Make the ArrayList add thread-safe without using thread safe collections

Question

I've made a code executing 4 threads that would add 5000 integers each to a list. At the end of the execution, I'm supposed to have 4 * 5000 elements in my list. But I end up with only 1190 (the number may change)

Here the code :

import java.util.ArrayList;
import java.util.List;

public class ConcurentList {
    public static void main(String[] args) throws InterruptedException {
        int range = 5000;
        int nbThreads = 4;

        List<Integer> integers = new ArrayList<>();
        ThreadSafeArrayList<Integer> integerThreadSafeArrayList = new ThreadSafeArrayList<>(integers);
        Thread[] arrayOfThread = new Thread[nbThreads];
        for (int i = 0; i < nbThreads; i++)
        {
            arrayOfThread[i] = new Thread(() -> {
                for (int j = 0; j < range; j++) {
                    integerThreadSafeArrayList.add(1);
                }
            });
            arrayOfThread[i].start();
            System.out.println("Le thread " + i + " ended.");
        }
        System.out.println("Size of the list " + integerThreadSafeArrayList.getSize());
    }
}

And ThreadSafeArrayList :

import java.util.List;

public class ThreadSafeArrayList<T> {
    private List<T> list;
    private final Object lock = new Object();

    public ThreadSafeArrayList(List<T> list) {
        this.list = list;
    }

    public boolean add(T element) {
        boolean add;
        synchronized (lock) {
            add = list.add(element);
        }
        return add;
    }

    public int getSize() {
        return list.size();
    }
}

I understand that arraylist isn't thread safe and therefor the predict the behaviour of the execution but since I used a synchronized on private final lock I expected to obtain the normal behaviour.

What am I missing ?

janardhan sharma · Accepted Answer · 2018-11-18 15:51:17Z

1

That is because, when this gets executed,

System.out.println("Size of the list " + integerThreadSafeArrayList.getSize());

the other threads are still running and doing the work that you have asked them to do.

To see the actual result, add this line just before the last statement.

TimeUnit.SECONDS.sleep(5);

The output I can see now is:

Le thread 0 ended.
Le thread 1 ended.
Le thread 2 ended.
Le thread 3 ended.
Size of the list 20000

answered Nov 18, 2018 at 15:51

janardhan sharma

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1 Comment

Omegaspard Over a year ago

I'm so dumb... Ty

Aleksandr Semyannikov · Accepted Answer · 2018-11-18 15:54:07Z

1

There are 2 issues:

You need to join started threads to let them finish their work before method main ends.
```
for (Thread thread : arrayOfThread) {
    thread.join();
}
```

Method getSize is not thread safe, thus you need to use synchronization:

public int getSize() {
    synchronized (lock) {
        return list.size();
    }
}

answered Nov 18, 2018 at 15:54

Aleksandr Semyannikov

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Comments

Ilya · Accepted Answer · 2018-11-18 15:54:40Z

0

wait result

for (int i = 0; i < nbThreads; i++) {
    arrayOfThread[i].join();
}

answered Nov 18, 2018 at 15:54

Ilya

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