UnhandledExceptionFilter.java

package servletutils;

import com.google.common.base.Preconditions;
import javax.servlet.*;
import javax.servlet.http.*;

public class UnhandledExceptionFilter implements Filter {
  
  public interface ErrorHandler {
    void handle(HttpServletRequest request, HttpServletResponse response, Throwable throwable);  
  }
  
  private ErrorHandler errorHandler;

	public UnhandledExceptionFilter()
	{	}

	public UnhandledExceptionFilter(ErrorHandler errorHandler)	{
		Preconditions.checkNotNull(errorHandler);
		this.errorHandler = errorHandler;
	}

	@Override
	public void init(FilterConfig filterConfig) throws ServletException	{
		// TODO: add init code to lookup errorHandler
	}

  // suppress calls to sendError() and just setStatus() instead
	private static class StatusCodeCaptureWrapper extends HttpServletResponseWrapper {

		public StatusCodeCaptureWrapper(HttpServletResponse response)	{
			super(response);
		}

		@Override
		public void sendError(int sc) throws IOException {
			// do NOT use sendError() otherwise per servlet spec the container will send an html error page
			this.setStatus(sc);
		}

		@Override
		public void sendError(int sc, String msg) throws IOException {
			// do NOT use sendError() otherwise per servlet spec the container will send an html error page
			this.setStatus(sc, msg);
		}
	}

	@Override
	public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
		doFilter((HttpServletRequest) request, (HttpServletResponse) response, chain);
	}

	public void doFilter(HttpServletRequest request, HttpServletResponse response, FilterChain chain) throws IOException, ServletException
	{
		StatusCodeCaptureWrapper responseWrapper = new StatusCodeCaptureWrapper(response);
		Throwable throwable = null;

		try
		{
			chain.doFilter(request, responseWrapper);
		}
		catch (ServletException e)
		{
			throwable = e.getRootCause();
		}
		catch (Throwable e)
		{
			throwable = e;
		}

		if (throwable != null)
		{
			errorHandler.handle(request, response, throwable);
		}
    
		// flush to prevent servletcontainer to add anymore  headers or content
		response.flushBuffer();
	}

	@Override
	public void destroy() {
		// noop
	}
}