Before you can use the java concurrent API, you need to add some dependencies to your "pom.xml" file.

Here's the minimal dependencies needed in order for the following examples to work:

<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-lang3</artifactId>
    <version>3.17.0</version>
</dependency>

First we will look at this example:

private Integer id = Integer.valueOf(0);

public void updateId(final Integer localId) {
    this.id = localId;
    System.out.println("localId: " + localId + " / id:" + this.id);
}

The problem with this example is if multiple threads call this method, the value of the instance variable "id" that we print at the end of the method can be different from the value of the parameter "localId".

If 5 concurrent threads call this method, the output could be something like the following:

localId: 2 / id:3
localId: 4 / id:3
localId: 1 / id:3
localId: 0 / id:3
localId: 3 / id:3

public class SampleReentrantLock {
    private final ReentrantLock lock = new ReentrantLock();

    private Integer id = Integer.valueOf(0);

    public void updateId(final Integer localId) {
        try {
            lock.lock();

            this.id = localId;

            // simulate a time-consuming code
            try {
                Thread.sleep(500l);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }

            System.out.println("localId: " + localId + " / id:" + this.id);
        } finally {
            lock.unlock();
        }
    }

    public static void main(String[] args) throws InterruptedException {
        final String poolName = "_newFixedThreadPool";

        final BasicThreadFactory basicThreadFactory = new BasicThreadFactory.Builder().namingPattern(poolName).daemon(true).build();

        final ExecutorService executorService = Executors.newFixedThreadPool(5, basicThreadFactory);

        final SampleReentrantLock instance = new SampleReentrantLock();

        IntStream.iterate(0, i -> i + 1).limit(5).forEach(t -> executorService.execute(() -> {
            try {
                // wait (random time) before calling the update method
                Thread.sleep(new Random().nextInt(500) + 1);
            } catch (Exception e) {
                e.printStackTrace();
            }

            instance.updateId(t);
        }));

        executorService.awaitTermination(10000l, TimeUnit.MILLISECONDS);

        executorService.shutdown();
    }
}

Output:

localId: 2 / id:2
localId: 0 / id:0
localId: 1 / id:1
localId: 4 / id:4
localId: 3 / id:3