Question

I have two threads, a consumer and a producer. The consumer thread is the main thread while the producer thread is created by a third party library I use.

You make a request for a List of data to the producer thread using ProducerChannel‘s requestData(), which returns immediately. Then, the producer thread will generate data one by one asynchronously and uses a call back method to send each of them. I want the method that requests data to return the result synchronously. The most straightforward way would be to use wait() and notify() like below.

public class DataFeed {
    boolean done;
    private List<Data> dataList;
    private ProducerChannel producerChannel;

    // This method should be synchronous.
    public List<Data> getDataList() {
        this.producerChannel.requestData();

        while (!done) {
            wait();
        }

        List<Data> dataList = this.dataList;
        this.dataList = null;

        return dataList;
    }

    // This is the call back method invoked by the producer thread.  
    public void generated(Data data) {
        if (data == null) {
            done = true; // End of data.
            notify();
        }
        else {
            this.dataList.add(data);
        }
    }
}

Note that there’s only one consumer thread in the entire application. That’s why DataFeed has only one List to hold the result for each request. I learned that the Executor framework is now the preferred way to manage threads. How can I refactor this class so that it does not use Thread objects explicitly while not creating additional threads?

Answer 1

I’m the OP. While BlockingQueue would certainly work, I learned that Semaphore would be a simpler solution.