Don’t check that a Gauge’s value is a Number until it’s being read

Spring Boot’s metrics require all values to be Numbers. A Dropwizard
Gauge can have a non-Number value. Previously, to prevent this causing
a problem, MetricRegistryMetricReader would check the value of a Gauge
when it’s being added and ignore it if it had a non-Number value.
Unfortunately, retrieving the value of a Gauge can take a non-trivial
amount of time (hence CachedGauge) so this approach, while functional,
could be improved.

This commit updates the filtering to happen when a Metric is being
retrieved from MetricRegistryMetricReader (via findOne or findAll)
when its value is required anyway. At this point, any Gauge with a
non-Number value is ignored.

Closes gh-4874

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/metrics/reader/MetricRegistryMetricReader.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2013-2015 the original author or authors.
+ * Copyright 2013-2016 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -87,9 +87,15 @@ public class MetricRegistryMetricReader implements MetricReader, MetricRegistryL
 			return new Metric<Number>(metricName, counter.getCount());
 		}
 		if (metric instanceof Gauge) {
-			@SuppressWarnings("unchecked")
-			Gauge<Number> value = (Gauge<Number>) metric;
-			return new Metric<Number>(metricName, value.getValue());
+			Object value = ((Gauge<?>) metric).getValue();
+			if (value instanceof Number) {
+				return new Metric<Number>(metricName, (Number) value);
+			}
+			if (logger.isDebugEnabled()) {
+				logger.debug("Ignoring gauge '" + name + "' (" + metric
+						+ ") as its value is not a Number");
+			}
+			return null;
 		}
 		if (metric instanceof Sampling) {
 			if (metricName.contains(".snapshot.")) {
@@ -129,13 +135,6 @@ public class MetricRegistryMetricReader implements MetricReader, MetricRegistryL
 
 	@Override
 	public void onGaugeAdded(String name, Gauge<?> gauge) {
-		if (!(gauge.getValue() instanceof Number)) {
-			if (logger.isDebugEnabled()) {
-				logger.debug("Ignoring gauge '" + name + "' (" + gauge
-						+ ") as its value is not a Number");
-			}
-			return;
-		}
 		this.names.put(name, name);
 		synchronized (this.monitor) {
 			this.reverse.add(name, name);

spring-boot-actuator/src/test/java/org/springframework/boot/actuate/metrics/reader/MetricRegistryMetricReaderTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2015 the original author or authors.
+ * Copyright 2012-2016 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.