In this post, we will discuss one of the most asked Spring interview questions. What happens if there are circular dependencies in Spring.

Problem

What if there are circular dependencies in Spring?
For example: Class A requires an instance of Class B and Class B requires an instance of Class A.

Let’s create an example for the same.
1. Create a simple java maven project.
2. Maven dependency
put spring and cglib maven dependency in pom.xml.

org.springframework
   spring-core
   ${spring.version}org.springframework
   spring-webmvc
   ${spring.version}cglib
            cglib
            3.04.2.1.RELEASE

So your pom.xml will look like:

org.arpit.java2blog
  SpringHelloWorldJavaBasedConfiguration
  0.0.1-SNAPSHOTSpringHelloWorldJavaBasedConfigurationIt provides hello world program for java based config
org.springframework
   spring-core
   ${spring.version}org.springframework
   spring-webmvc
   ${spring.version}cglib
            cglib
            3.04.2.1.RELEASE

3. Create Bean class

package org.arpit.java2blog;

public class A {

	B b;

	public A(B b) {
		super();
		this.b = b;
	}
	
	public B getB() {
		return b;
	}
	public void setB(B b) {
		this.b = b;
	}
	
	public void doSomeThing()
	{
		System.out.println("Doing some work");
	}
}

package org.arpit.java2blog;

public class B {
	A a;

	public B(A a) {
		super();
		this.a = a;
	}

	public A getA() {
		return a;
	}

	public void setA(A a) {
		this.a = a;
	}
}

4. ApplicationContext.xml
Create ApplicationContext.xml in src/main/resources as below.

Please note that we are using Contructor injection to inject dependencies here.
5. Create CircularDependencyMain.java

package org.arpit.java2blog;

import org.springframework.context.support.ClassPathXmlApplicationContext;

/**
 * Circular dependencies in Spring
 *
 */
public class CircularDependencyMain 
{
	public static void main( String[] args )
	{
		ClassPathXmlApplicationContext ac = new ClassPathXmlApplicationContext("ApplicationContext.xml");
		A a = (A) ac.getBean("a");
		a.doSomeThing();
		ac.close();
		
	}
}

6. Run it
When you run above program, you will get below output.

As you can see, we are getting BeanCurrentlyInCreationException.Requested bean is currently in creation: Is there an unresolvable circular reference?

Solution

There are many solution to this problem.

Redesign

Can you redesign your classes to avoid circular dependencies. Circular dependencies are bad and should be avoided if possible at all. There are very rare situations where circular dependencies can not be avoid.

Setter injection

If you use Setter injection instead of Constructor injection to solve Circular dependencies in Spring, there won’t be any issue.
Let’s change the above example and try Setter injection.
Update A.java

package org.arpit.java2blog;

public class A {

	B b;

	public B getB() {
		return b;
	}
	public void setB(B b) {
		this.b = b;
	}
	
	public void doSomeThing()
	{
		System.out.println("Doing some work");
	}
}

Update B.java

package org.arpit.java2blog;

public class B {
	A a;

	public A getA() {
		return a;
	}

	public void setA(A a) {
		this.a = a;
	}
}

Update ApplicationContext.xml

Run CircularDependencyMain again
Output

@Lazy with @Autowired annotation

You use @Lazy with @Autowired to solve circular dependencies in Spring.
Let’s change the above example and try with @Lazy annotation.
Update A.java
Please note that we have used @Autowired and @Lazy annotations here.

package org.arpit.java2blog;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.annotation.Lazy;

public class A {

	B b;

	@Autowired
	public A(@Lazy B b) {
		super();
		this.b = b;
	}
	
	public B getB() {
		return b;
	}
	public void setB(B b) {
		this.b = b;
	}
	
	public void doSomeThing()
	{
		System.out.println("Doing some work");
	}
}

Update B.java

package org.arpit.java2blog;

import org.springframework.beans.factory.annotation.Autowired;

public class B {
	A a;

	@Autowired
	public B(A a) {
		super();
		this.a = a;
	}

	public A getA() {
		return a;
	}

	public void setA(A a) {
		this.a = a;
	}
}

Update ApplicationContext.xml

Run CircularDependencyMain again
Output

As you can see, program works fine with @Lazy annotation as well.

@PostConstruct with @Autowired annotation

You use @PostConstruct with @Autowired to solve circular dependencies in Spring.
Let’s change the above example and try Setter injection.
Update A.java
Please note that we have used @PostConstruct and @Autowired annotations here.

package org.arpit.java2blog;

import javax.annotation.PostConstruct;

import org.springframework.beans.factory.annotation.Autowired;

public class A {

	@Autowired
	B b;
	
	public B getB() {
		return b;
	}
	public void setB(B b) {
		this.b = b;
	}
	
	
	@PostConstruct
	public void init()
	{
		b.setA(this);
	}
	
	public void doSomeThing()
	{
		System.out.println("Doing some work");
	}
}

Update B.java

package org.arpit.java2blog;

import org.springframework.beans.factory.annotation.Autowired;

public class B {
	A a;

	@Autowired
	public B(A a) {
		super();
		this.a = a;
	}

	public A getA() {
		return a;
	}

	public void setA(A a) {
		this.a = a;
	}
}

Update ApplicationContext.xml

Run CircularDependencyMain again
Output

As you can see, program works fine with @PostContruct annotation as well.

Thats’s all about Circular dependencies in Spring.

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