CODE EXAMPLE FOR JAVA

java responseentity

@GetMapping("/customHeader")
ResponseEntity<String> customHeader() {
    HttpHeaders headers = new HttpHeaders();
    headers.add("Custom-Header", "foo");
        
    return new ResponseEntity<>(
      "Custom header set", headers, HttpStatus.OK);
}
//Simple way
@GetMapping("/hello")
ResponseEntity<String> hello() {
    return ResponseEntity.ok("Hello World!");
}