Spring POST multipart/form-data, request parts always empty

I have a simple REST controller that I use for accepting a file being uploaded from a HTML form. The project is Spring Boot 2.6.1 and Java 17. But the problem was also to be found in Spring Boot 2.3.7 and Java 15.

@PostMapping(path = "/file", consumes = MediaType.MULTIPART_FORM_DATA_VALUE) public void handleFileUpload(@RequestParam("file") MultipartFile file) { fileService.upload(file.getInputStream(), file.getOriginalFilename()); } 

The problem is file is always NULL. I found a lot of different answers about setting a MultipartResolver bean or enabling spring.http.multipart.enabled = true but nothing helped. I have a logging filter as one of the first filters in the chain. After debugging in the filter chain I found out that making a call to request.getParts() made everything work. My filter look like this:

public class LoggingFilter extends GenericFilterBean { @Override public void doFilter(ServletRequest request, ServletResponse response, FilterChain filterChain) throws IOException, ServletException { HttpServletRequest httpServletRequest = (HttpServletRequest) request; BufferedRequestWrapper bufferedRequest = new BufferedRequestWrapper(httpServletRequest); BufferedResponseWrapper bufferedResponse = new BufferedResponseWrapper((HttpServletResponse) response); filterChain.doFilter(bufferedRequest, bufferedResponse); logRequest(httpServletRequest, bufferedRequest); logResponse(httpServletRequest, bufferedResponse); } 

I changed the filter to:

public class LoggingFilter extends GenericFilterBean { @Override public void doFilter(ServletRequest request, ServletResponse response, FilterChain filterChain) throws IOException, ServletException { HttpServletRequest httpServletRequest = (HttpServletRequest) request; if (request.getContentType() != null && request.getContentType().startsWith("multipart/form-data")) { httpServletRequest.getParts(); // Trigger initialization of multi-part. } BufferedRequestWrapper bufferedRequest = new BufferedRequestWrapper(httpServletRequest); BufferedResponseWrapper bufferedResponse = new BufferedResponseWrapper((HttpServletResponse) response); filterChain.doFilter(bufferedRequest, bufferedResponse); logRequest(httpServletRequest, bufferedRequest); logResponse(httpServletRequest, bufferedResponse); } 

and everything was working. My question is; why is this needed? And is there a better way of doing this?

Below is a complete example where only the actual logging is removed because we use a custom logging framework.

package com.unwire.ticketing.filter.logging; import lombok.Getter; import org.apache.commons.io.IOUtils; import org.apache.commons.io.output.TeeOutputStream; import org.springframework.web.filter.GenericFilterBean; import javax.servlet.*; import javax.servlet.http.*; import java.io.*; import java.nio.charset.StandardCharsets; import java.util.Collection; import java.util.Locale; import java.util.stream.Collectors; public class Log extends GenericFilterBean { @Override public void doFilter(ServletRequest request, ServletResponse response, FilterChain filterChain) throws IOException, ServletException { HttpServletRequest httpServletRequest = (HttpServletRequest) request; if (request.getContentType() != null && request.getContentType().startsWith("multipart/form-data")) { httpServletRequest.getParts(); // Trigger initialization of multi-part. } try { BufferedRequestWrapper bufferedRequest = new BufferedRequestWrapper(httpServletRequest); BufferedResponseWrapper bufferedResponse = new BufferedResponseWrapper((HttpServletResponse) response); filterChain.doFilter(bufferedRequest, bufferedResponse); logRequest(httpServletRequest, bufferedRequest); logResponse(httpServletRequest, bufferedResponse); } catch (Throwable t) { } } private void logRequest(HttpServletRequest request, BufferedRequestWrapper bufferedRequest) throws IOException { String body = bufferedRequest.getRequestBody(); // Log request } private void logResponse(HttpServletRequest httpServletRequest, BufferedResponseWrapper bufferedResponse) { // Log response } private static final class BufferedRequestWrapper extends HttpServletRequestWrapper { private final byte[] buffer; BufferedRequestWrapper(HttpServletRequest req) throws IOException { super(req); if (req.getContentType() == null || (req.getContentType() != null && !req.getContentType().startsWith("application/x-www-form-urlencoded"))) { // Read InputStream and store its content in a buffer. InputStream is = req.getInputStream(); ByteArrayOutputStream baos = new ByteArrayOutputStream(); byte[] buf = new byte[1024]; int read; while ((read = is.read(buf)) > 0) { baos.write(buf, 0, read); } this.buffer = baos.toByteArray(); } else { buffer = new byte[0]; } } @Override public ServletInputStream getInputStream() { return new BufferedServletInputStream(new ByteArrayInputStream(this.buffer)); } @Override public Collection<Part> getParts() throws IOException, ServletException { return super.getParts(); } String getRequestBody() throws IOException { return IOUtils.readLines(this.getInputStream(), StandardCharsets.UTF_8.name()).stream() .map(String::trim) .collect(Collectors.joining()); } } private static final class BufferedServletInputStream extends ServletInputStream { private final ByteArrayInputStream bais; BufferedServletInputStream(ByteArrayInputStream bais) { this.bais = bais; } @Override public int available() { return this.bais.available(); } @Override public int read() { return this.bais.read(); } @Override public int read(byte[] buf, int off, int len) { return this.bais.read(buf, off, len); } @Override public boolean isFinished() { return false; } @Override public boolean isReady() { return true; } @Override public void setReadListener(ReadListener readListener) { } } public static class TeeServletOutputStream extends ServletOutputStream { private final TeeOutputStream targetStream; TeeServletOutputStream(OutputStream one, OutputStream two) { targetStream = new TeeOutputStream(one, two); } @Override public void write(int arg0) throws IOException { this.targetStream.write(arg0); } public void flush() throws IOException { super.flush(); this.targetStream.flush(); } public void close() throws IOException { super.close(); this.targetStream.close(); } @Override public boolean isReady() { return false; } @Override public void setWriteListener(WriteListener writeListener) { } } public class BufferedResponseWrapper implements HttpServletResponse { HttpServletResponse original; TeeServletOutputStream tee; ByteArrayOutputStream bos; @Getter Long startTime; BufferedResponseWrapper(HttpServletResponse response) { this.original = response; this.startTime = System.currentTimeMillis(); } String getContent() { if (bos != null) { return bos.toString(); } else { return ""; } } @Override public PrintWriter getWriter() throws IOException { return original.getWriter(); } @Override public ServletOutputStream getOutputStream() throws IOException { if (tee == null) { bos = new ByteArrayOutputStream(); tee = new TeeServletOutputStream(original.getOutputStream(), bos); } return tee; } @Override public String getCharacterEncoding() { return original.getCharacterEncoding(); } @Override public void setCharacterEncoding(String charset) { original.setCharacterEncoding(charset); } @Override public String getContentType() { return original.getContentType(); } @Override public void setContentType(String type) { original.setContentType(type); } @Override public void setContentLength(int len) { original.setContentLength(len); } @Override public void setContentLengthLong(long l) { original.setContentLengthLong(l); } @Override public int getBufferSize() { return original.getBufferSize(); } @Override public void setBufferSize(int size) { original.setBufferSize(size); } @Override public void flushBuffer() throws IOException { if (tee != null) { tee.flush(); } } @Override public void resetBuffer() { original.resetBuffer(); } @Override public boolean isCommitted() { return original.isCommitted(); } @Override public void reset() { original.reset(); } @Override public Locale getLocale() { return original.getLocale(); } @Override public void setLocale(Locale loc) { original.setLocale(loc); } @Override public void addCookie(Cookie cookie) { original.addCookie(cookie); } @Override public boolean containsHeader(String name) { return original.containsHeader(name); } @Override public String encodeURL(String url) { return original.encodeURL(url); } @Override public String encodeRedirectURL(String url) { return original.encodeRedirectURL(url); } @Override public void sendError(int sc, String msg) throws IOException { original.sendError(sc, msg); } @Override public void sendError(int sc) throws IOException { original.sendError(sc); } @Override public void sendRedirect(String location) throws IOException { original.sendRedirect(location); } @Override public void setDateHeader(String name, long date) { original.setDateHeader(name, date); } @Override public void addDateHeader(String name, long date) { original.addDateHeader(name, date); } @Override public void setHeader(String name, String value) { original.setHeader(name, value); } @Override public void addHeader(String name, String value) { original.addHeader(name, value); } @Override public void setIntHeader(String name, int value) { original.setIntHeader(name, value); } @Override public void addIntHeader(String name, int value) { original.addIntHeader(name, value); } @Override public String getHeader(String arg0) { return original.getHeader(arg0); } @Override public Collection<String> getHeaderNames() { return original.getHeaderNames(); } @Override public Collection<String> getHeaders(String arg0) { return original.getHeaders(arg0); } @Override public int getStatus() { return original.getStatus(); } @Override public void setStatus(int sc) { original.setStatus(sc); } } }