Spring循环依赖正确性及Bean注入的顺序关系详解

2024-07-13 10:16:07

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一、前言

我们知道 Spring 可以是懒加载的，就是当真正使用到 Bean 的时候才实例化 Bean。当然也不全是这样，例如配置 Bean 的 lazy-init 属性，可以控制 Spring 的加载时机。现在机器的性能、内存等都比较高，基本上也不使用懒加载，在容器启动时候来加载bean，启动时间稍微长一点儿，这样在实际获取 bean 供业务使用时，就可以减轻不少负担，这个后面再做分析。我们使用到 Bean 的时候，最直接的方式就是从 Factroy 中获取，这个就是加载 Bean 实例的源头。

最近在做项目时候遇到一个奇葩问题，就是bean依赖注入的正确性与bean直接注入的顺序有关系，但是正常情况下明明是和顺序没关系的啊，究竟啥情况那，不急，让我一一道来。

二、普通Bean循环依赖-与注入顺序无关

2.1 循环依赖例子与原理

public class BeanA {private BeanB beanB;public BeanB getBeanB() { return beanB;}public void setBeanB(BeanB beanB) { this.beanB = beanB;}}

public class BeanB {private BeanA beanA;public BeanA getBeanA() { return beanA;}public void setBeanA(BeanA beanA) { this.beanA = beanA;}}

<bean id="beanA" class="com.alibaba.test.circle.BeanA"><property name="beanB"> <ref bean="beanB" /></property></bean>

<bean id="beanB" class="com.alibaba.test.circle.BeanB"><property name="beanA"> <ref bean="beanA" /></property></bean>

上述循环依赖注入能够正常工作，这是因为Spring提供了EarlyBeanReference功能，首先Spring里面有个名字为singletonObjects的并发map用来存放所有实例化并且初始化好的bean，singletonFactories则用来存放需要解决循环依赖的bean信息（beanName,和一个回调工厂）。当实例化beanA时候会触发getBean(“beanA”);首先看singletonObjects中是否有beanA有则返回:

（1）

Object sharedInstance = getSingleton(beanName);//getSingleton(beanName,true);if (sharedInstance != null && args == null) {if (logger.isDebugEnabled()) { if (isSingletonCurrentlyInCreation(beanName)) { logger.debug("Returning eagerly cached instance of singleton bean '" + beanName + "' that is not fully initialized yet - a consequence of a circular reference"); } else { logger.debug("Returning cached instance of singleton bean '" + beanName + "'"); }} // 如果是普通bean直接返回，工厂bean则返回sharedInstance.getObject();bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);}

protected Object getSingleton(String beanName, boolean allowEarlyReference) { Object singletonObject = this.singletonObjects.get(beanName); if (singletonObject == null) { synchronized (this.singletonObjects) { singletonObject = this.earlySingletonObjects.get(beanName); if (singletonObject == null && allowEarlyReference) { ObjectFactory singletonFactory = (ObjectFactory) this.singletonFactories.get(beanName); if (singletonFactory != null) {  singletonObject = singletonFactory.getObject();  this.earlySingletonObjects.put(beanName, singletonObject);  this.singletonFactories.remove(beanName); } } } } return (singletonObject != NULL_OBJECT ? singletonObject : null);}

一开始肯定没有所以会实例化beanA，如果设置了allowCircularReferences=true（默认为true）并且当前bean为单件并且该bean目前在创建中，则初始化属性前把该bean信息放入singletonFactories单件map里面：

（2）

boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&isSingletonCurrentlyInCreation(beanName));

if (earlySingletonExposure) {if (logger.isDebugEnabled()) { logger.debug("Eagerly caching bean '" + beanName + "' to allow for resolving potential circular references");}addSingletonFactory(beanName, new ObjectFactory() { public Object getObject() throws BeansException { return getEarlyBeanReference(beanName, mbd, bean); }});}

protected void addSingletonFactory(String beanName, ObjectFactory singletonFactory) {Assert.notNull(singletonFactory, "Singleton factory must not be null");synchronized (this.singletonObjects) { if (!this.singletonObjects.containsKey(beanName)) { this.singletonFactories.put(beanName, singletonFactory); this.earlySingletonObjects.remove(beanName); this.registeredSingletons.add(beanName); }}}

然后对该实例进行属性注入beanB，属性注入时候会getBean(“beanB”) ,发现beanB 不在singletonObjects中，就会实例化beanB,然后放入singletonFactories，然后进行属性注入beanA,然后触发getBean(“beanA”);这时候会到（1）getSingleton返回实例化的beanA。到此beanB初始化完毕添加beanB 到singletonObjects然后返回，然后beanA 初始化完毕，添加beanA到singletonObjects然后返回

2.2 允许循环依赖的开关

public class TestCircle2 {private final static ClassPathXmlApplicationContext moduleContext;private static Test test;static { moduleContext = new ClassPathXmlApplicationContext(new String[]{"beans-circile.xml"}); moduleContext.setAllowCircularReferences(false); test = (Test) moduleContext.getBean("test");}public static void main(String[] args) { System.out.println(test.name);}}

ClassPathXmlApplicationContext类中有个属性allowCircularReferences用来控制是否允许循环依赖默认为true，这里设置为false后发现循环依赖还是可以正常运行，翻看源码：

public ClassPathXmlApplicationContext(String[] configLocations) throws BeansException {this(configLocations, true, null);}

public ClassPathXmlApplicationContext(String[] configLocations, boolean refresh, ApplicationContext parent) throws BeansException {super(parent);setConfigLocations(configLocations);if (refresh) { refresh();}}

public ClassPathXmlApplicationContext(String[] configLocations, boolean refresh, ApplicationContext parent) throws BeansException {super(parent);setConfigLocations(configLocations);if (refresh) { refresh();}}

知道默认构造ClassPathXmlApplicationContext时候会刷新容器。

refresh方法会调用refreshBeanFactory：

protected final void refreshBeanFactory() throws BeansException {if (hasBeanFactory()) { destroyBeans(); closeBeanFactory();}try { // 创建bean工厂 DefaultListableBeanFactory beanFactory = createBeanFactory(); //定制bean工厂属性 customizeBeanFactory(beanFactory); loadBeanDefinitions(beanFactory); synchronized (this.beanFactoryMonitor) { this.beanFactory = beanFactory; }}catch (IOException ex) { throw new ApplicationContextException( "I/O error parsing XML document for application context [" + getDisplayName() + "]", ex);}}

protected void customizeBeanFactory(DefaultListableBeanFactory beanFactory) {if (this.allowBeanDefinitionOverriding != null) { beanFactory.setAllowBeanDefinitionOverriding(this.allowBeanDefinitionOverriding.booleanValue());}if (this.allowCircularReferences != null) { beanFactory.setAllowCircularReferences(this.allowCircularReferences.booleanValue());}}

到这里就知道了，我们在调用 moduleContext.setAllowCircularReferences(false)前，spring留出的设置bean工厂的回调customizeBeanFactory已经执行过了，最终原因是，调用设置前，bean工厂已经refresh了，所以测试代码改为：

public class TestCircle {private final static ClassPathXmlApplicationContext moduleContext;private static Test test;static { //初始化容器上下文，但是不刷新容器 moduleContext = new ClassPathXmlApplicationContext(new String[]{"beans-circile.xml"},false); moduleContext.setAllowCircularReferences(false); //刷新容器 moduleContext.refresh(); test = (Test) moduleContext.getBean("test");}public static void main(String[] args) { System.out.println(test.name);}}

现在测试就会抛出异常：

Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'beanA' defined in class path resource [beans-circile.xml]: Cannot resolve reference to bean 'beanB' while setting bean property 'beanB'; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'beanB' defined in class path resource [beans-circile.xml]: Cannot resolve reference to bean 'beanA' while setting bean property 'beanA'; nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'beanA': Requested bean is currently in creation: Is there an unresolvable circular reference?

三、工厂Bean与普通Bean循环依赖-与注入顺序有关

3.1 测试代码

工厂bean

public class MyFactoryBean implements FactoryBean,InitializingBean{private String name;private Test test;public String getName() { return name;}public void setName(String name) { this.name = name;}public DependentBean getDepentBean() { return depentBean;}public void setDepentBean(DependentBean depentBean) { this.depentBean = depentBean;}private DependentBean depentBean;public Object getObject() throws Exception { return test;}public Class getObjectType() { // TODO Auto-generated method stub return Test.class;}public boolean isSingleton() { // TODO Auto-generated method stub return true;}public void afterPropertiesSet() throws Exception {  System.out.println("name:" + this.name);  test = new Test();  test.name = depentBean.doSomething() + this.name;}}

为了简化，只写一个public的变量

public class Test {public String name;}

public class DependentBean {public String doSomething(){ return "hello:";}@Autowiredprivate Test test;}

xml配置

<bean id="test" class="com.alibaba.test.circle.MyFactoryBean"><property name="depentBean"> <bean class="com.alibaba.test.circle.DependentBean"></bean></property><property name="name" value="zlx"></property></bean>

其中工厂Bean MyFactoryBean作用是对Test类的包装，首先对MyFactoryBean设置属性，然后在MyFactoryBean的afterPropertiesSet方法中创建一个Test实例，并且设置属性，实例化MyFactoryBean最终会调用getObject方法返回创建的Test对象。这里MyFactoryBean依赖了DepentBean，而depentBean本身有依赖了Test，所以这是个循环依赖

测试：

public class TestCircle2 {private final static ClassPathXmlApplicationContext moduleContext;private static Test test;static { moduleContext = new ClassPathXmlApplicationContext(new String[]{"beans-circile.xml"}); test = (Test) moduleContext.getBean("test");}public static void main(String[] args) { System.out.println(test.name);}}

结果：

Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'com.alibaba.test.circle.DependentBean#1c701a27': Autowiring of fields failed; nested exception is org.springframework.beans.factory.BeanCreationException: Could not autowire field: private com.alibaba.test.circle.Test com.alibaba.test.circle.DependentBean.test; nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'test': FactoryBean which is currently in creation returned null from getObject

3.2 分析原因

当实例化test时候会触发getBean(“test”) ，会看当前bean是否存在

不存在则创建Test 的实例，创建完毕后会把当前bean信息放入singletonFactories单件map里面

然后对该实例进行属性注入depentBean，属性注入时候会getBean(“depentBean”) ,

发现depentBean 不存在，就会实例化depentBean,然后放入singletonFactories，

然后进行autowired注入test,然后触发getBean(“test”);这时候会到（1）getSingleton返回实例化的test。由于test是工厂bean所以返回test.getObject（）;

而MyFactoryBean的afterPropertiesSet还没被调用，所以test.getObject（）返回null.

下面列下Spring bean创建的流程：

getBean()->创建实例->autowired->set属性->afterPropertiesSet

也就是调用getObject方法早于afterPropertiesSet方法被调用了。

那么我们修改下MyFactoryBean为如下：

public Object getObject() throws Exception {// TODO Auto-generated method stubif(null == test){ afterPropertiesSet();}return test;}

public void afterPropertiesSet() throws Exception {if(null == test){ System.out.println("name:" + this.name); test = new Test(); test.name = depentBean.doSomething() + this.name;}}

也就是getObject内部先判断不如test==null那调用下afterPropertiesSet，然后afterPropertiesSet内部如果test==null在创建Test实例，看起来貌似不错，好想可以解决我们的问题。但是实际上还是不行的，因为afterPropertiesSet内部使用了depentBean，而此时depentBean=null。

3.3 思考如何解决

3.2分析原因是先创建了MyFactoryBean，并在在创建MyFactoryBean的过程中有创建了DepentBean，而创建DepentBean时候需要autowired MyFactoryBean的实例，然后要调用afterPropertiesSet前调用getObject方法所以返回null。

那如果先创建DepentBean，然后在创建MyFactoryBean那？下面分析下过程：

首先会实例化DepentBean，并且加入到singletonFactories

DepentBean实例会autowired Test,所以会先创建Test实例

创建Test实例，然后加入singletonFactories

Test实例会属性注入DepentBean实例，所以会getBean(“depentBean”);

getBean(“depentBean”) 发现singletonFactories中已经有depentBean了，则返回depentBean对象

因为depentBean不是工厂bean所以直接返回depentBean

Test实例会属性注入DepentBean实例成功，Test实例初始化OK

DepentBean实例会autowired Test实例OK

按照这分析先创建DepentBean，然后在实例化MyFactoryBean是可行的，修改xml为如下：

<bean id="dependentBean" class="com.alibaba.test.circle.DependentBean"></bean><bean id="test" class="com.alibaba.test.circle.MyFactoryBean"><property name="depentBean"> <ref bean="dependentBean" /> </property><property name="name" value="zlx"></property></bean>

测试运行结果：

name:zlx

hello:zlx

果真可以了，那按照这分析，上面XML配置如果调整了声明顺序，肯定也是会出错的，因为test创建比dependentBean早，测试下果然如此。另外可想而知工厂bean循环依赖工厂bean时候无论声明顺序如何必然也会失败。

3.3 一个思考

上面先注入了MyFactoryBean中需要使用的dependentBean，然后注入MyFactoryBean，问题就解决了。那么如果需要在另外一个Bean中使用创建的id=”test”的对象时候，这个Bean该如何注入那？
类似下面的方式，会成功？留给大家思考^^

public class UseTest {@Autowiredprivate Test test;}

<bean id="useTest" class="com.alibaba.test.circle.UseTest"></bean><bean id="dependentBean" class="com.alibaba.test.circle.DependentBean"></bean><bean id="test" class="com.alibaba.test.circle.MyFactoryBean"><property name="depentBean"> <ref bean="dependentBean" /> </property><property name="name" value="zlx"></property></bean>

四、总结

普通Bean之间相互依赖时候Bean注入顺序是没有关系的，但是工厂Bean与普通Bean相互依赖时候则必须先实例化普通bean,这是因为工厂Bean的特殊性，也就是其有个getObject方法的缘故。

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