再谈spring的循环依赖是怎么造成的？
老生常谈，循环依赖！顾名思义嘛，就是你依赖我，我依赖你，然后就造成了循环依赖了！由于A中注入B，B中注入A导致的吗？
看起来没毛病，然而，却没有说清楚问题！甚至会让你觉得你是不清楚spring的循环依赖的！
那么，spring的循环依赖到底是啥玩意？
来看个循环依赖注入失败的例子：
bean1: userService
@Servicepublic class LoginServiceImpl implements LoginService {// @Resourceprivate UserService userService;public LoginServiceImpl() {// 无参构造器是给默认创建的bean使用的}@Autowiredpublic LoginServiceImpl(UserService userService) {this.userService = userService;}public void setUserService(UserService userService) {this.userService = userService;}public Integer login(UserInfo userInfo) {System.out.println("login...");return 0;}public void logAction(String userId, String action) throws Exception {UserInfo userInfo = userService.getUser(userId);System.out.println("logged..." + userInfo + ",action: " + action);}}
bean2: loginService
@Servicepublic class UserServiceImpl implements UserService {// @Resourceprivate LoginService loginService;public UserServiceImpl() {// 无参构造器是给默认创建的bean使用的}// @Autowiredpublic UserServiceImpl(LoginService loginService) {this.loginService = loginService;}@Autowiredpublic void setLoginService(LoginService loginService) {this.loginService = loginService;}public Integer addUser(UserInfo userInfo) throws Exception {userInfo.setId("111");System.out.println("user added. " + userInfo);// 登录loginService.login(userInfo);return 1;}public UserInfo getUser(String id) throws Exception {UserInfo userInfo = null;if("111".equals(id)) {userInfo = new UserInfo();userInfo.setId(id);userInfo.setAge(22);userInfo.setName("mocking");return userInfo;}throw new Exception("err:" + ErrorCodeEnum.API_CALL_ERROR.getCode() + "," + ErrorCodeEnum.API_CALL_ERROR.getErrMsg());}}
然后再建一个测试类，来测试以上代码！
@RunWith(SpringJUnit4ClassRunner.class)@ContextConfiguration(locations = {"classpath:applicationContext.xml"})public class UserServiceTest {@Resourceprivate UserService userService;@Testpublic void testAddUserForLoopDI() throws Exception {UserInfo userInfo = new UserInfo();userInfo.setId("12");userInfo.setAddress("sdfds");userService.addUser(userInfo);System.out.println("ok");}}
applicationContext.xml 中只要开启扫描即可！
<context:annotation-config/><context:component-scan base-package="com.xx"></context:component-scan>
如上例子运行，就可以得到一个循环依赖的失败错误了！
不过，你不一定能运行进来，因为如例子还要依赖于一个事实，那就 loginService 要在 userService 之前被扫描到，而不同的jvm上，可能spring得到的扫描顺序不一致，如果想要100%失败，则换成两个类都是构造器注入就可以了！
错误堆栈样例如下：（Is there an unresolvable circular reference?）
java.lang.IllegalStateException: Failed to load ApplicationContextat org.springframework.test.context.cache.DefaultCacheAwareContextLoaderDelegate.loadContext(DefaultCacheAwareContextLoaderDelegate.java:124)at org.springframework.test.context.support.DefaultTestContext.getApplicationContext(DefaultTestContext.java:83)at org.springframework.test.context.support.DependencyInjectionTestExecutionListener.injectDependencies(DependencyInjectionTestExecutionListener.java:117)at org.springframework.test.context.support.DependencyInjectionTestExecutionListener.prepareTestInstance(DependencyInjectionTestExecutionListener.java:83)at org.springframework.test.context.TestContextManager.prepareTestInstance(TestContextManager.java:230)at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.createTest(SpringJUnit4ClassRunner.java:228)at org.springframework.test.context.junit4.SpringJUnit4ClassRunner$1.runReflectiveCall(SpringJUnit4ClassRunner.java:287)at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.methodBlock(SpringJUnit4ClassRunner.java:289)at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:247)at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:94)at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)at org.springframework.test.context.junit4.statements.RunBeforeTestClassCallbacks.evaluate(RunBeforeTestClassCallbacks.java:61)at org.springframework.test.context.junit4.statements.RunAfterTestClassCallbacks.evaluate(RunAfterTestClassCallbacks.java:70)at org.junit.runners.ParentRunner.run(ParentRunner.java:363)at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.run(SpringJUnit4ClassRunner.java:191)at org.junit.runner.JUnitCore.run(JUnitCore.java:137)at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:47)at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'loginService' defined in class path resource [applicationContext.xml]: Cannot resolve reference to bean 'userService' while setting constructor argument; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'userService' defined in class path resource [applicationContext.xml]: Cannot resolve reference to bean 'loginService' while setting bean property 'loginService'; nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'loginService': Requested bean is currently in creation: Is there an unresolvable circular reference?at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:359)at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveValueIfNecessary(BeanDefinitionValueResolver.java:108)at org.springframework.beans.factory.support.ConstructorResolver.resolveConstructorArguments(ConstructorResolver.java:648)at org.springframework.beans.factory.support.ConstructorResolver.autowireConstructor(ConstructorResolver.java:145)at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.autowireConstructor(AbstractAutowireCapableBeanFactory.java:1198)at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1100)at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:511)at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:481)at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:312)at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230)at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:308)at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:197)at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:761)at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:867)at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:543)at org.springframework.test.context.support.AbstractGenericContextLoader.loadContext(AbstractGenericContextLoader.java:128)at org.springframework.test.context.support.AbstractGenericContextLoader.loadContext(AbstractGenericContextLoader.java:60)at org.springframework.test.context.support.AbstractDelegatingSmartContextLoader.delegateLoading(AbstractDelegatingSmartContextLoader.java:281)at org.springframework.test.context.support.AbstractDelegatingSmartContextLoader.loadContext(AbstractDelegatingSmartContextLoader.java:249)at org.springframework.test.context.cache.DefaultCacheAwareContextLoaderDelegate.loadContextInternal(DefaultCacheAwareContextLoaderDelegate.java:98)at org.springframework.test.context.cache.DefaultCacheAwareContextLoaderDelegate.loadContext(DefaultCacheAwareContextLoaderDelegate.java:116)... 24 moreCaused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'userService' defined in class path resource [applicationContext.xml]: Cannot resolve reference to bean 'loginService' while setting bean property 'loginService'; nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'loginService': Requested bean is currently in creation: Is there an unresolvable circular reference?at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:359)at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveValueIfNecessary(BeanDefinitionValueResolver.java:108)at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.applyPropertyValues(AbstractAutowireCapableBeanFactory.java:1534)at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1281)at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:551)at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:481)at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:312)at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230)at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:308)at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:197)at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:351)... 44 moreCaused by: org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'loginService': Requested bean is currently in creation: Is there an unresolvable circular reference?at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.beforeSingletonCreation(DefaultSingletonBeanRegistry.java:347)at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:223)at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:308)at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:197)at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:351)... 54 morejava.lang.IllegalStateException: Failed to load ApplicationContextat org.springframework.test.context.cache.DefaultCacheAwareContextLoaderDelegate.loadContext(DefaultCacheAwareContextLoaderDelegate.java:124)at org.springframework.test.context.support.DefaultTestContext.getApplicationContext(DefaultTestContext.java:83)at org.springframework.test.context.support.DependencyInjectionTestExecutionListener.injectDependencies(DependencyInjectionTestExecutionListener.java:117)at org.springframework.test.context.support.DependencyInjectionTestExecutionListener.prepareTestInstance(DependencyInjectionTestExecutionListener.java:83)at org.springframework.test.context.TestContextManager.prepareTestInstance(TestContextManager.java:230)at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.createTest(SpringJUnit4ClassRunner.java:228)at org.springframework.test.context.junit4.SpringJUnit4ClassRunner$1.runReflectiveCall(SpringJUnit4ClassRunner.java:287)at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.methodBlock(SpringJUnit4ClassRunner.java:289)at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:247)at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:94)at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)at org.springframework.test.context.junit4.statements.RunBeforeTestClassCallbacks.evaluate(RunBeforeTestClassCallbacks.java:61)at org.springframework.test.context.junit4.statements.RunAfterTestClassCallbacks.evaluate(RunAfterTestClassCallbacks.java:70)at org.junit.runners.ParentRunner.run(ParentRunner.java:363)at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.run(SpringJUnit4ClassRunner.java:191)at org.junit.runner.JUnitCore.run(JUnitCore.java:137)at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:47)at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'loginService' defined in class path resource [applicationContext.xml]: Cannot resolve reference to bean 'userService' while setting constructor argument; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'userService' defined in class path resource [applicationContext.xml]: Cannot resolve reference to bean 'loginService' while setting bean property 'loginService'; nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'loginService': Requested bean is currently in creation: Is there an unresolvable circular reference?at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:359)at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveValueIfNecessary(BeanDefinitionValueResolver.java:108)at org.springframework.beans.factory.support.ConstructorResolver.resolveConstructorArguments(ConstructorResolver.java:648)at org.springframework.beans.factory.support.ConstructorResolver.autowireConstructor(ConstructorResolver.java:145)at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.autowireConstructor(AbstractAutowireCapableBeanFactory.java:1198)at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1100)at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:511)at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:481)at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:312)at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230)at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:308)at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:197)at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:761)at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:867)at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:543)at org.springframework.test.context.support.AbstractGenericContextLoader.loadContext(AbstractGenericContextLoader.java:128)at org.springframework.test.context.support.AbstractGenericContextLoader.loadContext(AbstractGenericContextLoader.java:60)at org.springframework.test.context.support.AbstractDelegatingSmartContextLoader.delegateLoading(AbstractDelegatingSmartContextLoader.java:281)at org.springframework.test.context.support.AbstractDelegatingSmartContextLoader.loadContext(AbstractDelegatingSmartContextLoader.java:249)at org.springframework.test.context.cache.DefaultCacheAwareContextLoaderDelegate.loadContextInternal(DefaultCacheAwareContextLoaderDelegate.java:98)at org.springframework.test.context.cache.DefaultCacheAwareContextLoaderDelegate.loadContext(DefaultCacheAwareContextLoaderDelegate.java:116)... 24 moreCaused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'userService' defined in class path resource [applicationContext.xml]: Cannot resolve reference to bean 'loginService' while setting bean property 'loginService'; nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'loginService': Requested bean is currently in creation: Is there an unresolvable circular reference?at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:359)at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveValueIfNecessary(BeanDefinitionValueResolver.java:108)at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.applyPropertyValues(AbstractAutowireCapableBeanFactory.java:1534)at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1281)at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:551)at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:481)at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:312)at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230)at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:308)at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:197)at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:351)... 44 moreCaused by: org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'loginService': Requested bean is currently in creation: Is there an unresolvable circular reference?at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.beforeSingletonCreation(DefaultSingletonBeanRegistry.java:347)at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:223)at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:308)at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:197)at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:351)... 54 more
好了，看完效果，是时候来分析下问题了。
1. 为什么会出现问题？
因为加载流程如下：
1. 首先加载 loginService, 然后构造器被调用，然后发现依赖了 userService;
2. 依赖注入，先去加载 userService，加载userService后，需要对其属性进行依赖注入，然后发现了 loginService 需要被注入；
3. 当去加载 loginService 的时候，发现 loginService 正在创建中，所以这个实例只能认为创建失败了，否则将会导致更多未知问题；
4. 同理，多个构造器互相注入失败问题更严重；
2. 循环依赖失败是不是只要 ABA 就一定会导致失败？（注：非单例对象一定不会导致循环依赖）
1. 按正常说是这样的，但是spring已经解决这个问题了；
2. spring 解决方案为，只为单例提供解决方案也只能为单例解决问题；
3. 创建A单例时，放入缓存，然后依赖注入B；
4. 依赖注入B时，发现需要依赖注入A，然后去加载A，此时从缓存中发现A正在加载中，于是直接从缓存得到A，完成自身的依赖注入；
5. B依赖完成后，返回给A，A再把B注入到自身域中；
6. B中的A也自然而然的完成了初始化动作；
其中，构造器注入的单例的循环依赖是无法解决的，因为在构造器注入时，本身的实例无法生成；如果强行使用，将导致不安全的发布，从而导致各种未知的问题！
3. 源码如何实现？
1. 首先，我们来看下什么情况下会抛出循环依赖异常？
// 只有单例的创建会存在循环依赖问题// org.springframework.beans.factory.support.DefaultSingletonBeanRegistry/** Set of registered singletons, containing the bean names in registration order */private final Set<String> registeredSingletons = new LinkedHashSet<String>(256);/** Names of beans that are currently in creation */private final Set<String> singletonsCurrentlyInCreation =Collections.newSetFromMap(new ConcurrentHashMap<String, Boolean>(16));/** Names of beans currently excluded from in creation checks */private final Set<String> inCreationCheckExclusions =Collections.newSetFromMap(new ConcurrentHashMap<String, Boolean>(16));protected void beforeSingletonCreation(String beanName) {if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.add(beanName)) {throw new BeanCurrentlyInCreationException(beanName);}}
从上面的代码可以看出，抛出循环依赖问题会抛出有两种前提:
1. 没有包含在创建检查的排除列表中；
2. 没有被排除，则检查是不是第一次被调用创建，如果单例不是第一次被调用创建，则不能再创建了（否则就不是单例了）；
所以，避免循环依赖有个出口，那就是提前把单例放到检查排除列表中！
那么什么时候会被加入到创建检查排除列表？
public void setCurrentlyInCreation(String beanName, boolean inCreation) {Assert.notNull(beanName, "Bean name must not be null");if (!inCreation) {this.inCreationCheckExclusions.add(beanName);}else {this.inCreationCheckExclusions.remove(beanName);}}
具体如何设置呢？其实普通的使用不会用到这个功能，只会在一些增强点作这些工作。
所以，spring 如何避免循环依赖失败？
// org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean()/*** Actually create the specified bean. Pre-creation processing has already happened* at this point, e.g. checking {@code postProcessBeforeInstantiation} callbacks.* <p>Differentiates between default bean instantiation, use of a* factory method, and autowiring a constructor.* @param beanName the name of the bean* @param mbd the merged bean definition for the bean* @param args explicit arguments to use for constructor or factory method invocation* @return a new instance of the bean* @throws BeanCreationException if the bean could not be created* @see #instantiateBean* @see #instantiateUsingFactoryMethod* @see #autowireConstructor*/protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args)throws BeanCreationException {// 第一步，创建bean实例// Instantiate the bean.BeanWrapper instanceWrapper = null;if (mbd.isSingleton()) {instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);}if (instanceWrapper == null) {instanceWrapper = createBeanInstance(beanName, mbd, args);}final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);Class<?> beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);mbd.resolvedTargetType = beanType;// Allow post-processors to modify the merged bean definition.synchronized (mbd.postProcessingLock) {if (!mbd.postProcessed) {try {applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);}catch (Throwable ex) {throw new BeanCreationException(mbd.getResourceDescription(), beanName,"Post-processing of merged bean definition failed", ex);}mbd.postProcessed = true;}}// 对于提前暴露的bean, 将它加入单例缓存中，从而使后续的依赖调用时，可以将此实例返回// Eagerly cache singletons to be able to resolve circular references// even when triggered by lifecycle interfaces like BeanFactoryAware.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");}// 针对提前暴露的 singleton, 添加一个 获取实例 工厂，以备后续获取实例时使用addSingletonFactory(beanName, new ObjectFactory<Object>() {@Overridepublic Object getObject() throws BeansException {return getEarlyBeanReference(beanName, mbd, bean);}});}// 第二步，初始化实例，完成依赖注入等工作// Initialize the bean instance.Object exposedObject = bean;try {populateBean(beanName, mbd, instanceWrapper);if (exposedObject != null) {exposedObject = initializeBean(beanName, exposedObject, mbd);}}catch (Throwable ex) {if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {throw (BeanCreationException) ex;}else {throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);}}if (earlySingletonExposure) {Object earlySingletonReference = getSingleton(beanName, false);if (earlySingletonReference != null) {if (exposedObject == bean) {exposedObject = earlySingletonReference;}else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {String[] dependentBeans = getDependentBeans(beanName);Set<String> actualDependentBeans = new LinkedHashSet<String>(dependentBeans.length);for (String dependentBean : dependentBeans) {if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {actualDependentBeans.add(dependentBean);}}if (!actualDependentBeans.isEmpty()) {throw new BeanCurrentlyInCreationException(beanName,"Bean with name '" + beanName + "' has been injected into other beans [" +StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +"] in its raw version as part of a circular reference, but has eventually been " +"wrapped. This means that said other beans do not use the final version of the " +"bean. This is often the result of over-eager type matching - consider using " +"'getBeanNamesOfType' with the 'allowEagerInit' flag turned off, for example.");}}}}// 第三步，对外暴露bean，如 singleton 入 容器中注册实例// Register bean as disposable.try {registerDisposableBeanIfNecessary(beanName, bean, mbd);}catch (BeanDefinitionValidationException ex) {throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);}return exposedObject;}
而在获取 singleton 时，会先尝试从工厂中获取！
// org.springframework.beans.factory.support.DefaultSingletonBeanRegistryprotected Object getSingleton(String beanName, boolean allowEarlyReference) {Object singletonObject = this.singletonObjects.get(beanName);// 针对正在创建中的 singleton, 将其放入 earlySingletonObjectsif (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) {// 将 singleton 实例从工厂中取出后，存入 提前暴露的缓存中，交将创建工厂删除，避免后续反复从中获取 singletonsingletonObject = singletonFactory.getObject();this.earlySingletonObjects.put(beanName, singletonObject);this.singletonFactories.remove(beanName);}}}}return (singletonObject != NULL_OBJECT ? singletonObject : null);}// org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.getEarlyBeanReference()protected Object getEarlyBeanReference(String beanName, RootBeanDefinition mbd, Object bean) {Object exposedObject = bean;if (bean != null && !mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {for (BeanPostProcessor bp : getBeanPostProcessors()) {if (bp instanceof SmartInstantiationAwareBeanPostProcessor) {SmartInstantiationAwareBeanPostProcessor ibp = (SmartInstantiationAwareBeanPostProcessor) bp;exposedObject = ibp.getEarlyBeanReference(exposedObject, beanName);if (exposedObject == null) {return null;}}}}return exposedObject;}
在获取bean实例时，会先尝试从缓存中直接获取，失败再进行真实的创建：
// org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean()// 会先尝试从单例缓存中获取bean，获取不到再进行创建/*** Return an instance, which may be shared or independent, of the specified bean.* @param name the name of the bean to retrieve* @param requiredType the required type of the bean to retrieve* @param args arguments to use when creating a bean instance using explicit arguments* (only applied when creating a new instance as opposed to retrieving an existing one)* @param typeCheckOnly whether the instance is obtained for a type check,* not for actual use* @return an instance of the bean* @throws BeansException if the bean could not be created*/@SuppressWarnings("unchecked")protected <T> T doGetBean(final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)throws BeansException {final String beanName = transformedBeanName(name);Object bean;// 先从缓存获取，失败则进行创建// Eagerly check singleton cache for manually registered singletons.Object sharedInstance = getSingleton(beanName);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 + "'");}}bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);}else {// Fail if we're already creating this bean instance:// We're assumably within a circular reference.if (isPrototypeCurrentlyInCreation(beanName)) {throw new BeanCurrentlyInCreationException(beanName);}// Check if bean definition exists in this factory.BeanFactory parentBeanFactory = getParentBeanFactory();if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {// Not found -> check parent.String nameToLookup = originalBeanName(name);if (args != null) {// Delegation to parent with explicit args.return (T) parentBeanFactory.getBean(nameToLookup, args);}else {// No args -> delegate to standard getBean method.return parentBeanFactory.getBean(nameToLookup, requiredType);}}if (!typeCheckOnly) {markBeanAsCreated(beanName);}try {final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);checkMergedBeanDefinition(mbd, beanName, args);// Guarantee initialization of beans that the current bean depends on.String[] dependsOn = mbd.getDependsOn();if (dependsOn != null) {for (String dep : dependsOn) {if (isDependent(beanName, dep)) {throw new BeanCreationException(mbd.getResourceDescription(), beanName,"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");}registerDependentBean(dep, beanName);getBean(dep);}}// Create bean instance.if (mbd.isSingleton()) {// 此时根据对象工厂，创建bean，创建时检测依赖情况sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {@Overridepublic Object getObject() throws BeansException {try {return createBean(beanName, mbd, args);}catch (BeansException ex) {// Explicitly remove instance from singleton cache: It might have been put there// eagerly by the creation process, to allow for circular reference resolution.// Also remove any beans that received a temporary reference to the bean.destroySingleton(beanName);throw ex;}}});bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);}else if (mbd.isPrototype()) {// It's a prototype -> create a new instance.Object prototypeInstance = null;try {beforePrototypeCreation(beanName);prototypeInstance = createBean(beanName, mbd, args);}finally {afterPrototypeCreation(beanName);}bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);}else {String scopeName = mbd.getScope();final Scope scope = this.scopes.get(scopeName);if (scope == null) {throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");}try {Object scopedInstance = scope.get(beanName, new ObjectFactory<Object>() {@Overridepublic Object getObject() throws BeansException {beforePrototypeCreation(beanName);try {return createBean(beanName, mbd, args);}finally {afterPrototypeCreation(beanName);}}});bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);}catch (IllegalStateException ex) {throw new BeanCreationException(beanName,"Scope '" + scopeName + "' is not active for the current thread; consider " +"defining a scoped proxy for this bean if you intend to refer to it from a singleton",ex);}}}catch (BeansException ex) {cleanupAfterBeanCreationFailure(beanName);throw ex;}}// Check if required type matches the type of the actual bean instance.if (requiredType != null && bean != null && !requiredType.isAssignableFrom(bean.getClass())) {try {return getTypeConverter().convertIfNecessary(bean, requiredType);}catch (TypeMismatchException ex) {if (logger.isDebugEnabled()) {logger.debug("Failed to convert bean '" + name + "' to required type '" +ClassUtils.getQualifiedName(requiredType) + "'", ex);}throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());}}return (T) bean;}// org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton()// 进行 singleton 创建时，做安全检查，成功后放入缓存中/*** Return the (raw) singleton object registered under the given name,* creating and registering a new one if none registered yet.* @param beanName the name of the bean* @param singletonFactory the ObjectFactory to lazily create the singleton* with, if necessary* @return the registered singleton object*/public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {Assert.notNull(beanName, "'beanName' must not be null");synchronized (this.singletonObjects) {Object singletonObject = this.singletonObjects.get(beanName);if (singletonObject == null) {if (this.singletonsCurrentlyInDestruction) {throw new BeanCreationNotAllowedException(beanName,"Singleton bean creation not allowed while singletons of this factory are in destruction " +"(Do not request a bean from a BeanFactory in a destroy method implementation!)");}if (logger.isDebugEnabled()) {logger.debug("Creating shared instance of singleton bean '" + beanName + "'");}// 创建前检查，循环依赖失败在此时检测到，即单例只能被创建一次beforeSingletonCreation(beanName);boolean newSingleton = false;boolean recordSuppressedExceptions = (this.suppressedExceptions == null);if (recordSuppressedExceptions) {this.suppressedExceptions = new LinkedHashSet<Exception>();}try {// createBean()singletonObject = singletonFactory.getObject();newSingleton = true;}catch (IllegalStateException ex) {// Has the singleton object implicitly appeared in the meantime ->// if yes, proceed with it since the exception indicates that state.singletonObject = this.singletonObjects.get(beanName);if (singletonObject == null) {throw ex;}}catch (BeanCreationException ex) {if (recordSuppressedExceptions) {for (Exception suppressedException : this.suppressedExceptions) {ex.addRelatedCause(suppressedException);}}throw ex;}finally {if (recordSuppressedExceptions) {this.suppressedExceptions = null;}// 最后，清除正在创建的标识afterSingletonCreation(beanName);}// 单例创建成功，则将其加入到缓存中，并清理之前的标记if (newSingleton) {addSingleton(beanName, singletonObject);}}return (singletonObject != NULL_OBJECT ? singletonObject : null);}}/*** Add the given singleton object to the singleton cache of this factory.* <p>To be called for eager registration of singletons.* @param beanName the name of the bean* @param singletonObject the singleton object*/protected void addSingleton(String beanName, Object singletonObject) {synchronized (this.singletonObjects) {this.singletonObjects.put(beanName, (singletonObject != null ? singletonObject : NULL_OBJECT));this.singletonFactories.remove(beanName);this.earlySingletonObjects.remove(beanName);this.registeredSingletons.add(beanName);}}
如上代码，说明了bean的创建过程，我们主要看 singleton。
1. 尝试从缓存中获取bean，可能是提前暴露的，也可能是已经完全初始化好的；（提前暴露解决循环依赖）
2. 如果获取不到，则进行一次完整的初始化，通过 对象工厂的 getObject() 进行对象创建；
3. 创建前，先将正在创建中标识写入，从而防止后续又有进行创建动作；（抛出创建异常）
4. 创建好 singleton 后，将多余标识删除，并将 singleton 放入缓存，以待下次直接使用；
最后，我们用一个示意图描述下整个过程：
思考题：
依赖注入使代码更简单和灵活，可以自动查找关联关系！当发现有依赖时，相当于递归生成里层bean实例！而每个bean的创建都经过n层方法的调用；
那么请问，在创建bean时有没有可能导致栈溢出？？
腾讯、阿里、滴滴后台面试题汇总总结 — （含答案）
面试：史上最全多线程面试题 ！
最新阿里内推Java后端面试题
JVM难学？那是因为你没认真看完这篇文章
关注作者微信公众号 —《JAVA烂猪皮》
了解更多java后端架构知识以及最新面试宝典
看完本文记得给作者点赞+在看哦~~~大家的支持，是作者源源不断出文的动力
作者：等你归去来
出处：https://www.cnblogs.com/yougewe/p/10146537.html
评论