ISO-8859-1 charset included in JSON response header when using HttpServletRespone direclty (Spring Boot 3.2.5) about spring-boot

[wilkinsona]

Thanks for the report. Unfortunately, I don't think there's anything that we can do about it as the behavior is determined by the Servlet specification with which Tomcat is complying. In the absence of the response's character encoding being set through setCharacterEncoding or setContentType, the default character is specified to be ISO-8859-1. That's what is happening here as you haven't called setCharacterEncoding and the content type that you have specified does not provide any charset information.

From the sample's README:

Also, there are integration tests demonstrating that issue does not reproduce when testing using MockMvc (no charset included at all)

This happens because Tomcat isn't involved so the default character encoding isn't being set. Strictly speaking, it could be argued that this is a bug in Spring Framework's mock response as it isn't completely compliant with the Servlet spec. Please open a Spring Framework issue if you would like to pursue this.