Add expectContinueThresholdInBytes config to S3

.changes/next-release/feature-AmazonS3-08f1e6b.json

@@ -0,0 +1,6 @@
+{
+    "type": "feature",
+    "category": "Amazon S3",
+    "contributor": "",
+    "description": "Add configurable `expectContinueThresholdInBytes` to S3Configuration (default 1 MB). The Expect: 100-continue header is now only added to PutObject and UploadPart requests when the content-length meets or exceeds the threshold, reducing latency overhead for small uploads."
+}

services/s3/src/main/java/software/amazon/awssdk/services/s3/S3Configuration.java

@@ -77,12 +77,19 @@ public final class S3Configuration implements ServiceConfiguration, ToCopyableBu
      */
     private static final boolean DEFAULT_EXPECT_CONTINUE_ENABLED = true;
 
+    /**
+     * The default minimum content-length in bytes at which the {@code Expect: 100-continue} header is added.
+     * Requests with a content-length below this threshold will not include the header.
+     */
+    private static final long DEFAULT_EXPECT_CONTINUE_THRESHOLD_IN_BYTES = 1_048_576L;
+
     private final FieldWithDefault<Boolean> pathStyleAccessEnabled;
     private final FieldWithDefault<Boolean> accelerateModeEnabled;
     private final FieldWithDefault<Boolean> dualstackEnabled;
     private final FieldWithDefault<Boolean> checksumValidationEnabled;
     private final FieldWithDefault<Boolean> chunkedEncodingEnabled;
     private final FieldWithDefault<Boolean> expectContinueEnabled;
+    private final FieldWithDefault<Long> expectContinueThresholdInBytes;
     private final Boolean useArnRegionEnabled;
     private final Boolean multiRegionEnabled;
     private final FieldWithDefault<Supplier<ProfileFile>> profileFile;
@@ -97,6 +104,13 @@ private S3Configuration(DefaultS3ServiceConfigurationBuilder builder) {
         this.chunkedEncodingEnabled = FieldWithDefault.create(builder.chunkedEncodingEnabled, DEFAULT_CHUNKED_ENCODING_ENABLED);
         this.expectContinueEnabled = FieldWithDefault.create(builder.expectContinueEnabled,
                                                              DEFAULT_EXPECT_CONTINUE_ENABLED);
+        this.expectContinueThresholdInBytes = FieldWithDefault.create(builder.expectContinueThresholdInBytes,
+                                                                      DEFAULT_EXPECT_CONTINUE_THRESHOLD_IN_BYTES);
+        if (this.expectContinueThresholdInBytes.value() < 0) {
+            throw new IllegalArgumentException(
+                "expectContinueThresholdInBytes must not be negative, but was: "
+                + this.expectContinueThresholdInBytes.value());
+        }
         this.profileFile = FieldWithDefault.create(builder.profileFile, ProfileFile::defaultProfileFile);
         this.profileName = FieldWithDefault.create(builder.profileName,
                                                    ProfileFileSystemSetting.AWS_PROFILE.getStringValueOrThrow());
@@ -247,6 +261,26 @@ public boolean expectContinueEnabled() {
         return expectContinueEnabled.value();
     }
 
+    /**
+     * Returns the minimum content-length in bytes at which the {@code Expect: 100-continue} header is added to
+     * {@link PutObjectRequest} and {@link UploadPartRequest}. Requests with a content-length below this threshold
+     * will not include the header.
+     * <p>
+     * The default value is 1048576 bytes (1 MB).
+     * <p>
+     * <b>Note:</b> When using the {@code ApacheHttpClient} (Apache 4), the Apache 4 client also independently adds the
+     * {@code Expect: 100-continue} header by default without any threshold via its own {@code expectContinueEnabled}
+     * setting. To benefit from the `expectContinueThresholdInBytes` you must disable {@code expectContinueEnabled}
+     * on the Apache4 HTTP client builder using {@code ApacheHttpClient.builder().expectContinueEnabled(false)}.
+     * This does NOT apply to the {@code Apache5HttpClient} which defaults {@code expectContinueEnabled} to false.
+     *
+     * @return The threshold in bytes.
+     * @see S3Configuration.Builder#expectContinueThresholdInBytes(Long)
+     */
+    public long expectContinueThresholdInBytes() {
+        return expectContinueThresholdInBytes.value();
+    }
+
     /**
      * Returns whether the client is allowed to make cross-region calls when an S3 Access Point ARN has a different
      * region to the one configured on the client.
@@ -278,6 +312,7 @@ public Builder toBuilder() {
                 .checksumValidationEnabled(checksumValidationEnabled.valueOrNullIfDefault())
                 .chunkedEncodingEnabled(chunkedEncodingEnabled.valueOrNullIfDefault())
                 .expectContinueEnabled(expectContinueEnabled.valueOrNullIfDefault())
+                .expectContinueThresholdInBytes(expectContinueThresholdInBytes.valueOrNullIfDefault())
                 .useArnRegionEnabled(useArnRegionEnabled)
                 .profileFile(profileFile.valueOrNullIfDefault())
                 .profileName(profileName.valueOrNullIfDefault());
@@ -407,6 +442,29 @@ public interface Builder extends CopyableBuilder<Builder, S3Configuration> {
          */
         Builder expectContinueEnabled(Boolean expectContinueEnabled);
 
+        Long expectContinueThresholdInBytes();
+
+        /**
+         * Option to configure the minimum content-length in bytes at which the {@code Expect: 100-continue} header
+         * is added to {@link PutObjectRequest} and {@link UploadPartRequest}. Requests with a content-length below
+         * this threshold will not include the header, reducing latency for small uploads where the round-trip cost
+         * of the 100-continue handshake outweighs the benefit.
+         * <p>
+         * The default value is 1048576 bytes (1 MB). Setting this to 0 restores the pre-threshold behavior where
+         * the header is added for all non-zero content-length requests.
+         * <p>
+         * This setting only takes effect when {@link #expectContinueEnabled(Boolean)} is {@code true} (the default).
+         * <p>
+         * <b>Note:</b> When using the {@code ApacheHttpClient} (Apache 4), the Apache 4 client also independently adds the
+         * {@code Expect: 100-continue} header by default via its own {@code expectContinueEnabled} setting. This threshold
+         * only controls the SDK's own header addition; it does not affect the Apache client's behavior.
+         *
+         * @param expectContinueThresholdInBytes The threshold in bytes, or {@code null} to use the default (1048576).
+         * @return This builder for method chaining.
+         * @see S3Configuration#expectContinueThresholdInBytes()
+         */
+        Builder expectContinueThresholdInBytes(Long expectContinueThresholdInBytes);
+
         Boolean useArnRegionEnabled();
 
         /**
@@ -476,6 +534,7 @@ static final class DefaultS3ServiceConfigurationBuilder implements Builder {
         private Boolean checksumValidationEnabled;
         private Boolean chunkedEncodingEnabled;
         private Boolean expectContinueEnabled;
+        private Long expectContinueThresholdInBytes;
         private Boolean useArnRegionEnabled;
         private Boolean multiRegionEnabled;
         private Supplier<ProfileFile> profileFile;
@@ -571,6 +630,21 @@ public void setExpectContinueEnabled(Boolean expectContinueEnabled) {
             expectContinueEnabled(expectContinueEnabled);
         }
 
+        @Override
+        public Long expectContinueThresholdInBytes() {
+            return expectContinueThresholdInBytes;
+        }
+
+        @Override
+        public Builder expectContinueThresholdInBytes(Long expectContinueThresholdInBytes) {
+            this.expectContinueThresholdInBytes = expectContinueThresholdInBytes;
+            return this;
+        }
+
+        public void setExpectContinueThresholdInBytes(Long expectContinueThresholdInBytes) {
+            expectContinueThresholdInBytes(expectContinueThresholdInBytes);
+        }
+
         @Override
         public Boolean useArnRegionEnabled() {
             return useArnRegionEnabled;

services/s3/src/main/java/software/amazon/awssdk/services/s3/internal/handlers/StreamingRequestInterceptor.java

@@ -37,6 +37,7 @@ public final class StreamingRequestInterceptor implements ExecutionInterceptor {
 
     private static final String DECODED_CONTENT_LENGTH_HEADER = "x-amz-decoded-content-length";
     private static final String CONTENT_LENGTH_HEADER = "Content-Length";
+    private static final long DEFAULT_EXPECT_CONTINUE_THRESHOLD_IN_BYTES = 1_048_576L;
 
     @Override
     public SdkHttpRequest modifyHttpRequest(Context.ModifyHttpRequest context,
@@ -55,22 +56,24 @@ private boolean shouldAddExpectContinueHeader(Context.ModifyHttpRequest context,
             return false;
         }
 
-        if (isExpect100ContinueDisabled(executionAttributes)) {
+        S3Configuration s3Config = getS3Configuration(executionAttributes);
+
+        if (s3Config != null && !s3Config.expectContinueEnabled()) {
             return false;
         }
 
+        long threshold = s3Config != null ? s3Config.expectContinueThresholdInBytes()
+                                          : DEFAULT_EXPECT_CONTINUE_THRESHOLD_IN_BYTES;
+
         return getContentLengthHeader(context.httpRequest())
             .map(Long::parseLong)
-            .map(length -> length != 0L)
+            .map(length -> length >= threshold && length != 0L)
             .orElse(true);
     }
 
-    private boolean isExpect100ContinueDisabled(ExecutionAttributes executionAttributes) {
+    private S3Configuration getS3Configuration(ExecutionAttributes executionAttributes) {
         ServiceConfiguration serviceConfig = executionAttributes.getAttribute(SdkExecutionAttribute.SERVICE_CONFIG);
-        if (serviceConfig instanceof S3Configuration) {
-            return !((S3Configuration) serviceConfig).expectContinueEnabled();
-        }
-        return false;
+        return serviceConfig instanceof S3Configuration ? (S3Configuration) serviceConfig : null;
     }
 
     /**

services/s3/src/test/java/software/amazon/awssdk/services/s3/S3ConfigurationTest.java

@@ -16,6 +16,7 @@
 package software.amazon.awssdk.services.s3;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
 import static software.amazon.awssdk.profiles.ProfileFileSystemSetting.AWS_CONFIG_FILE;
 import static software.amazon.awssdk.services.s3.S3SystemSetting.AWS_S3_DISABLE_MULTIREGION_ACCESS_POINTS;
 import static software.amazon.awssdk.services.s3.S3SystemSetting.AWS_S3_USE_ARN_REGION;
@@ -47,6 +48,7 @@ public void createConfiguration_minimal() {
         assertThat(config.pathStyleAccessEnabled()).isFalse();
         assertThat(config.useArnRegionEnabled()).isFalse();
         assertThat(config.expectContinueEnabled()).isTrue();
+        assertThat(config.expectContinueThresholdInBytes()).isEqualTo(1048576L);
     }
 
     @Test
@@ -116,5 +118,55 @@ public void useArnRegionEnabled_enabledInCProfile_shouldResolveToConfigCorrectly
         assertThat(config.useArnRegionEnabled()).isEqualTo(false);
     }
 
+    // -----------------------------------------------------------------------
+    // expectContinueThresholdInBytes
+    // -----------------------------------------------------------------------
+
+    @Test
+    public void expectContinueThresholdInBytes_defaultValue_is1MB() {
+        S3Configuration config = S3Configuration.builder().build();
+        assertThat(config.expectContinueThresholdInBytes()).isEqualTo(1048576L);
+    }
+
+    @Test
+    public void expectContinueThresholdInBytes_customValue_isPreserved() {
+        S3Configuration config = S3Configuration.builder()
+                                                .expectContinueThresholdInBytes(2_097_152L)
+                                                .build();
+        assertThat(config.expectContinueThresholdInBytes()).isEqualTo(2_097_152L);
+    }
+
+    @Test
+    public void expectContinueThresholdInBytes_toBuilder_preservesUserSetValue() {
+        S3Configuration config = S3Configuration.builder()
+                                                .expectContinueThresholdInBytes(512L)
+                                                .build();
+        S3Configuration rebuilt = config.toBuilder().build();
+        assertThat(rebuilt.expectContinueThresholdInBytes()).isEqualTo(512L);
+    }
+
+    @Test
+    public void expectContinueThresholdInBytes_toBuilder_returnsNullForDefault() {
+        S3Configuration config = S3Configuration.builder().build();
+        S3Configuration.Builder builder = config.toBuilder();
+        assertThat(builder.expectContinueThresholdInBytes()).isNull();
+    }
+
+    @Test
+    public void expectContinueThresholdInBytes_negativeValue_throwsException() {
+        assertThatThrownBy(() -> S3Configuration.builder()
+                                                .expectContinueThresholdInBytes(-1L)
+                                                .build())
+            .isInstanceOf(IllegalArgumentException.class)
+            .hasMessageContaining("expectContinueThresholdInBytes must not be negative");
+    }
+
+    @Test
+    public void expectContinueThresholdInBytes_zeroValue_isAccepted() {
+        S3Configuration config = S3Configuration.builder()
+                                                .expectContinueThresholdInBytes(0L)
+                                                .build();
+        assertThat(config.expectContinueThresholdInBytes()).isEqualTo(0L);
+    }
 
 }