With constructor autowiring Spring does not throw an exception when there are multiple implementation types - spring

Question

There are interfaces Work and You. There are implementation types WorkImpl and YouImpl and YouImpl2.
I use constructor autowire to inject a You implementation instance in a WorkImpl instance.
Because there are multiple You implementations types, I thought Spring would throw an exception. But Spring instantiates an instance of one of the implementation types, in my case it was YouImpl. This is what I do not understand.
The configuration file is partly,
<bean
id="work"
class="my.test.own.spring_book.WorkImpl"
autowire="constructor"
>
<property name="age" value="52"/>
<property" name="name" value="Foo Bar"></property>
</bean>
<bean
id="you"
class="my.test.own.spring_book.YouImpl"
>
</bean>
<bean
id="you2"
class="my.test.own.spring_book.YouImpl2"
>
</bean>
WorkImpl has one constructor,
public WorkImpl(You you) {
this.you=you;
}

Answer 1

There are few types of autowiring using configuration approach:
byName
byType
constructor
autodetect:- Similar to byType, but type applies to constructor arguments.
Spring container looks at the constructor of the beans on which autowire attribute is set to byType in the XML configuration file. It then tries to match and wire a property if its type matches with exactly one of the beans name in configuration file.
<bean id="you" class="my.test.own.spring_book.YouImpl">
</bean>
<bean id="you2" class="my.test.own.spring_book.YouImpl2">
</bean>
It will match with you as name of parameter used in constructor is you
public WorkImpl(You you) {
this.you=you;
}
In order to avoid this you can use autowire-candidate="false" hence that bean will not take part in autowiring
<!-- This bean will not be injected-->
<bean id="you" class="my.test.own.spring_book.YouImpl" autowiring-candidate="false">
</bean>
<bean id="you2" class="my.test.own.spring_book.YouImpl2">
</bean>
Above is the answer of your question. But I will try to explain more so I can use this answer for future if I forget.
Now suppose you don't give id attribute to the bean or value of id attribute is different than the constructor parameter name.
<bean id="you1" class="my.test.own.spring_book.YouImpl" autowiring-candidate="false">
</bean>
<bean id="you2" class="my.test.own.spring_book.YouImpl2">
</bean>
Spring container searches any bean with type You, yes found two. Do next step
Spring container sees any bean with name(i.e id="you") you. No
It throws exception Unsatisfied dependency Injection
org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'work' defined in class path resource [autowire-contructor.xml]: Unsatisfied dependency expressed through constructor argument with index 0 of type
[my.test.own.spring_book.You]: : No unique bean of type [my.test.own.spring_book.You] is defined: expected single matching bean but found 2: [you1, you2]; nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException