Allow @SpyBean to work with @Primary beans

Update `MockitoPostProcessor` so that `@SpyBean` will automatically
pick the `@Primary` bean when multiple candidates exist.

Fixes gh-7621

spring-boot-test/src/main/java/org/springframework/boot/test/mock/mockito/MockitoPostProcessor.java

@@ -37,6 +37,8 @@ import org.springframework.beans.factory.BeanFactory;
 import org.springframework.beans.factory.BeanFactoryAware;
 import org.springframework.beans.factory.BeanFactoryUtils;
 import org.springframework.beans.factory.FactoryBean;
+import org.springframework.beans.factory.NoSuchBeanDefinitionException;
+import org.springframework.beans.factory.NoUniqueBeanDefinitionException;
 import org.springframework.beans.factory.config.BeanDefinition;
 import org.springframework.beans.factory.config.BeanFactoryPostProcessor;
 import org.springframework.beans.factory.config.BeanPostProcessor;
@@ -250,7 +252,7 @@ public class MockitoPostProcessor extends InstantiationAwareBeanPostProcessorAda
 			createSpy(registry, definition, field);
 		}
 		else {
-			registerSpies(definition, field, existingBeans);
+			registerSpies(registry, definition, field, existingBeans);
 		}
 	}
 
@@ -307,15 +309,41 @@ public class MockitoPostProcessor extends InstantiationAwareBeanPostProcessorAda
 		registerSpy(definition, field, beanName);
 	}
 
-	private void registerSpies(SpyDefinition definition, Field field,
-			String[] existingBeans) {
-		Assert.state(field == null || existingBeans.length == 1,
-				"Unable to register spy bean " + definition.getTypeToSpy()
-						+ " expected a single existing bean to replace but found "
-						+ new TreeSet<String>(Arrays.asList(existingBeans)));
-		for (String beanName : existingBeans) {
-			registerSpy(definition, field, beanName);
+	private void registerSpies(BeanDefinitionRegistry registry, SpyDefinition definition,
+			Field field, String[] existingBeans) {
+		ResolvableType type = definition.getTypeToSpy();
+		try {
+			if (ObjectUtils.isEmpty(existingBeans)) {
+				throw new NoSuchBeanDefinitionException(type);
+			}
+			if (existingBeans.length > 1) {
+				existingBeans = new String[] {
+						determinePrimaryCandidate(registry, existingBeans, type) };
+			}
+			registerSpy(definition, field, existingBeans[0]);
 		}
+		catch (RuntimeException ex) {
+			throw new IllegalStateException(
+					"Unable to register spy bean " + definition.getTypeToSpy(), ex);
+		}
+	}
+
+	private String determinePrimaryCandidate(BeanDefinitionRegistry registry,
+			String[] candidateBeanNames, ResolvableType type) {
+		String primaryBeanName = null;
+		for (String candidateBeanName : candidateBeanNames) {
+			BeanDefinition beanDefinition = registry.getBeanDefinition(candidateBeanName);
+			if (beanDefinition.isPrimary()) {
+				if (primaryBeanName != null) {
+					throw new NoUniqueBeanDefinitionException(type.resolve(),
+							candidateBeanNames.length,
+							"more than one 'primary' bean found among candidates: "
+									+ Arrays.asList(candidateBeanNames));
+				}
+				primaryBeanName = candidateBeanName;
+			}
+		}
+		return primaryBeanName;
 	}
 
 	private void registerSpy(SpyDefinition definition, Field field, String beanName) {

spring-boot-test/src/test/java/org/springframework/boot/test/mock/mockito/SpyBeanOnTestFieldForMultipleExistingBeansIntegrationTests.java

@@ -0,0 +1,73 @@
+/*
+ * Copyright 2012-2016 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.test.mock.mockito;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.mockito.internal.util.MockUtil;
+
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.test.mock.mockito.example.ExampleGenericStringServiceCaller;
+import org.springframework.boot.test.mock.mockito.example.SimpleExampleStringGenericService;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.context.annotation.Import;
+import org.springframework.context.annotation.Primary;
+import org.springframework.test.context.junit4.SpringRunner;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.Mockito.verify;
+
+/**
+ * Test {@link SpyBean} on a test class field can be used to inject new spy instances.
+ *
+ * @author Phillip Webb
+ */
+@RunWith(SpringRunner.class)
+public class SpyBeanOnTestFieldForMultipleExistingBeansIntegrationTests {
+
+	@SpyBean
+	private SimpleExampleStringGenericService spy;
+
+	@Autowired
+	private ExampleGenericStringServiceCaller caller;
+
+	@Test
+	public void testSpying() throws Exception {
+		assertThat(this.caller.sayGreeting()).isEqualTo("I say two");
+		assertThat(new MockUtil().getMockName(this.spy).toString()).isEqualTo("two");
+		verify(this.spy).greeting();
+	}
+
+	@Configuration
+	@Import(ExampleGenericStringServiceCaller.class)
+	static class Config {
+
+		@Bean
+		public SimpleExampleStringGenericService one() {
+			return new SimpleExampleStringGenericService("one");
+		}
+
+		@Bean
+		@Primary
+		public SimpleExampleStringGenericService two() {
+			return new SimpleExampleStringGenericService("two");
+		}
+
+	}
+
+}

spring-boot-test/src/test/java/org/springframework/boot/test/mock/mockito/example/ExampleGenericStringServiceCaller.java

@@ -0,0 +1,41 @@
+/*
+ * Copyright 2012-2016 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.test.mock.mockito.example;
+
+/**
+ * Example bean for mocking tests that calls {@link ExampleGenericService}.
+ *
+ * @author Phillip Webb
+ */
+public class ExampleGenericStringServiceCaller {
+
+	private final ExampleGenericService<String> stringService;
+
+	public ExampleGenericStringServiceCaller(
+			ExampleGenericService<String> stringService) {
+		this.stringService = stringService;
+	}
+
+	public ExampleGenericService<String> getStringService() {
+		return this.stringService;
+	}
+
+	public String sayGreeting() {
+		return "I say " + this.stringService.greeting();
+	}
+
+}

spring-boot-test/src/test/java/org/springframework/boot/test/mock/mockito/example/SimpleExampleStringGenericService.java

@@ -23,9 +23,19 @@ package org.springframework.boot.test.mock.mockito.example;
  */
 public class SimpleExampleStringGenericService implements ExampleGenericService<String> {
 
+	private final String greeting;
+
+	public SimpleExampleStringGenericService() {
+		this("simple");
+	}
+
+	public SimpleExampleStringGenericService(String greeting) {
+		this.greeting = greeting;
+	}
+
 	@Override
 	public String greeting() {
-		return "simple";
+		return this.greeting;
 	}
 
 }