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Kiwi is a utility library. We really like Google's Guava, and also use Apache Commons. But if they don't have something we need, and we think it is useful, this is where we put it.
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package org.kiwiproject.collect;

import static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.collect.Lists.newArrayList;

import com.google.common.collect.ImmutableList;
import com.google.common.collect.Iterators;
import lombok.experimental.UtilityClass;

import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
import java.util.function.Consumer;
import java.util.function.Supplier;

/**
 * Utility methods for working with {@link Iterator} instances. Analogous to Guava's {@link Iterators}
 */
@UtilityClass
public class KiwiIterators {

    private static final String NOT_ENOUGH_VALUES_ERROR = "need at least 1 element to cycle";

    /**
     * Returns a thread-safe iterator that cycles indefinitely over the elements of {@code iterable}, base
     * on Guava's {@link com.google.common.collect.Iterables#cycle(Iterable)}. The differences from Guava is that the
     * returned iterator provides thread-safety; makes an immutable copy of the provided iterable; and does not
     * support element removal regardless of whether the original iterable does.
     * 
     * Typical use cases include round-robin scenarios, such as round-robin between replicated service registries
     * like Netflix Eureka.
     * 

     * The returned iterator does not support {@link Iterator#remove()} nor does it support
     * {@link Iterator#forEachRemaining(Consumer)}, as the entire point is to cycle forever.
     *
     * @param iterable the Iterable to cycle
     * @param  the type of objects in the Iterable
     * @return an Iterator that cycles through the given Iterable without terminating
     */
    public static  Iterator cycleForever(Iterable iterable) {
        var elements = ImmutableList.copyOf(iterable);
        checkArgument(!elements.isEmpty(), NOT_ENOUGH_VALUES_ERROR);
        return new ThreadSafeCyclicIterator<>(elements);
    }

    /**
     * Returns a thread-safe iterator that cycles indefinitely over the elements of {@code iterable}, base
     * on Guava's {@link com.google.common.collect.Iterables#cycle(Iterable)}. The differences from Guava is that the
     * returned iterator provides thread-safety; makes an immutable copy of the provided iterable; and does not
     * support element removal regardless of whether the original iterable does.
     * 

     * Typical use cases include round-robin scenarios, such as round-robin between replicated service registries
     * like Netflix Eureka.
     * 
     * The returned iterator does not support {@link Iterator#remove()} nor does it support
     * {@link Iterator#forEachRemaining(Consumer)}, as the entire point is to cycle forever.
     *
     * @param elements the values to cycle
     * @param  the type of the elements
     * @return an Iterator that cycles through the given iterable without terminating
     */
    @SafeVarargs
    public static  Iterator cycleForever(T... elements) {
        checkArgument(elements.length > 0, NOT_ENOUGH_VALUES_ERROR);
        return new ThreadSafeCyclicIterator<>(newArrayList(elements));
    }

    // TODO Should this be made public and/or a top-level class???
    private static class ThreadSafeCyclicIterator implements Iterator {

        private final Iterator cycler;
        private final Lock cyclerLock;

        ThreadSafeCyclicIterator(Iterable iterable) {
            this.cycler = Iterators.cycle(iterable);
            this.cyclerLock = new ReentrantLock();
        }

        @Override
        public boolean hasNext() {
            return withLock(cycler::hasNext);
        }

        @Override
        public E next() {
            return withLock(() -> {
                if (!hasNext()) {
                    throw new NoSuchElementException("there are no elements to cycle");
                }
                return cycler.next();
            });
        }

        @Override
        public void remove() {
            throw new UnsupportedOperationException("unsupported; cannot remove from an infinite cycling iterator");
        }

        @Override
        public void forEachRemaining(Consumer action) {
            throw new UnsupportedOperationException("unsupported; infinite loop would occur unless an exception is thrown");
        }

        private  T withLock(Supplier supplier) {
            try {
                cyclerLock.lock();
                return supplier.get();
            } finally {
                cyclerLock.unlock();
            }
        }
    }

}