---
title: The transport layer
description: ElasticsearchClient exposes the Elasticsearch APIs using high level objects and data structures. It relies on a Transport implementation for lower level...
url: https://www.elastic.co/elastic/docs-builder/docs/3028/reference/elasticsearch/clients/java/transport
products:
  - Elasticsearch Client
  - Elasticsearch Java Client
---

# The transport layer
`ElasticsearchClient` exposes the Elasticsearch APIs using high level objects and data structures. It relies on a `Transport` implementation for lower level concerns that include:
- JSON serialization and deserialization, by delegating to a `JsonpMapper`.
- network communication: HTTP, TLS, load-balancing among cluster nodes, etc.


## JSON mappers

The Java API client comes with two JSON mapping implementations:
- `JacksonJsonpMapper`, the default implementation based on the popular [Jackson](https://github.com/FasterXML/jackson) library.
- `Jackson3JsonpMapper`, based on the new [Jackson](https://github.com/FasterXML/jackson) library version 3.
- `JsonbJsonpMapper`, based on the JakartaEE JSONP specification, which allows using any implementation of this specification such as [Eclipse Parsson](https://github.com/eclipse-ee4j/parsson)

Which implementation should you use?
Unless you have specific requirements, use the default Jackson implementation.

## HTTP implementations

The Java API client comes with two HTTP implementations:
- [REST 5 Client](https://www.elastic.co/elastic/docs-builder/docs/3028/reference/elasticsearch/clients/java/transport/rest5-client), the default implementation, based on the Apache http client library version 5. It's a direct port of the previous version to this new library.
- [Rest Client](https://www.elastic.co/elastic/docs-builder/docs/3028/reference/elasticsearch/clients/java/transport/rest-client), the legacy implementation that has existed since Elasticsearch version 7, based on the Apache http client library version 4.

Which implementation should you use?
- if you're starting a new application, use the default Rest 5 Client.
- if you're upgrading an existing application, consider upgrading to Rest 5 Client. Adapting your client creation code to the newer Apache http library should be releatively straightforward.
- if you're upgrading an application with a heavily customized Rest Client configuration, then consider staying with this legacy version. Be mindful however that it may be removed in the next major release.