Circular dependency in Spring

Question

How does Spring resolve this  bean A is dependent on bean B  and bean B on bean A

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The Spring container is able to resolve Setter-based circular dependencies but gives a runtime exception BeanCurrentlyInCreationException in case of Constructor-based circular dependencies  In case of Setter-based circular dependency  the IOC container handles it differently from a typical scenario wherein it would fully configure the collaborating bean before injecting it   For eg   if Bean A has a dependency on Bean B and Bean B on Bean C  the container fully initializes C before injecting it to B and once B is fully initialized it is injected to A  But in case of circular dependency  one of the beans is injected to the other before it is fully initialized

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If two beans are dependent on each other then we should not use Constructor injection in both the bean definitions  Instead we have to use setter injection in any one of the beans   of course we can use setter injection n both the bean definitions  but constructor injections in both throw  BeanCurrentlyInCreationException   Refer Spring doc at  https   docs spring io spring docs current spring-framework-reference core html resources-resource

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Its clearly explained here  Thanks to Eugen Paraschiv   Circular dependency is a design smell  either fix it or use  Lazy for the dependency which causes problem to workaround it

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In the codebase I m working with  1 million   lines of code  we had a problem with long startup times  around 60 seconds  We were getting 12000  FactoryBeanNotInitializedException   What I did was set a conditional breakpoint in AbstractBeanFactory doGetBean  catch  BeansException ex          Explicitly remove instance from singleton cache  It might have been put there       eagerly by the creation process  to allow for circular reference resolution        Also remove any beans that received a temporary reference to the bean     destroySingleton beanName      throw ex      where it does destroySingleton beanName  I printed the exception with conditional breakpoint code      System out println ex      return false    Apparently this happens when FactoryBeans are involved in a cyclic dependency graph  We solved it by implementing ApplicationContextAware and  InitializingBean and manually injecting the beans    import org springframework beans BeansException  import org springframework beans factory InitializingBean  import org springframework context ApplicationContext  import org springframework context ApplicationContextAware   public class A implements ApplicationContextAware  InitializingBean       private B cyclicDepenency      private ApplicationContext ctx        Override     public void setApplicationContext ApplicationContext applicationContext              throws BeansException           ctx   applicationContext             Override     public void afterPropertiesSet   throws Exception           cyclicDepenency   ctx getBean B class              public void useCyclicDependency                 cyclicDepenency doSomething              This cut down the startup time to around 15 secs   So don t always assume that spring can be good at solving these references for you   For this reason I d recommend disabling cyclic dependency resolution with AbstractRefreshableApplicationContext setAllowCircularReferences false  to prevent many future problems

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Circular dependency in Spring   Dependency of one Bean to other  Bean A   Bean B   Bean A Solutions   Use  Lazy Annotation Redesign you class dependency Use Setter Field Injection Use  PostConstruct Annotation

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It just does it  It instantiates a and b  and injects each one into the other  using their setter methods    What s the problem

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Problem -   Class A       private final B b     must initialize in ctor instance block     public A B b    this b   b        Class B       private final A a     must initialize in ctor instance block     public B A a    this a   a            Caused by  org springframework beans factory BeanCurrentlyInCreationException  Error creating bean with name  A   Requested bean is currently in creation  Is there an unresolvable circular reference   Solution 1 -   Class A       private B b       public A                getter-setter for B b    Class B       private A a      public B                getter-setter for A a     Solution 2 -   Class A       private final B b     must initialize in ctor instance block     public A  Lazy B b    this b   b       Class B       private final A a     must initialize in ctor instance block     public B A a    this a   a

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Say A depends on B  then Spring will first instantiate A  then B  then set properties for B  then set B into A   But what if B also depends on A   My understanding is  Spring just found that A has been constructed  constructor executed   but not fully initialized  not all injections done   well  it thought  it s OK  it s tolerable that A is not fully initialized  just set this not-fully-initialized A instances into B for now  After B is fully initialized  it was set into A  and finally  A was fully initiated now   In other words  it just expose A to B in advance   For dependencies via constructor  Sprint just throw BeanCurrentlyInCreationException  to resolve this exception  set lazy-init to true for the bean which depends on others via constructor-arg way

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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    lt bean id  a  class  mypackage A  gt       lt property name  b  ref  b    gt   lt  bean gt    lt bean id  b  class  mypackage B  gt       lt property name  a  ref  a    gt   lt  bean gt    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

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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

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If you generally use constructor-injection and don t want to switch to property-injection then Spring s lookup-method-injection will let one bean lazily lookup the other and hence workaround the cyclic dependency  See here  http   docs spring io spring docs 1 2 9 reference beans html d0e1161

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Constructor Injection fails when there is Circular Dependency between spring beans  So in this case we Setter injection helps to resolve the issue   Basically  Constructor Injection is useful for Mandatory dependencies  for optional dependencies better to use Setter injection because we can do re-injection

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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

[java] Circular dependency in Spring

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