refactor: decompose & simplify streaming multipart body encoding

README.md

@@ -1471,11 +1471,22 @@ interface PizzaClient {
     );
 }
 
+Encoder jackson3Encoder = new ConditionalEncoder(
+        new Jackson3Encoder(),
+        EncoderPredicate.forContentType(MediaType.APPLICATION_JSON_VALUE)
+);
+PartBodyFactory partBodyFactory = new PartBodyFactory(encoders -> encoders.add(0, jackson3Encoder));
+PartFactory partFactory = PartFactory.builder()
+        .partBodyFactory(partBodyFactory)
+        .build();
+MultipartFormBodyFactory multipartFormBodyFactory = MultipartFormBodyFactory.builder()
+        .partFactory(partFactory)
+        .build();
+MultipartFormEncoder multipartFormEncoder = MultipartFormEncoder.builder()
+        .multipartFormBodyFactory(multipartFormBodyFactory)
+        .build();
 PizzaClient pizzaClient = Feign.builder()
-    .encoder(MultipartFormEncoder.builder()
-        .partBodyEncoders(List.of(new Jackson3Encoder()))
-        .build()
-    )
+    .encoder(multipartFormEncoder)
     .target(PizzaClient.class, "https://api.pizza.com");
 
 Pizza pizza = new Pizza();
@@ -1601,25 +1612,38 @@ public interface DownloadClient {
 #### Streaming Spring MultipartFile (Feign 14+)
 
 If you are using Spring Cloud OpenFeign and need to stream large `MultipartFile` payloads without consuming significant
-JVM memory, you can use `feign.form.spring.MultipartFileEncoder`. This encoder processes files as streams instead of
-loading their entire content into memory.
+JVM memory, configure `MultipartFormEncoder` with:
+
+- `MultipartFilePartContextResolver` to unwrap Spring `MultipartFile` metadata
+- `MultipartFileEncoder` as the part body encoder
 
 ```java
 @Configuration
 public class StreamingMultipartConfig {
 
   @Bean
   public Encoder feignFormEncoder() {
+    var partFactory = PartFactory.builder()
+            .partBodyFactory(new PartBodyFactory(List.of(new MultipartFileEncoder())))
+            .build();
+    var multipartFormBodyFactory = new MultipartFormBodyFactory(
+            new PartContextResolverChain(resolvers -> resolvers.addFirst(new MultipartFilePartContextResolver())),
+            partFactory
+    );
     return MultipartFormEncoder.builder()
-            .partEncoders(encoders -> encoders.addFirst(new MultipartFileEncoder()))
+            .multipartFormBodyFactory(multipartFormBodyFactory)
             .build();
   }
 }
 
 @FeignClient(name = "large-file-upload-service", configuration = StreamingMultipartConfig.class)
 public interface LargeFileUploadClient {
 
-  @RequestMapping(value = "/upload", method = RequestMethod.POST, consumes = MediaType.MULTIPART_FORM_DATA_VALUE)
+  @RequestMapping(
+          value = "/upload",
+          method = RequestMethod.POST,
+          consumes = MediaType.MULTIPART_FORM_DATA_VALUE
+  )
   void uploadLargeFile(@RequestBody MultipartFile file);
 }
 ```

api/src/main/java/feign/Request.java

@@ -725,6 +725,18 @@ public boolean isRepeatable() {
       return true;
     }
 
+    @Override
+    public boolean equals(Object object) {
+      if (!(object instanceof BodyImpl)) return false;
+      BodyImpl body = (BodyImpl) object;
+      return Objects.deepEquals(content, body.content) && Objects.equals(charset, body.charset);
+    }
+
+    @Override
+    public int hashCode() {
+      return Objects.hash(Arrays.hashCode(content), charset);
+    }
+
     @Override
     public String toString() {
       return new String(content, charset);

api/src/main/java/feign/codec/Encoder.java

@@ -31,7 +31,7 @@
  * void create(User user);
  * </pre>
  *
- * <p>Example implementation: <br>
+ * Example implementation: <br>
  *
  * <p>
  *
@@ -66,7 +66,6 @@
  * Session login(@Param(&quot;username&quot;) String username, @Param(&quot;password&quot;) String password);
  * </pre>
  */
-@FunctionalInterface
 public interface Encoder {
   /** Type literal for {@code Map<String, ?>}, indicating the object to encode is a form. */
   Type MAP_STRING_WILDCARD = Util.MAP_STRING_WILDCARD;
@@ -81,16 +80,4 @@ public interface Encoder {
    * @throws EncodeException when encoding failed due to a checked exception.
    */
   void encode(Object object, Type bodyType, RequestTemplate template) throws EncodeException;
-
-  /**
-   * Indicates whether this encoder supports encoding the given content type. Default implementation
-   * returns {@code true} for all content types.
-   *
-   * @param contentType the content type to check for support
-   * @return {@code true} if this encoder supports encoding the given content type, {@code false}
-   *     otherwise
-   */
-  default boolean supports(String contentType) {
-    return true;
-  }
 }

api/src/main/java/feign/codec/JsonEncoder.java

@@ -18,15 +18,4 @@
 import feign.Experimental;
 
 @Experimental
-public interface JsonEncoder extends Encoder {
-  /**
-   * {@inheritDoc}
-   *
-   * @param contentType {@inheritDoc}
-   * @return {@code true} if the given {@code contentType} is a JSON media type, {@code false}
-   */
-  @Override
-  default boolean supports(String contentType) {
-    return contentType != null && contentType.trim().matches("(?i)\\w+/(?:[\\w._-]+\\+)?json.*");
-  }
-}
+public interface JsonEncoder extends Encoder {}

form-spring/src/main/java/feign/form/spring/MultipartFileEncoder.java

@@ -16,44 +16,30 @@
 package feign.form.spring;
 
 import feign.Request;
-import feign.form.multipart.AbstractPartEncoder;
-import feign.form.multipart.Part;
-import feign.form.multipart.PartMetadata;
+import feign.RequestTemplate;
+import feign.form.multipart.ConditionalEncoder;
 import java.io.IOException;
 import java.io.OutputStream;
+import java.lang.reflect.Type;
 import lombok.NonNull;
 import org.springframework.web.multipart.MultipartFile;
 
 /** Encoder for {@link MultipartFile} instances. */
-public class MultipartFileEncoder extends AbstractPartEncoder {
+public class MultipartFileEncoder extends ConditionalEncoder {
   /**
-   * Encodes the content of the part using given {@link MultipartFile}.
-   *
-   * @param partMetadata {@inheritDoc}
-   * @return {@inheritDoc}
+   * Creates a new instance of {@link MultipartFileEncoder} that encodes {@link MultipartFile}
+   * instances.
    */
-  @Override
-  public Request.Body encode(PartMetadata partMetadata) {
-    var multipartFile = (MultipartFile) partMetadata.content().orElseThrow();
+  public MultipartFileEncoder() {
+    super(MultipartFileEncoder::doEncode, MultipartFileEncoder::isMultipartFile);
+  }
 
-    return new Part(
-        createHeaders(
-            multipartFile.getName(),
-            multipartFile.getOriginalFilename(),
-            multipartFile.getContentType()),
-        new MultipartFileBody(multipartFile));
+  private static void doEncode(Object object, Type bodyType, RequestTemplate template) {
+    template.body(new MultipartFileBody((MultipartFile) object));
   }
 
-  /**
-   * {@inheritDoc}
-   *
-   * @param partMetadata {@inheritDoc}
-   * @return {@code true} if the content of the part is an instance of {@link MultipartFile}, {@code
-   *     false} otherwise
-   */
-  @Override
-  public boolean supports(PartMetadata partMetadata) {
-    return partMetadata.content().filter(MultipartFile.class::isInstance).isPresent();
+  private static boolean isMultipartFile(Object object, Type bodyType, RequestTemplate template) {
+    return object instanceof MultipartFile;
   }
 
   private record MultipartFileBody(@NonNull MultipartFile multipartFile) implements Request.Body {

form-spring/src/main/java/feign/form/spring/MultipartFilePartContextResolver.java

@@ -0,0 +1,57 @@
+/*
+ * Copyright © 2012 The Feign Authors (feign@commonhaus.dev)
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package feign.form.spring;
+
+import feign.form.multipart.PartContext;
+import feign.form.multipart.PartContextResolver;
+import feign.form.multipart.PartContextResolverChain;
+import java.util.stream.Stream;
+import org.springframework.web.multipart.MultipartFile;
+
+/**
+ * A {@link PartContextResolver} implementation that resolves a {@link PartContext} containing a
+ * {@link MultipartFile} into a {@link PartContext} with the unwrapped content of the {@link
+ * MultipartFile}.
+ */
+public class MultipartFilePartContextResolver implements PartContextResolver {
+  /**
+   * {@inheritDoc}
+   *
+   * @param partContext {@inheritDoc}
+   * @param chain {@inheritDoc}
+   * @return {@inheritDoc}
+   */
+  @Override
+  public Stream<PartContext> resolve(PartContext partContext, PartContextResolverChain chain) {
+    var resolvedPartContext =
+        partContext
+            .content()
+            .filter(MultipartFile.class::isInstance)
+            .map(
+                content -> {
+                  var multipartFile = (MultipartFile) content;
+
+                  return partContext.toBuilder()
+                      .name(multipartFile.getName())
+                      .filename(multipartFile.getOriginalFilename())
+                      .contentType(multipartFile.getContentType())
+                      .build();
+                })
+            .orElse(partContext);
+
+    return chain.resolve(resolvedPartContext);
+  }
+}

form-spring/src/test/java/feign/form/feign/spring/SpringStreamingMultipartFormTest.java

@@ -29,8 +29,14 @@
 import feign.Util;
 import feign.codec.Encoder;
 import feign.form.MultipartFormEncoder;
+import feign.form.multipart.MultipartFormBodyFactory;
+import feign.form.multipart.PartBodyFactory;
+import feign.form.multipart.PartContextResolverChain;
+import feign.form.multipart.PartFactory;
 import feign.form.spring.MultipartFileEncoder;
+import feign.form.spring.MultipartFilePartContextResolver;
 import java.nio.charset.StandardCharsets;
+import java.util.List;
 import org.junit.jupiter.api.Test;
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.boot.test.context.SpringBootTest;
@@ -91,8 +97,17 @@ private interface SpringStreamingMultipartFormTestClient {
   private static class SpringStreamingMultipartFormTestConfiguration {
     @Bean
     public Encoder feignEncoder() {
+      var partFactory =
+          PartFactory.builder()
+              .partBodyFactory(new PartBodyFactory(List.of(new MultipartFileEncoder())))
+              .build();
+      var multipartFormBodyFactory =
+          new MultipartFormBodyFactory(
+              new PartContextResolverChain(
+                  resolvers -> resolvers.addFirst(new MultipartFilePartContextResolver())),
+              partFactory);
       return MultipartFormEncoder.builder()
-          .partEncoders(encoders -> encoders.addFirst(new MultipartFileEncoder()))
+          .multipartFormBodyFactory(multipartFormBodyFactory)
           .build();
     }
   }