Spring5源码 - 09 循环依赖解读

Myth丶恋晨 2022-12-10 08:43 184阅读 0赞

### 文章目录 ###

*  什么是循环依赖
 *  模拟循环依赖的问题
 *  Spring 的循环依赖解决方案（三级缓存）

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3lhbmdzaGFuZ3dlaQ_size_16_color_FFFFFF_t_70_pic_center]

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# 什么是循环依赖 #

举个简单的例子

A 依赖 B，B 又依赖 A，它们之间形成了循环依赖。

或者是 A 依赖 B，B 依赖 C，C 又依赖 A，同样的 也是循环依赖

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# 模拟循环依赖的问题 #

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3lhbmdzaGFuZ3dlaQ_size_16_color_FFFFFF_t_70_pic_center 1]

package com.artisan.cycleDepend;
    
    import org.springframework.beans.factory.annotation.Autowired;
    import org.springframework.stereotype.Component;
    
    import javax.annotation.PostConstruct;
    
    @Component
    public class A { 
    
      @Autowired
      private B b;
    
    	@PostConstruct
    	public void init(){ 
    		System.out.println("A");
    	}
    }

import javax.annotation.PostConstruct;
    
    @Component
    public class B { 
    
    	@Autowired
    	private A a ;
    
    	@PostConstruct
    	public void init(){ 
    		System.out.println("A");
    	}
    }

【测试代码】

package com.artisan.cycleDepend;
    
    import org.springframework.beans.factory.annotation.Autowired;
    import org.springframework.beans.factory.config.BeanDefinition;
    import org.springframework.beans.factory.support.RootBeanDefinition;
    
    import java.lang.reflect.Field;
    import java.util.Map;
    import java.util.concurrent.ConcurrentHashMap;
    
    /** * 模拟循环依赖的问题 */
    public class CycleDependTest { 
    
    	private static Map<String, BeanDefinition> beanDefinitionMap = new ConcurrentHashMap<>();
    
    	// 一级缓存
    	private static Map<String, Object> singletonObjects = new ConcurrentHashMap<>();
    
    	public static void main(String[] args) throws InstantiationException, IllegalAccessException { 
    		// 模拟Spring 的 解析对象为 BeanDefinition 并放到 BeanDefinitionMap中
    		RootBeanDefinition  a = new RootBeanDefinition(A.class);
    		RootBeanDefinition  b = new RootBeanDefinition(B.class);
    
    		beanDefinitionMap.put("a",a);
    		beanDefinitionMap.put("b",b);
    
    
    		// 模拟Spring 的 getBean 循环创建Bean
    		for (String beanName : beanDefinitionMap.keySet()){ 
    			getBean(beanName);
    		}
    
    	}
    
    
    	public static Object getBean(String beanName) throws IllegalAccessException, InstantiationException { 
    
    		// 实例化
    		RootBeanDefinition  beanDefinition = (RootBeanDefinition) beanDefinitionMap.get(beanName);
    		Class<?> beanClass = beanDefinition.getBeanClass();
    		Object o = beanClass.newInstance();
    
    
    		// 属性填充
    
    		// 获取声明的字段
    		Field[] declaredFields = o.getClass().getDeclaredFields();
    		// 遍历
    		for (Field declaredField : declaredFields) { 
    			// 检查是否有Autowired注解
    			Autowired annotation = declaredField.getAnnotation(Autowired.class);
    			if (annotation != null){ 
    				// private属性 需要设置为true
    				declaredField.setAccessible(true);
    				// 获取属性的名字
    				String name = declaredField.getName();
    				// 递归调用getBean获取实例
    				Object bean = getBean(name);
    				// 字段设置属性
    				 declaredField.set(o,bean);
    
    			}
    
    		}
    
    		// 初始化 init-method 。。。。
    
    		// 存入一级缓存
    		singletonObjects.put(beanName,o);
    
    		return o;
    	}
    }

【运行结果】

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3lhbmdzaGFuZ3dlaQ_size_16_color_FFFFFF_t_70_pic_center 2]

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分析下原因

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3lhbmdzaGFuZ3dlaQ_size_16_color_FFFFFF_t_70_pic_center 3]

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# Spring 的循环依赖解决方案（三级缓存） #

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3lhbmdzaGFuZ3dlaQ_size_16_color_FFFFFF_t_70_pic_center 4]

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