LogRequestFilter Sample Code

LogRequestFilter.java

import javax.servlet.*;
import javax.servlet.http.HttpServletRequest;

public class LogRequestFilter implements Filter
{
  public static final String kDebugHeaderName = "DEBUG";
  private final static ThreadLocal<String> threadLocalLoggingTag = new ThreadLocal<String>();

  /* Nothing needed in init/destroy methods for the javax Filter interface. Just included for completeness */
  @Override public void init(FilterConfig config) { }
  @Override public void destroy() { }

  @Override
  public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws java.io.IOException, ServletException
  {
    try {
      String loggingKey = ((HttpServletRequest) request).getHeader(kDebugHeaderName);

      // If a special header is present, we will tag this thread to start logging.
      if(loggingKey != null && !loggingKey.isEmpty()) {
        startLogging(loggingKey);
      }

      chain.doFilter(request, response);
    } finally {
      // Be sure to close out logging once this thread is finished processing the request.
      endLogging();
    }
  }

  public void startLogging(String loggingKey)
  {
    // Only set the tag if one is not already present.
    // Note: You can also enforce a strict check to only allow logging when this Filter's doFilter method has been called.
    //       This would ensure that the DEBUG logging can only be called on request threads which will automatically call endLogging
    //       The stricter check is not included in this sample for brevity.
    if(threadLocalLoggingTag.get() == null) {
        threadLocalLoggingTag.set(loggingKey);
    }
  }

  private void endLogging()
  {
    threadLocalLoggingTag.remove();
  }

  public static String getLoggingKey()
  {
    return threadLocalLoggingTag.get();
  }
}