Add i18n module: MessageSource, LocaleResolver and localized ProblemDetail

English, Marathi and Hindi bundles behind one small web app, with tests that run it on a real port and write the transcripts (01-11) quoted in the article, plus a jar-metadata capture (12). Co-Authored-By: Claude Sonnet 5 <[email protected]> Claude-Session: https://claude.ai/code/session_01Uu7q8vPeREyT4218EJPzz1
2026-09-24 09:54:55 +00:00
parent e59ff029a1
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+# GET /api/greeting under different Accept-Language headers (Spring Boot's default LocaleResolver)
+
+Accept-Language: mr-IN                -> status=200  locale=mr_IN  message=नमस्कार, Asha!
+Accept-Language: hi                   -> status=200  locale=hi  message=नमस्ते, Asha!
+Accept-Language: mr;q=0.4, hi;q=0.9   -> status=200  locale=hi  message=नमस्ते, Asha!
+Accept-Language: en-GB                -> status=200  locale=en_GB  message=Hello, Asha!
+Accept-Language: fr                   -> status=200  locale=fr  message=Hello, Asha!
+Accept-Language: (no header)          -> status=200  locale=en_US  message=Hello, Asha!
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+# The JVM's default locale is Hindi: what does a French or an English request get?
+
+Locale.getDefault() while these ran: hi
+
+--- spring.messages.fallback-to-system-locale left at its default ---
+Accept-Language: fr  -> status=200  locale=fr  message=नमस्ते, Asha!
+Accept-Language: en  -> status=200  locale=en  message=नमस्ते, Asha!
+
+--- spring.messages.fallback-to-system-locale=false ---
+Accept-Language: fr  -> status=200  locale=fr  message=Hello, Asha!
+Accept-Language: en  -> status=200  locale=en  message=Hello, Asha!
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+# Letting the user pick a language: ?lang=mr with a cookie resolver, and two ways to get it wrong
+
+
+--- A. CookieLocaleResolver named localeResolver + LocaleChangeInterceptor (profile 'cookie') ---
+no cookie yet, Accept-Language: hi        -> status=200  locale=en  message=Hello, Asha!
+?lang=mr, Accept-Language: hi             -> status=200  locale=mr  message=नमस्कार, Asha!
+Set-Cookie: lang=mr; Path=/; Max-Age=2592000; SameSite=Lax
+Cookie: lang=mr, Accept-Language: hi      -> status=200  locale=mr  message=नमस्कार, Asha!
+
+--- B. the same resolver under the bean name cookieLocaleResolver (profile 'cookiewrongname') ---
+Accept-Language: hi                       -> status=200  locale=hi  message=नमस्ते, Asha!
+?lang=mr                                  -> status=500 (Tomcat's own HTML error page)
+bean 'cookieLocaleResolver' exists: true
+bean 'localeResolver' is a: AcceptHeaderLocaleResolver
+what the interceptor triggers: UnsupportedOperationException: Cannot change HTTP Accept-Language header - use a different locale resolution strategy
+
+--- C. only the interceptor, Spring Boot's default resolver (profile 'interceptoronly') ---
+?lang=mr                                  -> status=500 (Tomcat's own HTML error page)
+bean 'localeResolver' is a: AcceptHeaderLocaleResolver
+what the interceptor triggers: UnsupportedOperationException: Cannot change HTTP Accept-Language header - use a different locale resolution strategy
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+# The same sentence with a doubled and a single apostrophe, with and without arguments
+
+apos.doubled = Order {0} can''t be shipped yet.
+apos.single  = Order {0} can't be shipped yet.
+
+with an argument:    doubled -> Order A-1 can't be shipped yet.
+with an argument:    single  -> Order A-1 cant be shipped yet.
+without arguments:   doubled -> Order {0} can''t be shipped yet.
+without arguments:   single  -> Order {0} can't be shipped yet.
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+# GET /api/cart?items=1234567 in three languages, and two ways to keep Latin digits
+
+Accept-Language: en  -> You have 1,234,567 items in your cart.
+Accept-Language: hi  -> आपकी कार्ट में 1,234,567 वस्तुएँ हैं।
+Accept-Language: mr  -> तुमच्या कार्टमध्ये १,२३४,५६७ वस्तू आहेत.
+
+--- fix 1: the client asks for Latin digits (Unicode extension -u-nu-latn) ---
+Accept-Language: mr-IN-u-nu-latn -> locale=mr_IN_#u-nu-latn  तुमच्या कार्टमध्ये 1,234,567 वस्तू आहेत.
+
+--- fix 2: the server formats the number itself and passes text (GET /api/cart-latin) ---
+Accept-Language: mr -> तुमच्या कार्टमध्ये 1,234,567 वस्तू आहेत.
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+# A key that exists only in English, and a key that exists nowhere
+
+only.english.key, locale mr -> This sentence has not been translated yet.
+no.such.key, locale mr        -> org.springframework.context.NoSuchMessageException: No message found under code 'no.such.key' for locale 'mr'.
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+# The same Marathi greeting, with Spring Boot's default encoding and with spring.messages.encoding=ISO-8859-1
+
+default (UTF-8):   नमस्कार, Asha!
+ISO-8859-1:        à¤¨à¤®à¤¸à¥à¤à¤¾à¤°, Asha!
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+# POST /api/customers with an invalid body, four Accept-Language values
+
+body: {"name":"","email":"x","age":10}
+
+--- Accept-Language: en ---
+status=400  Content-Type=application/problem+json
+title:  Validation failed
+detail: The request has invalid fields.
+  error: age: You must be at least 18 years old.
+  error: city: must not be null
+  error: email: Enter a valid email address.
+  error: name: Name is required.
+
+--- Accept-Language: mr ---
+status=400  Content-Type=application/problem+json
+title:  पडताळणी अयशस्वी
+detail: विनंतीतील काही फील्ड चुकीची आहेत.
+  error: age: तुमचे वय किमान 18 वर्षे असणे आवश्यक आहे.
+  error: city: रिक्त नसावे.
+  error: email: कृपया वैध ईमेल पत्ता द्या.
+  error: name: नाव आवश्यक आहे.
+
+--- Accept-Language: hi ---
+status=400  Content-Type=application/problem+json
+title:  सत्यापन विफल
+detail: अनुरोध के कुछ फ़ील्ड अमान्य हैं।
+  error: age: आपकी आयु कम से कम 18 वर्ष होनी चाहिए।
+  error: city: खाली नहीं होना चाहिए।
+  error: email: कृपया सही ईमेल पता दर्ज करें।
+  error: name: नाम आवश्यक है।
+
+--- Accept-Language: fr ---
+status=400  Content-Type=application/problem+json
+title:  Validation failed
+detail: The request has invalid fields.
+  error: age: You must be at least 18 years old.
+  error: city: ne doit pas être nul
+  error: email: Enter a valid email address.
+  error: name: Name is required.
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+# Three kinds of exception: an ErrorResponse, an ordinary exception handled through the MessageSource, and one handled with English text in the source
+
+
+--- an ErrorResponse (OrderNotFoundException) ---
+Accept-Language: en
+status=404  Content-Type=application/problem+json
+title:  Order not found
+detail: No order with id 7 exists.
+Accept-Language: mr
+status=404  Content-Type=application/problem+json
+title:  ऑर्डर सापडली नाही
+detail: ७ क्रमांकाची ऑर्डर अस्तित्वात नाही.
+Accept-Language: hi
+status=404  Content-Type=application/problem+json
+title:  ऑर्डर नहीं मिला
+detail: क्रमांक 7 वाला ऑर्डर मौजूद नहीं है।
+
+--- an ordinary exception, translated in the handler (PaymentDeclinedException) ---
+Accept-Language: en
+status=402  Content-Type=application/problem+json
+title:  Payment declined
+detail: Payment of 499.00 was declined.
+Accept-Language: mr
+status=402  Content-Type=application/problem+json
+title:  पेमेंट नाकारले
+detail: 499.00 चे पेमेंट नाकारले गेले.
+Accept-Language: hi
+status=402  Content-Type=application/problem+json
+title:  भुगतान अस्वीकृत
+detail: 499.00 का भुगतान अस्वीकार कर दिया गया।
+
+--- an ordinary exception, English in the handler (LegacyStateException) ---
+Accept-Language: en
+status=409  Content-Type=application/problem+json
+title:  Conflict
+detail: The order is in a state that does not allow this.
+Accept-Language: mr
+status=409  Content-Type=application/problem+json
+title:  Conflict
+detail: The order is in a state that does not allow this.
+Accept-Language: hi
+status=409  Content-Type=application/problem+json
+title:  Conflict
+detail: The order is in a state that does not allow this.
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+# The same four lookups with spring.messages.always-use-message-format=true
+
+with an argument:    doubled -> Order A-1 can't be shipped yet.
+with an argument:    single  -> Order A-1 cant be shipped yet.
+without arguments:   doubled -> Order {0} can't be shipped yet.
+without arguments:   single  -> Order {0} cant be shipped yet.
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+# spring.web.locale on its own, and with spring.web.locale-resolver=fixed
+
+
+--- spring.web.locale=hi (spring.web.locale-resolver left at accept-header) ---
+Accept-Language: (no header) -> status=200  locale=hi  message=नमस्ते, Asha!
+Accept-Language: mr          -> status=200  locale=mr  message=नमस्कार, Asha!
+Accept-Language: fr          -> status=200  locale=fr  message=Hello, Asha!
+
+--- spring.web.locale=hi and spring.web.locale-resolver=fixed ---
+Accept-Language: (no header) -> status=200  locale=hi  message=नमस्ते, Asha!
+Accept-Language: mr          -> status=200  locale=hi  message=नमस्ते, Asha!
+Accept-Language: fr          -> status=200  locale=hi  message=नमस्ते, Asha!
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+# spring.messages.* and spring.web.locale* in the configuration metadata shipped with Spring Boot 4.1.1
+
+jar: spring-boot-autoconfigure-4.1.1.jar
+
+spring.messages.always-use-message-format
+    type:    java.lang.Boolean
+    default: false
+spring.messages.basename
+    type:    java.util.List<java.lang.String>
+    default: ["messages"]
+spring.messages.cache-duration
+    type:    java.time.Duration
+    default: (none)
+spring.messages.common-messages
+    type:    java.util.List<org.springframework.core.io.Resource>
+    default: (none)
+spring.messages.encoding
+    type:    java.nio.charset.Charset
+    default: "UTF-8"
+spring.messages.fallback-to-system-locale
+    type:    java.lang.Boolean
+    default: true
+spring.messages.use-code-as-default-message
+    type:    java.lang.Boolean
+    default: false
+spring.web.locale
+    type:    java.util.Locale
+    default: (none)
+spring.web.locale-resolver
+    type:    org.springframework.boot.autoconfigure.web.WebProperties$LocaleResolver
+    default: "accept-header"