Deep understanding of circular dependencies in spring
Cyclic dependence
Definition: circular dependency refers to circular reference, that is, two or more beans hold each other. For example, circularitya refers to circularityb, circularityb refers to circularityc, and circularityc refers to circularitya. Form a circular reference relationship.
When using spring, if you mainly use the constructor based dependency injection method, you may encounter circular dependency. In short, the constructor of bean a depends on bean B, and the constructor of bean B depends on bean a. In this case, spring will throw a beancurrentyincreationexception at compile time.
Class A
Class B
test
reason
At this time, running the test will find that a beancurrentyincreationexception exception is thrown. The reason for this is that when spring creates a bean, it will first instantiate the object and then inject dependencies. If spring first creates class A, it will find that there is a dependency on class B in the constructor, so it will turn to create class B, and it will find a dependency on Class A in the constructor of class B. at this time, class A has not been initialized, so it will turn to create class a, which will fall into an endless loop.
resolvent
This problem can be solved by using setter based dependency injection. Because setter based dependency injection will first call the default constructor to instantiate the object, and then call setter to implement dependency injection. In this way, there is no dependency in the object instantiation stage. Therefore, class a will call class B after the instantiation is completed. After the instantiation of class B is completed, the value will be set. At this time, class A has been instantiated, so class a can be referenced successfully.
summary
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