Spring 源码学习(五) 循环依赖

public static void main(String[] args) {
	// 报错原因： Requested bean is currently in creation: Is there an unresolvable circular reference?
	ApplicationContext context = new ClassPathXmlApplicationContext("circle/circle.xml");
}

<!--注释 5.3 setter 方法注入-->
<bean id="circleA" class="base.circle.CircleA">
	<property name="circleB" ref="circleB"/>
</bean>

<bean id="circleB" class="base.circle.CircleB">
	<property name="circleA" ref="circleA"/>
</bean>

public static void main(String[] args) {
	ApplicationContext context = new ClassPathXmlApplicationContext("circle/circle.xml");
	CircleA circleA = (CircleA) context.getBean("circleA");
	circleA.a();
}

在 a 方法中，输出 A，在 b 方法中，输出B，下面是执行 demo 输出的结果：
错误提示是因为两个方法互相调用进行输出，然后打印到一定行数提示 main 函数栈溢出了=-=

A
B
A
B
*** java.lang.instrument ASSERTION FAILED ***: "!errorOutstanding" with message transform method call failed at JPLISAgent.c line: 844
*** java.lang.instrument ASSERTION FAILED ***: "!errorOutstanding" with message transform method call failed at JPLISAgent.c line: 844
*** java.lang.instrument ASSERTION FAILED ***: "!errorOutstanding" with message transform method call failed at JPLISAgent.c line: 844
Exception in thread "main" java.lang.StackOverflowError

/** Cache of singleton objects: bean name to bean instance. */
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);

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

/** Cache of early singleton objects: bean name to bean instance. */
private final Map<String, Object> earlySingletonObjects = new HashMap<>(16);

// 是否需要提前曝光，用来解决循环依赖时使用
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");
	}
	// 注释 5.2 解决循环依赖 第二个参数是回调接口，实现的功能是将切面动态织入 bean
	addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}

protected void addSingletonFactory(String beanName, ObjectFactory<?> singletonFactory) {
Assert.notNull(singletonFactory, "Singleton factory must not be null");
	synchronized (this.singletonObjects) {
		// 判断 singletonObjects 不存在 beanName
		if (!this.singletonObjects.containsKey(beanName)) {
		// 注释 5.4 放入 beanName -> beanFactory，到时在 getSingleton() 获取单例时，可直接获取创建对应 bean 的工厂，解决循环依赖
		this.singletonFactories.put(beanName, singletonFactory);
		// 从提前曝光的缓存中移除，之前在 getSingleton() 放入的
		this.earlySingletonObjects.remove(beanName);
		// 往注册缓存中添加 beanName
		this.registeredSingletons.add(beanName);
	}
}
}

BeanWrapper instanceWrapper = createBeanInstance(beanName, mbd, args);
// 原始 bean
final Object bean = instanceWrapper.getWrappedInstance();

// 注释 5.2 解决循环依赖 第二个参数是回调接口，实现的功能是将切面动态织入 bean
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));

protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {
    ...
    if (pvs != null) {
		// 将属性应用到 bean 中，使用深拷贝，将子类的属性一并拷贝
		applyPropertyValues(beanName, mbd, bw, pvs);
	}
}

protected void applyPropertyValues(String beanName, BeanDefinition mbd, BeanWrapper bw, PropertyValues pvs) {
    ...
    String propertyName = pv.getName();
	Object originalValue = pv.getValue();
	// 注释 5.5 解析参数，如果是引用对象，将会进行提前加载
	Object resolvedValue = valueResolver.resolveValueIfNecessary(pv, originalValue);
	...
}

public Object resolveValueIfNecessary(Object argName, @Nullable Object value) {
	// 我们必须检查每个值，看看它是否需要一个运行时引用，然后来解析另一个 bean
	if (value instanceof RuntimeBeanReference) {
		// 注释 5.6 在这一步中，如果判断是引用类型，需要解析引用，加载另一个 bean
		RuntimeBeanReference ref = (RuntimeBeanReference) value;
		return resolveReference(argName, ref);
	}
	...
}

// 注释 5.7 在这里加载引用的 bean
bean = this.beanFactory.getBean(refName);
this.beanFactory.registerDependentBean(refName, this.beanName);

protected Object getSingleton(String beanName, boolean allowEarlyReference) {
	Object singletonObject = this.singletonObjects.get(beanName);
	// 检查缓存中是否存在实例
	if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
		// 记住，公共变量都需要加锁操作，避免多线程并发修改
		synchronized (this.singletonObjects) {
			// 如果此 bean 正在加载则不处理
			singletonObject = this.earlySingletonObjects.get(beanName);
			if (singletonObject == null && allowEarlyReference) {
				// 当某些方法需要提前初始化，调用 addSingletonFactory 方法将对应的
				// objectFactory 初始化策略存储在 earlySingletonObjects，并且从 singletonFactories 移除
				ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
				if (singletonFactory != null) {
					singletonObject = singletonFactory.getObject();
					this.earlySingletonObjects.put(beanName, singletonObject);
					this.singletonFactories.remove(beanName);
				}
			}
		}
	}
	return singletonObject;
}