### [Fast Fallback](https://square.github.io/okhttp/5.x/okhttp/okhttp3/-ok-http-client/-builder/fast-fallback)

Since version 5.0, `OkHttpClient` supports fast fallback, which is our implementation of Happy Eyeballs [RFC 6555](https://datatracker.ietf.org/doc/html/rfc6555).

     * Execute a callable through the circuit breaker with fallback
     *
     * @param <T> return type
     * @param callable the callable to execute
     * @param fallback fallback supplier if circuit is open
     * @return the result or fallback value
     * @throws Exception if execution fails and no fallback provided
     */
    public <T> T call(Callable<T> callable, Callable<T> fallback) throws Exception {

 * and performant but introduces some testing difficulties. This test exercises the two fallback
 * strategies.
 *
 * <ul>
 *   <li>SafeAtomicHelper: uses Atomic FieldsUpdaters to implement synchronization
 *   <li>SynchronizedHelper: uses {@code synchronized} blocks for synchronization
 * </ul>
 *
 * To force selection of our fallback strategies we load {@link AggregateFutureState} (and all of

import java.util.EnumSet;
import java.util.Set;

/**
 * Interface for RDMA provider implementations.
 *
 * This interface abstracts different RDMA implementations such as
 * InfiniBand, iWARP, RoCE, or TCP fallback providers.
 */
public interface RdmaProvider {

    /**
     * Check if RDMA is available on this system
     *
     * @return true if RDMA can be used, false otherwise
     */
    boolean isAvailable();

   *
   * @param future The future attach the callback to.
   * @param callback The callback to invoke when {@code future} is completed.
   * @param executor The executor to run {@code callback} when the future completes.
   * @since 10.0
   */
  public static <V extends @Nullable Object> void addCallback(
      ListenableFuture<V> future, FutureCallback<? super V> callback, Executor executor) {

        assertTrue(ex.getMessage().contains("Cannot use netbios/short names"));
    }

    @Test
    @DisplayName("createContext: no Kerberos in initial token and no fallback -> throws")
    void createContext_noKerberosNoFallback_throws() throws CIFSException {
        Kerb5Authenticator auth = new Kerb5Authenticator((Subject) null);

 *
 * @author Kevin Damm
 * @author Kyle Maddison
 */
final class LittleEndianByteArray {

  /**
   * The instance that actually does the work; delegates to VarHandle, Unsafe, or a Java-8
   * compatible pure-Java fallback.
   */
  private static final LittleEndianBytes byteArray = makeGetter();

  /**
   * Load 8 bytes into long in a little endian manner, from the substring between position and

    }

    @Test
    @DisplayName("Test fallback used when circuit is open")
    void testFallbackWhenOpen() throws Exception {
        // Open the circuit
        circuitBreaker.tripBreaker();

        String result = circuitBreaker.call(() -> "primary", () -> "fallback");

        assertEquals("fallback", result);
    }

    @Test

 * selected in the static initializer of AbstractFuture. This is convenient and performant but
 * introduces some testing difficulties. This test exercises the fallback strategies.
 *
 * <p>To force selection of our fallback strategies, we load {@link AbstractFuture} (and all of
 * {@code com.google.common.util.concurrent}) in degenerate class loaders which make certain
 * platform classes unavailable. Then we construct a test suite so we can run the normal

 */
package okhttp3.internal

import java.net.InetAddress
import okhttp3.Dns

/**
 * A network that always resolves two IP addresses per host. Use this when testing route selection
 * fallbacks to guarantee that a fallback address is available.
 */
class DoubleInetAddressDns : Dns {
  override fun lookup(hostname: String): List<InetAddress> {
    val addresses = Dns.SYSTEM.lookup(hostname)