ref vs idref attributes in spring bean declaration

Question

Can someone tell me the difference between

<bean id="b1" class="" />

<bean id="" class="">
 <property name="b1" ref="b1" />
</bean>

and

<bean id="" class="">
 <property name="b1" idref="b1" />
</bean>

and which one has to be used when?

Answer 1

Here is a little more verbose example, suppose you have two beans A and B:

<bean class="A" id="a" />

<bean class="B"/>
    <constructor-arg>
        <ref bean="a"/>
        <idref bean="a"/>
    </constructor-arg>
</bean>

In this case B would have a constructor that would look like this:

public B(A a, String string) {
     string.equals("a"); //true
}

So with ref you can reference an object and with idref you just reference the name of the bean