一、Spring不能解决的三种循环依赖问题

1. 构造器注入类型循环依赖

1）代码示例

@Component
class A1 {

    private B1 b1;

    public A1(B1 b1) {
        this.b1 = b1;
    }
}

@Component
class B1 {

    private A1 a1;

    public B1(A1 a1) {
        this.a1 = a1;
    }
}

2）错误信息

2. @Async类型循环依赖

1）代码示例

@Component
class A2 {

    @Resource
    private B2 b2;

    @Async
    public void hello() {
        System.out.println("hello I'm a2! ");
    }
}

@Component
class B2 {

    @Resource
    @Lazy
    private A2 a2;

    public void hello() {
        System.out.println("hello I'm b2! ");
    }
}

2）错误信息

3. prototype类型循环依赖

1）代码示例

@Component
@Scope("prototype")
class A3 {
    @Resource
    private B3 b3;
}

@Component
@Scope("prototype")
class B3 {
    @Resource
    private A3 a3;
}

2）错误信息

二、解决方案

在需要循环注入的属性上加上@Lazy 来解决循环依赖问题

三、单元测试

package com.example.demo5;

import org.junit.jupiter.api.Test;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.context.annotation.Lazy;
import org.springframework.context.annotation.Scope;
import org.springframework.scheduling.annotation.Async;
import org.springframework.stereotype.Component;

import javax.annotation.Resource;

/**
 * @Author lcb
 * @Date 2023/03/14
 **/
@SpringBootTest
public class CirculateDependencyTest {

    @Resource
    private A1 a1;

    @Test
    void constructorTest() {
    }

    @Resource
    private B2 b2;

    @Test
    void asyncTest() {
    }

    @Resource
    private A3 a3;

    @Test
    void prototypeTest() {
    }
}

//1、构造器注入问题
@Component
class A1 {

    private B1 b1;

    @Lazy
    public A1(B1 b1) {
        this.b1 = b1;
    }
}

@Component
class B1 {

    private A1 a1;

    public B1(A1 a1) {
        this.a1 = a1;
    }
}

//2、@Async
@Component
class A2 {

    @Resource
    private B2 b2;

    @Async
    public void hello() {
        System.out.println("hello I'm a2! ");
    }
}

@Component
class B2 {

    @Resource
    @Lazy
    private A2 a2;

    public void hello() {
        System.out.println("hello I'm b2! ");
    }
}

//3、prototype
@Component
@Scope("prototype")
class A3 {
    @Resource
    @Lazy
    private B3 b3;
}

@Component
@Scope("prototype")
class B3 {
    @Resource
    private A3 a3;
}