Spring循环依赖的原理系列(二)--打断点分析

秒速五厘米 2023-09-28 10:52 4阅读 0赞

原文网址： [Spring循环依赖的原理系列(二)--打断点分析\_IT利刃出鞘的博客-CSDN博客][Spring_--_IT_-CSDN]

## 简介 ##

**说明**

本文打断点来分析Spring（SpringBoot）是如何解决循环依赖的。

**相关网址**

调用到此处代码的流程见：[SpringBoot原理--启动流程\_][SpringBoot_--][IT利刃出鞘][IT][的博客-CSDN博客][SpringBoot_--]  
Bean初始化流程：[Spring--Bean的创建过程/获取流程--SpringBoot\_IT利刃出鞘的博客-CSDN博客][Spring--Bean_--SpringBoot_IT_-CSDN]

**定位代码位置**

由上边两篇博客，可以定位到实例化的代码位置：

AbstractAutowireCapableBeanFactory\#doCreateBean(beanName, mbdToUse, args)

**系列文章**

1.  [Spring循环依赖的原理系列(一)--什么是循环依赖\_IT利刃出鞘的博客-CSDN博客][Spring_--_IT_-CSDN 1]
2.  [Spring循环依赖的原理系列(二)--打断点分析\_IT利刃出鞘的博客-CSDN博客][Spring_--_IT_-CSDN]
3.  [Spring循环依赖的原理系列(三)--原理概述\_IT利刃出鞘的博客-CSDN博客][Spring_--_IT_-CSDN 2]
4.  [Spring循环依赖的原理系列(四)--为什么用三级缓存，而不是二级\_IT利刃出鞘的博客-CSDN博客][Spring_--_IT_-CSDN 3]

## 1.总入口：populateBean ##

### **总结** ###

经过下边的定位，可确定，核心代码在：AbstractAutowireCapableBeanFactory\#populateBean(beanName, mbd, instanceWrapper);

**doGetBean** 中有两个 **getSingleton** 方法会先后执行。第一个是尝试从缓存中获取，若缓存中没有 A，则执行第二个，通过工厂获得。

public Object getSingleton(String beanName) 。  
public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory)。

第一次通过getSingleton(String beanName)获取bean的时候，缓存中是没有的，于是走到getSingleton(String beanName, ObjectFactory<?> singletonFactory)

### **流程概述** ###

getBean(beanName) //AbstractBeanFactory抽象类（实现的BeanFactory接口）  
doGetBean(name, null, null, false) //AbstractBeanFactory抽象类。自己的方法。  
// 先从单例的缓存中取，取不到则走下一个getSingleton  
getSingleton(beanName);  
getSingleton(beanName, () -> \{  
try \{  
return createBean(beanName, mbd, args);  
\}\}  
)  
  
createBean(beanName, mbd, args) //AbstractAutowireCapableBeanFactory抽象类（实现的AbstractBeanFactory）  
doCreateBean(beanName, mbdToUse, args)//AbstractAutowireCapableBeanFactory抽象类。以下为此类方法  
createBeanInstance(beanName, mbd, args); //创建实例  
applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName) //post-processors修改bean的定义  
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean)); //解决循环依赖  
populateBean(beanName, mbd, instanceWrapper); //填充属性

### **断点调试** ###

打条件断点（DefaultListableBeanFactory\#doCreateBean(beanName, mbdToUse, args)

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2ZlaXlpbmcwY2FuZ2xhbmc_size_16_color_FFFFFF_t_70][]

### **定位代码** ###

**实例化A时去实例化B并填充到A对象**

在populateBean前后打断点。

**结果：**

1.  populateBean前（A的）：A已经实例化，其字段b还是null。（图1）
2.  populateBean后（A的）：没走到下一行，而是去实例化B去了，再次到了populateBean前。（图2）
    
    1.  此时，B已经实例化，其字段a为null
3.  populateBean后（B的）：的对象里已经给字段a赋值了，但a的字段b仍然为null。（图3）
4.  populateBean后（A的）：在第3步执行完之后，堆栈执行到A的populateBean后边。（图4）
    
    1.  此时，A对象里有B对象，B对象里边有A对象，一直循环

图1：

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2ZlaXlpbmcwY2FuZ2xhbmc_size_16_color_FFFFFF_t_70 1][]

图2：

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2ZlaXlpbmcwY2FuZ2xhbmc_size_16_color_FFFFFF_t_70 2][]

图3：

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2ZlaXlpbmcwY2FuZ2xhbmc_size_16_color_FFFFFF_t_70 3][]

图4：

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2ZlaXlpbmcwY2FuZ2xhbmc_size_16_color_FFFFFF_t_70 4][]

## 2.后置处理器注入属性 ##

**简介**

经过上边的定位，可以确定，核心代码在：AbstractAutowireCapableBeanFactory\#populateBean(beanName, mbd, instanceWrapper);

打条件断点：

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2ZlaXlpbmcwY2FuZ2xhbmc_size_16_color_FFFFFF_t_70 5][]

**总结（代码流程）**

getBeanPostProcessors() // 其中有：AutowiredAnnotationBeanPostProcessor （图1）  
ibp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);  
// AutowiredAnnotationBeanPostProcessor  
postProcessProperties(PropertyValues pvs, Object bean, String beanName) （图2）  
InjectionMetadata metadata = findAutowiringMetadata(beanName, bean.getClass(), pvs);  
metadata.inject(bean, beanName, pvs);  
inject(bean, beanName, pvs) //InjectionMetadata  
element.inject(target, beanName, pvs); //InjectionMetadata （图3）  
// AutowiredAnnotationBeanPostProcessor  
// 第一次进来时，this.cached为false。去解决依赖（B对象）（图4）  
value = beanFactory.resolveDependency(desc, beanName, autowiredBeanNames, typeConverter)  
// 上边获得之后，使用反射，设置字段的值  
field.set(bean, value);

图1：

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2ZlaXlpbmcwY2FuZ2xhbmc_size_16_color_FFFFFF_t_70 6][]

图2：

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2ZlaXlpbmcwY2FuZ2xhbmc_size_16_color_FFFFFF_t_70 7][]

图3：

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2ZlaXlpbmcwY2FuZ2xhbmc_size_16_color_FFFFFF_t_70 8][]

图4：

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2ZlaXlpbmcwY2FuZ2xhbmc_size_16_color_FFFFFF_t_70 9][]

## 3.解决依赖 ##

**总结**

实例化B时，由于之前实例化过A，这次 getSingleton(String beanName) 中发生了不一样的事：能够获得 A 的缓存。

**简介**

从上一步可以看到，最终调用DefaultListableBeanFactory\#resolveDependency，来解决 A => B 的依赖，需要去获取 B，仍然通过 getBean 获取，和之前说 getBean 获取 A 的过程类似，只是这次换成了 B。

调用栈如下：getBean=> doGetBean=> getSingleton，又是熟悉的步骤，但这次 getSingleton(String beanName) 中发生了不一样的事：能够获得 A 的缓存。

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2ZlaXlpbmcwY2FuZ2xhbmc_size_16_color_FFFFFF_t_70 10][]

## 4.三级缓存 ##

**总结**

可以在第3级缓存中获得到bean。

**简介**

在AbstractBeanFactory\#doGetBean()上打两个条件断点：beanName.equals("a") || beanName.equals("b")

// 为什么不在getSingleton里边打断点？因为getSingleton除了获取bean之外，其他也有地方调用了，会影响我们本处的分析。

**断点1**：

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2ZlaXlpbmcwY2FuZ2xhbmc_size_16_color_FFFFFF_t_70 11][]

**断点2**：

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2ZlaXlpbmcwY2FuZ2xhbmc_size_16_color_FFFFFF_t_70 12][]

**第1次到达断点1：Controller对象获取A对象，三级缓存中都没有**

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2ZlaXlpbmcwY2FuZ2xhbmc_size_16_color_FFFFFF_t_70 13][]

**第1次到达断点2：缓存中都没有对象A，所以去创建**

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2ZlaXlpbmcwY2FuZ2xhbmc_size_16_color_FFFFFF_t_70 14][]

**第2次到达断点1： 因为A类依赖了B类，所以获取B对象。但也是：三级缓存中都没有B对象**

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2ZlaXlpbmcwY2FuZ2xhbmc_size_16_color_FFFFFF_t_70 15][]

**第2次到达断点2： 缓存中都没有对象B，所以去创建**

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2ZlaXlpbmcwY2FuZ2xhbmc_size_16_color_FFFFFF_t_70 16][]

**第3次到达断点1： 因为B类依赖了A类，所以获取A对象。此时：第1级缓存中已经有A对象了**

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2ZlaXlpbmcwY2FuZ2xhbmc_size_16_color_FFFFFF_t_70 17][]

**第4次到达断点1：A用@Component修饰，要扫描进来。此时缓存中已经有了**

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2ZlaXlpbmcwY2FuZ2xhbmc_size_16_color_FFFFFF_t_70 18][]

**第5次到达断点1：B用@Component修饰，要扫描进来。此时缓存中已经有了**

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2ZlaXlpbmcwY2FuZ2xhbmc_size_16_color_FFFFFF_t_70 19][]

[Spring_--_IT_-CSDN]: https://knife.blog.csdn.net/article/details/125332501
[SpringBoot_--]: https://blog.csdn.net/feiying0canglang/article/details/110534892
[IT]: https://blog.csdn.net/feiying0canglang/article/details/108322129
[Spring--Bean_--SpringBoot_IT_-CSDN]: https://knife.blog.csdn.net/article/details/120962057
[Spring_--_IT_-CSDN 1]: https://knife.blog.csdn.net/article/details/108711870
[Spring_--_IT_-CSDN 2]: https://knife.blog.csdn.net/article/details/125332666
[Spring_--_IT_-CSDN 3]: https://knife.blog.csdn.net/article/details/125332773
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