Add ConditionalOnSingleCandidate

Add a new ConditionalOnSingleCandidate condition that determines if the
condition should match only if autowiring by type is guaranteed to
succeed. Used by auto-configuration that relies on a single candidate of
a given type (for instance, the JdbcTemplate auto-configuration relies on
the presence of a DataSource).

Such wiring by type will succeed if only one bean of that type is present
or if one matching instance is flagged "primary" amongst the candidates.

ConditionalOnSingleCandidate is a basic version of ConditionalOnBean that
only accepts a single type and does not determine a defaut based on its
presence on a bean definition.

Closes gh-1702

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/condition/ConditionalOnSingleCandidate.java

@@ -0,0 +1,74 @@
+/*
+ * Copyright 2012-2015 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.autoconfigure.condition;
+
+import java.lang.annotation.Documented;
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+import org.springframework.beans.factory.BeanFactory;
+import org.springframework.context.ApplicationContext;
+import org.springframework.context.annotation.Conditional;
+
+/**
+ * {@link Conditional} that only matches when the specified bean class is already
+ * contained in the {@link BeanFactory} and a single candidate can be determined.
+ * <p>
+ * The conditional will also match if multiple matching bean instances are already
+ * contained in the {@link BeanFactory} but a primary candidate has been defined;
+ * essentially, the condition match if auto-wiring a bean with the defined type
+ * will succeed.
+ *
+ * @author Stephane Nicoll
+ * @since 1.3.0
+ */
+@Target({ElementType.TYPE, ElementType.METHOD})
+@Retention(RetentionPolicy.RUNTIME)
+@Documented
+@Conditional(OnBeanCondition.class)
+public @interface ConditionalOnSingleCandidate {
+
+	/**
+	 * The class type of bean that should be checked. The condition match if the class
+	 * specified is contained in the {@link ApplicationContext} and a primary candidate
+	 * exists in case of multiple instances.
+	 * <p>This attribute may <strong>not</strong> be used in conjunction with
+	 * {@link #type()}, but it may be used instead of {@link #type()}.
+	 * @return the class type of the bean to check
+	 */
+	Class<?> value() default Object.class;
+
+	/**
+	 * The class type name of bean that should be checked. The condition matches if the
+	 * class specified is contained in the {@link ApplicationContext} and a primary
+	 * candidate exists in case of multiple instances.
+	 * <p>This attribute may <strong>not</strong> be used in conjunction with
+	 * {@link #value()}, but it may be used instead of {@link #value()}.
+	 * @return the class type name of the bean to check
+	 */
+	String type() default "";
+
+	/**
+	 * Strategy to decide if the application context hierarchy (parent contexts) should be
+	 * considered.
+	 * @return the search strategy
+	 */
+	SearchStrategy search() default SearchStrategy.ALL;
+
+}

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/condition/OnBeanCondition.java

@@ -24,13 +24,16 @@ import java.util.Collection;
 import java.util.Collections;
 import java.util.LinkedHashSet;
 import java.util.List;
+import java.util.ListIterator;
 import java.util.Set;
 
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
+
 import org.springframework.beans.factory.BeanFactory;
 import org.springframework.beans.factory.HierarchicalBeanFactory;
 import org.springframework.beans.factory.ListableBeanFactory;
+import org.springframework.beans.factory.config.BeanDefinition;
 import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Condition;
@@ -53,6 +56,7 @@ import org.springframework.util.StringUtils;
  * @author Phillip Webb
  * @author Dave Syer
  * @author Jakub Kubrynski
+ * @author Stephane Nicoll
  */
 @Order(Ordered.LOWEST_PRECEDENCE)
 public class OnBeanCondition extends SpringBootCondition implements
@@ -88,6 +92,24 @@ public class OnBeanCondition extends SpringBootCondition implements
 			matchMessage.append("@ConditionalOnBean " + spec + " found the following "
 					+ matching);
 		}
+		if (metadata.isAnnotated(ConditionalOnSingleCandidate.class.getName())) {
+			BeanSearchSpec spec = new SingleCandidateBeanSearchSpec(context, metadata,
+					ConditionalOnSingleCandidate.class);
+			List<String> matching = getMatchingBeans(context, spec);
+			if (matching.isEmpty()) {
+				return ConditionOutcome.noMatch("@ConditionalOnSingleCandidate " + spec
+						+ " found no beans");
+			}
+			else if (hasSingleAutowireCandidate(context.getBeanFactory(), matching)) {
+				matchMessage.append("@ConditionalOnSingleCandidate " + spec + " found a primary " +
+						"candidate amongst the following " + matching);
+			}
+			else {
+				return ConditionOutcome.noMatch("@ConditionalOnSingleCandidate " + spec
+						+ " found no primary candidate amongst the"
+						+ " following " + matching);
+			}
+		}
 		if (metadata.isAnnotated(ConditionalOnMissingBean.class.getName())) {
 			BeanSearchSpec spec = new BeanSearchSpec(context, metadata,
 					ConditionalOnMissingBean.class);
@@ -199,8 +221,29 @@ public class OnBeanCondition extends SpringBootCondition implements
 		}
 	}
 
+	private boolean hasSingleAutowireCandidate(ConfigurableListableBeanFactory beanFactory,
+			List<String> beans) {
+
+		if (beans.size() == 1) {
+			return true;
+		}
+		boolean primaryFound = false;
+		for (String bean : beans) {
+			BeanDefinition beanDefinition = beanFactory.getBeanDefinition(bean);
+			if (beanDefinition != null && beanDefinition.isPrimary()) {
+				if (primaryFound) {
+					return false;
+				}
+				primaryFound = true;
+			}
+		}
+		return primaryFound;
+	}
+
 	private static class BeanSearchSpec {
 
+		private final Class<?> annotationType;
+
 		private final List<String> names = new ArrayList<String>();
 
 		private final List<String> types = new ArrayList<String>();
@@ -211,6 +254,7 @@ public class OnBeanCondition extends SpringBootCondition implements
 
 		public BeanSearchSpec(ConditionContext context, AnnotatedTypeMetadata metadata,
 				Class<?> annotationType) {
+			this.annotationType = annotationType;
 			MultiValueMap<String, Object> attributes = metadata
 					.getAllAnnotationAttributes(annotationType.getName(), true);
 			collect(attributes, "name", this.names);
@@ -220,11 +264,15 @@ public class OnBeanCondition extends SpringBootCondition implements
 			if (this.types.isEmpty() && this.names.isEmpty()) {
 				addDeducedBeanType(context, metadata, this.types);
 			}
-			Assert.isTrue(hasAtLeastOne(this.types, this.names, this.annotations),
-					annotationName(annotationType) + " annotations must "
-							+ "specify at least one bean (type, name or annotation)");
 			this.strategy = (SearchStrategy) metadata.getAnnotationAttributes(
 					annotationType.getName()).get("search");
+			validate();
+		}
+
+		protected void validate() {
+			Assert.isTrue(hasAtLeastOne(this.types, this.names, this.annotations),
+					annotationName() + " annotations must "
+							+ "specify at least one bean (type, name or annotation)");
 		}
 
 		private boolean hasAtLeastOne(List<?>... lists) {
@@ -236,16 +284,23 @@ public class OnBeanCondition extends SpringBootCondition implements
 			return false;
 		}
 
-		private String annotationName(Class<?> annotationType) {
-			return "@" + ClassUtils.getShortName(annotationType);
+		protected String annotationName() {
+			return "@" + ClassUtils.getShortName(this.annotationType);
 		}
 
 		@SuppressWarnings({ "unchecked", "rawtypes" })
 		private void collect(MultiValueMap<String, Object> attributes, String key,
 				List<String> destination) {
-			List<String[]> valueList = (List) attributes.get(key);
-			for (String[] valueArray : valueList) {
-				Collections.addAll(destination, valueArray);
+			List<?> values = attributes.get(key);
+			if (values != null) {
+				for (Object value : values) {
+					if (value instanceof String[]) {
+						Collections.addAll(destination, (String[]) value);
+					}
+					else {
+						destination.add((String) value);
+					}
+				}
 			}
 		}
 
@@ -326,4 +381,27 @@ public class OnBeanCondition extends SpringBootCondition implements
 
 	}
 
+	private static class SingleCandidateBeanSearchSpec extends BeanSearchSpec {
+
+		public SingleCandidateBeanSearchSpec(ConditionContext context,
+				AnnotatedTypeMetadata metadata, Class<?> annotationType) {
+			super(context, metadata, annotationType);
+		}
+
+		@Override
+		protected void validate() {
+			List<String> types = getTypes();
+			ListIterator<String> it = types.listIterator();
+			while (it.hasNext()) {
+				String value = it.next();
+				if (!StringUtils.hasText(value) || Object.class.getName().equals(value)) {
+					it.remove();
+				}
+			}
+			Assert.isTrue(types.size() == 1, annotationName() + " annotations must "
+					+ "specify only one type (got " + types + ")");
+
+		}
+	}
+
 }

spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/condition/ConditionalOnSingleCandidateTests.java

@@ -0,0 +1,164 @@
+/*
+ * Copyright 2012-2015 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.autoconfigure.condition;
+
+import org.junit.After;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.ExpectedException;
+
+import org.springframework.context.annotation.AnnotationConfigApplicationContext;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.context.annotation.Primary;
+
+import static org.hamcrest.CoreMatchers.isA;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
+
+/**
+ * Tests for {@link ConditionalOnSingleCandidate}.
+ *
+ * @author Stephane Nicoll
+ */
+public class ConditionalOnSingleCandidateTests {
+
+	@Rule
+	public final ExpectedException thrown = ExpectedException.none();
+
+	private final AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext();
+
+	@After
+	public void close() {
+		if (this.context != null) {
+			this.context.close();
+		}
+	}
+
+	@Test
+	public void singleCandidateNoCandidate() {
+		load(OnBeanSingleCandidateConfiguration.class);
+		assertFalse(this.context.containsBean("baz"));
+	}
+
+	@Test
+	public void singleCandidateOneCandidate() {
+		load(FooConfiguration.class,
+				OnBeanSingleCandidateConfiguration.class);
+		assertTrue(this.context.containsBean("baz"));
+		assertEquals("foo", this.context.getBean("baz"));
+	}
+
+	@Test
+	public void singleCandidateMultipleCandidates() {
+		load(FooConfiguration.class, BarConfiguration.class,
+				OnBeanSingleCandidateConfiguration.class);
+		assertFalse(this.context.containsBean("baz"));
+	}
+
+	@Test
+	public void singleCandidateMultipleCandidatesOnePrimary() {
+		load(FooPrimaryConfiguration.class, BarConfiguration.class,
+				OnBeanSingleCandidateConfiguration.class);
+		assertTrue(this.context.containsBean("baz"));
+		assertEquals("foo", this.context.getBean("baz"));
+	}
+
+	@Test
+	public void singleCandidateMultipleCandidatesMultiplePrimary() {
+		load(FooPrimaryConfiguration.class, BarPrimaryConfiguration.class,
+				OnBeanSingleCandidateConfiguration.class);
+		assertFalse(this.context.containsBean("baz"));
+	}
+
+	@Test
+	public void invalidAnnotationTwoTypes() {
+		thrown.expect(IllegalStateException.class);
+		thrown.expectCause(isA(IllegalArgumentException.class));
+		thrown.expectMessage(OnBeanSingleCandidateTwoTypesConfiguration.class.getName());
+		load(OnBeanSingleCandidateTwoTypesConfiguration.class);
+	}
+
+	@Test
+	public void invalidAnnotationNoType() {
+		thrown.expect(IllegalStateException.class);
+		thrown.expectCause(isA(IllegalArgumentException.class));
+		thrown.expectMessage(OnBeanSingleCandidateNoTypeConfiguration.class.getName());
+		load(OnBeanSingleCandidateNoTypeConfiguration.class);
+	}
+
+	private void load(Class<?>... classes) {
+		this.context.register(classes);
+		this.context.refresh();
+	}
+
+	@Configuration
+	@ConditionalOnSingleCandidate(value = String.class)
+	protected static class OnBeanSingleCandidateConfiguration {
+		@Bean
+		public String baz(String s) {
+			return s;
+		}
+	}
+
+	@Configuration
+	@ConditionalOnSingleCandidate(value = String.class, type = "java.lang.String")
+	protected static class OnBeanSingleCandidateTwoTypesConfiguration {
+
+	}
+
+	@Configuration
+	@ConditionalOnSingleCandidate
+	protected static class OnBeanSingleCandidateNoTypeConfiguration {
+
+	}
+
+	@Configuration
+	protected static class FooConfiguration {
+		@Bean
+		public String foo() {
+			return "foo";
+		}
+	}
+
+	@Configuration
+	protected static class FooPrimaryConfiguration {
+		@Bean
+		@Primary
+		public String foo() {
+			return "foo";
+		}
+	}
+
+	@Configuration
+	protected static class BarConfiguration {
+		@Bean
+		public String bar() {
+			return "bar";
+		}
+	}
+
+	@Configuration
+	protected static class BarPrimaryConfiguration {
+		@Bean
+		@Primary
+		public String bar() {
+			return "bar";
+		}
+	}
+}