When you provide OkHttp with an HTTP request, you’re describing the request at a high-level: _“fetch me this URL with these headers.”_ For correctness and efficiency, OkHttp rewrites your request before transmitting it.

/**
 * The concrete route used by a connection to reach an abstract origin server. When creating a
 * connection the client has many options:
 *
 *  * **HTTP proxy:** a proxy server may be explicitly configured for the client. Otherwise, the
 *    [proxy selector][java.net.ProxySelector] is used. It may return multiple proxies to attempt.
 *  * **IP address:** whether connecting directly to an origin server or a proxy, opening a socket

 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package okhttp3

import okio.IOException

/**
 * Returns the trailers that follow an HTTP response, blocking if they aren't ready yet.

_2019-05-19_

 *  Fix: Lock in a route when recovering from an HTTP/2 connection error. We had a bug where two
    calls that failed at the same time could cause OkHttp to crash with a `NoSuchElementException`
    instead of the expected `IOException`.

 *  Fix: Don't crash with a `NullPointerException` when formatting an error message describing a
    truncated response from an HTTPS proxy.


## Version 3.14.1

_2019-04-10_

    whether an HTTP request permits a body.
 *  Fix: `HttpUrl` now uses the canonical form for IPv6 addresses.
 *  Fix: Use `HttpUrl` internally.
 *  Fix: Recover from Android 4.2.2 EBADF crashes.
 *  Fix: Don't crash with an `IllegalStateException` if an HTTP/2 or SPDY
    write fails, leaving the connection in an inconsistent state.
 *  Fix: Make sure the default user agent is ASCII.

    fix insecure hosts crash with an `IllegalArgumentException` on Android.


## Version 4.7.0

_2020-05-17_

 *  New: `HandshakeCertificates.Builder.addInsecureHost()` makes it easy to turn off security in
    private development environments that only carry test data. Prefer this over creating an
    all-trusting `TrustManager` because only hosts on the allowlist are insecure. From

 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package okhttp3

 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

/** Returns an immutable wrap of this. */
@Suppress("NOTHING_TO_INLINE")
internal inline fun <T> List<T>.unmodifiable(): List<T> = Collections.unmodifiableList(this)

/** Returns an immutable wrap of this. */
@Suppress("NOTHING_TO_INLINE")
internal inline fun <T> Set<T>.unmodifiable(): Set<T> = Collections.unmodifiableSet(this)

/** Returns an immutable wrap of this. */
@Suppress("NOTHING_TO_INLINE")

   * the event source has already been closed or canceled.
   */
  fun cancel()

  fun interface Factory {
    /**
     * Creates a new event source and immediately returns it. Creating an event source initiates an
     * asynchronous process to connect the socket. Once that succeeds or fails, `listener` will be
     * notified. The caller must cancel the returned event source when it is no longer in use.
     */