Break GaugeWriter dependency cycle in MetricExportAutoConfiguration

Previously, MetricExportAutoConfiguration consumed
ExportMetricWriter-annotated GaugeWriter beans in its constructor and
also produced such a bean from one of its @Bean methods. This cycle
caused a BeanCurrentlyInCreationException to be thrown when the
bean method was active (the spring.metrics.export.statsd.host property
was set).

This commit break the cycle by moving the bean method into a separate,
nested configuration class. It also updates the existing test for
auto-configuration of a Statsd writer to catch any possible cycles
and to verify that the writer has be registered with the
MetricsExporter.

Closes gh-6544

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

@@ -55,8 +55,6 @@ import org.springframework.util.CollectionUtils;
 @EnableConfigurationProperties
 public class MetricExportAutoConfiguration {
 
-	private final MetricExportProperties properties;
-
 	private final MetricsEndpointMetricReader endpointReader;
 
 	private final List<MetricReader> readers;
@@ -70,7 +68,6 @@ public class MetricExportAutoConfiguration {
 			@ExportMetricReader ObjectProvider<List<MetricReader>> readersProvider,
 			@ExportMetricWriter ObjectProvider<Map<String, GaugeWriter>> writersProvider,
 			ObjectProvider<Map<String, Exporter>> exportersProvider) {
-		this.properties = properties;
 		this.endpointReader = endpointReaderProvider.getIfAvailable();
 		this.readers = readersProvider.getIfAvailable();
 		this.writers = writersProvider.getIfAvailable();
@@ -79,7 +76,8 @@ public class MetricExportAutoConfiguration {
 
 	@Bean
 	@ConditionalOnMissingBean(name = "metricWritersMetricExporter")
-	public SchedulingConfigurer metricWritersMetricExporter() {
+	public SchedulingConfigurer metricWritersMetricExporter(
+			MetricExportProperties properties) {
 		Map<String, GaugeWriter> writers = new HashMap<String, GaugeWriter>();
 		MetricReader reader = this.endpointReader;
 		if (reader == null && !CollectionUtils.isEmpty(this.readers)) {
@@ -89,7 +87,7 @@ public class MetricExportAutoConfiguration {
 		if (reader == null && CollectionUtils.isEmpty(this.exporters)) {
 			return new NoOpSchedulingConfigurer();
 		}
-		MetricExporters exporters = new MetricExporters(this.properties);
+		MetricExporters exporters = new MetricExporters(properties);
 		if (reader != null) {
 			if (!CollectionUtils.isEmpty(this.writers)) {
 				writers.putAll(this.writers);
@@ -102,16 +100,21 @@ public class MetricExportAutoConfiguration {
 		return exporters;
 	}
 
+	@Configuration
+	static class StatsdConfiguration {
+
 		@Bean
 		@ExportMetricWriter
 		@ConditionalOnMissingBean
 		@ConditionalOnProperty(prefix = "spring.metrics.export.statsd", name = "host")
-	public StatsdMetricWriter statsdMetricWriter() {
-		MetricExportProperties.Statsd statsdProperties = this.properties.getStatsd();
+		public StatsdMetricWriter statsdMetricWriter(MetricExportProperties properties) {
+			MetricExportProperties.Statsd statsdProperties = properties.getStatsd();
 			return new StatsdMetricWriter(statsdProperties.getPrefix(),
 					statsdProperties.getHost(), statsdProperties.getPort());
 		}
 
+	}
+
 	@Configuration
 	protected static class MetricExportPropertiesConfiguration {
 

spring-boot-actuator/src/test/java/org/springframework/boot/actuate/autoconfigure/MetricExportAutoConfigurationTests.java

@@ -16,6 +16,8 @@
 
 package org.springframework.boot.actuate.autoconfigure;
 
+import java.util.Map;
+
 import org.junit.After;
 import org.junit.Rule;
 import org.junit.Test;
@@ -30,6 +32,7 @@ import org.springframework.boot.actuate.metrics.Metric;
 import org.springframework.boot.actuate.metrics.export.MetricCopyExporter;
 import org.springframework.boot.actuate.metrics.export.MetricExporters;
 import org.springframework.boot.actuate.metrics.statsd.StatsdMetricWriter;
+import org.springframework.boot.actuate.metrics.writer.GaugeWriter;
 import org.springframework.boot.actuate.metrics.writer.MetricWriter;
 import org.springframework.boot.autoconfigure.PropertyPlaceholderAutoConfiguration;
 import org.springframework.boot.test.util.EnvironmentTestUtils;
@@ -41,6 +44,8 @@ import org.springframework.messaging.Message;
 import org.springframework.messaging.MessageHandler;
 import org.springframework.messaging.MessagingException;
 import org.springframework.messaging.SubscribableChannel;
+import org.springframework.scheduling.annotation.SchedulingConfigurer;
+import org.springframework.test.util.ReflectionTestUtils;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
@@ -137,16 +142,24 @@ public class MetricExportAutoConfigurationTests {
 		this.context.getBean(StatsdMetricWriter.class);
 	}
 
+	@SuppressWarnings("unchecked")
 	@Test
 	public void statsdWithHost() throws Exception {
 		this.context = new AnnotationConfigApplicationContext();
 		EnvironmentTestUtils.addEnvironment(this.context,
 				"spring.metrics.export.statsd.host=localhost");
-		this.context.register(WriterConfig.class, MetricEndpointConfiguration.class,
+		this.context.register(MetricEndpointConfiguration.class,
 				MetricExportAutoConfiguration.class,
 				PropertyPlaceholderAutoConfiguration.class);
 		this.context.refresh();
-		assertThat(this.context.getBean(StatsdMetricWriter.class)).isNotNull();
+		StatsdMetricWriter statsdWriter = this.context.getBean(StatsdMetricWriter.class);
+		assertThat(statsdWriter).isNotNull();
+		SchedulingConfigurer schedulingConfigurer = this.context
+				.getBean(SchedulingConfigurer.class);
+		Map<String, GaugeWriter> exporters = (Map<String, GaugeWriter>) ReflectionTestUtils
+				.getField(schedulingConfigurer, "writers");
+		assertThat(exporters).containsValue(statsdWriter);
+
 	}
 
 	@Configuration