FlashMapFilter.java

package com.github.ryoasai.springmvc.flashmap;

import java.io.IOException;
import java.util.Map;

import javax.servlet.FilterChain;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

import org.springframework.web.filter.OncePerRequestFilter;

public class FlashMapFilter extends OncePerRequestFilter {


	@Override
	protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response, FilterChain filterChain)
		throws ServletException, IOException {
		HttpSession session = request.getSession(false);
		if (session != null) {
			synchronized (session) {
				@SuppressWarnings("unchecked")
				Map<String, ?> flash = (Map<String, ?>) session.getAttribute(FlashMap.FLASH_SCOPE_ATTRIBUTE);
				if (flash != null) {
					for (Map.Entry<String, ?> entry : flash.entrySet()) {
						Object currentValue = request.getAttribute(entry.getKey());
						if (currentValue == null) {
							request.setAttribute(entry.getKey(), entry.getValue());
						}					
					}

					session.removeAttribute(FlashMap.FLASH_SCOPE_ATTRIBUTE);
				}
			}
		}
		
		filterChain.doFilter(request, response);
	}
}