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Common Headers

The common header specifies the size and type of a header. Every header must begin with the common header. The common header must not appear by itself.

Content-Type

The Content-Type signals the media type of the request/response entity. The Content-Type header on requests is read via HttpHeaders#getMediaType() method. The Content-Type is set for responses when doing a javax.ws.rs.core.Response.ResponseBuilder#type() method or a javax.ws.rs.core.Response.ResponseBuilder#variant() method.

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Common Request HTTP Headers

An HTTP Request Header is a line of text that a client software (i.e. Web Browser) sends to a server together with other request data.

Accept

The Accept header is used to determine the possible response representations that the client prefers such as XML over JSON but not plain text. When a resource method is effectively annotated with a @Produces, any incoming request must have a compatible Accept header value for the resource method to be selected. Clients can set quality parameters (priority ordering) for the best possible response and services generally try to honor the request. To get the best representation of a response, use either the HttpHeaders#getAcceptableMediaTypes() or Request#selectVariant() methods.

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Like ETags, If-Modified-Since and If-Unmodified-Since are based on the Last-Modified date. Using either request header with a date, will set up a conditional request (if the server application supports the headers). For instance, a GET with an If-Modified-Since header of an old known date would allow the server to send back a response with just a HTTP status code of 304 (Not Modified). By sending back a HTTP status code of 304, the server is telling the client that the resource has not changed so there is no need to re-download the resource representation. This could save precious bandwidth for the client. javax.ws.rs.core.Request#evaluatePreconditions() may help evaluate the If-Modified-Since and If-Unmodified-Since headers.

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