情况1 set注入

bean1

@Repository
public class Bean1 {
    private Bean2 bean2;
    public Bean1() {
        System.out.println("Bean1()");
    }
    public Bean1(Bean2 bean2) {
        System.out.println("Bean1(Bean2 bean2)"+bean2.getClass());
        this.bean2 = bean2;
    }
    @Autowired
    public void setBean2(Bean2 bean2) {
        System.out.println("setBean2()");
        this.bean2 = bean2;
    }
}

bean2

@Repository
public class Bean2 {
    private Bean1 bean1;

    public Bean2() {
        System.out.println("Bean2()");
    }

    public Bean2(Bean1 bean1) {
        System.out.println("Bean2(Bean1 bean1)"+bean1.getClass());
        this.bean1 = bean1;
    }

    @Autowired
    public void setBean1(Bean1 bean1) {
        System.out.println("setBean1()");
        this.bean1 = bean1;
    }


}

Test

@Test
    public void test() {
        AbstractApplicationContext context = new ClassPathXmlApplicationContext("applicationContext.xml");
        Bean1 bean1 = context.getBean("bean1", Bean1.class);
        Bean2 bean2 = context.getBean("bean2", Bean2.class);
    }

结果：
Bean1()
Bean2()
setBean1()
setBean2()
没有问题,Spring三级缓存会帮我们解决循环依赖问题。

情况2 构造函数注入

bean1

@Repository
public class Bean1 {
    private Bean2 bean2;
    public Bean1(Bean2 bean2) {
        System.out.println("Bean1(Bean2 bean2)"+bean2.getClass());
        this.bean2 = bean2;
    }

}

bean2

@Repository
public class Bean2 {
    private Bean1 bean1;
    public Bean2(Bean1 bean1) {
        System.out.println("Bean2(Bean1 bean1)"+bean1.getClass());
        this.bean1 = bean1;
    }
}

结果报错:

Is there an unresolvable circular reference?是否存在无法解析的循环引用

解决@Lazy

public Bean1(@Lazy Bean2 bean2) {
        System.out.println("Bean1(Bean2 bean2)"+bean2.getClass());
        this.bean2 = bean2;
    }

public Bean2(@Lazy Bean1 bean1) {
        System.out.println("Bean2(Bean1 bean1)"+bean1.getClass());
        this.bean1 = bean1;
    }

运行结果:

Bean1(Bean2 bean2)class com.kuang.dao.Bean2$$EnhancerBySpringCGLIB$$9f101a2 
Bean2(Bean1 bean1)class com.kuang.dao.Bean1$$EnhancerBySpringCGLIB$$f65578ff

使用动态代理延迟bean创建

情况3 bean1采用set注入，bean2采用构造器注入

@Repository
public class Bean1 {
    private Bean2 bean2;

    public Bean1() {
        System.out.println("Bean1()");
    }

    public Bean1( Bean2 bean2) {
        System.out.println("Bean1(Bean2 bean2)"+bean2.getClass());
        this.bean2 = bean2;
    }

    @Autowired
    public void setBean2(Bean2 bean2) {
        System.out.println("setBean2()");
        this.bean2 = bean2;
    }


}

@Repository
public class Bean2 {
    private Bean1 bean1;

    public Bean2( Bean1 bean1) {
        System.out.println("Bean2(Bean1 bean1)"+bean1.getClass());
        this.bean1 = bean1;
    }

    public Bean1 getBean1() {
        return bean1;
    }


}

@Test
    public void test() {
        AbstractApplicationContext context = new ClassPathXmlApplicationContext("applicationContext.xml");

        Bean2 bean2 = context.getBean("bean2", Bean2.class);
        Bean1 bean1 = context.getBean("bean1", Bean1.class);
    }
}

结果：
Bean1()
Bean2(Bean1 bean1)class com.kuang.dao.Bean1
setBean2()

当循环依赖bean1采用set注入，bean2采用构造器注入时，Spring可以解决，无论bean1，bean2的创建顺序。