This issue has been filed to capture an accepted proposal that arose during JEP review on
http://mail.openjdk.java.net/pipermail/net-dev/2018-April/011314.html

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The one area that I do see some room for improvement is the possibility of providing a Publishing handler out of the box, so one does not need to reach beyond Java SE for such echo use-cases. Daniel already has this code in the repository as a test [1], so it would not take much work to provide an API similar to:

BodyHandlers {
..
public static BodyHandler<Publisher<List<ByteBuffer>>> ofPublisher() {...}
..
}

[1] http://hg.openjdk.java.net/jdk/sandbox/file/c59f684f1eda/test/jdk/java/net/httpclient/ResponsePublisher.java