java - Spring中的循环依赖

Question

Spring 如何解决这个问题:bean A 依赖于 bean B，bean B 依赖于 bean A。

Answer 1

Spring reference manual解释了如何解决循环依赖。 Bean 首先被实例化，然后相互注入(inject)。

考虑这个类:

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;
    }

}

还有一个类似的类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;
    }

}

如果您有此配置文件:

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

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

使用此配置创建上下文时，您将看到以下输出:

Creating instance of A
Creating instance of B
Setting property a of B instance
Setting property b of A instance

请注意，当a注入(inject)b时，a尚未完全初始化。