Improve usage of ConcurrentMap

- Call get rather than containsKey then get
- Only call putIfAbsent after get has returned null to avoid unnecessary
  object creation

Closes gh-6382

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/autoconfigure/EndpointWebMvcHypermediaManagementContextConfiguration.java

@@ -223,7 +223,7 @@ public class EndpointWebMvcHypermediaManagementContextConfiguration {
 		@Autowired
 		private List<RequestMappingHandlerAdapter> handlerAdapters;
 
-		private Map<MediaType, HttpMessageConverter<?>> converterCache = new ConcurrentHashMap<MediaType, HttpMessageConverter<?>>();
+		private final Map<MediaType, HttpMessageConverter<?>> converterCache = new ConcurrentHashMap<MediaType, HttpMessageConverter<?>>();
 
 		@Override
 		public boolean supports(MethodParameter returnType,
@@ -278,8 +278,10 @@ public class EndpointWebMvcHypermediaManagementContextConfiguration {
 		private HttpMessageConverter<Object> findConverter(
 				Class<? extends HttpMessageConverter<?>> selectedConverterType,
 				MediaType mediaType) {
-			if (this.converterCache.containsKey(mediaType)) {
-				return (HttpMessageConverter<Object>) this.converterCache.get(mediaType);
+			HttpMessageConverter<Object> cached = (HttpMessageConverter<Object>) this.converterCache
+					.get(mediaType);
+			if (cached != null) {
+				return cached;
 			}
 			for (RequestMappingHandlerAdapter handlerAdapter : this.handlerAdapters) {
 				for (HttpMessageConverter<?> converter : handlerAdapter

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/cache/AbstractJmxCacheStatisticsProvider.java

@@ -50,7 +50,7 @@ public abstract class AbstractJmxCacheStatisticsProvider<C extends Cache>
 
 	private MBeanServer mBeanServer;
 
-	private Map<String, ObjectNameWrapper> caches = new ConcurrentHashMap<String, ObjectNameWrapper>();
+	private final Map<String, ObjectNameWrapper> caches = new ConcurrentHashMap<String, ObjectNameWrapper>();
 
 	@Override
 	public CacheStatistics getCacheStatistics(CacheManager cacheManager, C cache) {

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/metrics/buffer/BufferGaugeService.java

@@ -48,8 +48,9 @@ public class BufferGaugeService implements GaugeService {
 	}
 
 	private String wrap(String metricName) {
-		if (this.names.containsKey(metricName)) {
-			return this.names.get(metricName);
+		String cached = this.names.get(metricName);
+		if (cached != null) {
+			return cached;
 		}
 		if (metricName.startsWith("gauge") || metricName.startsWith("histogram")
 				|| metricName.startsWith("timer")) {

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/metrics/jmx/JmxMetricWriter.java

@@ -109,9 +109,13 @@ public class JmxMetricWriter implements MetricWriter {
 	}
 
 	private MetricValue getValue(String name) {
-		if (!this.values.containsKey(name)) {
-			this.values.putIfAbsent(name, new MetricValue());
-			MetricValue value = this.values.get(name);
+		MetricValue value = this.values.get(name);
+		if (value == null) {
+			value = new MetricValue();
+			MetricValue oldValue = this.values.putIfAbsent(name, value);
+			if (oldValue != null) {
+				value = oldValue;
+			}
 			try {
 				this.exporter.registerManagedResource(value, getName(name, value));
 			}
@@ -119,7 +123,7 @@ public class JmxMetricWriter implements MetricWriter {
 				// Could not register mbean, maybe just a race condition
 			}
 		}
-		return this.values.get(name);
+		return value;
 	}
 
 	private ObjectName getName(String name, MetricValue value)

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/metrics/util/SimpleInMemoryRepository.java

@@ -59,13 +59,7 @@ public class SimpleInMemoryRepository<T> {
 	}
 
 	public void set(String name, T value) {
-		T current = this.values.get(name);
-		if (current != null) {
-			this.values.replace(name, current, value);
-		}
-		else {
-			this.values.putIfAbsent(name, value);
-		}
+		this.values.put(name, value);
 	}
 
 	public long count() {
@@ -77,10 +71,7 @@ public class SimpleInMemoryRepository<T> {
 	}
 
 	public T findOne(String name) {
-		if (this.values.containsKey(name)) {
-			return this.values.get(name);
-		}
-		return null;
+		return this.values.get(name);
 	}
 
 	public Iterable<T> findAll() {

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/metrics/writer/DefaultGaugeService.java

@@ -46,8 +46,9 @@ public class DefaultGaugeService implements GaugeService {
 	}
 
 	private String wrap(String metricName) {
-		if (this.names.containsKey(metricName)) {
-			return this.names.get(metricName);
+		String cached = this.names.get(metricName);
+		if (cached != null) {
+			return cached;
 		}
 		if (metricName.startsWith("gauge") || metricName.startsWith("histogram")
 				|| metricName.startsWith("timer")) {