Servlet Filter: How to get all the headers from servletRequest?

Here is how my WebFilter looks like

@WebFilter("/rest/*")
public class AuthTokenValidatorFilter implements Filter {

	@Override
	public void init(final FilterConfig filterConfig) throws ServletException {
	}

	@Override
	public void doFilter(final ServletRequest servletRequest, final ServletResponse servletResponse, final FilterChain filterChain) throws IOException, ServletException {
		final Enumeration<String> attributeNames = servletRequest.getAttributeNames();
		while (attributeNames.hasMoreElements()) {
			System.out.println("{attribute} " + servletRequest.getParameter(attributeNames.nextElement()));
		}

		final Enumeration<String> parameterNames = servletRequest.getParameterNames();
		while (parameterNames.hasMoreElements()) {
			System.out.println("{parameter} " + servletRequest.getParameter(parameterNames.nextElement()));
		}
		filterChain.doFilter(servletRequest, servletResponse);
	}

	@Override
	public void destroy() {
	}
}

I tried to find out online as to how to get values for HTTP headers coming from request.

I did not find anything, so I tried to enumerate on servletRequest.getAttributeNames() and servletRequest.getParameterNames() without knowing anything, but I do not get any headers.

Question
How can I get all the headers coming from the request?

Typecast ServletRequest into HttpServletRequest (only if ServletRequest request is an instanceof HttpServletRequest).

Then you can use HttpServletRequest.getHeader() and HttpServletRequest.getHeaderNames() method.

Something like this:

@Override
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
    HttpServletRequest httpRequest = (HttpServletRequest) request;
    Enumeration<String> headerNames = httpRequest.getHeaderNames();

    if (headerNames != null) {
            while (headerNames.hasMoreElements()) {
                    System.out.println("Header: " + httpRequest.getHeader(headerNames.nextElement()));
            }
    }
    
    //doFilter
    chain.doFilter(httpRequest, response);
}

With Java 8+ you can use a stream to collect request headers:

HttpServletRequest httpRequest = (HttpServletRequest) request;

Map<String, String> headers = Collections.list(httpRequest.getHeaderNames())
    .stream()
    .collect(Collectors.toMap(h -> h, httpRequest::getHeader));

UPDATED

@Matthias reminded me that headers can have multiple values:

Map<String, List<String>>

Map<String, List<String>> headersMap = Collections.list(httpRequest.getHeaderNames())    
    .stream()
    .collect(Collectors.toMap(
        Function.identity(), 
        h -> Collections.list(httpRequest.getHeaders(h))
    ));

org.springframework.http.HttpHeaders

HttpHeaders httpHeaders = Collections.list(httpRequest.getHeaderNames())
    .stream()
    .collect(Collectors.toMap(
        Function.identity(),
        h -> Collections.list(httpRequest.getHeaders(h)),
        (oldValue, newValue) -> newValue,
        HttpHeaders::new
    ));

https://gist.github.com/Cepr0/fd5d9459f17da13b29126cf313328fe3

You should consider that the same HTTP header can occur multiple times with different values:

Map<String, Serializable> headers = Collections.list(request.getHeaderNames()).stream().collect(Collectors.toMap(h -> h, h -> {
    ArrayList<String> headerValues = Collections.list(request.getHeaders(h));
    return headerValues.size() == 1 ? headerValues.get(0) : headerValues;
}));