HTTP trailer is a collection of a special type of HTTP headers that comes after the response body. The trailer response header allows the sender to include additional fields at the end of chunked messages in order to supply metadata that might be dynamically generated while the message body is sent, such as a message integrity check, digital signature, or post-processing status.

If trailer headers are ready for reading, isTrailerFieldsReady() will return true. Then a servlet can read trailer headers of the HTTP request using the getTrailerFields method of the HttpServletRequest interface. If trailer headers are not ready for reading, isTrailerFieldsReady() returns false and will cause an IllegalStateException.

A servlet can write trailer headers to the response by providing a supplier to the setTrailerFields() method of the HttpServletResponse interface. The following headers and types of headers must not be included in the set of keys in the map passed to setTrailerFields(): Transfer-Encoding, Content-Length, Host, controls and conditional headers, authentication headers, Content-Encoding, Content-Type, Content-Range, and Trailer. When sending response trailers, you must include a regular header, called Trailer, whose value is a comma-separated list of all the keys in the map that is supplied to the setTrailerFields() method. The value of the Trailer header lets the client know what trailers to expect.

The supplier of the trailer headers can be obtained by accessing the getTrailerFields() method of the HttpServletResponse interface.

See the javadoc for getTrailerFields() and isTrailerFieldsReady() in HttpServletRequest, and getTrailerFields() and setTrailerFields() in HttpServletResponse.