Java SynchronousQueue is a blocking queue that is designed to support the exchange of data between two threads. It has a capacity of zero, which means that each put() operation must be paired with a corresponding take() operation before the data can be exchanged.

SynchronousQueue provides a simple way for two threads to communicate and exchange data in a thread-safe manner. One thread can put data into the queue using the put() method, and the other thread can take the data from the queue using the take() method. The put() operation will block until another thread calls take() to remove the data, and vice versa.

Here's an example of how to use SynchronousQueue in Java to exchange data between two threads:

SynchronousQueue<String> queue = new SynchronousQueue<>();

// Thread 1
new Thread(() -> {
    try {
        String data = "Hello, world!";
        queue.put(data); // block until Thread 2 takes the data
        System.out.println("Thread 1: data has been sent");
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
}).start();

// Thread 2
new Thread(() -> {
    try {
        String data = queue.take(); // block until Thread 1 puts the data
        System.out.println("Thread 2: received data: " + data);
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
}).start();

In this example, a SynchronousQueue is created to exchange data between two threads. Thread 1 puts the data into the queue using the put() method, which blocks until Thread 2 calls take() to remove the data. Thread 2 then takes the data from the queue using the take() method, which blocks until Thread 1 calls put() to insert the data into the queue.

SynchronousQueue can be a useful tool for implementing a simple messaging system between two threads in a thread-safe manner. Since it has a capacity of zero, it doesn't store any data, which means that it can be more efficient than other blocking queues in certain scenarios.