spring解决循环依赖

2020-06-08 约 736 字预计阅读 2 分钟次阅读

原文：一文告诉你Spring是如何利用“三级缓存“巧妙解决Bean的循环依赖问题的【享学Spring】

// todo

总结：通过三级缓存技术将对象实例化和初始化分离

具体步骤：a->b->a

创建a对象
1. 实例化a，从singletonFactories移除，放入earlySingletonObjects
2. 由于a依赖b，所以需要创建b对象
创建b对象
1. 实例化b 从singletonFactories移除，放入earlySingletonObjects
2. 初始化b 由于b依赖a 所以在容器内找a，从缓存中拿到a
3. b初始化成功，从earlySingletonObjects移除放入singletonObjects
由于a依赖b， b初始化成功，a也就随之初始化成功
此时，a和b互相持有的引用指向的实例都是初始化好的，并且都保存在一级缓存中。

三级缓存源码：

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18


public class DefaultSingletonBeanRegistry extends SimpleAliasRegistry implements SingletonBeanRegistry {
	...
	// 从上至下 分表代表这“三级缓存”
	private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256); //一级缓存
	private final Map<String, Object> earlySingletonObjects = new HashMap<>(16); // 二级缓存
	private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16); // 三级缓存
	...
	
	/** Names of beans that are currently in creation. */
	// 这个缓存也十分重要：它表示bean创建过程中都会在里面呆着~
	// 它在Bean开始创建时放值，创建完成时会将其移出~
	private final Set<String> singletonsCurrentlyInCreation = Collections.newSetFromMap(new ConcurrentHashMap<>(16));

	/** Names of beans that have already been created at least once. */
	// 当这个Bean被创建完成后，会标记为这个 注意：这里是set集合 不会重复
	// 至少被创建了一次的  都会放进这里~~~~
	private final Set<String> alreadyCreated = Collections.newSetFromMap(new ConcurrentHashMap<>(256));
}

获取单例bean源码：

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34


public class DefaultSingletonBeanRegistry extends SimpleAliasRegistry implements SingletonBeanRegistry {
	...
	@Override
	@Nullable
	public Object getSingleton(String beanName) {
		return getSingleton(beanName, true);
	}
	@Nullable
	protected Object getSingleton(String beanName, boolean allowEarlyReference) {
		Object singletonObject = this.singletonObjects.get(beanName);
		if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
			synchronized (this.singletonObjects) {
				singletonObject = this.earlySingletonObjects.get(beanName);
				if (singletonObject == null && allowEarlyReference) {
					ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
					if (singletonFactory != null) {
						singletonObject = singletonFactory.getObject();
						this.earlySingletonObjects.put(beanName, singletonObject);
						this.singletonFactories.remove(beanName);
					}
				}
			}
		}
		return singletonObject;
	}
	...
	public boolean isSingletonCurrentlyInCreation(String beanName) {
		return this.singletonsCurrentlyInCreation.contains(beanName);
	}
	protected boolean isActuallyInCreation(String beanName) {
		return isSingletonCurrentlyInCreation(beanName);
	}
	...
}

目录

spring解决循环依赖