Added changes to handle EmptyString and Empty list to add SDK User Agent

Author: joviegas

core/sdk-core/src/main/java/software/amazon/awssdk/core/internal/http/pipeline/stages/ApplyUserAgentStage.java

@@ -68,7 +68,7 @@ public ApplyUserAgentStage(HttpClientDependencies dependencies) {
     public SdkHttpFullRequest.Builder execute(SdkHttpFullRequest.Builder request,
                                               RequestExecutionContext context) throws Exception {
 
-        if (hasNonNullUserAgentHeader(request)) {
+        if (request.firstMatchingHeader(HEADER_USER_AGENT).isPresent()) {
             return request;
         }
         String headerValue = finalizeUserAgent(context);

core/sdk-core/src/test/java/software/amazon/awssdk/core/internal/http/pipeline/stages/ApplyUserAgentStageTest.java

@@ -182,7 +182,7 @@ public void when_userAgentHeaderPresentButEmpty_EmptyHeaderIsPreserved() throws
 
         List<String> userAgentHeaders = result.headers().get(HEADER_USER_AGENT);
         assertThat(userAgentHeaders).isNotNull().hasSize(1);
-        assertThat(userAgentHeaders.get(0)).doesNotContain("aws-sdk-java");
+        assertThat(userAgentHeaders.get(0)).contains("aws-sdk-java");
     }
 
     @Test

test/codegen-generated-classes-test/src/test/java/software/amazon/awssdk/services/useragent/CustomUserAgentHeaderTest.java

@@ -18,6 +18,8 @@
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatThrownBy;
 
+import java.util.Arrays;
+import java.util.Collections;
 import java.util.List;
 import java.util.Map;
 import java.util.stream.Stream;
@@ -34,48 +36,63 @@
 import software.amazon.awssdk.regions.Region;
 import software.amazon.awssdk.services.restjsonendpointproviders.RestJsonEndpointProvidersClient;
 import software.amazon.awssdk.services.restjsonendpointproviders.RestJsonEndpointProvidersClientBuilder;
+import software.amazon.awssdk.utils.StringUtils;
 
 /**
- * Functional tests to verify that custom User-Agent headers provided via
- * {@link software.amazon.awssdk.core.client.config.ClientOverrideConfiguration.Builder#putHeader(String, String)}
- * are preserved and not overwritten by the SDK's default User-Agent generation logic.
+ * Functional tests verifying custom User-Agent header preservation.
+ *
+ * <p>Tests ensure that User-Agent headers provided via
+ * {@link software.amazon.awssdk.core.client.config.ClientOverrideConfiguration.Builder#putHeader(String, String)} are preserved
+ * and not overwritten by SDK's default User-Agent generation logic.
  */
 class CustomUserAgentHeaderTest {
 
     private static final String USER_AGENT_HEADER = "User-Agent";
     private static final String SDK_USER_AGENT_PREFIX = "aws-sdk-java";
     private static final String TEST_API_NAME = "TestApiName";
     private static final String TEST_API_VERSION = "1.0";
+    private static final String INTERCEPTOR_STOP_MESSAGE = "stop";
 
     private CapturingInterceptor interceptor;
 
+    private static Stream<Arguments> customUserAgentValues() {
+        return Stream.of(
+            Arguments.of("CustomUserAgentHeaderValue"),
+            Arguments.of("MyApplication/1.0.0"),
+            Arguments.of("CustomClient/2.0 (Linux; x86_64)")
+        );
+    }
+
+    // ========== Default Behavior Tests ==========
+
+    private static Stream<Arguments> customUserAgentListValues() {
+        return Stream.of(
+            Arguments.of(Arrays.asList("Agent1")),
+            Arguments.of(Arrays.asList("Agent1", "Agent2")),
+            Arguments.of(Arrays.asList("CustomClient/1.0", "MyApp/2.0"))
+        );
+    }
+
+    // ========== Custom User-Agent Preservation Tests ==========
+
     @BeforeEach
     void setUp() {
         interceptor = new CapturingInterceptor();
     }
 
     @Test
-    void execute_withoutCustomUserAgent_shouldAddSdkDefaultUserAgent() {
-
+    void executeRequest_withoutCustomUserAgent_shouldAddSdkDefaultUserAgent() {
         RestJsonEndpointProvidersClient client = defaultClientBuilder().build();
         executeRequestExpectingInterception(client);
-        String userAgent = getCapturedUserAgent();
-        assertThat(userAgent).contains(SDK_USER_AGENT_PREFIX);
-    }
-
-    @Test
-    void execute_withEmptyCustomUserAgent_shouldPreserveEmptyValue() {
 
-        RestJsonEndpointProvidersClient client = clientWithCustomUserAgent("");
-        executeRequestExpectingInterception(client);
-        String userAgent = getCapturedUserAgent();
-        assertThat(userAgent).isEmpty();
+        assertUserAgentContains(SDK_USER_AGENT_PREFIX);
     }
 
-    @ParameterizedTest(name = "{index}: userAgent={0}")
-    @MethodSource("customUserAgentValues")
-    void execute_withCustomUserAgent_shouldPreserveAndNotOverwrite(String customUserAgent) {
+    // ========== API Name Handling Tests ==========
 
+    @ParameterizedTest(name = "Custom User-Agent ''{0}'' should be preserved without SDK prefix")
+    @MethodSource("customUserAgentValues")
+    void executeRequest_withCustomUserAgent_shouldPreserveAndNotOverwrite(String customUserAgent) {
         RestJsonEndpointProvidersClient client = clientWithCustomUserAgent(customUserAgent);
         executeRequestExpectingInterception(client);
 
@@ -85,33 +102,120 @@ void execute_withCustomUserAgent_shouldPreserveAndNotOverwrite(String customUser
             .doesNotContain(SDK_USER_AGENT_PREFIX);
     }
 
-    private static Stream<Arguments> customUserAgentValues() {
-        return Stream.of(
-            Arguments.of("CustomUserAgentHeaderValue"),
-            Arguments.of("MyApplication/1.0.0"),
-            Arguments.of("CustomClient/2.0 (Linux; x86_64)")
-        );
+    @ParameterizedTest(name = "Custom User-Agent list {0} should be preserved")
+    @MethodSource("customUserAgentListValues")
+    void executeRequest_withCustomUserAgentList_shouldPreserveAllValues(List<String> customUserAgentList) {
+        RestJsonEndpointProvidersClient client = clientWithCustomUserAgentList(customUserAgentList);
+        executeRequestExpectingInterception(client);
+
+        List<String> userAgentList = getCapturedUserAgentList();
+        assertThat(userAgentList).isEqualTo(customUserAgentList);
     }
 
-    @Test
-    void execute_withCustomUserAgentAndApiName_shouldNotAppendApiName() {
+    // ========== Edge Case Tests ==========
 
+    @Test
+    void executeRequest_withCustomUserAgentAndApiName_shouldNotAppendApiName() {
         String customUserAgent = "CustomUserAgentHeaderValue";
         RestJsonEndpointProvidersClient client = clientWithCustomUserAgent(customUserAgent);
         executeRequestWithApiName(client);
+
         String userAgent = getCapturedUserAgent();
         assertThat(userAgent)
             .isEqualTo(customUserAgent)
             .doesNotContain(TEST_API_NAME);
     }
 
     @Test
-    void execute_withoutCustomUserAgentAndWithApiName_shouldAppendApiName() {
-
+    void executeRequest_withoutCustomUserAgentAndWithApiName_shouldAppendApiName() {
         RestJsonEndpointProvidersClient client = defaultClientBuilder().build();
         executeRequestWithApiName(client);
-        String userAgent = getCapturedUserAgent();
-        assertThat(userAgent).contains(TEST_API_NAME + "/" + TEST_API_VERSION);
+
+        assertUserAgentContains(TEST_API_NAME + "/" + TEST_API_VERSION);
+    }
+
+    /**
+     * Verifies that null User-Agent list throws NullPointerException.
+     *
+     * <p>This ensures the SDK fails fast with clear error rather than allowing
+     * invalid configuration.
+     */
+    @Test
+    void buildClient_withNullListUserAgent_shouldThrowNullPointerException() {
+        assertThatThrownBy(() -> clientWithCustomUserAgentList(null))
+            .isInstanceOf(NullPointerException.class)
+            .hasMessageContaining("values must not be null");
+    }
+
+    /**
+     * Verifies that empty User-Agent list results in SDK default User-Agent.
+     *
+     * <p><b>Behavioral Change:</b> Previously as in when UserAgentApplyStage was done before MergeCustomHeaderStage, explicitly
+     * setting User-Agent Header to empty String or empty list would delete the SDK User-Agent. Current behavior ensures SDK
+     * User-Agent is always present when User-Agent Header is emptyString/EmptyList/Null.
+     */
+    @Test
+    void executeRequest_withEmptyListUserAgent_shouldResultInSdkUserAgentHeader() {
+        RestJsonEndpointProvidersClient client = clientWithCustomUserAgentList(Collections.emptyList());
+        executeRequestExpectingInterception(client);
+
+        List<String> userAgentList = getCapturedUserAgentList();
+        assertThat(userAgentList)
+            .hasSize(1)
+            .anyMatch(ua -> ua.startsWith(SDK_USER_AGENT_PREFIX));
+    }
+
+    @Test
+    void executeRequest_withEmptyCustomUserAgent_shouldStoreSdkUserAgent() {
+        RestJsonEndpointProvidersClient client = clientWithCustomUserAgent("");
+        executeRequestExpectingInterception(client);
+
+        assertUserAgentContains(SDK_USER_AGENT_PREFIX);
+    }
+
+    @Test
+    void executeRequest_withNullStringUserAgent_shouldStoreAsSdkUserAgent() {
+        RestJsonEndpointProvidersClient client = clientWithCustomUserAgent(null);
+        executeRequestExpectingInterception(client);
+
+        List<String> userAgentList = getCapturedUserAgentList();
+        assertThat(userAgentList).hasSize(1);
+        assertThat(userAgentList)
+            .hasSize(1)
+            .allSatisfy(ua -> {
+                assertThat(ua).isNotNull();
+                assertThat(ua).startsWith(SDK_USER_AGENT_PREFIX);
+            });
+
+    }
+
+    private void assertUserAgentContains(String expected) {
+        assertThat(getCapturedUserAgent()).contains(expected);
+    }
+
+    private void executeRequestExpectingInterception(RestJsonEndpointProvidersClient client) {
+        assertThatThrownBy(() -> client.allTypes(r -> {
+        }))
+            .hasMessageContaining(INTERCEPTOR_STOP_MESSAGE);
+    }
+
+    private void executeRequestWithApiName(RestJsonEndpointProvidersClient client) {
+        assertThatThrownBy(() -> client.allTypes(r -> r
+            .overrideConfiguration(o -> o.addApiName(api -> api
+                .name(TEST_API_NAME)
+                .version(TEST_API_VERSION)))))
+            .hasMessageContaining(INTERCEPTOR_STOP_MESSAGE);
+    }
+
+    private String getCapturedUserAgent() {
+        Map<String, List<String>> headers = interceptor.context.httpRequest().headers();
+        assertThat(headers).containsKey(USER_AGENT_HEADER);
+        return headers.get(USER_AGENT_HEADER).get(0);
+    }
+
+    private List<String> getCapturedUserAgentList() {
+        Map<String, List<String>> headers = interceptor.context.httpRequest().headers();
+        return headers.get(USER_AGENT_HEADER);
     }
 
     private RestJsonEndpointProvidersClientBuilder defaultClientBuilder() {
@@ -133,23 +237,15 @@ private RestJsonEndpointProvidersClient clientWithCustomUserAgent(String customU
                                               .build();
     }
 
-    private void executeRequestExpectingInterception(RestJsonEndpointProvidersClient client) {
-        assertThatThrownBy(() -> client.allTypes(r -> {}))
-            .hasMessageContaining("stop");
-    }
-
-    private void executeRequestWithApiName(RestJsonEndpointProvidersClient client) {
-        assertThatThrownBy(() -> client.allTypes(r -> r
-            .overrideConfiguration(o -> o.addApiName(api -> api
-                .name(TEST_API_NAME)
-                .version(TEST_API_VERSION)))))
-            .hasMessageContaining("stop");
-    }
-
-    private String getCapturedUserAgent() {
-        Map<String, List<String>> headers = interceptor.context.httpRequest().headers();
-        assertThat(headers).containsKey(USER_AGENT_HEADER);
-        return headers.get(USER_AGENT_HEADER).get(0);
+    private RestJsonEndpointProvidersClient clientWithCustomUserAgentList(List<String> customUserAgentList) {
+        return RestJsonEndpointProvidersClient.builder()
+                                              .region(Region.US_WEST_2)
+                                              .credentialsProvider(StaticCredentialsProvider.create(
+                                                  AwsBasicCredentials.create("akid", "skid")))
+                                              .overrideConfiguration(c -> c
+                                                  .addExecutionInterceptor(interceptor)
+                                                  .putHeader(USER_AGENT_HEADER, customUserAgentList))
+                                              .build();
     }
 
     private static class CapturingInterceptor implements ExecutionInterceptor {
@@ -158,7 +254,7 @@ private static class CapturingInterceptor implements ExecutionInterceptor {
         @Override
         public void beforeTransmission(Context.BeforeTransmission context, ExecutionAttributes executionAttributes) {
             this.context = context;
-            throw new RuntimeException("stop");
+            throw new RuntimeException(INTERCEPTOR_STOP_MESSAGE);
         }
     }
 }