RoundRobinInetAddressResolver Class — netty Architecture

Source Code

resolver/src/main/java/io/netty/resolver/RoundRobinInetAddressResolver.java lines 37–100

public class RoundRobinInetAddressResolver extends InetNameResolver {
    private final NameResolver<InetAddress> nameResolver;

    /**
     * @param executor the {@link EventExecutor} which is used to notify the listeners of the {@link Future} returned by
     * {@link #resolve(String)}
     * @param nameResolver the {@link NameResolver} used for name resolution
     */
    public RoundRobinInetAddressResolver(EventExecutor executor, NameResolver<InetAddress> nameResolver) {
        super(executor);
        this.nameResolver = nameResolver;
    }

    @Override
    protected void doResolve(final String inetHost, final Promise<InetAddress> promise) throws Exception {
        // hijack the doResolve request, but do a doResolveAll request under the hood.
        // Note that InetSocketAddress.getHostName() will never incur a reverse lookup here,
        // because an unresolved address always has a host name.
        nameResolver.resolveAll(inetHost).addListener((FutureListener<List<InetAddress>>) future -> {
            if (future.isSuccess()) {
                List<InetAddress> inetAddresses = future.getNow();
                int numAddresses = inetAddresses.size();
                if (numAddresses > 0) {
                    // if there are multiple addresses: we shall pick one by one
                    // to support the round robin distribution
                    promise.setSuccess(inetAddresses.get(randomIndex(numAddresses)));
                } else {
                    promise.setFailure(new UnknownHostException(inetHost));
                }
            } else {
                promise.setFailure(future.cause());
            }
        });
    }

    @Override
    protected void doResolveAll(String inetHost, final Promise<List<InetAddress>> promise) throws Exception {
        nameResolver.resolveAll(inetHost).addListener((FutureListener<List<InetAddress>>) future -> {
            if (future.isSuccess()) {
                List<InetAddress> inetAddresses = future.getNow();
                if (!inetAddresses.isEmpty()) {
                    // create a copy to make sure that it's modifiable random access collection
                    List<InetAddress> result = new ArrayList<InetAddress>(inetAddresses);
                    // rotate by different distance each time to force round robin distribution
                    Collections.rotate(result, randomIndex(inetAddresses.size()));
                    promise.setSuccess(result);
                } else {
                    promise.setSuccess(inetAddresses);
                }
            } else {
                promise.setFailure(future.cause());
            }
        });
    }

    private static int randomIndex(int numAddresses) {
        return numAddresses == 1 ? 0 : ThreadLocalRandom.current().nextInt(numAddresses);
    }

    @Override
    public void close() {
        nameResolver.close();
    }
}