DaoService.java

package daoservice;

import java.util.function.Consumer;

@SuppressWarnings("unused")
public final class DaoService {
    private ConnectionPool pool = new ConnectionPool(1);

    public void updateTimestamp(long id) {
        perform(conn -> {
            if (!conn.recordExists(id))
                create(id);

            conn.updateTimestamp(id, System.currentTimeMillis());
        });
    }

    private void create(long id) {
        perform(conn -> {
            try {
                conn.createRecord(id);
            } catch (DuplicateKeyException ignored) {

            }
        });
    }

    private void perform(Consumer<Connection> c) {
        Connection conn = pool.borrowConnection();
        try {
            c.accept(conn);
        }
        finally {
            pool.returnConnection(conn);
        }
    }
}

XmlService.java

package xmlservice;

import lombok.extern.slf4j.Slf4j;
import org.w3c.dom.Node;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

@SuppressWarnings("unused")
@Slf4j
public class XmlService {
    static final class Request {
        String originalXmlRequest;
        Node root;
    }

    private final ExecutorService executor = Executors.newFixedThreadPool(1);

    public void enqueue(Request req) {
        log.info("Got XML: {}", req.originalXmlRequest);
        executor.execute(() -> process(req.root));
    }

    private void process(Node root) {
        // do stuff on parsed xml DOM tree starting on 'root'
        try {
            Thread.sleep(5000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}