Fix synchronization issue in OpenTSDBMetricWriter

Closes gh-4010

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/metrics/opentsdb/OpenTsdbMetricWriter.java

@@ -18,6 +18,7 @@ package org.springframework.boot.actuate.metrics.opentsdb;
 
 import java.util.ArrayList;
 import java.util.Arrays;
+import java.util.Collections;
 import java.util.List;
 import java.util.Map;
 
@@ -43,6 +44,7 @@ import org.springframework.web.client.RestTemplate;
  * task to flush periodically.
  *
  * @author Dave Syer
+ * @author Thomas Badie
  * @since 1.3.0
  */
 public class OpenTsdbMetricWriter implements MetricWriter {
@@ -67,7 +69,7 @@ public class OpenTsdbMetricWriter implements MetricWriter {
 	 */
 	private MediaType mediaType = MediaType.APPLICATION_JSON;
 
-	private List<OpenTsdbData> buffer = new ArrayList<OpenTsdbData>(this.bufferSize);
+	private final List<OpenTsdbData> buffer = new ArrayList<OpenTsdbData>(this.bufferSize);
 
 	private OpenTsdbNamingStrategy namingStrategy = new DefaultOpenTsdbNamingStrategy();
 
@@ -105,35 +107,42 @@ public class OpenTsdbMetricWriter implements MetricWriter {
 		OpenTsdbData data = new OpenTsdbData(
 				this.namingStrategy.getName(value.getName()), value.getValue(), value
 						.getTimestamp().getTime());
-		this.buffer.add(data);
-		if (this.buffer.size() >= this.bufferSize) {
-			flush();
+		synchronized (this.buffer) {
+			this.buffer.add(data);
+			if (this.buffer.size() >= this.bufferSize) {
+				flush();
+			}
 		}
 	}
 
 	/**
 	 * Flush the buffer without waiting for it to fill any further.
 	 */
+	@SuppressWarnings("rawtypes")
 	public void flush() {
-		if (this.buffer.isEmpty()) {
+		List<OpenTsdbData> snapshot = getBufferSnapshot();
+		if (snapshot.isEmpty()) {
 			return;
 		}
-		List<OpenTsdbData> temp = new ArrayList<OpenTsdbData>();
-		synchronized (this.buffer) {
-			temp.addAll(this.buffer);
-			this.buffer.clear();
-		}
 		HttpHeaders headers = new HttpHeaders();
 		headers.setAccept(Arrays.asList(this.mediaType));
 		headers.setContentType(this.mediaType);
-		HttpEntity<List<OpenTsdbData>> request = new HttpEntity<List<OpenTsdbData>>(temp,
-				headers);
-		@SuppressWarnings("rawtypes")
-		ResponseEntity<Map> response = this.restTemplate.postForEntity(this.url, request,
-				Map.class);
+		ResponseEntity<Map> response = this.restTemplate.postForEntity(this.url,
+				new HttpEntity<List<OpenTsdbData>>(snapshot, headers), Map.class);
 		if (!response.getStatusCode().is2xxSuccessful()) {
-			logger.warn("Cannot write metrics (discarded " + temp.size() + " values): "
-					+ response.getBody());
+			logger.warn("Cannot write metrics (discarded " + snapshot.size()
+					+ " values): " + response.getBody());
+		}
+	}
+
+	private List<OpenTsdbData> getBufferSnapshot() {
+		synchronized (this.buffer) {
+			if (this.buffer.isEmpty()) {
+				return Collections.emptyList();
+			}
+			List<OpenTsdbData> snapshot = new ArrayList<OpenTsdbData>(this.buffer);
+			this.buffer.clear();
+			return snapshot;
 		}
 	}
 

spring-boot-actuator/src/test/java/org/springframework/boot/actuate/metrics/opentsdb/OpenTsdbMetricWriterTests.java

@@ -40,6 +40,7 @@ import static org.mockito.Mockito.verify;
 public class OpenTsdbMetricWriterTests {
 
 	private OpenTsdbMetricWriter writer;
+
 	private RestOperations restTemplate = Mockito.mock(RestOperations.class);
 
 	@Before