Seminar 10 - Threads

In this seminar, you will learn about thread safety in java, principles of synchronization, and bad practices to avoid.

Task 1: The Deadlock Empire

• Enter The DeadLock Empire and solve as many tasks as possible for the given time period.
• You can see that the code is written in C# but it will serve well for the purposes of this seminar. However, you may find a functionality that is C# specific.
• Examples:
  • Monitor.Enter(mutex) and Monitor.Exit(mutex) is similar to synchronized block or lock() and unlock().
  • Monitor.TryEnter(mutex) is similar to tryLock().
  • Others can be found at the Concurrent documentation.

Task 2: Thread Quiz

• Look at the code, discuss it with your partner and answer these questions
  • Is this code thread-safe?
  • Is the scope of the synchronization appropriate?
• If the answer to any question above is no, explain why.
• Discuss your answers with tutors.

Question A:

public final class QuestionA {

    private static int id = 0;

    public synchronized int getId() {
        return ++id;
    }
}

Question B:

public final class QuestionB {

    private static final Object LOCK = new Object();
    private int id = 0;

    public int getId() {
        synchronized (LOCK) {
            return ++id;
        }
    }
}

Question C:

public final class QuestionC {

    private String text;

    public void addLine(String line) {
        synchronized (text) {
            text += line;
        }
    }
}

Question D:

public final class QuestionD {

    private final Set<Integer> set = Collections.synchronizedSet(new HashSet<Integer>());

    public boolean addValue(int value) {
        return set.add(value);
    }

    public int count() {
        int value = 0;
        for (int i : set) {
            value += i;
        }
        return value;
    }
}

Task 3: Prepare the repository

Every team needs just a single repository for this seminar.

Pick up a teammate who forks the repository and sets it up for the benefit of the whole team.

Task 3.1: Fork the repository and make it private

• Fork the Counter repository into your personal namespace.

• Make it private, so that it is visible only to explicitly assigned individuals.

  Go to Settings > General and expand Visibility, project features, permissions. Then change the Project visibility and scroll down (to be able) to click on Save changes.

Task 3.2: Make the repository accessible for designated people

• Add all your team members under the Maintainer role.

  Go to Project information > Members and click on Invite Members. Fill in the FI login names of your teammates and choose Maintainer in Select a role. Finally, click on Invite.

• Add both your seminar tutors under the Reporter role.

  You can find the FI login names of your seminar group tutors in Seminars page.

Task 3.3: Clone the repository and create a branch for your work

Each pair clones the repository into a single computer as they are working together as one person during the seminar.

Pair 1 creates a new branch seminar10-pair1:

git switch -c seminar10-pair1

Pair 2 creates a new branch seminar10-pair2:

git switch -c seminar10-pair2

Task 4: Multi-thread counter

Task 4.1: Implement a 3-thread Counter

• Fill in the implementation of Main class to spawn 3 threads. These threads cooperate on generating unique Fibonacci numbers from 0 to 40 and printing them on standard output (not necessarily in the correct order) in the following format:

  Thread-0 0: 0
  Thread-1 1: 1
  Thread-2 2: 1
  Thread-0 3: 2
  …
  Thread-2 39: 63245986
  Thread-0 40: 102334155

Hint: take a look at the Thread documentation.

Task 4.2: Commit and push your implementation

Once you are finished, please commit your implementation and push it to the remote repository.