Inconsistency between property Injection and setter Injection causes NoUniqueBeanDefinitionException [SPR-14179]

[spring-projects-issues]

Christian Hersevoort opened SPR-14179 and commented

The case is as follows:

Beans:

• MyParentBean
• MyChildBeanA implements MyChildBean
• MyChildBeanB implements MyChildBean

XML configuration:

<bean id="myChildBeanA" class="com.hersevoort.java.test.MyChildBeanA">
</bean>
<bean id="myChildBeanB" class="com.hersevoort.java.test.MyChildBeanB">
</bean>
<bean id="myParentBean" class="com.hersevoort.java.test.MyParentBean">
	<property name="myChild" ref="myChildBeanB"/>
</bean>

Case #1: This works:

public class MyParentBean
{
	private MyChildBean myChild;

	@Inject
	public void setMyChild(MyChildBean myChild)
	{
		this.myChild = myChild;
	}
}

Case #2: This doesn't work:

public class MyParentBean
{
	@Inject
	private MyChildBean myChild;
}

I expect Spring to handle both cases exactly the same, but Case #2 throws an unexpected NoUniqueBeanDefinitionException. (see attachment)

Am I wrong to assume this is a bug?

---

Affects: 4.2.1, 4.2.5, 4.3 RC1

Attachments:

• stacktrace.txt (7.43 kB)

Issue Links:

• Should @Configuration mark @Inject fields and setters as satisfied (injected)? [SPR-14180] #18751 Should @Configuration mark @Inject fields and setters as satisfied (injected)?

[spring-projects-issues]

Juergen Hoeller commented

I'm afraid that this is by design: <property> elements in XML bean definitions specify values for bean properties, going through a JavaBean setter method just like setter injection does. This is completely decoupled from field injection which reflectively populates instance fields directly.

In the end, it's a kind of special feature of setter injection: It allows for overriding through external property values, similar to programmatic calling of a public setter method. Whereas field injection unconditionally operates on (typically) private fields, with no programmatic overriding possible either.