![Events Diagram](../assets/images/******@****.***)

Here’s a [sample event listener](https://github.com/square/okhttp/blob/master/samples/guide/src/main/java/okhttp3/recipes/PrintEventsNonConcurrent.java) that prints each event with a timestamp.

```java
class PrintingEventListener extends EventListener {
  private long callStartNanos;

## Version 4.0.0

_2019-06-26_

**This release upgrades OkHttp to Kotlin.** We tried our best to make fast and safe to upgrade
from OkHttp 3.x. We wrote an [upgrade guide][upgrading_to_okhttp_4] to help with the migration and a
[blog post][okhttp4_blog_post] to explain it.

 *  Fix: Target Java 8 bytecode for Java and Kotlin.


## Version 4.0.0-RC3

_2019-06-24_

      }
    ```

 [CustomTrustJava]: https://github.com/square/okhttp/blob/master/samples/guide/src/main/java/okhttp3/recipes/CustomTrust.java
 [CustomTrustKotlin]: https://github.com/square/okhttp/blob/master/samples/guide/src/main/java/okhttp3/recipes/kt/CustomTrust.kt
 [CertificatePinningJava]: https://github.com/square/okhttp/blob/master/samples/guide/src/main/java/okhttp3/recipes/CertificatePinning.java

 * 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.guide;

import java.io.IOException;
import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.Response;

public class GetExample {
  final OkHttpClient client = new OkHttpClient();

 [AccessHeadersJava]: https://github.com/square/okhttp/blob/master/samples/guide/src/main/java/okhttp3/recipes/AccessHeaders.java
 [AccessHeadersKotlin]: https://github.com/square/okhttp/blob/master/samples/guide/src/main/java/okhttp3/recipes/kt/AccessHeaders.kt
 [PostStringJava]: https://github.com/square/okhttp/blob/master/samples/guide/src/main/java/okhttp3/recipes/PostString.java

 *  * BER: Basic Encoding Rules.
 *
 * This class was implemented according to the [X.690 spec][[x690]], and under the advice of
 * [Lets Encrypt's ASN.1 and DER][asn1_and_der] guide.
 *
 * [x690]: https://www.itu.int/rec/T-REC-X.690
 * [asn1_and_der]: https://letsencrypt.org/docs/a-warm-welcome-to-asn1-and-der/
 */
internal class DerReader(source: Source) {

    platform.assumeJdk9()
  }

  @Test
  fun testTlsv13Works() {
    // https://docs.oracle.com/en/java/javase/14/security/java-secure-socket-extension-jsse-reference-guide.html
    // TODO test jdk.tls.client.enableSessionTicketExtension
    // TODO check debugging information

    enableTls()

    server.enqueue(MockResponse(body = "abc"))

    val request = Request(server.url("/"))

private val prefix = if (File("samples").exists()) "" else "../../"

private fun mainFiles(): List<File> {
  val directories =
    listOf(
      "$prefix/samples/guide/src/main/java/okhttp3/guide",
      "$prefix/samples/guide/src/main/java/okhttp3/recipes",
      "$prefix/samples/guide/src/main/java/okhttp3/recipes/kt",
    ).map { File(it) }

  return directories.flatMap {
    it.listFiles().orEmpty().filter { f -> f.isFile }.toList()
  }
}

 [changelog]: https://square.github.io/okhttp/changelog/
 [conscrypt]: https://github.com/google/conscrypt/
 [get_example]: https://raw.github.com/square/okhttp/master/samples/guide/src/main/java/okhttp3/guide/GetExample.java
 [kotlin]: https://kotlinlang.org/
 [okhttp3_pro]: https://raw.githubusercontent.com/square/okhttp/master/okhttp/src/main/resources/META-INF/proguard/okhttp3.pro

 * 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.guide;

import java.io.IOException;
import okhttp3.MediaType;
import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.RequestBody;
import okhttp3.Response;

public class PostExample {