The BodyPublisher interface extends Flow.Publisher<ByteBuffer>, which means that a BodyPublisher acts as a publisher of byte buffers.

When sending a request that contains a body, the HTTP Client subscribes to the request's BodyPublisher in order to receive the flow of outgoing request body data. The normal semantics of Flow.Subscriber and Flow.Publisher are implemented by the HTTP Client and are expected from BodyPublisher implementations. Each outgoing request results in one HTTP Client Subscriber subscribing to the BodyPublisher in order to provide the sequence of byte buffers containing the request body. Instances of ByteBuffer published by the publisher must be allocated by the publisher, and must not be accessed after being published to the HTTP Client. These subscriptions complete normally when the request body is fully sent, and can be canceled or terminated early through error. If a request needs to be resent for any reason, then a new subscription is created which is expected to generate the same data as before.

A BodyPublisher that reports a content length of 0 may not be subscribed to by the HTTP Client, as it has effectively no data to publish.