Automatically autowire a bean with one constructor

Previously, if a managed bean had only one non-default constructor, we
should still annotate it with `@Autowired` to properly use constructor
injection. Not doing so resulted in an error as the container was
trying to call the default (non-existing) constructor.

This commit updates this behaviour to automatically applyed the
autowiring semantic to any bean that has only one constructor. As
before, if more than one constructor is defined, `@Autowired` must be
specified to teach the container the constructor it has to use.

Issue: SPR-12278

spring-beans/src/main/java/org/springframework/beans/factory/annotation/AutowiredAnnotationBeanPostProcessor.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2014 the original author or authors.
+ * Copyright 2002-2015 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.
@@ -107,6 +107,7 @@ import org.springframework.util.StringUtils;
  *
  * @author Juergen Hoeller
  * @author Mark Fisher
+ * @author Stephane Nicoll
  * @since 2.5
  * @see #setAutowiredAnnotationType
  * @see Autowired
@@ -312,6 +313,9 @@ public class AutowiredAnnotationBeanPostProcessor extends InstantiationAwareBean
 						}
 						candidateConstructors = candidates.toArray(new Constructor<?>[candidates.size()]);
 					}
+					else if (rawCandidates.length == 1 && rawCandidates[0].getParameterTypes().length > 0) {
+						candidateConstructors = new Constructor<?>[] {rawCandidates[0]};
+					}
 					else {
 						candidateConstructors = new Constructor<?>[0];
 					}

spring-context/src/test/java/org/springframework/context/annotation/Spr12278Tests.java

@@ -0,0 +1,115 @@
+/*
+ * Copyright 2002-2015 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.context.annotation;
+
+import org.junit.After;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.ExpectedException;
+
+import org.springframework.beans.factory.BeanCreationException;
+
+import static org.hamcrest.core.Is.*;
+import static org.junit.Assert.*;
+
+/**
+ * @author Stephane Nicoll
+ */
+public class Spr12278Tests {
+
+	@Rule
+	public final ExpectedException thrown = ExpectedException.none();
+
+	private AnnotationConfigApplicationContext context;
+
+	@After
+	public void close() {
+		if (context != null) {
+			context.close();
+		}
+	}
+
+	@Test
+	public void componentSingleConstructor() {
+		this.context = new AnnotationConfigApplicationContext(BaseConfiguration.class,
+				SingleConstructorComponent.class);
+		assertThat(this.context.getBean(SingleConstructorComponent.class).autowiredName, is("foo"));
+	}
+
+	@Test
+	public void componentTwoConstructorsNoHint() {
+		this.context = new AnnotationConfigApplicationContext(BaseConfiguration.class,
+				TwoConstructorsComponent.class);
+		assertThat(this.context.getBean(TwoConstructorsComponent.class).name, is("fallback"));
+	}
+
+	@Test
+	public void componentTwoSpecificConstructorsNoHint() {
+		thrown.expect(BeanCreationException.class);
+		thrown.expectMessage(NoSuchMethodException.class.getName());
+		new AnnotationConfigApplicationContext(BaseConfiguration.class,
+				TwoSpecificConstructorsComponent.class);
+	}
+
+
+	@Configuration
+	static class BaseConfiguration {
+
+		@Bean
+		public String autowiredName() {
+			return "foo";
+		}
+	}
+
+	private static class SingleConstructorComponent {
+
+		private final String autowiredName;
+
+		// No @Autowired - implicit wiring
+		public SingleConstructorComponent(String autowiredName) {
+			this.autowiredName = autowiredName;
+		}
+
+	}
+
+	private static class TwoConstructorsComponent {
+
+		private final String name;
+
+		public TwoConstructorsComponent(String name) {
+			this.name = name;
+		}
+
+		public TwoConstructorsComponent() {
+			this("fallback");
+		}
+	}
+
+	private static class TwoSpecificConstructorsComponent {
+
+		private final Integer counter;
+
+		public TwoSpecificConstructorsComponent(Integer counter) {
+			this.counter = counter;
+		}
+
+		public TwoSpecificConstructorsComponent(String name) {
+			this(Integer.valueOf(name));
+		}
+	}
+
+}

spring-context/src/test/java/org/springframework/context/annotation/configuration/AutowiredConfigurationTests.java

@@ -89,19 +89,17 @@ public class AutowiredConfigurationTests {
 		assertThat(context.getBean(TestBean.class).getName(), equalTo(""));
 	}
 
-	/**
-	 * {@link Autowired} constructors are not supported on {@link Configuration} classes
-	 * due to CGLIB constraints
-	 */
-	@Test(expected = BeanCreationException.class)
-	public void testAutowiredConfigurationConstructorsAreNotSupported() {
-		DefaultListableBeanFactory context = new DefaultListableBeanFactory();
-		new XmlBeanDefinitionReader(context).loadBeanDefinitions(
+	@Test
+	public void testAutowiredConfigurationConstructorsAreSupported() {
+		DefaultListableBeanFactory factory = new DefaultListableBeanFactory();
+		new XmlBeanDefinitionReader(factory).loadBeanDefinitions(
 				new ClassPathResource("annotation-config.xml", AutowiredConstructorConfig.class));
-		GenericApplicationContext ctx = new GenericApplicationContext(context);
+		GenericApplicationContext ctx = new GenericApplicationContext(factory);
 		ctx.registerBeanDefinition("config1", new RootBeanDefinition(AutowiredConstructorConfig.class));
 		ctx.registerBeanDefinition("config2", new RootBeanDefinition(ColorConfig.class));
-		ctx.refresh(); // should throw
+		ctx.refresh();
+		assertSame(ctx.getBean(AutowiredConstructorConfig.class).colour,
+				ctx.getBean(Colour.class));
 	}
 
 	@Test