How to Resolve the Feign Client Error: 'Body Parameters Cannot Be Used with Form Parameters' When Using Headers and JSON Data

What does the error 'Body parameters cannot be used with form parameters' mean in a Feign client context?

@FormUrlEncoded\n@POST("/example")\nResponse submitData(@Field("field1") String field1, @Body MyRequestBody body);\n

The error message 'Body parameters cannot be used with form parameters' in a Feign client arises when you mistakenly attempt to send both body parameters and form parameters in the same request. Feign, a popular declarative web service client for Java, does not support sending both types together because they are fundamentally different in how data is sent over HTTP.

@FeignClient(name = "exampleClient")\npublic interface ExampleClient {\n    @FormUrlEncoded\n    @POST("/submit")\n    Response submitFormData(@Field("field1") String field1);\n    \n    @POST("/submitJson")\n    Response submitJsonData(@Body MyRequestBody body);\n}

Causes

• Attempting to use @Body and @FormUrlEncoded together in a single request.
• Inconsistent use of parameter annotations for the same request method.
• Misunderstanding of how Feign handles request payloads.

Solutions

• Use @FormUrlEncoded with @Field annotations for form data only.
• Use @Body for JSON or other body data, without form parameter annotations.
• Separate requests into different methods if both types of parameters are needed.

Mistake: Using both @Body and @FormUrlEncoded in the same method.

Solution: Choose one of the two; do not combine them in the same request.

Mistake: Not correctly defining the object for @Body when sending JSON.

Solution: Ensure the class used with @Body is properly annotated and defined.

Mistake: Inconsistent use of HTTP methods (POST instead of PUT or vice versa).

Solution: Check to ensure the correct HTTP methods are applied as per the API’s design.