Register dropwizard gauges once and then update them

The previous implementation would remove and add a new Gauge each time
a metric was written. After this change the Gauge is registered once
and the value is updated on subsequent calls.

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/metrics/dropwizard/DropwizardMetricServices.java

@@ -51,7 +51,7 @@ public class DropwizardMetricServices implements CounterService, GaugeService {
 
 	private final MetricRegistry registry;
 
-	private final ConcurrentMap<String, Object> gaugeLocks = new ConcurrentHashMap<String, Object>();
+	private final ConcurrentMap<String, SimpleGauge> gauges = new ConcurrentHashMap<String, SimpleGauge>();
 
 	private final ConcurrentHashMap<String, String> names = new ConcurrentHashMap<String, String>();
 
@@ -99,15 +99,22 @@ public class DropwizardMetricServices implements CounterService, GaugeService {
 		}
 		else {
 			name = wrapGaugeName(name);
-			final double gauge = value;
-			// Ensure we synchronize to avoid another thread pre-empting this thread after
-			// remove causing an error in Dropwizard metrics
-			// NOTE: Dropwizard provides no way to do this atomically
-			synchronized (getGaugeLock(name)) {
-				this.registry.remove(name);
-				this.registry.register(name, new SimpleGauge(gauge));
+			setGaugeValue(name, value);
+		}
+	}
+
+	private void setGaugeValue(final String name, final double value) {
+		// NOTE: Dropwizard provides no way to do this atomically
+		SimpleGauge gauge = this.gauges.get(name);
+		if (gauge == null) {
+			final SimpleGauge newGauge = new SimpleGauge(value);
+			gauge = this.gauges.putIfAbsent(name, newGauge);
+			if (gauge == null) {
+				gauge = newGauge;
+				this.registry.register(name, gauge);
 			}
 		}
+		gauge.setValue(value);
 	}
 
 	private String wrapGaugeName(String metricName) {
@@ -130,16 +137,6 @@ public class DropwizardMetricServices implements CounterService, GaugeService {
 		return name;
 	}
 
-	private Object getGaugeLock(String name) {
-		Object lock = this.gaugeLocks.get(name);
-		if (lock == null) {
-			Object newLock = new Object();
-			lock = this.gaugeLocks.putIfAbsent(name, newLock);
-			lock = (lock == null ? newLock : lock);
-		}
-		return lock;
-	}
-
 	@Override
 	public void reset(String name) {
 		if (!name.startsWith("meter")) {
@@ -153,7 +150,7 @@ public class DropwizardMetricServices implements CounterService, GaugeService {
 	 */
 	private final static class SimpleGauge implements Gauge<Double> {
 
-		private final double value;
+		private volatile double value;
 
 		private SimpleGauge(double value) {
 			this.value = value;
@@ -163,6 +160,10 @@ public class DropwizardMetricServices implements CounterService, GaugeService {
 		public Double getValue() {
 			return this.value;
 		}
+
+		public void setValue(final double value) {
+			this.value = value;
+		}
 	}
 
 }