In section 5.2 of servlet spec, we have:
"To be successfully transmitted back to the client, headers must be set
before the response is committed. Headers set after the response is
committed will be ignored by the servlet container"
So servlet does not support trailer headers, as specified in RFC7230
[1]. This is an attempt to provide better HTTP support for protocols built on
top of it that rely on this trailer feature (such as gRPC).
In the following we will investigate the various options of supporting
trailer headers in servlet.
A. Reading the request trailer headers.
a) StandardHeader:
Request trailer headers are standard http request headers.
This can be accessed by using standard header APIs in HttpServletRequest.
No new API is added in this case.
b) HttpServletRequest:
Add the following API to HttpServletRequest:
public String getTrailer(String name);
public Collection<String> getTrailers(String name);
public Collection<String> getTrailerNames();
Note: existing methods in HttpServletRequest interface use Enumeration here.
c) RequestTrailerMap (from Simone Bordet):
Add the following API to HttpServletRequest:
public Map<String, String> getTrailers();
d) RequestTrailerField (from Simone Bordet):
Add the following API to HttpServletRequest:
public TrailerFields getTrailers();
TrailerFields is a class representing a list of trailer fields
Note: For (b)-(d) above, then the trailer headers will "not" be
available to HttpServletRequest#getHeader, #getHeaders, #getHeaderNames.
B. Writing the response trailer headers.
a) HttpServletResponse:
public void addTrailer(String name, String value)
public void getTrailer(String name)
public Collection<String> getTrailers(String name)
public Collection<String> getTrailerNames()
b) Supplier (from Simone Bordet):
public void setTrailers(Supplier<TrailerFields>);
where TrailerFields is defined in A.d above.
c) SupplierMap (cf. A.c, above)
public void setTrailers(Supplier<Map<String, String>>);
ACTION: Please reply by April 11, 2017, COB. In the absence of responses, I am inclined
to select A.a HttpServletRequest and B.a HttpServletResponse.
Shing Wai Chan
ps.
[1]
https://tools.ietf.org/rfc/rfc7230.txt