Explain Circular Dependency scenario in Spring.

Consider a scenario where Class A requires an instance of class B through constructor injection, and class B requires an instance of class A through constructor injection. If you configure beans for the classes A and B to be injected into each other, the Spring IoC container detects this circular reference at runtime and throws a BeanCurrentlyInCreationException.

The easiest solution to resolve the circular dependency issues would be editing the classes to have the dependency set by setters rather than constructor.

public class A {

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

    private B b;

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

}

public class B {

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

    private A a;

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

}

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

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

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