This is the simplest Spring REST Controller illustrating how to get Locale from HTTP Request. All we need to do is to add Locale parameter to the method receiving HTTP Request:

package com.farenda.spring.tutorial.rest.locale;

import org.springframework.web.bind.annotation.*;

import java.util.Locale;

import static org.springframework.http.MediaType.TEXT_PLAIN_VALUE;
import static org.springframework.web.bind.annotation.RequestMethod.GET;

@RestController
public class LocalizedController {

    @RequestMapping(value = "/echo", method = GET,
            produces = TEXT_PLAIN_VALUE)
    public String getLocaleName(Locale locale) {
        return "Locale from request: " + locale;
    }
}

Why that works? When a request comes in it goes to Spring’s DespatcherServlet, which tries to find a LocaleResolver that can provide locale based on some strategy – “accept-language” header, cookies, user’s session, etc. Locale from request is then available through RequestContext.getLocale(), but also can be injected as we did here.

We can pretty easily test the above controller using MockMvcBuilders from the Spring Test package. The only thing we need to do is to call locale(Locale) method, when we build GET Request, to set our Locale on the request – you can pass more locales if you want.

package com.farenda.spring.tutorial.rest.locale

import org.springframework.test.web.servlet.setup.MockMvcBuilders
import spock.lang.Specification
import spock.lang.Subject

import static org.springframework.http.MediaType.APPLICATION_JSON
import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.get
import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.content
import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.status

class LocalizedControllerTest extends Specification {

    @Subject
    def localeApi = new LocalizedController()

    def mockMvc = MockMvcBuilders.standaloneSetup(localeApi).build()

    def 'should return locale from request'() {
        given:
        def myLocale = Locale.CHINA
        def expectedResponse = 'Locale from request: ' + myLocale

        when:
        def result = mockMvc.perform(
                get('/echo').contentType(APPLICATION_JSON)
                        .locale(myLocale))

        then:
        result.andExpect(status().isOk())
                .andExpect(content().string(expectedResponse))
    }
}

Experiment with the above code and use different locales, pass more than one locale, or even add parameters to the API method.

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