Make MetricRegistryMetricReader thread-safe

MetricRegistryMetricReader’s fields where neither final, nor volatile
but could be accessed on multiple threads. This lead to visibility
problems where the value of a field would unexpectedly be null, causing
an NPE.

This commit updates all of the fields to declare them as final, thereby
ensuring that their values are guaranteed to be visible across different
threads.

Fixes gh-2590

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

 /*
- * Copyright 2013-2104 the original author or authors.
+ * Copyright 2013-2015 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.
@@ -48,17 +48,17 @@ import com.codahale.metrics.Timer;
  * of type Number.
  *
  * @author Dave Syer
- *
+ * @author Andy Wilkinson
  */
 public class MetricRegistryMetricReader implements MetricReader, MetricRegistryListener {
 
-	private static Map<Class<?>, Set<String>> numberKeys = new ConcurrentHashMap<Class<?>, Set<String>>();
+	private static final Map<Class<?>, Set<String>> NUMBER_KEYS = new ConcurrentHashMap<Class<?>, Set<String>>();
 
-	private MetricRegistry registry;
+	private final MetricRegistry registry;
 
-	private Map<String, String> names = new HashMap<String, String>();
+	private final Map<String, String> names = new HashMap<String, String>();
 
-	private MultiValueMap<String, String> reverse = new LinkedMultiValueMap<String, String>();
+	private final MultiValueMap<String, String> reverse = new LinkedMultiValueMap<String, String>();
 
 	public MetricRegistryMetricReader(MetricRegistry registry) {
 		this.registry = registry;
@@ -67,11 +67,11 @@ public class MetricRegistryMetricReader implements MetricReader, MetricRegistryL
 
 	@Override
 	public Metric<?> findOne(String metricName) {
-		if (!names.containsKey(metricName)) {
+		if (!this.names.containsKey(metricName)) {
 			return null;
 		}
-		com.codahale.metrics.Metric metric = registry.getMetrics().get(
-				names.get(metricName));
+		com.codahale.metrics.Metric metric = this.registry.getMetrics().get(
+				this.names.get(metricName));
 		if (metric instanceof Counter) {
 			Counter counter = (Counter) metric;
 			return new Metric<Number>(metricName, counter.getCount());
@@ -107,13 +107,13 @@ public class MetricRegistryMetricReader implements MetricReader, MetricRegistryL
 
 	@Override
 	public long count() {
-		return names.size();
+		return this.names.size();
 	}
 
 	@Override
 	public void onGaugeAdded(String name, Gauge<?> gauge) {
-		names.put(name, name);
-		reverse.add(name, name);
+		this.names.put(name, name);
+		this.reverse.add(name, name);
 	}
 
 	@Override
@@ -123,8 +123,8 @@ public class MetricRegistryMetricReader implements MetricReader, MetricRegistryL
 
 	@Override
 	public void onCounterAdded(String name, Counter counter) {
-		names.put(name, name);
-		reverse.add(name, name);
+		this.names.put(name, name);
+		this.reverse.add(name, name);
 	}
 
 	@Override
@@ -136,13 +136,13 @@ public class MetricRegistryMetricReader implements MetricReader, MetricRegistryL
 	public void onHistogramAdded(String name, Histogram histogram) {
 		for (String key : getNumberKeys(histogram)) {
 			String metricName = name + "." + key;
-			names.put(metricName, name);
-			reverse.add(name, metricName);
+			this.names.put(metricName, name);
+			this.reverse.add(name, metricName);
 		}
 		for (String key : getNumberKeys(histogram.getSnapshot())) {
 			String metricName = name + ".snapshot." + key;
-			names.put(metricName, name);
-			reverse.add(name, metricName);
+			this.names.put(metricName, name);
+			this.reverse.add(name, metricName);
 		}
 	}
 
@@ -155,8 +155,8 @@ public class MetricRegistryMetricReader implements MetricReader, MetricRegistryL
 	public void onMeterAdded(String name, Meter meter) {
 		for (String key : getNumberKeys(meter)) {
 			String metricName = name + "." + key;
-			names.put(metricName, name);
-			reverse.add(name, metricName);
+			this.names.put(metricName, name);
+			this.reverse.add(name, metricName);
 		}
 	}
 
@@ -169,13 +169,13 @@ public class MetricRegistryMetricReader implements MetricReader, MetricRegistryL
 	public void onTimerAdded(String name, Timer timer) {
 		for (String key : getNumberKeys(timer)) {
 			String metricName = name + "." + key;
-			names.put(metricName, name);
-			reverse.add(name, metricName);
+			this.names.put(metricName, name);
+			this.reverse.add(name, metricName);
 		}
 		for (String key : getNumberKeys(timer.getSnapshot())) {
 			String metricName = name + ".snapshot." + key;
-			names.put(metricName, name);
-			reverse.add(name, metricName);
+			this.names.put(metricName, name);
+			this.reverse.add(name, metricName);
 		}
 	}
 
@@ -185,10 +185,10 @@ public class MetricRegistryMetricReader implements MetricReader, MetricRegistryL
 	}
 
 	private void remove(String name) {
-		for (String key : reverse.get(name)) {
-			names.remove(name + "." + key);
+		for (String key : this.reverse.get(name)) {
+			this.names.remove(name + "." + key);
 		}
-		reverse.remove(name);
+		this.reverse.remove(name);
 	}
 
 	private class MetricRegistryIterator implements Iterator<Metric<?>> {
@@ -202,12 +202,12 @@ public class MetricRegistryMetricReader implements MetricReader, MetricRegistryL
 
 		@Override
 		public boolean hasNext() {
-			return iterator.hasNext();
+			return this.iterator.hasNext();
 		}
 
 		@Override
 		public Metric<?> next() {
-			String name = iterator.next();
+			String name = this.iterator.next();
 			return MetricRegistryMetricReader.this.findOne(name);
 		}
 
@@ -220,7 +220,7 @@ public class MetricRegistryMetricReader implements MetricReader, MetricRegistryL
 	}
 
 	private static Set<String> getNumberKeys(Object metric) {
-		Set<String> result = numberKeys.containsKey(metric.getClass()) ? numberKeys
+		Set<String> result = NUMBER_KEYS.containsKey(metric.getClass()) ? NUMBER_KEYS
 				.get(metric.getClass()) : new HashSet<String>();
 		if (result.isEmpty()) {
 			for (PropertyDescriptor descriptor : BeanUtils.getPropertyDescriptors(metric
@@ -229,7 +229,7 @@ public class MetricRegistryMetricReader implements MetricReader, MetricRegistryL
 					result.add(descriptor.getName());
 				}
 			}
-			numberKeys.put(metric.getClass(), result);
+			NUMBER_KEYS.put(metric.getClass(), result);
 		}
 		return result;
 	}