Circular dependency in spring

Question

How does spring resolve this. Bean A is dependent on bean b, and bean b on bean a.

Answer 1

It just does it. It instantiates a and b, and injects each one into the other (using their setter methods).

What's the problem?

Answer 2

The Spring reference manual explains how circular dependencies are resolved. The beans are instantiated first, then injected into each other.

Consider this class:

package mypackage;

public class A {

    public A() {
        System.out.println("Creating instance of A");
    }

    private B b;

    public void setB(B b) {
        System.out.println("Setting property b of A instance");
        this.b = b;
    }

}

And a similar class B:

package mypackage;

public class B {

    public B() {
        System.out.println("Creating instance of B");
    }

    private A a;

    public void setA(A a) {
        System.out.println("Setting property a of B instance");
        this.a = a;
    }

}

If you then had this configuration file:

<bean id="a" class="mypackage.A">
    <property name="b" ref="b" />
</bean>

<bean id="b" class="mypackage.B">
    <property name="a" ref="a" />
</bean>

You would see the following output when creating a context using this configuration:

Creating instance of A
Creating instance of B
Setting property a of B instance
Setting property b of A instance

Note that when a is injected into b, a is not yet fully initialised.

Answer 3

From the Spring Reference:

You can generally trust Spring to do the right thing. It detects configuration problems, such as references to non-existent beans and circular dependencies, at container load-time. Spring sets properties and resolves dependencies as late as possible, when the bean is actually created.

Answer 4

As the other answers have said, Spring just takes care of it, creating the beans and injecting them as required.

One of the consequences is that bean injection / property setting might occur in a different order to what your XML wiring files would seem to imply. So you need to be careful that your property setters don't do initialization that relies on other setters already having been called. The way to deal with this is to declare beans as implementing the InitializingBean interface. This requires you to implement the afterPropertiesSet() method, and this is where you do the critical initialization. (I also include code to check that important properties have actually been set.)