Spring循环依赖正确性及Bean注入的顺序关系详解

时间:2021-04-03 人气:0

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:";
}
@Autowired
private 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 stub
if(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 {
@Autowired
private 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。

好了，以上就是这篇文章的全部内容了，希望本文的内容对大家的学习或者工作具有一定的参考学习价值，如果有疑问大家可以留言交流，谢谢大家对的支持。

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