Especially when we use Grails to create a RESTful API we want to enable the request header
Accept, so Grails can do content negotiation based on the value in the header. For example we could use the request header
Accept: application/json to get a JSON response. Grails will look at the
boolean configuration property
grails.mime.use.accept.header to see if the
Accept header must be parsed. The default value is
true so the
Accept header is used. But there is another property which determines if the
Accept header is used:
grails.mime.disable.accept.header.userAgents. The value must contain a list or a regular expression pattern of user agents which are ignored for the
Accept header. The default value is
~/(Gecko(?i)|WebKit(?i)|Presto(?i)|Trident(?i))/. So for any request from these user agents, mostly our web browser, the
Accept header is ignored.
If we use REST web services test tools from our browser, for example Postman, then any
Accept header we set in the tool is ignored by Grails. We must change the value of the configuration property
grails.mime.disable.accept.header.userAgents to for example an empty list. Then we know for every request send to our Grails application, either from a web browser or command-line tool like curl, the
Accept header is interpreted by Grails.
The following sample shows how we make sure the
Accept header is always used:
// File: grails-app/conf/Config.groovy ... grails.mime.use.accept.header = true // Default value is true. grails.mime.disable.accept.header.userAgents =  ...
Written with Grails 2.4.1.