other, and the result is not deterministic. For example, imagine that p reads the value of n and gets, say, 0, then q reads the same value and gets 0 too, then p sets value to 1 and q also sets it to 1 (because it still thinks that it has value 0). Now the value of n is increased by 1, even though both counters "incremented" it once. You need to use synchronized block to make sure the counters won't interfere with each other. See https://docs.oracle.com/javase/tutorial/essential/concurrency/locksync.html for more.
The problem here is that you allow for race conditions. Consider the block inside the loop:
temp = n.getN();
n.setN(temp + 1);
The code context switch between the time you get the current N and by the time you increment it, making you set an "old" value. One way around this is to ensure the inner part of the loop runs in a synchronized block:
for (int i = 0; i < 200000; i++) {
synchronized (n) { / Here!
temp = n.getN();
n.setN(temp + 1);
}
}
At work training, I'm writing a Java (in which I have 0 experience) program that should meet the following criteria:
Write a program that replicates distributed computing application
Create central 'scheduler' object which contains a list of M random numbers
Create N processor threads that retrieve a number from the scheduler then loop that many times before requesting another number
If no numbers are available from the scheduler, wait to request another number.
If no more numbers are left, all the threads should end.
So far, I created an object with an array of random numbers, but I really don't know how to proceed with multithreading. Could someone please guide me through it? This is what I have so far, along with comments indicating pseudo code.
public class ThreadDemo extends Thread
{
//create new array of arbitrary size 5
static int SIZE = 5;
static int[] myIntArray = new int[SIZE];
public ThreadDemo()
{
start();
}
class RunnableThread implements Runnable {
Thread runner;
public RunnableThread() {
}
public RunnableThread(String threadName) {
runner = new Thread(this, threadName); // (1) Create a new thread.
System.out.println(runner.getName());
runner.start(); // (2) Start the thread.
}
public void run() {
//Display info about this particular thread
System.out.println(Thread.currentThread());
}
}
public static void main(String[] args)
{
for(int i=0; i<SIZE; i++)
{
myIntArray[i] = (int)(Math.random() * 10);
}
ThreadDemo scheduler = new ThreadDemo();
//create M processor threads that retrieve number from scheduler
//for(int i=0; i<SIZE; i++)
//
//if no threads available
//make the scheduler thread wait() ??
//if empty
//stop() the scheduler thread ??
}
}
Could anyone steer me in the right direction?
Thank you!
As a first pointer: don't start threads in a constructor and don't use the Runnable object to start a thread using itself. It's very confusing to whoever reads the code.
Here's my take on this problem (hope I didn't get carried away):
class Scheduler {
private int[] numbers;
private AtomicInteger current = new AtomicInteger();
public Scheduler(int count) {
Random rand = new Random();
numbers = new int[count];
for(int i = 0; i < count; i++) {
numbers[i] = rand.nextInt();
if(numbers[i] < 0) numbers[i] *= -1;
}
}
public int getNextNumber() {
int local = current.incrementAndGet();
if(local >= numbers.length) {
return -1;
}
return numbers[local];
}
}
First, we define the Scheduler class that holds an array of random (positive) integers and returns a number from the array on-demand, based on an atomically incrementing counter.
class Task implements Runnable {
private Scheduler scheduler;
public Task(Scheduler scheduler) {
this.scheduler = scheduler;
}
public void run() {
while(true) {
int limit = scheduler.getNextNumber(); // get next number
if(limit == -1) return; // no more numbers
System.out.println(limit);
for(int i = 0; i < limit; i++) {
// spin
}
}
}
}
The Task class holds the code that each thread executes. Each thread loops indefinitely requesting numbers from the Scheduler, until the array is exhausted.
public class Test {
public static void main(String[] args) throws InterruptedException {
Scheduler s = new Scheduler(100);
ExecutorService exec = Executors.newFixedThreadPool(4);
for(int i = 0; i < 4; i++) {
exec.submit(new Task(s));
}
exec.shutdown();
exec.awaitTermination(Long.MAX_VALUE, TimeUnit.DAYS);
}
}
In the main class we set up a thread pool and execute 4 threads to do the aforementioned tasks.
This is a good place to start. IT will also help to look at a executor service. Here is an example.
You might also want to take a look at some of the concurrent collections. It might be worth using a queue instead of an array so its a little cleaner to tell when something has been pulled out of it.
As per my understanding of your Homework, you need to create a producer and worker thread units. Please refer the below link, which will suits your requirement.
http://www.exampledepot.com/egs/java.lang/WorkQueue.html
Thanks
Thanikachalan
You might want to take a look at te ThreadPoolExecutor
You should end up with something like this.
public static void main(){
ThreadPoolExecutor tpe = new ThreadPoolExecutor(...);
List<Integer> numbers = getNumberList();
for(Integer i : numbers){
tpe.submit(new MyRunnable(i) {
Integer i;
public MyRunnable(Integer i){
this.i=i;
}
#Override
public void run() {
dosomethingWith(i);
}
}
}
}
This program in Java creates a list of 15 numbers and creates 3 threads to search for the maximum in a given interval. I want to create another thread that takes those 3 numbers and get the maximum. but i don't know how to get those values in the other thread.
public class apple implements Runnable{
String name;
int time, number, first, last, maximum;
int[] array = {12, 32, 54 ,64, 656, 756, 765 ,43, 34, 54,5 ,45 ,6 , 5, 65};
public apple(String s, int f, int l){
name = s;
first = f;
last = l;
maximum = array[0];
}
public void run(){
try{
for(int i = first; i < last; i++ )
{
if(maximum < array[i])
{
maximum = array[i];
}
}
System.out.println("Thread"+ name + "maximum = " + maximum);
}catch(Exception e){}
}
public static void main(String[] args){
Thread t1 = new Thread(new apple("1 ", 0, 5));
Thread t2 = new Thread(new apple("2 ", 5, 10 ));
Thread t3 = new Thread(new apple("3 ", 10, 15));
try{
t1.start();
t2.start();
t3.start();
}catch(Exception e){}
}
}
Here is how ExecutorService and ExecutorCompletionService can solve it:
public class MaxFinder {
private int[] values;
private int threadsCount;
public MaxFinder(int[] values, int threadsCount) {
this.values = values;
this.threadsCount = threadsCount;
}
public int find() throws InterruptedException {
ExecutorService executor = Executors.newFixedThreadPool(threadsCount);
ExecutorCompletionService<Integer> cs = new ExecutorCompletionService<Integer>(executor);
// Split the work
int perThread = values.length / threadsCount;
int from = 0;
for(int i = 0; i < threadsCount - 1; i++) {
cs.submit(new Worker(from, from + perThread));
from += perThread;
}
cs.submit(new Worker(from,values.length));
// Start collecting results as they arrive
int globalMax = values[0];
try {
for(int i = 0; i < threadsCount; i++){
int v = cs.take().get();
if (v > globalMax)
globalMax = v;
}
} catch (ExecutionException e) {
throw new RuntimeException(e);
}
executor.shutdown();
return globalMax;
}
private class Worker implements Callable<Integer> {
private int fromIndex;
private int toIndex;
public Worker(int fromIndex, int toIndex) {
this.fromIndex = fromIndex;
this.toIndex = toIndex;
}
#Override
public Integer call() {
int max = values[0];
for(int i = fromIndex; i<toIndex; i++){
if (values[i] > max)
max = values[i];
}
return max;
}
}
}
In this solution, N threads work concurrently, each on its portion of the array. The caller thread is responsible for gathering the local maximums as they arrive, and find the global maximum. This solution uses some non-trivial concurrency tools from java.util.concurrent package.
If you prefer a solution that only uses primitive synchronization tools, then you should use a synchronized block in the worker threads, that sets the maximum in some data member and then notifies the collector thread. The collector thread should be in a loop, waiting for notification and then examining the new number, and updating the global maximum if needed. This "consumer producer" model requires careful synchronization.
Based on the code you have, the simplest solution is to join the main thread to each instance thread and then get the max value from them for comparison purposes. Like so:
int globalMax;
try{
t1.start();
t2.start();
t3.start();
t1.join();
globalMax = t1.maximum;
t2.join();
if (t2.maximum > globalMax) {
globalMax = t2.maximum;
}
t3.join();
if (t3.maximum > globalMax) {
globalMax = t3.maximum;
}
} catch(Exception e){
}
Instead of implementing Runnable, try implementing Callable, which is capable of returning a result. The tutorial given here is a good source for describing how to do this.
Another approach to your problem could be to create an object which each apple instance (not sure why you've called it this) could register its maximum with the object. This new class could be passed into each apple constructor, then the apple could call a method, passing its own maximum into this.
For instance:
public class MaximumOfMaximumsFinder implements Runnable {
private List<Integer> maximums = new ArrayList<Integer>();
public void registerSingleMaximum(Integer max) {
maximums.add(max);
}
public void run() {
// use similar logic to find the maximum
}
}
There are several issues around making sure this is coordinated with the other threads, I'll leave this to you, since there's some interesting things to think about.