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fix(java): serialize Feign Jackson JSON headers #25530

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b94b3c8
fix(java): serialize Feign Jackson JSON headers
AndreyVMarkelov Nov 3, 2026
e66f5fa
fix(java): address Feign JSON header review
AndreyVMarkelov Nov 3, 2026
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32 changes: 32 additions & 0 deletions .github/workflows/test-java-feign-jackson-headers.yaml
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@@ -0,0 +1,32 @@
name: Java Feign Jackson headers
on:
push:
paths:
- 'modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/JavaClientCodegen.java'
- 'modules/openapi-generator/src/main/resources/Java/**'
- 'modules/openapi-generator/src/test/resources/3_1/java/feign-jackson-json-header.yaml'
- 'modules/openapi-generator/src/test/resources/3_1/java/FeignJacksonHeaderRuntimeTest.java'
- '.github/workflows/test-java-feign-jackson-headers.yaml'
pull_request:
paths:
- 'modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/JavaClientCodegen.java'
- 'modules/openapi-generator/src/main/resources/Java/**'
- 'modules/openapi-generator/src/test/resources/3_1/java/feign-jackson-json-header.yaml'
- 'modules/openapi-generator/src/test/resources/3_1/java/FeignJacksonHeaderRuntimeTest.java'
- '.github/workflows/test-java-feign-jackson-headers.yaml'
jobs:
runtime:
runs-on: ubuntu-latest
steps:
- uses: actions/checkout@v7
- uses: actions/setup-java@v6
with:
distribution: temurin
java-version: '17'
cache: maven
- run: ./mvnw -B -pl modules/openapi-generator-cli -am -DskipTests package
- run: |
java -jar modules/openapi-generator-cli/target/openapi-generator-cli.jar generate -g java --library feign -i modules/openapi-generator/src/test/resources/3_1/java/feign-jackson-json-header.yaml -o /tmp/feign-test --additional-properties=serializationLibrary=jackson --global-property=apis,models,supportingFiles,apiTests=false,modelTests=false
mkdir -p /tmp/feign-test/src/test/java
cp modules/openapi-generator/src/test/resources/3_1/java/FeignJacksonHeaderRuntimeTest.java /tmp/feign-test/src/test/java/
mvn -B -f /tmp/feign-test/pom.xml -Dtest=FeignJacksonHeaderRuntimeTest test
19 changes: 19 additions & 0 deletions ...openapi-generator/src/main/java/org/openapitools/codegen/languages/JavaClientCodegen.java
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Expand Up @@ -17,6 +17,7 @@

package org.openapitools.codegen.languages;

import io.swagger.v3.oas.models.OpenAPI;
import io.swagger.v3.oas.models.Operation;
import io.swagger.v3.oas.models.media.Schema;
import io.swagger.v3.oas.models.servers.Server;
Expand Down Expand Up @@ -947,6 +948,17 @@ public void processOpts() {
public OperationsMap postProcessOperationsWithModels(OperationsMap objs, List<ModelMap> allModels) {
super.postProcessOperationsWithModels(objs, allModels);

if (isLibrary(FEIGN) && jackson && objs.getOperations() != null) {
for (CodegenOperation operation : objs.getOperations().getOperation()) {
if (operation.allParams.stream().anyMatch(p -> p.isHeaderParam && p.headerIsJsonMimeType)) {
additionalProperties.put("hasFeignJsonHeader", true);
if (operation.allParams.stream().anyMatch(p -> p.isHeaderParam && p.headerIsJsonMimeType && p.isArray)) {
additionalProperties.put("hasFeignJsonHeaderArrays", true);
}
}
}
}

if (this.getSingleRequestParameter() && (isLibrary(JERSEY2) || isLibrary(JERSEY3) || isLibrary(OKHTTP_GSON) || isLibrary(NATIVE))) {
// loop through operations to set x-group-parameters extension to true if useSingleRequestParameter option is enabled
OperationMap operations = objs.getOperations();
Expand Down Expand Up @@ -1141,6 +1153,13 @@ private static boolean isMultipartType(List<Map<String, String>> consumes) {
return false;
}

@Override
public void processOpenAPI(OpenAPI openAPI) {
super.processOpenAPI(openAPI);
additionalProperties.remove("hasFeignJsonHeader");
additionalProperties.remove("hasFeignJsonHeaderArrays");
}

@Override
public void postProcessModelProperty(CodegenModel model, CodegenProperty property) {
super.postProcessModelProperty(model, property);
Expand Down
110 changes: 110 additions & 0 deletions modules/openapi-generator/src/main/resources/Java/libraries/feign/ApiClient.mustache
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Expand Up @@ -61,6 +61,103 @@ import feign.Retryer;
{{>generatedAnnotation}}

public class ApiClient {
{{#hasFeignJsonHeader}}
/** Serializes JSON-content headers with the client's Jackson configuration. */
public static class JsonHeaderParamExpander implements feign.Param.Expander {
private final ApiClient client;

public JsonHeaderParamExpander() {
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throw new IllegalStateException("JSON header expander requires a configured ApiClient contract");
}

private JsonHeaderParamExpander(ApiClient client) {
this.client = client;
}

@Override
public String expand(Object value) {
try {
if (value instanceof JsonHeaderIterable) value = ((JsonHeaderIterable) value).value;
return client.objectMapper.copy()
.setSerializationInclusion(com.fasterxml.jackson.annotation.JsonInclude.Include.NON_NULL)
.writer()
.with(com.fasterxml.jackson.core.JsonGenerator.Feature.ESCAPE_NON_ASCII)
.writeValueAsString(value);
} catch (com.fasterxml.jackson.core.JsonProcessingException e) {
throw new IllegalArgumentException("Failed to serialize JSON header", e);
}
}
}

private static final class JsonHeaderIterable {
private final Object value;
private JsonHeaderIterable(Object value) { this.value = value; }
}

{{#hasFeignJsonHeaderArrays}}
private static final class JsonHeaderInvocationHandler implements java.lang.reflect.InvocationHandler {
private final Object target;
private final java.lang.reflect.InvocationHandler delegate;

private JsonHeaderInvocationHandler(Object target) {
this.target = target;
this.delegate = java.lang.reflect.Proxy.getInvocationHandler(target);
}

@Override
public Object invoke(Object proxy, java.lang.reflect.Method method, Object[] args) throws Throwable {
if (method.getDeclaringClass() == Object.class && method.getName().equals("equals")) {
Object other = args[0];
if (other == proxy) return true;
if (other != null && java.lang.reflect.Proxy.isProxyClass(other.getClass())) {
java.lang.reflect.InvocationHandler otherHandler = java.lang.reflect.Proxy.getInvocationHandler(other);
if (otherHandler instanceof JsonHeaderInvocationHandler) {
return delegate.equals(((JsonHeaderInvocationHandler) otherHandler).delegate);
}
}
return false;
}
if (args != null) {
args = args.clone();
java.lang.annotation.Annotation[][] annotations = method.getParameterAnnotations();
for (int i = 0; i < args.length; i++) {
if (!(args[i] instanceof Iterable)) continue;
for (java.lang.annotation.Annotation annotation : annotations[i]) {
if (annotation instanceof feign.Param
&& ((feign.Param) annotation).expander() == JsonHeaderParamExpander.class) {
args[i] = new JsonHeaderIterable(args[i]);
break;
}
}
}
}
return delegate.invoke(target, method, args);
}
}
{{/hasFeignJsonHeaderArrays}}

private feign.Contract jsonHeaderContract() {
feign.Contract delegate = new feign.Contract.Default();
return type -> {
java.util.List<feign.MethodMetadata> methods = delegate.parseAndValidateMetadata(type);
for (feign.MethodMetadata method : methods) {
java.util.Map<Integer, feign.Param.Expander> expanders = new java.util.LinkedHashMap<>();
if (method.indexToExpander() != null) expanders.putAll(method.indexToExpander());
java.util.Iterator<java.util.Map.Entry<Integer, Class<? extends feign.Param.Expander>>> entries =
method.indexToExpanderClass().entrySet().iterator();
while (entries.hasNext()) {
java.util.Map.Entry<Integer, Class<? extends feign.Param.Expander>> entry = entries.next();
if (entry.getValue() == JsonHeaderParamExpander.class) {
expanders.put(entry.getKey(), new JsonHeaderParamExpander(this));
entries.remove();
}
}
method.indexToExpander(expanders);
}
return methods;
};
}
{{/hasFeignJsonHeader}}
protected static final Logger log = Logger.getLogger(ApiClient.class.getName());

public interface Api {}
Expand Down Expand Up @@ -89,6 +186,9 @@ public class ApiClient {
{{#feign-hc5}}
.client(APACHE_HTTP5_CLIENT)
{{/feign-hc5}}
{{#hasFeignJsonHeader}}
.contract(jsonHeaderContract())
{{/hasFeignJsonHeader}}
.encoder(new FormEncoder(new JacksonEncoder(objectMapper)))
.decoder(new ApiResponseDecoder(objectMapper))
{{#hasOAuthMethods}}
Expand Down Expand Up @@ -188,6 +288,9 @@ public class ApiClient {

public ApiClient setFeignBuilder(Feign.Builder feignBuilder) {
this.feignBuilder = feignBuilder;
{{#hasFeignJsonHeader}}
this.feignBuilder.contract(jsonHeaderContract());
{{/hasFeignJsonHeader}}
return this;
}

Expand Down Expand Up @@ -255,7 +358,14 @@ public class ApiClient {
* @return The Client
*/
public <T extends Api> T buildClient(Class<T> clientClass) {
{{#hasFeignJsonHeaderArrays}}
T target = feignBuilder.target(clientClass, basePath);
return clientClass.cast(java.lang.reflect.Proxy.newProxyInstance(clientClass.getClassLoader(),
new Class<?>[] { clientClass }, new JsonHeaderInvocationHandler(target)));
{{/hasFeignJsonHeaderArrays}}
{{^hasFeignJsonHeaderArrays}}
return feignBuilder.target(clientClass, basePath);
{{/hasFeignJsonHeaderArrays}}
}

/**
Expand Down
8 changes: 4 additions & 4 deletions modules/openapi-generator/src/main/resources/Java/libraries/feign/api.mustache
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Expand Up @@ -53,7 +53,7 @@ public interface {{classname}} extends ApiClient.Api {
"{{baseName}}: {{=<% %>=}}{<%paramName%>}<%={{ }}=%>"{{^-last}},
{{/-last}}{{/headerParams}}
})
{{#returnType}}{{{.}}} {{/returnType}}{{^returnType}}void {{/returnType}}{{nickname}}({{#allParams}}{{^isBodyParam}}{{^isFormParam}}{{^legacyDates}}@Param("{{paramName}}") {{/legacyDates}}{{#legacyDates}}@Param(value="{{paramName}}", expander=ParamExpander.class) {{/legacyDates}}{{/isFormParam}}{{#isFormParam}}@Param("{{baseName}}") {{/isFormParam}}{{/isBodyParam}}{{>nullable_var_annotations}} {{{dataType}}} {{paramName}}{{^-last}}, {{/-last}}{{/allParams}});
{{#returnType}}{{{.}}} {{/returnType}}{{^returnType}}void {{/returnType}}{{nickname}}({{#allParams}}{{^isBodyParam}}{{^isFormParam}}{{#jackson}}{{#headerIsJsonMimeType}}@Param(value="{{paramName}}", expander=ApiClient.JsonHeaderParamExpander.class) {{/headerIsJsonMimeType}}{{/jackson}}{{^headerIsJsonMimeType}}{{^legacyDates}}@Param("{{paramName}}") {{/legacyDates}}{{#legacyDates}}@Param(value="{{paramName}}", expander=ParamExpander.class) {{/legacyDates}}{{/headerIsJsonMimeType}}{{^jackson}}{{#headerIsJsonMimeType}}{{^legacyDates}}@Param("{{paramName}}") {{/legacyDates}}{{#legacyDates}}@Param(value="{{paramName}}", expander=ParamExpander.class) {{/legacyDates}}{{/headerIsJsonMimeType}}{{/jackson}}{{/isFormParam}}{{#isFormParam}}@Param("{{baseName}}") {{/isFormParam}}{{/isBodyParam}}{{>nullable_var_annotations}} {{{dataType}}} {{paramName}}{{^-last}}, {{/-last}}{{/allParams}});
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/**
* {{summary}}
Expand Down Expand Up @@ -83,7 +83,7 @@ public interface {{classname}} extends ApiClient.Api {
"{{baseName}}: {{=<% %>=}}{<%paramName%>}<%={{ }}=%>"{{^-last}},
{{/-last}}{{/headerParams}}
})
ApiResponse<{{#returnType}}{{{.}}}{{/returnType}}{{^returnType}}Void{{/returnType}}> {{nickname}}WithHttpInfo({{#allParams}}{{^isBodyParam}}{{^isFormParam}}{{^legacyDates}}@Param("{{paramName}}") {{/legacyDates}}{{#legacyDates}}@Param(value="{{paramName}}", expander=ParamExpander.class) {{/legacyDates}}{{/isFormParam}}{{#isFormParam}}@Param("{{baseName}}") {{/isFormParam}}{{/isBodyParam}}{{>nullable_var_annotations}} {{{dataType}}} {{paramName}}{{^-last}}, {{/-last}}{{/allParams}});
ApiResponse<{{#returnType}}{{{.}}}{{/returnType}}{{^returnType}}Void{{/returnType}}> {{nickname}}WithHttpInfo({{#allParams}}{{^isBodyParam}}{{^isFormParam}}{{#jackson}}{{#headerIsJsonMimeType}}@Param(value="{{paramName}}", expander=ApiClient.JsonHeaderParamExpander.class) {{/headerIsJsonMimeType}}{{/jackson}}{{^headerIsJsonMimeType}}{{^legacyDates}}@Param("{{paramName}}") {{/legacyDates}}{{#legacyDates}}@Param(value="{{paramName}}", expander=ParamExpander.class) {{/legacyDates}}{{/headerIsJsonMimeType}}{{^jackson}}{{#headerIsJsonMimeType}}{{^legacyDates}}@Param("{{paramName}}") {{/legacyDates}}{{#legacyDates}}@Param(value="{{paramName}}", expander=ParamExpander.class) {{/legacyDates}}{{/headerIsJsonMimeType}}{{/jackson}}{{/isFormParam}}{{#isFormParam}}@Param("{{baseName}}") {{/isFormParam}}{{/isBodyParam}}{{>nullable_var_annotations}} {{{dataType}}} {{paramName}}{{^-last}}, {{/-last}}{{/allParams}});


{{#hasQueryParams}}
Expand Down Expand Up @@ -128,7 +128,7 @@ public interface {{classname}} extends ApiClient.Api {
"{{baseName}}: {{=<% %>=}}{<%paramName%>}<%={{ }}=%>"{{^-last}},
{{/-last}}{{/headerParams}}
})
{{#returnType}}{{{.}}} {{/returnType}}{{^returnType}}void {{/returnType}}{{nickname}}({{#allParams}}{{^isQueryParam}}{{^isBodyParam}}{{^isFormParam}}{{^legacyDates}}@Param("{{paramName}}") {{/legacyDates}}{{#legacyDates}}@Param(value="{{paramName}}", expander=ParamExpander.class) {{/legacyDates}}{{/isFormParam}}{{#isFormParam}}@Param("{{baseName}}") {{/isFormParam}}{{/isBodyParam}}{{>nullable_var_annotations}} {{{dataType}}} {{paramName}}, {{/isQueryParam}}{{/allParams}}@QueryMap {{operationIdCamelCase}}QueryParams queryParams);
{{#returnType}}{{{.}}} {{/returnType}}{{^returnType}}void {{/returnType}}{{nickname}}({{#allParams}}{{^isQueryParam}}{{^isBodyParam}}{{^isFormParam}}{{#jackson}}{{#headerIsJsonMimeType}}@Param(value="{{paramName}}", expander=ApiClient.JsonHeaderParamExpander.class) {{/headerIsJsonMimeType}}{{/jackson}}{{^headerIsJsonMimeType}}{{^legacyDates}}@Param("{{paramName}}") {{/legacyDates}}{{#legacyDates}}@Param(value="{{paramName}}", expander=ParamExpander.class) {{/legacyDates}}{{/headerIsJsonMimeType}}{{^jackson}}{{#headerIsJsonMimeType}}{{^legacyDates}}@Param("{{paramName}}") {{/legacyDates}}{{#legacyDates}}@Param(value="{{paramName}}", expander=ParamExpander.class) {{/legacyDates}}{{/headerIsJsonMimeType}}{{/jackson}}{{/isFormParam}}{{#isFormParam}}@Param("{{baseName}}") {{/isFormParam}}{{/isBodyParam}}{{>nullable_var_annotations}} {{{dataType}}} {{paramName}}, {{/isQueryParam}}{{/allParams}}@QueryMap {{operationIdCamelCase}}QueryParams queryParams);

/**
* {{summary}}
Expand Down Expand Up @@ -168,7 +168,7 @@ public interface {{classname}} extends ApiClient.Api {
"{{baseName}}: {{=<% %>=}}{<%paramName%>}<%={{ }}=%>"{{^-last}},
{{/-last}}{{/headerParams}}
})
ApiResponse<{{#returnType}}{{{.}}}{{/returnType}}{{^returnType}}Void{{/returnType}}> {{nickname}}WithHttpInfo({{#allParams}}{{^isQueryParam}}{{^isBodyParam}}{{^isFormParam}}{{^legacyDates}}@Param("{{paramName}}") {{/legacyDates}}{{#legacyDates}}@Param(value="{{paramName}}", expander=ParamExpander.class) {{/legacyDates}}{{/isFormParam}}{{#isFormParam}}@Param("{{baseName}}") {{/isFormParam}}{{/isBodyParam}}{{>nullable_var_annotations}} {{{dataType}}} {{paramName}}, {{/isQueryParam}}{{/allParams}}@QueryMap {{operationIdCamelCase}}QueryParams queryParams);
ApiResponse<{{#returnType}}{{{.}}}{{/returnType}}{{^returnType}}Void{{/returnType}}> {{nickname}}WithHttpInfo({{#allParams}}{{^isQueryParam}}{{^isBodyParam}}{{^isFormParam}}{{#jackson}}{{#headerIsJsonMimeType}}@Param(value="{{paramName}}", expander=ApiClient.JsonHeaderParamExpander.class) {{/headerIsJsonMimeType}}{{/jackson}}{{^headerIsJsonMimeType}}{{^legacyDates}}@Param("{{paramName}}") {{/legacyDates}}{{#legacyDates}}@Param(value="{{paramName}}", expander=ParamExpander.class) {{/legacyDates}}{{/headerIsJsonMimeType}}{{^jackson}}{{#headerIsJsonMimeType}}{{^legacyDates}}@Param("{{paramName}}") {{/legacyDates}}{{#legacyDates}}@Param(value="{{paramName}}", expander=ParamExpander.class) {{/legacyDates}}{{/headerIsJsonMimeType}}{{/jackson}}{{/isFormParam}}{{#isFormParam}}@Param("{{baseName}}") {{/isFormParam}}{{/isBodyParam}}{{>nullable_var_annotations}} {{{dataType}}} {{paramName}}, {{/isQueryParam}}{{/allParams}}@QueryMap {{operationIdCamelCase}}QueryParams queryParams);


/**
Expand Down
33 changes: 33 additions & 0 deletions .../openapi-generator/src/test/java/org/openapitools/codegen/java/JavaClientCodegenTest.java
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Expand Up @@ -81,6 +81,39 @@ public class JavaClientCodegenTest {

private static final String JAVA_GENERATOR = "java";

@Test
public void testFeignJacksonJsonHeader() {
final Path output = newTempFolder();
final CodegenConfigurator configurator = new CodegenConfigurator()
.setGeneratorName(JAVA_GENERATOR)
.setLibrary(FEIGN)
.addAdditionalProperty("serializationLibrary", "jackson")
.setInputSpec("src/test/resources/3_1/java/feign-jackson-json-header.yaml")
.setOutputDir(output.toString());
List<File> files = new DefaultGenerator().opts(configurator.toClientOptInput()).generate();
validateJavaSourceFiles(files);
Path java = output.resolve("src/main/java/org/openapitools/client");
assertThat(java.resolve("api/DefaultApi.java")).content()
.contains("@Param(value=\"xJsonArg\", expander=ApiClient.JsonHeaderParamExpander.class)",
"@Param(\"xPlain\")");
assertThat(java.resolve("ApiClient.java")).content()
.contains("class JsonHeaderParamExpander", "ESCAPE_NON_ASCII", "writeValueAsString(value)");
}

@Test
public void testFeignJacksonResponseHeaderDoesNotEnableRequestExpander() {
Path output = newTempFolder();
CodegenConfigurator configurator = new CodegenConfigurator()
.setGeneratorName(JAVA_GENERATOR)
.setLibrary(FEIGN)
.addAdditionalProperty("serializationLibrary", "jackson")
.setInputSpec("src/test/resources/3_1/java/feign-jackson-response-header.yaml")
.setOutputDir(output.toString());
new DefaultGenerator().opts(configurator.toClientOptInput()).generate();
assertThat(output.resolve("src/main/java/org/openapitools/client/ApiClient.java"))
.content().doesNotContain("JsonHeaderParamExpander");
}

// This is the kind of information that ideally would be defined and available system-wide
@Getter
enum Library {
Expand Down
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