code: @Test public void filterHasError(){
    this.filter.setErrorAttributes(new DefaultErrorAttributes());
    MockHttpServletRequest request=new MockHttpServletRequest("GET","/foo");
    MockHttpServletResponse response=new MockHttpServletResponse();
    response.setStatus(500);
    request.setAttribute("javax.servlet.error.exception",new IllegalStateException("Foo"));
    response.addHeader("Content-Type","application/json");
    Map<String,Object> trace=this.filter.getTrace(request);
    this.filter.enhanceTrace(trace,response);
    @SuppressWarnings("unchecked") Map<String,Object> map=(Map<String,Object>)trace.get("error");
    System.err.println(map);
    assertEquals("Foo",map.get("message").toString());
  }
  