前言

1. 代码写的太乱，需要重构。
2. 确实没办法，需要使用循环依赖。

什么是循环依赖？

Bean A 依赖 B，Bean B 依赖 A这种情况下出现循环依赖。
Bean A → Bean B → Bean A
更复杂的间接依赖造成的循环依赖如下。
Bean A → Bean B → Bean C → Bean D → Bean E → Bean A

e.g.

Description:

The dependencies of some of the beans in the application context form a cycle:

┌─────┐
|  browserSecurityConfig defined in file [D:\CODE\Java\IdeaProjects\mango-security\mango-security-browser\target\classes\stu\mango\security\browser\BrowserSecurityConfig.class]
↑     ↓
|  myUserDetailService defined in file [D:\CODE\Java\IdeaProjects\mango-security\mango-security-browser\target\classes\stu\mango\security\browser\MyUserDetailService.class]
└─────┘

该例中，BrowserSecurityConfig 通过构造函数注入 UserDetailsService实例，而 UserDetailsService由通过构造函数注入在BrowserSecurityConfig 中声明的PasswordEncoder。

循环依赖会产生什么结果？

当Spring正在加载所有Bean时，Spring尝试以能正常创建Bean的顺序去创建Bean。
例如，有如下依赖:
Bean A → Bean B → Bean C
Spring先创建beanC，接着创建bean B（将C注入B中)，最后创建bean A(将B注入A中)。

但当存在循环依赖时，Spring将无法决定先创建哪个bean。这种情况下，Spring将产生异常BeanCurrentlyInCreationException。

当使用构造器注入时经常会发生循环依赖问题。如果使用其它类型的注入方式能够避免这种问题。

不同场景下不同的解决办法

解决办法1-SpringContextHolder

如果确实是需要使用循环依赖的话，可以写一个工具类SpringContextHolder，去操作Bean，从容器中获取Bean：

package com.lzhpo.common.utils;

import com.lzhpo.common.enums.ResultStatus;
import com.lzhpo.common.exception.CustomException;
import org.springframework.beans.BeansException;
import org.springframework.beans.factory.DisposableBean;
import org.springframework.context.ApplicationContext;
import org.springframework.context.ApplicationContextAware;
import org.springframework.context.ApplicationEvent;
import org.springframework.context.annotation.Lazy;
import org.springframework.stereotype.Service;

@Service
@Lazy(false)
public class SpringContextHolder implements ApplicationContextAware, DisposableBean {

    private static ApplicationContext applicationContext = null;

    /**
     * 获取applicationContext
     *
     * @return ApplicationContext
     */
    public static ApplicationContext getApplicationContext() {
        if (applicationContext == null) {
            throw new CustomException(ResultStatus.FAIL.getCode(), "applicationContext为空！");
        }
        return applicationContext;
    }

    @Override
    public void destroy() throws Exception {
        applicationContext = null;
    }

    @Override
    public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
        SpringContextHolder.applicationContext = applicationContext;
    }

    /**
     * 发布事件
     *
     * @param event
     */
    public static void publishEvent(ApplicationEvent event) {
        if (applicationContext == null) {
            return;
        }
        applicationContext.publishEvent(event);
    }
}

使用：

// 使用SpringContextHolder获取到上下文之后，根据类去获取Bean
TokenStore tokenStore = SpringContextHolder.getApplicationContext().getBean(TokenStore.class);

// 之后就可以正常使用了
Collection<OAuth2AccessToken> oAuth2AccessToken = tokenStore.findTokensByClientId("lzhpo");
log.info("oAuth2AccessToken：{}", oAuth2AccessToken);

解决办法2-使用注解 @Lazy

一种最简单的消除循环依赖的方式是通过延迟加载。在注入依赖时，先注入代理对象，当首次使用时再创建对象完成注入。

@Component
public class CircularDependencyA {
 
    private CircularDependencyB circB;
 
    @Autowired
    public CircularDependencyA(@Lazy CircularDependencyB circB) {
        this.circB = circB;
    }
}

使用@Lazy后，运行代码，可以看到异常消除。

解决办法3-使用Setter/Field注入

Spring文档建议的一种方式是使用setter注入。当依赖最终被使用时才进行注入。

@Component
public class CircularDependencyA {
 
    private CircularDependencyB circB;
 
    @Autowired
    public void setCircB(CircularDependencyB circB) {
        this.circB = circB;
    }
 
    public CircularDependencyB getCircB() {
        return circB;
    }
}

@Component
public class CircularDependencyB {
 
    private CircularDependencyA circA;
 
    private String message = "Hi!";
 
    @Autowired
    public void setCircA(CircularDependencyA circA) {
        this.circA = circA;
    }
 
    public String getMessage() {
        return message;
    }
}

@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(classes = { TestConfig.class })
public class CircularDependencyTest {
 
    @Autowired
    ApplicationContext context;
 
    @Bean
    public CircularDependencyA getCircularDependencyA() {
        return new CircularDependencyA();
    }
 
    @Bean
    public CircularDependencyB getCircularDependencyB() {
        return new CircularDependencyB();
    }
 
    @Test
    public void givenCircularDependency_whenSetterInjection_thenItWorks() {
        CircularDependencyA circA = context.getBean(CircularDependencyA.class);

        Assert.assertEquals("Hi!", circA.getCircB().getMessage());
    }
}

解决办法4-使用@PostConstruct

@Component
public class CircularDependencyA {
 
    @Autowired
    private CircularDependencyB circB;
 
    @PostConstruct
    public void init() {
        circB.setCircA(this);
    }
 
    public CircularDependencyB getCircB() {
        return circB;
    }
}

@Component
public class CircularDependencyB {
 
    private CircularDependencyA circA;
     
    private String message = "Hi!";
 
    public void setCircA(CircularDependencyA circA) {
        this.circA = circA;
    }
     
    public String getMessage() {
        return message;
    }

}

解决办法5-实现ApplicationContextAware与InitializingBean

@Component
public class CircularDependencyA implements ApplicationContextAware, InitializingBean {
 
    private CircularDependencyB circB;
 
    private ApplicationContext context;
 
    public CircularDependencyB getCircB() {
        return circB;
    }
 
    @Override
    public void afterPropertiesSet() throws Exception {
        circB = context.getBean(CircularDependencyB.class);
    }
 
    @Override
    public void setApplicationContext(final ApplicationContext ctx) throws BeansException {
        context = ctx;
    }
}

@Component
public class CircularDependencyB {
 
    private CircularDependencyA circA;
 
    private String message = "Hi!";
 
    @Autowired
    public void setCircA(CircularDependencyA circA) {
        this.circA = circA;
    }
 
    public String getMessage() {
        return message;
    }
}

参考文章：https://www.baeldung.com/circular-dependencies-in-spring