Welcome back to our advanced concurrency journey! Previously, we explored how to coordinate threads using the CyclicBarrier. Today, we will take that concept further by learning about the Phaser, a versatile synchronization tool. Unlike CyclicBarrier, the Phaser allows more flexible handling of phased activities, especially in dynamic scenarios where threads may join or leave at different stages.

In this lesson, you will:

• Understand the role of the Phaser in coordinating multithreaded tasks.
• Learn about the Phaser lifecycle, including registration, arrival, and phase advancement.
• Explore how to dynamically add and remove participants.
• Implement a multi-phase race simulation using the Phaser.

By the end of this lesson, you'll be equipped to manage tasks that require multiple phases and varying numbers of threads.

The Phaser is a synchronization utility that allows a set of threads to wait for each other to reach specific points (or barriers) in the program before continuing their execution. Unlike simpler tools like CountDownLatch or CyclicBarrier, the Phaser can handle multiple phases of execution and dynamically register or deregister participants. This makes it ideal for complex workflows with evolving thread participation.

In essence, the Phaser ensures that all registered threads arrive at a synchronization point (the barrier) before moving to the next step. Each participating thread must register itself with the Phaser instance. Once a thread reaches the barrier, it signals its arrival. When all registered threads have arrived, the Phaser advances to the next phase, allowing all threads to proceed.

Now, let's look at an example where we simulate a race, with participants reaching multiple checkpoints before completing the race.

We'll start by creating the RaceParticipant class, which will represent the participants in our race. Each participant will synchronize with others at each checkpoint.

Java
Copy to clipboard
1import java.util.concurrent.Phaser;
2
3public class RaceParticipant implements Runnable {
4    private Phaser phaser;
5
6    public RaceParticipant(Phaser phaser) {
7        this.phaser = phaser;
8        this.phaser.register(); // Registering the participant with the Phaser
9    }
10
11    @Override
12    public void run() {
13        reachCheckpoint("Checkpoint 1");
14        phaser.arriveAndAwaitAdvance(); // Waiting for all participants to reach Checkpoint 1
15
16        reachCheckpoint("Checkpoint 2");
17        phaser.arriveAndAwaitAdvance(); // Waiting for all participants to reach Checkpoint 2
18
19        finishRace();
20    }
21}

Here, each RaceParticipant thread registers with the Phaser during initialization by calling the register() method. This ensures that the Phaser tracks the participant. After registering, each participant proceeds to its first checkpoint and calls arriveAndAwaitAdvance() to wait for other participants before moving forward.

Before moving on with our implementation, let's look at some of they key methods of Phaser:

• register(): This method registers a new participant with the Phaser. It increases the number of registered parties that the Phaser tracks, ensuring that the Phaser accounts for the new participant during synchronization.

• arriveAndAwaitAdvance(): This method is called when a participant reaches a checkpoint and is waiting for others to arrive. The participant calls this method to notify the Phaser that it has arrived and to wait until all other participants reach the same phase (or checkpoint). Once all participants have arrived, they advance to the next phase together.

• arrive(): Unlike arriveAndAwaitAdvance(), this method simply signals the Phaser that the participant has arrived at a phase, without waiting for others to complete it. It is useful when the participant doesn’t need to wait for synchronization but needs to inform the Phaser that it has reached a milestone.

• awaitAdvance(): This method allows a thread to wait for the Phaser to advance to the next phase without arriving at the current phase. It is useful when a thread is observing the phasing process without actively participating.

These methods are crucial for coordinating concurrent tasks in scenarios like races or stages where participants must synchronize at specific checkpoints.

Next, we'll add the logic to simulate reaching the checkpoints and finishing the race.

Java
Copy to clipboard
1private void reachCheckpoint(String checkpoint) {
2    System.out.println(Thread.currentThread().getName() + " is reaching " + checkpoint);
3    try {
4        Thread.sleep((long) (Math.random() * 2000) + 500); // Simulate time to reach the checkpoint
5    } catch (InterruptedException e) {
6        e.printStackTrace();
7    }
8    System.out.println(Thread.currentThread().getName() + " reached " + checkpoint);
9}
10
11private void finishRace() {
12    phaser.arriveAndDeregister(); // Deregistering the participant after finishing the race
13    System.out.println(Thread.currentThread().getName() + " has finished the race.");
14}

In the reachCheckpoint() method, we simulate the time it takes for each participant to reach a checkpoint. After reaching a checkpoint, the participant waits for all others to arrive at the same point. The finishRace() method is called after all phases are completed, and the participant deregisters from the Phaser using arriveAndDeregister().

The Phaser method arriveAndAwaitAdvance() ensures that each participant waits for others before moving to the next phase. Once all participants reach the checkpoint, they advance to the next phase, simulating the progress of the race.

Now, let's create the main class, where the race is coordinated, and the participants (threads) are started.

Java
Copy to clipboard
1import java.util.concurrent.Phaser;
2
3public class Main {
4    public static void main(String[] args) {
5        final int NUM_RACERS = 4; // Number of race participants
6
7        // Create a Phaser with 1 party for the main thread
8        Phaser phaser = new Phaser(1);
9
10        // Create and start racer threads
11        for (int i = 0; i < NUM_RACERS; i++) {
12            new Thread(new RaceParticipant(phaser), "Racer " + (i + 1)).start();
13        }
14
15        // Main thread controls the start of each checkpoint
16        System.out.println("Race starts! Participants are heading to Checkpoint 1...");
17        phaser.arriveAndAwaitAdvance(); // Wait for all participants to reach Checkpoint 1
18
19        System.out.println("Participants are heading to Checkpoint 2...");
20        phaser.arriveAndAwaitAdvance(); // Wait for all participants to reach Checkpoint 2
21
22        // Deregister the main thread after all participants have finished
23        phaser.arriveAndDeregister();
24        System.out.println("All participants finished the race! Main thread finished.");
25    }
26}

In the Main class, the Phaser is initialized with one party for the main thread itself, which controls the race. After starting all participant threads, the main thread uses arriveAndAwaitAdvance() to synchronize with the participants, ensuring that all participants reach each checkpoint before continuing.

Once the race is complete, the main thread calls arriveAndDeregister() to signal that it's finished coordinating the race.

Understanding and implementing phased synchronization using the Phaser is essential for developers working with complex, multithreaded programs. Phasers provide a versatile and dynamic approach to managing thread coordination, especially in scenarios where the number of participants may vary or where tasks are divided into multiple stages.

By using Phaser, you can manage coordination between threads efficiently, ensuring that all participants move through the program's phases at the right pace. This approach is particularly useful in simulations, games, workflows with interdependent tasks, or any system requiring synchronized multistep processing.

With these concepts in hand, you're now ready to apply Phaser to real-world tasks in the upcoming practice section. Let's put this knowledge to the test!