Circular dependencies in spring

Cyclic dependency

When using spring, you may encounter cyclic dependencies if you are primarily using a dependency injection, and in short, constructors that depend on 基于构造器 Bean A Bean B it Bean B Bean A . In this case, spring will be thrown at compile time BeanCurrentlyInCreationException .

Class A

@Componentpublicclass ClassA {    private ClassB classB;    @Autowired    publicClassA(ClassB classB) {        this.classB = classB;    }    publicvoidprintClass() {        System.out.println("Class A = "this);        System.out.println("Class B = " + classB);    }}

Class B

@Componentpublicclass ClassB {    private ClassA classA;    @Autowired    publicClassB(ClassA classA) {        this.classA = classA;    }    publicvoidprintClass() {        System.out.println("Class A = " + classA);        System.out.println("Class B = "this);    }}

Test

@ContextConfiguration(classes = {ClassA.class, ClassB.class})@RunWith(SpringRunner.class)publicclass MyTest {    @Autowired    private ClassA classA;    @Autowired    private ClassB classB;    @Test    publicvoidname() {        classA.printClass();        classB.printClass();    }}

Reason

Running the test at this time will find that the exception was thrown BeanCurrentlyInCreationException . The reason for this is thatwhen spring creates the bean, it instantiates the object first and then injects the dependency . Assuming spring is created first, it will Class A find dependencies in the constructor, Class B so it will go to create Class B , and in Class B the constructor find the Class A dependency, and this Class A is not initialized, and therefore will be transferred to create Class A , So we get into a dead loop.

Workaround

Replace 基于setter the dependency injection to solve this problem. Because 基于setter the dependency injection will first call the default constructor to instantiate the object, and then invoke the setter implementation dependency injection. This way, there is no dependency at the stage of object instantiation, so the Class A instantiation is completed and then called Class B , and when the instantiation is complete, Class B the value is set, and Class A it is already instantiated, so it can be successfully referenced Class A .

Circular dependencies in spring