Improve documentation for @Autowired constructors

[farazdurrani]

When multiple constructors are marked by @Autowired, it doesn't work. Although Spring documentation clearly says that it should:

Only one annotated constructor per class can be marked as required, but multiple non-required constructors can be annotated. In that case, each is considered among the candidates and Spring uses the greediest constructor whose dependencies can be satisfied — that is, the constructor that has the largest number of arguments. The constructor resolution algorithm is the same as for non-annotated classes with overloaded constructors, just narrowing the candidates to annotated constructors.

I get this error when I mark multiple constructors with @Autowired with one being required = true and other being marked as required = false:

org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'constructorInjectComponent': Invalid autowire-marked constructors: [public com.example.demo.ConstructorInjectComponent(com.example.demo.InjectionServiceThree)]. Found constructor with 'required' Autowired annotation: public com.example.demo.ConstructorInjectComponent(com.example.demo.InjectionServiceOne,com.example.demo.InjectionServiceTwo)
	at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.determineCandidateConstructors(AutowiredAnnotationBeanPostProcessor.java:322) ~[spring-beans-5.1.8.RELEASE.jar:5.1.8.RELEASE]
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.determineConstructorsFromBeanPostProcessors(AbstractAutowireCapableBeanFactory.java:1269) ~[spring-beans-5.1.8.RELEASE.jar:5.1.8.RELEASE]
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1184) ~[spring-beans-5.1.8.RELEASE.jar:5.1.8.RELEASE]
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:555) ~[spring-beans-5.1.8.RELEASE.jar:5.1.8.RELEASE]
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:515) ~[spring-beans-5.1.8.RELEASE.jar:5.1.8.RELEASE]
	at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:320) ~[spring-beans-5.1.8.RELEASE.jar:5.1.8.RELEASE]
	at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:222) ~[spring-beans-5.1.8.RELEASE.jar:5.1.8.RELEASE]
	at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:318) ~[spring-beans-5.1.8.RELEASE.jar:5.1.8.RELEASE]
	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:199) ~[spring-beans-5.1.8.RELEASE.jar:5.1.8.RELEASE]
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:845) ~[spring-beans-5.1.8.RELEASE.jar:5.1.8.RELEASE]
	at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:877) ~[spring-context-5.1.8.RELEASE.jar:5.1.8.RELEASE]
	at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:549) ~[spring-context-5.1.8.RELEASE.jar:5.1.8.RELEASE]
	at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.refresh(ServletWebServerApplicationContext.java:140) ~[spring-boot-2.1.6.RELEASE.jar:2.1.6.RELEASE]
	at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:742) [spring-boot-2.1.6.RELEASE.jar:2.1.6.RELEASE]
	at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:389) [spring-boot-2.1.6.RELEASE.jar:2.1.6.RELEASE]
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:311) [spring-boot-2.1.6.RELEASE.jar:2.1.6.RELEASE]
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1213) [spring-boot-2.1.6.RELEASE.jar:2.1.6.RELEASE]
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1202) [spring-boot-2.1.6.RELEASE.jar:2.1.6.RELEASE]
	at com.example.demo.MultipleConstructionInjectionApplication.main(MultipleConstructionInjectionApplication.java:10) [main/:na]

This is the code:

@Component
@EnableScheduling
public class ConstructorInjectComponent {

	private InjectionServiceOne injectionServiceOne;

	private InjectionServiceTwo injectionServiceTwo;
	
	private InjectionServiceThree injectionServiceThree;


	@Autowired(required = true)
	public ConstructorInjectComponent(InjectionServiceOne injectionServiceOne,
			InjectionServiceTwo injectionServiceTwo) {
		this.injectionServiceOne = injectionServiceOne;
		this.injectionServiceTwo = injectionServiceTwo;
	}

	@Autowired(required = false)
	public ConstructorInjectComponent(InjectionServiceThree injectionServiceThree) {
		this.injectionServiceThree = injectionServiceThree;
	}
	
	@Scheduled(fixedRate = 1000L)
	public void allFieldsConstructorInjectionTest() {
		System.err.println("constructorInjection " + injectionServiceOne.method() + " " + injectionServiceTwo.method() + injectionServiceThree.method());
	}
}

I have created a simple github repo to reproduce the issue.

OS: Ubuntu 18.04
JDK 8.

Spring Boot version 2.1.6

[sbrannen]

Related to gh-21832