Custom OncePerRequestFilter reads empty request body – why is the body already consumed?

I'm developing a Spring Boot application (with Spring Security) and need to add default headers (specifically, Content-Type and Content-Length) to requests targeting the endpoint /api/v1/mpos/set-token when those headers are missing. To achieve this, I wrote a custom filter that extends OncePerRequestFilter and wraps the incoming HttpServletRequest so that I can read and cache its body. I use the following code:

@Component@Order(Ordered.HIGHEST_PRECEDENCE)public class SetTokenDefaultHeadersFilter extends OncePerRequestFilter {    private static final String DEFAULT_CONTENT_TYPE = "application/json";    private static final String TARGET_URI = "/api/v1/mpos/set-token";    @Override    protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response, FilterChain filterChain)            throws ServletException, IOException {        if (TARGET_URI.equals(request.getRequestURI())&& "POST".equalsIgnoreCase(request.getMethod())&& (request.getHeader("Content-Type") == null || request.getHeader("Content-Length") == null)) {            HttpServletRequest wrappedRequest = new HttpServletRequestWrapper(request) {                private final byte[] cachedBody = toByteArray(request.getInputStream());                private byte[] toByteArray(InputStream input) throws IOException {                    ByteArrayOutputStream baos = new ByteArrayOutputStream();                    byte[] buffer = new byte[1024];                    int len;                    while ((len = input.read(buffer)) != -1) {                        baos.write(buffer, 0, len);                    }                    return baos.toByteArray();                }                @Override                public ServletInputStream getInputStream() {                    final ByteArrayInputStream bais = new ByteArrayInputStream(cachedBody);                    return new ServletInputStream() {                        @Override                        public int read() {                            return bais.read();                        }                        @Override                        public boolean isFinished() {                            return bais.available() == 0;                        }                        @Override                        public boolean isReady() {                            return true;                        }                        @Override                        public void setReadListener(ReadListener readListener) {                            // Not implemented                        }                    };                }                @Override                public BufferedReader getReader() {                    return new BufferedReader(new InputStreamReader(getInputStream(), StandardCharsets.UTF_8));                }                @Override                public int getContentLength() {                    return cachedBody.length;                }                @Override                public long getContentLengthLong() {                    return cachedBody.length;                }                @Override                public String getHeader(String name) {                    if ("Content-Type".equalsIgnoreCase(name)) {                        String header = super.getHeader(name);                        return header == null ? DEFAULT_CONTENT_TYPE : header;                    }                    if ("Content-Length".equalsIgnoreCase(name)) {                        String header = super.getHeader(name);                        return header == null ? String.valueOf(cachedBody.length) : header;                    }                    return super.getHeader(name);                }            };            filterChain.doFilter(wrappedRequest, response);        } else {            filterChain.doFilter(request, response);        }    }}

I have verified (via logging and FilterRegistrationBean) that my filter is registered with the highest precedence and should run first. However, when I call input.read(buffer), it immediately returns -1 (i.e. EOF), and the cached body is an empty array.

What I've tried so far:

Verified the filter order – my filter is running first.

Checked for other filters that might be reading the body; the logs indicate that my filter is before them.

My questions:

What could be consuming the request body before my filter even gets a chance to read it?

How can I ensure that my filter has access to the full request body so I can wrap it and add the default headers?

Is there a recommended approach in Spring Boot to cache the request body before any processing?

Any insights or suggestions would be greatly appreciated.