*
     * @param factory The URL stream handler factory.
     */
    public static void setURLStreamHandlerFactory(final URLStreamHandlerFactory factory) {
        synchronized (PROTOCOL_HANDLERS) {
            if (Handler.factory != null) {
                throw new IllegalStateException("URLStreamHandlerFactory already set.");
            }
            PROTOCOL_HANDLERS.clear();
            Handler.factory = factory;
        }
    }

     *            The URL stream handler factory.
     */
    public static void setURLStreamHandlerFactory(final URLStreamHandlerFactory factory) {
        synchronized (PROTOCOL_HANDLERS) {
            if (Handler.factory != null) {
                throw new IllegalStateException("URLStreamHandlerFactory already set.");
            }
            PROTOCOL_HANDLERS.clear();
            Handler.factory = factory;
        }
    }

        // This test ensures that attempting to set the URLStreamHandlerFactory more than once
        // throws an IllegalStateException as per the contract.
        URLStreamHandlerFactory mockFactory1 = mock(URLStreamHandlerFactory.class);
        URLStreamHandlerFactory mockFactory2 = mock(URLStreamHandlerFactory.class);

        Handler.setURLStreamHandlerFactory(mockFactory1); // First call, should succeed.

import static org.mockito.Mockito.when;

import java.lang.reflect.Field;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLStreamHandler;
import java.net.URLStreamHandlerFactory;
import java.util.ArrayList;
import java.util.List;

import org.junit.jupiter.api.BeforeAll;
import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.DisplayName;

   the way the hostname verifier was selected.
 * Fix: Locking bug in SpdyConnection.
 * Fix: Ignore null header values (for compatibility with HttpURLConnection).
 * Add URLStreamHandlerFactory support so that `URL.openConnection()` uses
   OkHttp.
 * Expose the transport ("http/1.1", "spdy/3", etc.) via magic request headers.
   Use `X-Android-Transports` to write the preferred transports and