QueueWorker.java

package bo.gotthardt.queue;

import com.google.common.base.Preconditions;

import java.io.Closeable;
import java.io.IOException;

/**
 * A worker that runs a processing function on messages from a specific message queue.
 * Subclass this to create workers for specific tasks.
 *
 * Having this as a concrete class rather than passing the processing function directly to the
 * queue lets us make the link between queue and function explicit. It also allows the worker to be instantiated by
 * Guice/HK2 without TypeLiteral binding configuration craziness.
 *
 * @author Bo Gotthardt
 */
public abstract class QueueWorker<T> implements Runnable {
    private final MessageQueue<T> queue;
    private Closeable cancel;

    public QueueWorker(MessageQueue<T> queue) {
        this.queue = queue;
    }

    protected abstract Void process(T message);

    @Override
    public void run() {
        cancel = queue.consume(this::process);
    }

    public void cancel() {
        Preconditions.checkNotNull(cancel, "Cannot cancel before run.");
        try {
            cancel.close();
        } catch (IOException e) {
            throw new MessageQueueException("Unable to close.", e);
        }
    }
}