Welcome back to our journey into advanced concurrency! In the previous lesson, we explored phased activities using the Phaser. Today, we'll dive into a different synchronization tool: the Exchanger. This utility is designed for two threads to exchange data at a synchronization point, making it perfect for direct communication between threads in producer-consumer patterns.

In this lesson, we will cover:

• The role of the Exchanger in facilitating thread communication.
• How to use the exchange() method for data swapping.
• Practical applications like pipeline designs where the Exchanger excels.

By the end of this lesson, you'll be able to implement efficient thread communication using the Exchanger, ensuring synchronized data exchange between threads.

The Exchanger is a synchronization tool that enables two threads to swap objects at a specific point. It provides a one-to-one synchronization, where each thread waits until its partner thread is ready to exchange data. This tool is useful in scenarios where two threads need to communicate directly, like in producer-consumer setups, without the complexity of multiple threads interacting at once.

The key method in Exchanger is exchange(). When one thread calls this method, it sends data and waits to receive data from the other thread. If one thread arrives first, it will wait for the other thread, ensuring a synchronized exchange of information.

Let’s break this down through a practical example.

First, we need to set up the Exchanger that will allow our two threads (producer and consumer) to exchange data.

Java
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1import java.util.concurrent.Exchanger;
2
3public class Main {
4    public static void main(String[] args) {
5        Exchanger<String> exchanger = new Exchanger<>();
6        
7        // Rest of the implementation
8    }
9}

Here, the Exchanger is created, which will hold the string data that the producer and consumer will swap. This object acts as the meeting point for both threads to exchange their data.

Next, let’s create the Producer, which will generate messages and pass them to the consumer through the Exchanger.

Java
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1// Producer Runnable
2Runnable producer = () -> {
3    String[] messages = { "Message 1", "Message 2", "Message 3" };
4    String data = "";
5
6    for (int i = 0; i < messages.length; i++) {
7        data = messages[i];
8        System.out.println("Producer is producing data: " + data);
9
10        try {
11            // Exchange data with consumer
12            data = exchanger.exchange(data);
13        } catch (InterruptedException e) {
14            e.printStackTrace();
15        }
16    }
17};

In this Producer role, we simulate the production of three messages. For each message, the exchange(data) method is called, passing the message to the consumer. If the consumer isn’t ready to receive, the producer waits until the exchange is possible.

Now, we’ll implement the Consumer that will receive the messages produced by the Producer through the Exchanger.

Java
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1// Consumer Runnable
2Runnable consumer = () -> {
3    String data = "";
4
5    for (int i = 0; i < 3; i++) {
6        try {
7            // Exchange data with producer
8            data = exchanger.exchange("");
9            System.out.println("Consumer received data: " + data);
10        } catch (InterruptedException e) {
11            e.printStackTrace();
12        }
13    }
14};

The Consumer waits to receive data from the Producer by calling exchange(""). This method will block the consumer until the producer is ready to exchange data. Once the producer sends a message, the consumer will receive and print it.

Finally, let's start both the Producer and Consumer threads to see how they interact.

Java
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1// Start Producer and Consumer threads
2new Thread(producer).start();
3new Thread(consumer).start();

This snippet launches both threads, allowing them to synchronize at the Exchanger point and swap data back and forth. The producer sends its messages, and the consumer receives them, simulating a simple data exchange scenario.

Using an Exchanger helps efficiently manage direct data flow between threads, especially in tasks that rely on tight coordination, such as producer-consumer setups. By swapping data at a precise synchronization point, you avoid race conditions and ensure that both threads proceed only when they have successfully exchanged their data.

Understanding how to use the Exchanger enables you to design cleaner, more efficient multithreaded systems where thread communication is essential. This technique is commonly applied in pipeline designs, parallel processing tasks, or stages of a workflow where direct handoff of data is required between two threads.

Get ready to practice this concept in upcoming exercises, where you'll implement real-world scenarios using the Exchanger. Let's put these new skills into action!