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Processes given SQL query in parallel in multiple data sources (assuming that all data source contain the same data). Combined results from multiple queries are exposed to application as java.util.Iterator. Worker thread use spring's JdbcTemplate with named parameters support.

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package com.alexkasko.springjdbc.parallel.accessor;

import org.springframework.jdbc.core.namedparam.NamedParameterJdbcOperations;
import org.springframework.jdbc.core.namedparam.SqlParameterSource;

import java.util.Collection;
import java.util.concurrent.atomic.AtomicInteger;

/**
 * Round-robin collection accessor. Copies provided collection into inner immutable one on creation.
 * Element is chosen in circular order ignoring provided parameter.
 * Thread-safe.
 *
 * @author  alexkasko
 * Date: 6/11/12
 * @see DataSourceAccessor
 */
public class RoundRobinAccessor extends ImmutableAccessor {
    private AtomicInteger index = new AtomicInteger(0);

    /**
     * Protected constructors
     *
     * @param target collection to wrap
     */
    public RoundRobinAccessor(Collection target) {
        super(target);
    }

    /**
     * Generic friendly factory method
     *
     * @param target collection to wrap
     * @param  JdbcOperations implementation
     * @return new accessor instance
     */
    public static  RoundRobinAccessor of(Collection target) {
        return new RoundRobinAccessor(target);
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public T get(SqlParameterSource params) {
        return delegate.get(incrementAndGet());
    }

    /**
     * Returns current index value
     *
     * @return current index
     */
    public int getIndex() {
        return index.get();
    }

    /**
     * Atomically increments index using target list size modulus
     *
     * @return incremented index
     */
    private int incrementAndGet() {
        for (;;) {
            int current = index.get();
            int next = (current < delegate.size() - 1) ? current + 1 : 0;
            if (index.compareAndSet(current, next)) return current;
        }
    }
}