public class TestThread2 {
static int count = 0;
public static void main(String[] args) {
Thread t = new Thread(new Runnable(){
public void run()
{
for (int i=1; i<=100000; i++) {
count++;
}
}
});
Thread t1 = new Thread(new Runnable(){
public void run()
{
for (int i=1; i<=100000; i++) {
count++;
}
}
});
t.start();
t1.start();
try{
t.join();
t1.join();
}
catch(InterruptedException e){
e.printStackTrace();
}
System.out.println(count);
}
}
The above code prints various values of count such as 131938, 127518 etc. But I think it should always print 20000 as after join() is called, main thread cannot move to next statement until current thread dies. I know I'm missing a basic concept here, but I'm not able to figure it out so please help.
i++ isn't atomic. It's effectively:
int j = i;
i = j + 1;
If two threads are trying to do this at the same time, the reads and writes from different threads could interleave, meaning that the value isn't strictly incrementing in each thread.
Additionally, there is no guarantee that the increased value from one thread is visible to the other thread.
Instead of using int, use AtomicInteger, and addAndGet. AtomicInteger guarantees both the atomicity of the incrementation and the visibility of the value between threads.
static AtomicInteger count = new AtomicInteger();
// In the thread:
count.incrementAndGet();
Note that something very similar to your example is described in the Oracle Java Tutorial.
You are not synchronizing the variables. Instead of static int count, use an AtomicInteger.
Yes main thread blocks on join(). But the main thread does not write to the variable count, so it does not matter what main thread is doing. You have 2 threads, t and t1, which read and write the same variable count without any synchronization, and indeed it causes unstable results.
Related
I'm well aware that this might be considered a duplicate, however I ran through many answers considering my problem here I can't come up with a solution.
I synchronized my runnable with an object shared by multiple threads and explicitly synchronized the method I am using inside, but the outcome of the program is always 3000.
I tried locking the Counter class but it won't change a thing.
Could anyone explain me why none of my actions work in this particular example?
public static void zad3() {
var counter = new Counter();
var toRun = new Runnable() {
#Override
public void run() {
synchronized (counter) {
for (var i = 0; i < 1000; i++) {
counter.add(1);
}
}
}
};
var t1 = new Thread(toRun);
var t2 = new Thread(toRun);
var t3 = new Thread(toRun);
t1.start();
t2.start();
t3.start();
try {
t1.join();
t2.join();
t3.join();
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("counter = " + counter.getCount());
}
public class Counter {
protected long count_ = 0;
public synchronized void add(long value) {
count_ += value;
}
public long getCount() {
return count_;
}
}
edit:
As suggested the problem was in the loop being constantly ran a 1000 times by each of the threads.
My solution:
var toRun = new Runnable() {
#Override
public void run() {
synchronized (counter) {
for (var i = counter.getCount(); i < 1000; i++) {
counter.add(1);
}
}
}
};
Well you have synchronized the complete for loop around the "counter" variable which means that each thread will run tthe block once. 3 X 1000 = 3000
this block will be executed once per thread
for (var i = 0; i < 1000; i++) {
counter.add(1);
}
UPDATE: judging from your comments that you want interrupt on 1000 example code can be:
t1.start();
t2.start();
t3.start();
while(counter.getValue()<1000) {
Thread.sleep(20)
}
Annother suggestion:
public class Incremetor extends Runnable {
Counter counter;
public Incremetor(Counter counter) {
this.counter = counter;
}
public void run() {
counter.increment();
}
}
ExecutorService executorService = Executors.newFixedThreadPool(8); // this mean 8 threads in total to do your runnables.
for (int i=0;i<1000;++i) {
executorService.submit(new Incrementor(counter));
}
So the problem is that you let each thread attempt 1000 increments, so you'll need something like this instead:
while (counter.getCount() < 1000) {
counter.add(1);
}
The solution you have provided may give you the correct result, but you're actually only incrementing the counter from 1 thread. When you make a synchronized block with synchronized(object) { }, all threads will attempt to get the lock for this block, but only one will. That means in your solution, that the first thread which gets the lock, will do all 1000 increments. When the thread releases the lock and lets the others get it, the work is already done. A solution that actually distributes the increments amongst the 3 threads, should therefore not synchronize the entire for-loop.
If you run the while-loop I suggested, you will get a lot closer to 1000, but it may actually be more than 1000. Remember to run your program 10 times or set up a test-function which runs it 100 times and reports back. The problem, is that from the point of reading counter.getCount(), the value may already have changed by another thread. To reliably always get 1000, you could ensure exclusive rights to both reading and writing to the counter:
while (true) {
synchronized (counter) {
if (counter.getCount() < 1000) {
counter.add(1);
} else {
break;
}
}
}
Notice that incrementing one variable like this, is slow. You're only doing 1000, but try with one billion. In fact, the 3-threaded version takes (on my PC) 1m17s, whereas a simple sequential loop takes ~1.2 seconds. You can solve this by splitting the workload amongst the threads and letting them work on a local counter with exclusive rights and then finally add the results.
Im trying to understand threads.
I wrote simple program.
public class Main {
static int counter = 0;
public static void main(String[] args) throws InterruptedException {
Thread t1 = new Thread(() -> {
for (int i = 0; i < 1000; i++) {
counter++;
}
});
Thread t2 = new Thread(() -> {
for (int i = 0; i < 1000; i++) {
counter++;
}
});
t1.start();
t2.start();
t1.join();
t2.join();
System.out.println(counter);
}
}
The result is always 2000, but I do not know understand why. Any of the run methods are not synchronized so why it is giving me always the same result.
If I write:
t1.start();
t1.join();
System.out.println(counter);
t2.start();
System.out.println(counter);
then I got result: 1000,1000. Why it is always equals to 1000?
Your loops are so short that t1 finishes before t2 gets going. Try 100,000 instead. Lack of synchronization does not guarantee you will have concurrency issues but correctly incorporating synchronization will prevent them.
The result is always 2000, but I do not know understand why.
because you were lucky enough to not have any race condition between the two threads. Just because a race condition can happen doesn't mean that it's guaranteed to happen.
Why it is always equals to 1000?
Because the second thread has not executed yet when you print the counter for the second time. Or because it has executed, but since there is no visibilty guarantee due to the lack of synchronization, the main thread can still see the latest value.
The following code is shows how no race condition in thread works, but I don't get the difference between with the synchronized and without it. I thought the static variable counter will be added to 20000 anyway but it turned out that without synchronized counter would be less than 20000. Can you please explain how threads work in this case? Also, in Java, are threads are actually not running "concurrently", instead are they taking turns to run for a while?
public class NoRaceCondition implements Runnable {
private static int counter = 0;
private static Object gateKeeper = new Object();
public static void main(String[] args) {
Thread t1 = new Thread(new NoRaceCondition());
Thread t2 = new Thread(new NoRaceCondition());
t1.start();
t2.start();
try {
t1.join();
t2.join();
} catch (InterruptedException e) { e.printStackTrace(); }
System.out.printf("counter = %d\n", counter);
}
public void run() {
synchronized (gateKeeper) {
for (int i = 0; i < 10000; i++) {
{
counter++;
}
}
}
}
}
You can think of count++ as 3 separate steps.
read the value of count
increment the value
override count with the new incremented value
When multiple thread execute the above steps concurrently, race conditions may occur. 1 example of race condition is
Let count = 1
Let there be 2 threads named A and B
Thread A reads the value of count and get 1
Thread B reads the value of count and get 1
Thread A increments its value and get 2
Thread B increments its value and get 2
Thread A writes the value to count
count is now 2
Thread B writes the value to count
count is now 2 again when it's expected to be 3 after 2 increments
I was reading about AtomicInteger and how its operations are atomic and how these properties make it useful for multithreading.
I wrote the following program to test the same.
I am expecting the final size of the set should be 1000, since each thread loops 500 times and assuming each time a thread calls getNext() it should get a unique number.
But the output is always less than 1000. What am i missing here?
public class Sequencer {
private final AtomicInteger i = new AtomicInteger(0);
public int getNext(){
return i.incrementAndGet();
}
public static void main(String[] args) {
final Sequencer seq = new Sequencer();
final Set<Integer> set = new HashSet<Integer>();
Thread t1 = new Thread(new Runnable() {
#Override
public void run() {
for (int i=0; i<500; i++)
set.add(seq.getNext());
}
},"T1");
t1.start();
Thread t2 = new Thread(new Runnable() {
#Override
public void run() {
for (int i=0; i<500; i++)
set.add(seq.getNext());
}
},"T2");
t2.start();
try {
t1.join();
t2.join();
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(set.size());
}
}
You are missing that HashSet is not thread-safe. In addition the properties of a set would erase all duplicated numbers, so your test would fail if AtomicInteger was not thread-safe.
Try using a ConcurrentLinkedQueue instead.
Edit: Because it has been asked twice: Using a synchronized set works, but it destroys the idea behind using a lock-free algorithm like the Atomic-classes. If in your code above you replace the set with a synchronized set, then the threads will have to block each time add is called.
This will effectively reduce your application to single-threaded, because the only work done happens synchronized. Actually it will even be slower than single-threaded, because synchronized takes its toll as well. So if you want to actually utilize threading, try to avoid synchronized at all cost.
HashSet is not thread safe so you are facing problem.You can use Vector or any collection class which is thread safe or run two thread sequentially if you stricly need to use HashSet.
t1.start();
t1.join();
t2.start();
t2.join();
As mentioned in several answers, it fails due to HashSet not being thread safe.
First, lets verify for the sake of your test, that AtomicInteger is indeed thread-safe then proceed to see why your test failed. Modify your test slightly. Use two hashsets, one for each thread. Finally, after joins, merge the second set into the first, by iterating over the second and adding it to the first which will eliminate duplicates(set property). Then do a count on the first set.
The count will be what you expect. This proves that it is HashSet that is not thread safe and not the AtomicInteger.
So lets look at what aspect is not thread safe. You're doing onlyf add()s, so clearly add() is the operation that is not thread safe causing the loss of numbers. Lets look at an example pseudo-code non-thread safe HashMap add() that would lose numbers(this is obviously not how it implemented, just trying to state one way in which it could be non-thread safe):
class HashMap {
int valueToAdd;
public add(int valueToAdd) {
this.valueToAdd = valueToAdd;
addToBackingStore(this.valueToAdd);
}
}
If multiple threads call add() and they all reach the addToBackingStore() after they've changed this.valueToAdd, only the final value of valueToAdd is added, all other values are overwritten and lost.
Something similar to this is probably what happened in your test.
Try do it in that way using Collections synchronized.
public class Sequencer {
private final AtomicInteger i = new AtomicInteger(0);
public static void main(String[] args) {
final Sequencer seq = new Sequencer();
final Set<Integer> notSafe = new HashSet<Integer>();
final Set<Integer> set = Collections.synchronizedSet(notSafe);
Thread t1 = new Thread(new Runnable() {
#Override
public void run() {
for (int i = 0; i < 500; i++)
set.add(seq.getNext());
}
}, "T1");
t1.start();
Thread t2 = new Thread(new Runnable() {
#Override
public void run() {
for (int i = 0; i < 500; i++)
set.add(seq.getNext());
}
}, "T2");
t2.start();
try {
t1.join();
t2.join();
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(set.size());
}
public int getNext() {
return i.incrementAndGet();
}
}
class ThreadSafe implements Runnable {
int arr[]=new int[]{1,2,3,4,5};
int sum=0;
public void run() {
int result=sum();
System.out.println("for "+Thread.currentThread().getName()+"the value is"+result);
}
public int sum() {
for(int i=0;i<5;i++) {
sum=sum+arr[i];
System.out.println("calculating sum for thread"+Thread.currentThread().getName()+"sum ="+sum);
try {
Thread.sleep(10);
} catch(Exception e) {}
}
return sum;
}
public static void main(String...d) {
ThreadSafe ts=new ThreadSafe();
ThreadSafe ts1=new ThreadSafe();
Thread t=new Thread(ts);
Thread t1=new Thread(ts1);
t1.start();
t.start();
}
}
I was expecting the output not to come 15. because the sum method is not synchronized so more then one thread can execute the sum method at the same time
What I was expecting that because the 2 thread's will execute the sum method instantly so the output should not be 15 because the first thread will update the value of sum to some value which will be read by the another thread.
So my question is why the output of the program come out the way I'm expecting even though i haven't synchronized the sum() method?
You're creating two instances of ThreadSafe, each with their own sum instance variable.
If you want them to overwrite each other (I'm assuming this is just playing around), create two Threads on the same instance of ThreadSafe. Also, mark your sum variable as volatile to indicate that it can be changed by another thread (to avoid internal caching of the value if the compiler detects that it is not changed elsewhere within the method).
For example, change your main method to this:
public static void main(String...d) {
ThreadSafe ts=new ThreadSafe();
Thread t=new Thread(ts);
Thread t1=new Thread(ts);
t1.start();
t.start();
}
And the beginning of your ThreadSafe class definition to this:
class ThreadSafe implements Runnable {
int arr[]=new int[]{1,2,3,4,5};
volatile int sum=0;
Because you have two instances of ThreadSafe being handled by different threads. For producing your desired result it should be like one instance and multiple threads working on that same instance