Prevent empty json for parameters in trace endpoint

Fixes gh-8883

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/trace/WebRequestTraceFilter.java

@@ -132,7 +132,7 @@ public class WebRequestTraceFilter extends OncePerRequestFilter implements Order
 		add(trace, Include.USER_PRINCIPAL, "userPrincipal",
 				(userPrincipal == null ? null : userPrincipal.getName()));
 		if (isIncluded(Include.PARAMETERS)) {
-			trace.put("parameters", request.getParameterMap());
+			trace.put("parameters", getParameterMap(request));
 		}
 		add(trace, Include.QUERY_STRING, "query", request.getQueryString());
 		add(trace, Include.AUTH_TYPE, "authType", request.getAuthType());
@@ -170,6 +170,12 @@ public class WebRequestTraceFilter extends OncePerRequestFilter implements Order
 		return headers;
 	}
 
+	private Map<String, String[]> getParameterMap(HttpServletRequest request) {
+		Map<String, String[]> map = new LinkedHashMap<String, String[]>();
+		map.putAll(request.getParameterMap());
+		return map;
+	}
+
 	/**
 	 * Post process request headers before they are added to the trace.
 	 * @param headers a mutable map containing the request headers to trace

spring-boot-samples/spring-boot-sample-actuator/src/test/java/sample/actuator/SampleActuatorApplicationTests.java

@@ -187,6 +187,22 @@ public class SampleActuatorApplicationTests {
 		assertThat(map.get("status")).isEqualTo("200");
 	}
 
+	@Test
+	public void traceWithParameterMap() throws Exception {
+		this.restTemplate.getForEntity("/health?param1=value1", String.class);
+		@SuppressWarnings("rawtypes")
+		ResponseEntity<List> entity = this.restTemplate
+				.withBasicAuth("user", getPassword()).getForEntity("/trace", List.class);
+		assertThat(entity.getStatusCode()).isEqualTo(HttpStatus.OK);
+		@SuppressWarnings("unchecked")
+		List<Map<String, Object>> list = entity.getBody();
+		Map<String, Object> trace = list.get(0);
+		@SuppressWarnings("unchecked")
+		Map<String, Object> map = (Map<String, Object>) ((Map<String, Object>)trace
+				.get("info")).get("parameters");
+		assertThat(map.get("param1")).isNotNull();
+	}
+
 	@Test
 	public void testErrorPageDirectAccess() throws Exception {
 		@SuppressWarnings("rawtypes")