getHeaderString was returning an empty string for missing headers (#341)

* getHeaderString was returning an empty string for missing headers

This is not as per the definition.
This change makes it return null when it it missing.

https://jakarta.ee/specifications/platform/8/apidocs/javax/ws/rs/core/httpheaders#getHeaderString-java.lang.String-

* Defend against null, add happy path test

jaxrs-server/src/main/java/io/micronaut/jaxrs/runtime/ext/bind/JaxRsHttpHeaders.java

@@ -16,6 +16,7 @@
 package io.micronaut.jaxrs.runtime.ext.bind;
 
 import io.micronaut.core.annotation.Internal;
+import io.micronaut.core.util.CollectionUtils;
 import io.micronaut.jaxrs.runtime.core.Weighted;
 import io.micronaut.jaxrs.runtime.ext.impl.CookieHeaderDelegate;
 import jakarta.ws.rs.core.Cookie;
@@ -58,7 +59,12 @@ public List<String> getRequestHeader(String name) {
 
     @Override
     public String getHeaderString(String name) {
-        return String.join(",", httpHeaders.getAll(name));
+        List<String> all = httpHeaders.getAll(name);
+        if (CollectionUtils.isEmpty(all)) {
+            return null;
+        } else {
+            return String.join(",", all);
+        }
     }
 
     @Override

jaxrs-server/src/test/java/io/micronaut/jaxrs/runtime/HeadersResource.java

@@ -5,6 +5,7 @@
 import jakarta.ws.rs.HeaderParam;
 import jakarta.ws.rs.POST;
 import jakarta.ws.rs.Path;
+import jakarta.ws.rs.PathParam;
 import jakarta.ws.rs.Produces;
 import jakarta.ws.rs.core.CacheControl;
 import jakarta.ws.rs.core.Cookie;
@@ -81,4 +82,14 @@ public Response headers(HttpHeaders httpHeaders) {
         });
         return ok.build();
     }
+
+    @GET
+    @Path("/header")
+    @Produces("text/plain")
+    public Response header(HttpHeaders httpHeaders) {
+        final Response.ResponseBuilder ok = Response.ok();
+        String headerString = httpHeaders.getHeaderString("test-value");
+        ok.entity(headerString == null ? "null" : headerString);
+        return ok.build();
+    }
 }

jaxrs-server/src/test/java/io/micronaut/jaxrs/runtime/HeadersTest.java

@@ -21,6 +21,7 @@
 import java.util.Locale;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 
 @MicronautTest
@@ -188,4 +189,24 @@ void testAcceptLanguageHeader() {
                 acceptableLanguages.get(0).getLanguage()
         );
     }
+
+    @Test
+    void testUnknownHeaderValueIsNull() {
+        final MutableHttpRequest<Object> request = HttpRequest.GET("/headers/header");
+        request.header(HttpHeaders.ACCEPT_LANGUAGE, "fr;q=0.7,en;q=0.9");
+
+        final HttpResponse<String> response = httpClient.toBlocking().exchange(request, String.class);
+        assertEquals("null", response.body());
+    }
+
+    @Test
+    void testKnownHeaderValueString() {
+        final MutableHttpRequest<Object> request = HttpRequest.GET("/headers/header");
+        request.header(HttpHeaders.ACCEPT_LANGUAGE, "fr;q=0.7,en;q=0.9");
+        request.header("test-value", "one");
+        request.header("test-value", "two");
+
+        final HttpResponse<String> response = httpClient.toBlocking().exchange(request, String.class);
+        assertEquals("one,two", response.body());
+    }
 }