开启循环依赖参数allowCircularReferences

骑猪看日落 2024-03-24 09:04 20阅读 0赞

### 使用springboot测试循环依赖问题 ###

### 问题出现 ###

编写了两个类来测试一下循环依赖:

TestA类

@Component
    public class TestA {
        
        @Autowired
        private TestB testB;
    
        public void aFunc() {
        
            System.out.println("TestA");
        }
    }

TestB类

@Component
    public class TestB {
        
        @Autowired
        private TestA a;
    
        public void bFunc() {
        
            System.out.println("TestB");
        }
    }

没想到启动报错了

Description:
    
    The dependencies of some of the beans in the application context form a cycle:
    
    ┌─────┐
    |  testA (field private com.example.springtest.TestB com.example.springtest.TestA.testB)
    ↑     ↓
    |  testB (field private com.example.springtest.TestA com.example.springtest.TestB.a)
    └─────┘
    
    
    Action:
    
    Relying upon circular references is discouraged and they are prohibited by default. Update your application to remove the dependency cycle between beans. As a last resort, it may be possible to break the cycle automatically by setting spring.main.allow-circular-references to true

可是印象里，spring不是默认支持循环依赖的吗？哎？

### 问题溯源 ###

首先按照之前对[spring循环依赖][spring]的理解，我们知道spring有三级缓存来解决循环依赖的问题，假设先创建的是TestA，那么存在以下的推导：

doCreateBean（TestA） -> populateBean（TestA） -> doCreateBean（TestB） -> populateBean(TestB) -> TestB添加到IOC容器中 -> TestA添加的IOC容器中
    1

那为啥还是出现了循环依赖问题呢？根据报错提示我们可以设置`spring.main.allow-circular-references`为true来解决循环依赖问题，这个配置在spring中不说默认为true吗，难道是springboot对这个配置进行了修改？

在doCreateBean中有一段关键的代码

// Eagerly cache singletons to be able to resolve circular references
    // even when triggered by lifecycle interfaces like BeanFactoryAware.
    boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
          isSingletonCurrentlyInCreation(beanName));
    if (earlySingletonExposure) {
        
       if (logger.isTraceEnabled()) {
        
          logger.trace("Eagerly caching bean '" + beanName +
                "' to allow for resolving potential circular references");
       }
       addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
    }

通过断点调试可以看到this.allowCircularReferences的值为false  
![在这里插入图片描述][fe38e74be4f146f7a33b8fff13aba6ef.png]  
earlySingletonExposure为false，那[三级缓存][Link 1]中就没有TestA，在populateBean(TestB)中又会走getSingleton()，最终在beforeSingletonCreation抛出了循环依赖的异常。

/** Cache of singleton factories: bean name to ObjectFactory. */
    private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);

至于this.allowCircularReferences的值为什么默认是false，我的springboot的版本使用的是2.7.x，然后  
![在这里插入图片描述][9ccfa8a052c14435810a32fc6e8cdaa8.png]  
2.6版本默认不支持循环依赖了

### 解决方法 ###

在application.properties中，设置`spring.main.allow-circular-references=true`  
![在这里插入图片描述][0551c88bb51f4e02b175010015b89cff.png]

[spring]: https://so.csdn.net/so/search?q=spring%E5%BE%AA%E7%8E%AF%E4%BE%9D%E8%B5%96&spm=1001.2101.3001.7020
[fe38e74be4f146f7a33b8fff13aba6ef.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/24/fca9ab14a590432b879ea5f810a97701.png
[Link 1]: https://so.csdn.net/so/search?q=%E4%B8%89%E7%BA%A7%E7%BC%93%E5%AD%98&spm=1001.2101.3001.7020
[9ccfa8a052c14435810a32fc6e8cdaa8.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/24/7a70cb4205894ccab32a7dd812bcff0e.png
[0551c88bb51f4e02b175010015b89cff.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/24/3b4c6a7923844522aaf31ce5ca263e0e.png