Support property based MeterFilters

Add per-meter property support for `enabled`, `percentiles-histogram`,
`percentiles` and `sla`.

Fixes gh-11800

spring-boot-project/spring-boot-actuator-autoconfigure/src/main/java/org/springframework/boot/actuate/autoconfigure/metrics/MetricsAutoConfiguration.java

@@ -49,6 +49,7 @@ import org.springframework.boot.context.properties.EnableConfigurationProperties
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.context.annotation.Import;
+import org.springframework.core.annotation.Order;
 import org.springframework.integration.config.EnableIntegrationManagement;
 import org.springframework.integration.support.management.IntegrationManagementConfigurer;
 
@@ -96,6 +97,12 @@ public class MetricsAutoConfiguration {
 		return Clock.SYSTEM;
 	}
 
+	@Bean
+	@Order(0)
+	public PropertiesMeterFilter propertiesMeterFilter(MetricsProperties properties) {
+		return new PropertiesMeterFilter(properties);
+	}
+
 	/**
 	 * Binds metrics from Spring Integration.
 	 */

spring-boot-project/spring-boot-actuator-autoconfigure/src/main/java/org/springframework/boot/actuate/autoconfigure/metrics/MetricsProperties.java

@@ -16,7 +16,11 @@
 
 package org.springframework.boot.actuate.autoconfigure.metrics;
 
+import java.util.LinkedHashMap;
+import java.util.Map;
+
 import org.springframework.boot.context.properties.ConfigurationProperties;
+import org.springframework.util.Assert;
 
 /**
  * {@link ConfigurationProperties} for configuring Micrometer-based metrics.
@@ -27,8 +31,6 @@ import org.springframework.boot.context.properties.ConfigurationProperties;
 @ConfigurationProperties("management.metrics")
 public class MetricsProperties {
 
-	private final Web web = new Web();
-
 	/**
 	 * Whether auto-configured MeterRegistry implementations should be bound to the global
 	 * static registry on Metrics. For testing, set this to 'false' to maximize test
@@ -36,6 +38,16 @@ public class MetricsProperties {
 	 */
 	private boolean useGlobalRegistry = true;
 
+	/**
+	 * Whether meter IDs starting-with the specified name should be enabled. The longest
+	 * match wins, the key `all` can also be used to configure all meters.
+	 */
+	private Map<String, Boolean> enable = new LinkedHashMap<>();
+
+	private final Web web = new Web();
+
+	private final Distribution distribution = new Distribution();
+
 	public boolean isUseGlobalRegistry() {
 		return this.useGlobalRegistry;
 	}
@@ -44,10 +56,23 @@ public class MetricsProperties {
 		this.useGlobalRegistry = useGlobalRegistry;
 	}
 
+	public Map<String, Boolean> getEnable() {
+		return this.enable;
+	}
+
+	public void setEnable(Map<String, Boolean> enable) {
+		Assert.notNull(enable, "enable must not be null");
+		this.enable = enable;
+	}
+
 	public Web getWeb() {
 		return this.web;
 	}
 
+	public Distribution getDistribution() {
+		return this.distribution;
+	}
+
 	public static class Web {
 
 		private final Client client = new Client();
@@ -118,13 +143,6 @@ public class MetricsProperties {
 			 */
 			private boolean autoTimeRequests = true;
 
-			/**
-			 * Whether or not instrumented requests record percentiles histogram buckets
-			 * by default. Can be overridden by adding '@Timed' to a request endpoint and
-			 * setting 'percentiles' to true.
-			 */
-			private boolean recordRequestPercentiles;
-
 			/**
 			 * Name of the metric for received requests.
 			 */
@@ -138,14 +156,6 @@ public class MetricsProperties {
 				this.autoTimeRequests = autoTimeRequests;
 			}
 
-			public boolean isRecordRequestPercentiles() {
-				return this.recordRequestPercentiles;
-			}
-
-			public void setRecordRequestPercentiles(boolean recordRequestPercentiles) {
-				this.recordRequestPercentiles = recordRequestPercentiles;
-			}
-
 			public String getRequestsMetricName() {
 				return this.requestsMetricName;
 			}
@@ -158,4 +168,59 @@ public class MetricsProperties {
 
 	}
 
+	public static class Distribution {
+
+		/**
+		 * Whether meter IDs starting-with the specified name should be publish percentile
+		 * histograms. Monitoring systems that support aggregable percentile calculation
+		 * based on a histogram be set to true. For other systems, this has no effect. The
+		 * longest match wins, the key `all` can also be used to configure all meters.
+		 */
+		private Map<String, Boolean> percentilesHistogram = new LinkedHashMap<>();
+
+		/**
+		 * Specific computed non-aggregable percentiles to ship to the backend for meter
+		 * IDs starting-with the specified name. The longest match wins, the key `all` can
+		 * also be used to configure all meters.
+		 */
+		private Map<String, double[]> percentiles = new LinkedHashMap<>();
+
+		/**
+		 * Specific SLA boundaries for meter IDs starting-with the specified name. The
+		 * longest match wins, the key `all` can also be used to configure all meters.
+		 * Counters will be published for each sepecified boundary. Values can be
+		 * specified as a long or as a Duration value (for timer meters, defaulting to ms
+		 * if no unit specified).
+		 */
+		private Map<String, ServiceLevelAgreementBoundary[]> sla = new LinkedHashMap<>();
+
+		public Map<String, Boolean> getPercentilesHistogram() {
+			return this.percentilesHistogram;
+		}
+
+		public void setPercentilesHistogram(Map<String, Boolean> percentilesHistogram) {
+			Assert.notNull(percentilesHistogram, "PercentilesHistogram must not be null");
+			this.percentilesHistogram = percentilesHistogram;
+		}
+
+		public Map<String, double[]> getPercentiles() {
+			return this.percentiles;
+		}
+
+		public void setPercentiles(Map<String, double[]> percentiles) {
+			Assert.notNull(percentiles, "Percentiles must not be null");
+			this.percentiles = percentiles;
+		}
+
+		public Map<String, ServiceLevelAgreementBoundary[]> getSla() {
+			return this.sla;
+		}
+
+		public void setSla(Map<String, ServiceLevelAgreementBoundary[]> sla) {
+			Assert.notNull(sla, "SLA must not be null");
+			this.sla = sla;
+		}
+
+	}
+
 }

spring-boot-project/spring-boot-actuator-autoconfigure/src/main/java/org/springframework/boot/actuate/autoconfigure/metrics/PropertiesMeterFilter.java

+/*
+ * Copyright 2012-2018 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.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.actuate.autoconfigure.metrics;
+
+import java.util.Arrays;
+import java.util.Map;
+import java.util.Objects;
+
+import io.micrometer.core.instrument.Meter;
+import io.micrometer.core.instrument.Meter.Id;
+import io.micrometer.core.instrument.config.MeterFilter;
+import io.micrometer.core.instrument.config.MeterFilterReply;
+import io.micrometer.core.instrument.histogram.HistogramConfig;
+
+import org.springframework.boot.actuate.autoconfigure.metrics.MetricsProperties.Distribution;
+import org.springframework.util.Assert;
+import org.springframework.util.StringUtils;
+
+/**
+ * {@link MeterFilter} to apply settings from {@link MetricsProperties}.
+ *
+ * @author Jon Schneider
+ * @author Phillip Webb
+ * @since 2.0.0
+ */
+public class PropertiesMeterFilter implements MeterFilter {
+
+	private static final ServiceLevelAgreementBoundary[] EMPTY_SLA = {};
+
+	private MetricsProperties properties;
+
+	public PropertiesMeterFilter(MetricsProperties properties) {
+		Assert.notNull(properties, "Properties must not be null");
+		this.properties = properties;
+	}
+
+	@Override
+	public MeterFilterReply accept(Meter.Id id) {
+		boolean enabled = lookup(this.properties.getEnable(), id, true);
+		return (enabled ? MeterFilterReply.NEUTRAL : MeterFilterReply.DENY);
+	}
+
+	@Override
+	public HistogramConfig configure(Meter.Id id, HistogramConfig config) {
+		HistogramConfig.Builder builder = HistogramConfig.builder();
+		Distribution distribution = this.properties.getDistribution();
+		builder.percentilesHistogram(lookup(distribution.getPercentilesHistogram(), id, null));
+		builder.percentiles(lookup(distribution.getPercentiles(), id, null));
+		builder.sla(convertSla(id.getType(), lookup(distribution.getSla(), id, null)));
+		return builder.build().merge(config);
+	}
+
+	private long[] convertSla(Meter.Type meterType, ServiceLevelAgreementBoundary[] sla) {
+		long[] converted = Arrays.stream(sla == null ? EMPTY_SLA : sla)
+				.map((candidate) -> candidate.getValue(meterType))
+				.filter(Objects::nonNull).mapToLong(Long::longValue).toArray();
+		return (converted.length == 0 ? null : converted);
+	}
+
+	private <T> T lookup(Map<String, T> values, Id id, T defaultValue) {
+		String name = id.getName();
+		while (StringUtils.hasLength(name)) {
+			T result = values.get(name);
+			if (result != null) {
+				return result;
+			}
+			int lastDot = name.lastIndexOf('.');
+			name = lastDot == -1 ? "" : name.substring(0, lastDot);
+		}
+		return values.getOrDefault("all", defaultValue);
+	}
+
+}

spring-boot-project/spring-boot-actuator-autoconfigure/src/main/java/org/springframework/boot/actuate/autoconfigure/metrics/ServiceLevelAgreementBoundary.java

+/*
+ * Copyright 2012-2018 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.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.actuate.autoconfigure.metrics;
+
+import java.time.Duration;
+import java.util.concurrent.TimeUnit;
+
+import io.micrometer.core.instrument.Meter;
+import io.micrometer.core.instrument.Meter.Type;
+
+import org.springframework.boot.context.properties.bind.convert.DurationConverter;
+
+/**
+ * A service level agreement boundary for use when configuring micrometer. Can be
+ * specified as either a {@link Long} (applicable to timers and distribution summaries) or
+ * a {@link Long} (applicable to only timers).
+ *
+ * @author Phillip Webb
+ * @since 2.0.0
+ */
+public final class ServiceLevelAgreementBoundary {
+
+	private final Object value;
+
+	ServiceLevelAgreementBoundary(long value) {
+		this.value = value;
+	}
+
+	ServiceLevelAgreementBoundary(Duration value) {
+		this.value = value;
+	}
+
+	/**
+	 * Return the underlying value of the SLA in form suitable to apply to the given meter
+	 * type.
+	 * @param meterType the meter type
+	 * @return the value or {@code null} if the value cannot be applied
+	 */
+	public Long getValue(Meter.Type meterType) {
+		if (meterType == Type.DISTRIBUTION_SUMMARY) {
+			return getDistributionSummaryValue();
+		}
+		if (meterType == Type.TIMER) {
+			return getTimerValue();
+		}
+		return null;
+	}
+
+	private Long getDistributionSummaryValue() {
+		if (this.value instanceof Long) {
+			return (Long) this.value;
+		}
+		return null;
+	}
+
+	private Long getTimerValue() {
+		if (this.value instanceof Long) {
+			return TimeUnit.MILLISECONDS.toNanos((long) this.value);
+		}
+		if (this.value instanceof Duration) {
+			return ((Duration) this.value).toNanos();
+		}
+		return null;
+	}
+
+	public static ServiceLevelAgreementBoundary valueOf(String value) {
+		if (isNumber(value)) {
+			return new ServiceLevelAgreementBoundary(Long.parseLong(value));
+		}
+		return new ServiceLevelAgreementBoundary(
+				DurationConverter.toDuration(value, null));
+	}
+
+	/**
+	 * Return a new {@link ServiceLevelAgreementBoundary} instance for the given long
+	 * value.
+	 * @param value the source value
+	 * @return a {@link ServiceLevelAgreementBoundary} instance
+	 */
+	public static ServiceLevelAgreementBoundary valueOf(long value) {
+		return new ServiceLevelAgreementBoundary(value);
+	}
+
+	/**
+	 * Return a new {@link ServiceLevelAgreementBoundary} instance for the given String
+	 * value. The value may contain a simple number, or a {@link DurationConverter
+	 * duration formatted value}
+	 * @param value the source value
+	 * @return a {@link ServiceLevelAgreementBoundary} instance
+	 */
+	private static boolean isNumber(String value) {
+		return value.chars().allMatch(Character::isDigit);
+	}
+
+}

spring-boot-project/spring-boot-actuator-autoconfigure/src/test/java/org/springframework/boot/actuate/autoconfigure/metrics/MetricsAutoConfigurationTests.java

@@ -104,6 +104,16 @@ public class MetricsAutoConfigurationTests {
 				});
 	}
 
+	@Test
+	public void propertyBasedMeterFilter() {
+		this.contextRunner.withPropertyValues("management.metrics.enable.my.org=false")
+				.run(context -> {
+					MeterRegistry registry = context.getBean(MeterRegistry.class);
+					registry.timer("my.org.timer");
+					assertThat(registry.find("my.org.timer").timer()).isNull();
+				});
+	}
+
 	@Configuration
 	static class TwoDataSourcesConfiguration {
 

spring-boot-project/spring-boot-actuator-autoconfigure/src/test/java/org/springframework/boot/actuate/autoconfigure/metrics/ServiceLevelAgreementBoundaryTests.java

+/*
+ * Copyright 2012-2018 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.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.actuate.autoconfigure.metrics;
+
+import io.micrometer.core.instrument.Meter.Type;
+import org.junit.Test;
+
+import org.springframework.boot.context.properties.bind.Bindable;
+import org.springframework.boot.context.properties.bind.Binder;
+import org.springframework.boot.test.util.TestPropertyValues;
+import org.springframework.mock.env.MockEnvironment;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link ServiceLevelAgreementBoundary}.
+ *
+ * @author Phillip Webb
+ */
+public class ServiceLevelAgreementBoundaryTests {
+
+	@Test
+	public void getValueForDistributionSummaryWhenFromLongShouldReturnLongValue() {
+		ServiceLevelAgreementBoundary sla = ServiceLevelAgreementBoundary.valueOf(123L);
+		assertThat(sla.getValue(Type.DISTRIBUTION_SUMMARY)).isEqualTo(123);
+	}
+
+	@Test
+	public void getValueForDistributionSummaryWhenFromNumberStringShouldReturnLongValue() {
+		ServiceLevelAgreementBoundary sla = ServiceLevelAgreementBoundary.valueOf("123");
+		assertThat(sla.getValue(Type.DISTRIBUTION_SUMMARY)).isEqualTo(123);
+	}
+
+	@Test
+	public void getValueForDistributionSummaryWhenFromDurationStringShouldReturnNull() {
+		ServiceLevelAgreementBoundary sla = ServiceLevelAgreementBoundary
+				.valueOf("123ms");
+		assertThat(sla.getValue(Type.DISTRIBUTION_SUMMARY)).isNull();
+	}
+
+	@Test
+	public void getValueForTimerWhenFromLongShouldReturnMsToNanosValue() {
+		ServiceLevelAgreementBoundary sla = ServiceLevelAgreementBoundary.valueOf(123L);
+		assertThat(sla.getValue(Type.TIMER)).isEqualTo(123000000);
+	}
+
+	@Test
+	public void getValueForTimerWhenFromNumberStringShouldMsToNanosValue() {
+		ServiceLevelAgreementBoundary sla = ServiceLevelAgreementBoundary.valueOf("123");
+		assertThat(sla.getValue(Type.TIMER)).isEqualTo(123000000);
+	}
+
+	@Test
+	public void getValueForTimerWhenFromDurationStringShouldReturnDrationNanos() {
+		ServiceLevelAgreementBoundary sla = ServiceLevelAgreementBoundary
+				.valueOf("123ms");
+		assertThat(sla.getValue(Type.TIMER)).isEqualTo(123000000);
+	}
+
+	@Test
+	public void getValueForOthersShouldReturnNull() {
+		ServiceLevelAgreementBoundary sla = ServiceLevelAgreementBoundary.valueOf("123");
+		assertThat(sla.getValue(Type.COUNTER)).isNull();
+		assertThat(sla.getValue(Type.GAUGE)).isNull();
+		assertThat(sla.getValue(Type.LONG_TASK_TIMER)).isNull();
+		assertThat(sla.getValue(Type.OTHER)).isNull();
+	}
+
+	@Test
+	public void valueOfShouldWorkInBinder() {
+		MockEnvironment environment = new MockEnvironment();
+		TestPropertyValues.of("duration=10ms", "long=20").applyTo(environment);
+		assertThat(Binder.get(environment)
+				.bind("duration", Bindable.of(ServiceLevelAgreementBoundary.class)).get()
+				.getValue(Type.TIMER)).isEqualTo(10000000);
+		assertThat(Binder.get(environment)
+				.bind("long", Bindable.of(ServiceLevelAgreementBoundary.class)).get()
+				.getValue(Type.TIMER)).isEqualTo(20000000);
+	}
+
+}

spring-boot-project/spring-boot-docs/src/main/asciidoc/appendix-application-properties.adoc

@@ -1378,6 +1378,10 @@ content into your application. Rather, pick only the properties that you need.
 	management.metrics.web.server.auto-time-requests=true # Whether requests handled by Spring MVC or WebFlux should be automatically timed.
 	management.metrics.web.server.record-request-percentiles=false # Whether instrumented requests record percentiles histogram buckets by default.
 	management.metrics.web.server.requests-metric-name=http.server.requests # Name of the metric for received requests.
+	management.metrics.enabled.<name>= # Whether meter IDs starting-with the specified name should be enabled. The longest match wins, the key `all` can also be used to configure all meters.
+	management.metrics.distribution.percentiles-histogram.<name>= # Whether meter IDs starting-with the specified name should be publish percentile histograms.
+	management.metrics.distribution.percentiles.<name>= # Specific computed non-aggregable percentiles to ship to the backend for meter IDs starting-with the specified name.
+	management.metrics.distribution.sla.<name>= Specific SLA boundaries for meter IDs starting-with the specified name. The longest match wins, the key `all` can also be used to configure all meters.
 
 
 	# ----------------------------------------

spring-boot-project/spring-boot-docs/src/main/asciidoc/index.adoc

@@ -65,6 +65,7 @@ Phillip Webb; Dave Syer; Josh Long; Stéphane Nicoll; Rob Winch; Andy Wilkinson;
 :gradle-user-guide: https://docs.gradle.org/4.2.1/userguide
 :hibernate-documentation: https://docs.jboss.org/hibernate/orm/5.2/userguide/html_single/Hibernate_User_Guide.html
 :jetty-documentation: https://www.eclipse.org/jetty/documentation/9.4.x
+:micrometer-concepts-documentation: https://micrometer.io/docs/concepts
 :tomcat-documentation: https://tomcat.apache.org/tomcat-8.5-doc
 // ======================================================================================
 

spring-boot-project/spring-boot-docs/src/main/asciidoc/production-ready-features.adoc

@@ -1294,6 +1294,59 @@ Auto-configuration enables binding of a number of Spring Integration-related met
 
 
 
+[[production-ready-metrics-per-meter-properties]]
+=== Customizing individual meters
+If you need to apply customizations to specific `Meter` instances you can use the
+`io.micrometer.core.instrument.config.MeterFilter` interface. By default, all
+`MeterFilter` beans will be automatically applied to the micrometer
+`MeterRegistry.Config`.
+
+For example, if you want to rename the `mytag.region` tag to `mytag.area` for
+all meter IDs beginning with `com.example`, you can do the following:
+
+[source,java,indent=0]
+----
+include::{code-examples}/actuate/metrics/MetricsFilterBeanExample.java[tag=configuration]
+----
+
+==== Per-meter properties
+In addition to `MeterFilter` beans, it's also possible to apply a limited set of
+customization on a per-meter basis using properties. Per-meter customizations apply to
+any all meter IDs that start with the given name. For example, the following will disable
+any meters that have an ID starting with `example.remote`
+
+[source,properties,indent=0]
+----
+	management.metrics.enable.example.remote=false
+----
+
+The following properties allow per-meter customization:
+
+.Per-meter customizations
+|===
+| Property | Description
+
+| `management.metrics.enable`
+| Whether to deny meters from emitting any metrics.
+
+| `management.metrics.distribution.percentiles-histogram`
+| Whether to publish a histogram suitable for computing aggregable (across dimension)
+percentile approximations.
+
+| `management.metrics.distribution.percentiles`
+| Publish percentile values computed in your application
+
+| `management.metrics.distribution.sla`
+| Publish a cumulative histogram with buckets defined by your SLAs.
+
+|===
+
+For more details on concepts behind `percentiles-histogram`, `percentiles` and `sla`
+refer to the {micrometer-concepts-documentation}#_histograms_and_percentiles["Histograms
+and percentiles" section] of the micrometer documentation.
+
+
+
 [[production-ready-auditing]]
 == Auditing
 Once Spring Security is in play, Spring Boot Actuator has a flexible audit framework that

spring-boot-project/spring-boot-docs/src/main/java/org/springframework/boot/actuate/metrics/MetricsFilterBeanExample.java

+/*
+ * Copyright 2012-2018 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.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.actuate.metrics;
+
+import io.micrometer.core.instrument.config.MeterFilter;
+
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+
+/**
+ * Example to show a {@link MeterFilter}.
+ *
+ * @author Phillip Webb
+ */
+public class MetricsFilterBeanExample {
+
+	@Configuration
+	static class MetricsFilterExampleConfiguration {
+
+		// tag::configuration[]
+		@Bean
+		public MeterFilter renameRegionTagMeterFilter() {
+			return MeterFilter.renameTag("com.example", "mytag.region", "mytag.area");
+		}
+		// end::configuration[]
+
+	}
+
+}

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/context/properties/bind/convert/DurationConverter.java

@@ -37,8 +37,9 @@ import org.springframework.util.StringUtils;
  * {@link Duration#parse(CharSequence)} as well a more readable {@code 10s} form.
  *
  * @author Phillip Webb
+ * @since 2.0.0
  */
-class DurationConverter implements GenericConverter {
+public class DurationConverter implements GenericConverter {
 
 	private static final Set<ConvertiblePair> TYPES;
 
@@ -78,11 +79,20 @@ class DurationConverter implements GenericConverter {
 		if (source == null) {
 			return null;
 		}
+		DefaultDurationUnit defaultUnit = targetType
+				.getAnnotation(DefaultDurationUnit.class);
 		return toDuration(source.toString(),
-				targetType.getAnnotation(DefaultDurationUnit.class));
+				(defaultUnit == null ? null : defaultUnit.value()));
 	}
 
-	private Duration toDuration(String source, DefaultDurationUnit defaultUnit) {
+	/**
+	 * Convert the specified source to a {@link Duration}.
+	 * @param source the source to convert
+	 * @param defaultUnit the default unit to use ({@code null} is treated as
+	 * milliseconds)
+	 * @return the duration
+	 */
+	public static Duration toDuration(String source, ChronoUnit defaultUnit) {
 		try {
 			if (!StringUtils.hasLength(source)) {
 				return null;
@@ -103,9 +113,9 @@ class DurationConverter implements GenericConverter {
 		}
 	}
 
-	private ChronoUnit getUnit(String value, DefaultDurationUnit defaultUnit) {
+	private static ChronoUnit getUnit(String value, ChronoUnit defaultUnit) {
 		if (StringUtils.isEmpty(value)) {
-			return (defaultUnit != null ? defaultUnit.value() : ChronoUnit.MILLIS);
+			return (defaultUnit != null ? defaultUnit : ChronoUnit.MILLIS);
 		}
 		ChronoUnit unit = UNITS.get(value.toLowerCase());
 		Assert.state(unit != null, () -> "Unknown unit '" + value + "'");