• Spring循环依赖

    目录

    一、Spring循环依赖

    1、什么是循环依赖？

    构造器方式注入依赖

    setter方式注入依赖

    Spring容器

    2、Spring循环依赖源码

    一、Spring循环依赖

    1. 面试题：
    2. 1、你解释下spring中的三级缓存？
    3.  
    4. 2、三级缓存分别是什么？三个Map有什么异同？
    5.  
    6. 3、什么是循环依赖？请你谈谈？看过spring源码吗？一般我们说的spring容器是什么？
    7.  
    8. 4、如何检测是否存在循环依赖？实际开发中见过循环依赖的异常吗？
    9.  
    10. 5、多例的情况下，循环依赖问题为什么无法解决？
    11.  
    12. 6、为什么一定要用3级缓存？用2级可否？1级可否？
    13. 是否可以关闭循环依赖？

    1、什么是循环依赖？

            1、多个bean之间相互依赖，形成了一个闭环。

            比如：A依赖于B、B依赖于C、C依赖于A

    1. public class T1 {
    2.  
    3. 	class A {
    4. 		B b;
    5. 	}
    6.  
    7. 	class B {
    8. 		C c;
    9. 	}
    10.  
    11. 	class C {
    12. 		A a;
    13. 	}
    14. }

            2、通常来说，如果问spring容器内部如何解决循环依赖，一定是指默认的单例Bean中，属性互相引用的场景。

    1. @Component
    2. public class A {
    3. 	@Autowired
    4. 	private B b;
    5. }
    6.  
    7. @Component
    8. public class B {
    9. 	@Autowired
    10. 	private C c;
    11. }
    12.  
    13. @Component
    14. public class C {
    15. 	@Autowired
    16. 	private A a;
    17. }

    spring官方文档

    1. Circular dependencies
    2. If you use predominantly constructor injection, it is possible to create an 
    3. unresolvable circular dependency scenario.
    4.  
    5. For example: Class A requires an instance of class B through constructor injection, 
    6. and class B requires an instance of class A through constructor injection. If you
    7.  configure beans for classes A and B to be injected into each other, the Spring IoC
    8.  container detects this circular reference at runtime, and throws a 
    9. BeanCurrentlyInCreationException.
    10.  
    11. One possible solution is to edit the source code of some classes to be configured by
    12.  setters rather than constructors. Alternatively, avoid constructor injection and use
    13.  setter injection only. In other words, although it is not recommended, you can 
    14. configure circular dependencies with setter injection.
    15.  
    16. Unlike the typical case (with no circular dependencies), a circular dependency between
    17.  bean A and bean B forces one of the beans to be injected into the other prior to 
    18. being fully initialized itself (a classic chicken-and-egg scenario).
    19.  
    20. ==================================
    21. 循环依赖项
    22. 如果主要使用构造函数注入，则有可能创建一个无法解决的循环依赖场景。
    23.  
    24. 例如：类A通过构造函数注入需要类B的实例，而类B通过构造函数注入要求类A的实例。如果为类A
    25. 和类B配置bean以相互注入，那么Spring IoC容器会在运行时检测到这个循环引用，并抛出
    26. BeanCurrentlyInCreationException。
    27.  
    28. 一种可能的解决方案是编辑一些类的源代码，由setter而不是构造函数进行配置。或者，避免构造函数
    29. 注入，只使用setter注入。换句话说，尽管不建议这样做，但您可以使用setter注入来配置循环依赖关系。
    30.  
    31. 与典型的情况（没有循环依赖关系）不同，bean a和bean B之间的循环依赖关系迫使其中一个bean在
    32. 完全初始化之前注入另一个bean（典型的先有鸡后有蛋的场景）。
    33.  
    34. ==================================
    35. 结论：AB循环依赖问题，只要A的注入方式是setter且singleton,就不会有循环依赖问题

    spring容器循环依赖报错演示BeanCurrentlyInCreationException

    构造器方式注入依赖

    ServiceA 

    1. import org.springframework.stereotype.Component;
    2.  
    3. @Component
    4. public class ServiceA {
    5.  
    6. 	private ServiceB serviceB;
    7.  
    8. 	public ServiceA(ServiceB serviceB) {
    9. 		this.serviceB = serviceB;
    10. 	}
    11.  
    12. }

    ServiceB 

    1. import org.springframework.stereotype.Component;
    2.  
    3. @Component
    4. public class ServiceB {
    5.  
    6. 	private ServiceA serviceA;
    7.  
    8. 	public ServiceB(ServiceA serviceA) {
    9. 		this.serviceA = serviceA;
    10. 	}
    11.  
    12. }

    Test

    1. public class Test {
    2.  
    3. 	public static void main(String[] args) {
    4. 		new ServiceA(new ServiceB(new ServiceA(....)));
    5. 	}
    6.  
    7. }

    结论：构造器循环依赖是无法解决的，你想让构造器注入支持循环依赖，是不存在的

    setter方式注入依赖

    ServiceAA 

    1. import org.springframework.stereotype.Component;
    2.  
    3. @Component
    4. public class ServiceAA {
    5.  
    6. 	private ServiceBB serviceBB;
    7.  
    8. 	public void setServiceBB(ServiceBB serviceBB) {
    9. 		this.serviceBB = serviceBB;
    10. 		System.out.println("A里面设置B");
    11. 	}
    12.  
    13. }

    ServiceBB 

    1. import org.springframework.stereotype.Component;
    2.  
    3. @Component
    4. public class ServiceBB {
    5.  
    6. 	private ServiceAA serviceAA;
    7.  
    8. 	public void setServiceAA(ServiceAA serviceAA) {
    9. 		this.serviceAA = serviceAA;
    10. 		System.out.println("B里面设置A");
    11. 	}
    12.  
    13. }

    Test

    1. public class Test {
    2.  
    3. 	public static void main(String[] args) {
    4. 		ServiceAA aa = new ServiceAA();
    5. 		ServiceBB bb = new ServiceBB();
    6. 		bb.setServiceAA(aa);
    7. 		aa.setServiceBB(bb);
    8. 	}
    9. }

    结论：setter方式可以解决循环依赖

    Spring容器

    默认的单例(singleton)的场景是支持循环依赖的，不报错

    pom

    1. "1.0" encoding="UTF-8"?>
    2. <project xmlns="http://maven.apache.org/POM/4.0.0"
    3.          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    4.          xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    5.     <modelVersion>4.0.0modelVersion>
    6.  
    7.     <groupId>com.lwzgroupId>
    8.     <artifactId>springboot_aopartifactId>
    9.     <version>1.0-SNAPSHOTversion>
    10.  
    11.     <properties>
    12.     	<java.version>1.8java.version>
    13.     properties>
    14.  
    15.    <dependencies>
    16.        <dependency>
    17.             <groupId>org.springframeworkgroupId>
    18.             <artifactId>spring-contextartifactId>
    19.             <version>5.1.6.RELEASEversion>
    20.         dependency>
    21.    dependencies>
    22.     <build>
    23.         <plugins>
    24.             <plugin>
    25.                 <groupId>org.springframework.bootgroupId>
    26.                 <artifactId>spring-boot-maven-pluginartifactId>
    27.             plugin>
    28.         plugins>
    29.     build>
    30. project>

    src\main\resources\applicationContext.xml

    1. "1.0" encoding="UTF-8"?>
    2. <beans xmlns="http://www.springframework.org/schema/beans"
    3.        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    4.        xmlns:p="http://www.springframework.org/schema/p"
    5.        xmlns:aop="http://www.springframework.org/schema/aop"
    6.        xmlns:tx="http://www.springframework.org/schema/tx"
    7.        xmlns:context="http://www.springframework.org/schema/context"
    8.        xsi:schemaLocation="http://www.springframework.org/schema/beans
    9.         http://www.springframework.org/schema/beans/spring-beans.xsd
    10.         http://www.springframework.org/schema/context
    11.         http://www.springframework.org/schema/context/spring-context.xsd
    12.         http://www.springframework.org/schema/aop
    13.         http://www.springframework.org/schema/aop/spring-aop.xsd
    14.         http://www.springframework.org/schema/tx
    15.         http://www.springframework.org/schema/tx/spring-tx.xsd">
    16.  
    17.     <bean id="a" class="com.lwz.part3.A" scope="singleton">
    18.     	<property name="B" ref="b"/>
    19.     bean>
    20.     <bean id="b" class="com.lwz.part3.B" scope="singleton">
    21.     	<property name="a" ref="a"/>
    22.     bean>
    23.  
    24. beans>

    A

    1. package com.lwz.part3;
    2.  
    3. public class A {
    4.  
    5. 	private B b;
    6.  
    7. 	public A() {
    8. 		System.out.println("A...created success");
    9. 	}
    10.  
    11. 	public B getB() {
    12. 		return b;
    13. 	}
    14.  
    15. 	public void setB(B b) {
    16. 		this.b = b;
    17. 	}
    18.  
    19. }

    B

    1. package com.lwz.part3;
    2.  
    3. public class B {
    4.  
    5. 	private A a;
    6.  
    7. 	public B() {
    8. 		System.out.println("B...created success");
    9. 	}
    10.  
    11. 	public A getA() {
    12. 		return a;
    13. 	}
    14.  
    15. 	public void setA(A a) {
    16. 		this.a = a;
    17. 	}
    18.  
    19. }

    TestSpringContainer 

    1. package com.lwz.part3;
    2.  
    3. import org.springframework.context.ApplicationContext;
    4. import org.springframework.context.support.ClassPathXmlApplicationContext;
    5.  
    6. public class TestSpringContainer {
    7.  
    8. 	public static void main(String[] args) {
    9. 		ApplicationContext context = new ClassPathXmlApplicationContext("applicationContext.xml");
    10. 		A a = context.getBean("a", A.class);
    11. 		B b = context.getBean("b", B.class);
    12. 	}
    13. }

    运行结果

    1. A...created success
    2. B...created success

    原型(prototype)的场景是不支持循环依赖的，会报错

    src\main\resources\applicationContext.xml   修改为prototype

    1. "1.0" encoding="UTF-8"?>
    2. <beans xmlns="http://www.springframework.org/schema/beans"
    3.        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    4.        xmlns:p="http://www.springframework.org/schema/p"
    5.        xmlns:aop="http://www.springframework.org/schema/aop"
    6.        xmlns:tx="http://www.springframework.org/schema/tx"
    7.        xmlns:context="http://www.springframework.org/schema/context"
    8.        xsi:schemaLocation="http://www.springframework.org/schema/beans
    9.         http://www.springframework.org/schema/beans/spring-beans.xsd
    10.         http://www.springframework.org/schema/context
    11.         http://www.springframework.org/schema/context/spring-context.xsd
    12.         http://www.springframework.org/schema/aop
    13.         http://www.springframework.org/schema/aop/spring-aop.xsd
    14.         http://www.springframework.org/schema/tx
    15.         http://www.springframework.org/schema/tx/spring-tx.xsd">
    16.  
    17.     <bean id="a" class="com.lwz.part3.A" scope="prototype">
    18.     	<property name="B" ref="b"/>
    19.     bean>
    20.     <bean id="b" class="com.lwz.part3.B" scope="prototype">
    21.     	<property name="a" ref="a"/>
    22.     bean>
    23.  
    24. beans>

    测试类

    1. package com.lwz.part3;
    2.  
    3. import org.springframework.context.ApplicationContext;
    4. import org.springframework.context.support.ClassPathXmlApplicationContext;
    5.  
    6. public class TestSpringContainer {
    7.  
    8. 	public static void main(String[] args) {
    9. 		ApplicationContext context = new ClassPathXmlApplicationContext("applicationContext.xml");
    10. 		A a = context.getBean("a", A.class);
    11. 		B b = context.getBean("b", B.class);
    12. 	}
    13. }

    运行结果

    1. A...created success
    2. B...created success
    3. Exception in thread "main" org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'a' defined in class path resource [applicationContext.xml]: Cannot resolve reference to bean 'b' while setting bean property 'B'; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'b' defined in class path resource [applicationContext.xml]: Cannot resolve reference to bean 'a' while setting bean property 'a'; nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'a': Requested bean is currently in creation: Is there an unresolvable circular reference?
    4. 	at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:378)
    5. 	at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveValueIfNecessary(BeanDefinitionValueResolver.java:110)
    6. 	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.applyPropertyValues(AbstractAutowireCapableBeanFactory.java:1681)
    7. 	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1433)
    8. 	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:592)
    9. 	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:515)
    10. 	at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:338)
    11. 	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:204)
    12. 	at org.springframework.context.support.AbstractApplicationContext.getBean(AbstractApplicationContext.java:1111)
    13. 	at com.lwz.part3.TestSpringContainer.main(TestSpringContainer.java:10)
    14. Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'b' defined in class path resource [applicationContext.xml]: Cannot resolve reference to bean 'a' while setting bean property 'a'; nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'a': Requested bean is currently in creation: Is there an unresolvable circular reference?
    15. 	at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:378)
    16. 	at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveValueIfNecessary(BeanDefinitionValueResolver.java:110)
    17. 	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.applyPropertyValues(AbstractAutowireCapableBeanFactory.java:1681)
    18. 	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1433)
    19. 	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:592)
    20. 	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:515)
    21. 	at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:338)
    22. 	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:199)
    23. 	at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:367)
    24. 	... 9 more
    25. Caused by: org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'a': Requested bean is currently in creation: Is there an unresolvable circular reference?
    26. 	at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:264)
    27. 	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:199)
    28. 	at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:367)
    29. 	... 17 more

    报org.springframework.beans.factory.BeanCurrentlyInCreationException异常

    结论：Spring内部通过3级缓存来解决循环依赖：spring-beans-5.1.6.RELEASE.jar包下org.springframework.beans.factory.support.DefaultSingletonBeanRegistry

    1. package org.springframework.beans.factory.support;
    2. ...
    3. public class DefaultSingletonBeanRegistry extends SimpleAliasRegistry implements SingletonBeanRegistry {
    4.  
    5.     //一级缓存
    6. 	/** Cache of singleton objects: bean name to bean instance. */
    7. 	private final Map singletonObjects = new ConcurrentHashMap<>(256);
    8.     //三级缓存
    9. 	/** Cache of singleton factories: bean name to ObjectFactory. */
    10. 	private final Map> singletonFactories = new HashMap<>(16);
    11.     //二级缓存
    12. 	/** Cache of early singleton objects: bean name to bean instance. */
    13. 	private final Map earlySingletonObjects = new HashMap<>(16);

    第一级缓存：（也叫单例池）singletonObjects ：存放已经经历了完整生命周期的Bean对象

    第二级缓存：earlySingletonObjects ，存放早期暴漏出来的Bean对象，Bean的生命周期未结束（属性还未填充完）

    第三级缓存：Map> singletonFactories，存放可以生成Bean的工厂

    结论：只有单例的bean会通过三级缓存提前暴漏来解决循环依赖的问题，而非单例的bean，每次从容器中获取都是一个新的对象，都会重新创建，所以非单例的bean是没有缓存的，不会将其放到三级缓存中。

    前置知识

    实例化：内存中申请一块内存空间

    初始化：属性填充，完成属性的各种赋值

    三个Map和四个方法：三个Map看上面，四个方法如下

    1. getSingleton() ：获取Bean
    2.  
    3. doCreateBean()：创建Bean
    4.  
    5. populateBean()：属性填充
    6.  
    7. addSingleton()：添加Bean

    第一层singletonObjects存放的是已经初始化好了的Bean。

    第二层earlySingletonObjects存放的是实例化了，但是未初始化的Bean。

    第三层singletonFactories存放的是FactoryBean。假如A类实现了FactoryBean，那么依赖注入的时候不是A类，而是A类产生的Bean。

    A/B两对象在三级缓存中的迁移说明

    1、A创建过程中需要B，于是A将自己放到三级缓存里面，去实例化B

    2、B实例化的时候发现需要A，于是B先查一级缓存，没有，再查二级缓存，还是没有，再查三级缓存，找到了A，然后把三级缓存里面的这个A放到二级缓存里面，并删除三级缓存里面的A。

    3、B顺利初始化完毕，将自己放到一级缓存里面（此时B里面的A依然是创建中状态），然后回来接着创建A，此时B已经创建结束，直接从一级缓存里面拿到了B，然后完成创建，并将A自己放到一级缓存里面。

    2、Spring循环依赖源码

    src\main\resources\applicationContext.xml   修改为singleton

    1. "1.0" encoding="UTF-8"?>
    2. <beans xmlns="http://www.springframework.org/schema/beans"
    3.        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    4.        xmlns:p="http://www.springframework.org/schema/p"
    5.        xmlns:aop="http://www.springframework.org/schema/aop"
    6.        xmlns:tx="http://www.springframework.org/schema/tx"
    7.        xmlns:context="http://www.springframework.org/schema/context"
    8.        xsi:schemaLocation="http://www.springframework.org/schema/beans
    9.         http://www.springframework.org/schema/beans/spring-beans.xsd
    10.         http://www.springframework.org/schema/context
    11.         http://www.springframework.org/schema/context/spring-context.xsd
    12.         http://www.springframework.org/schema/aop
    13.         http://www.springframework.org/schema/aop/spring-aop.xsd
    14.         http://www.springframework.org/schema/tx
    15.         http://www.springframework.org/schema/tx/spring-tx.xsd">
    16.  
    17.     <bean id="a" class="com.lwz.part3.A" scope="singleton">
    18.     	<property name="B" ref="b"/>
    19.     bean>
    20.     <bean id="b" class="com.lwz.part3.B" scope="singleton">
    21.     	<property name="a" ref="a"/>
    22.     bean>
    23.  
    24. beans>

    测试类第一行进行打断点

    1. package com.lwz.part3;
    2.  
    3. import org.springframework.context.ApplicationContext;
    4. import org.springframework.context.support.ClassPathXmlApplicationContext;
    5.  
    6. public class TestSpringContainer {
    7.  
    8. 	public static void main(String[] args) {
    9. 		ApplicationContext context = new ClassPathXmlApplicationContext("applicationContext.xml");
    10. 		A a = context.getBean("a", A.class);
    11. 		B b = context.getBean("b", B.class);
    12. 	}
    13. }

    打断点的位置//F5

    1. package org.springframework.context.support;
    2. ...
    3. public class ClassPathXmlApplicationContext extends AbstractXmlApplicationContext {
    4. ...
    5.  
    6. 	public ClassPathXmlApplicationContext(
    7. 			String[] configLocations, boolean refresh, @Nullable ApplicationContext parent)
    8. 			throws BeansException {
    9.  
    10. 		super(parent);
    11. 		setConfigLocations(configLocations);
    12. 		if (refresh) {
    13. 			refresh(); //F5
    14. 		}
    15. 	}

    1. package org.springframework.context.support;
    2. ...
    3. public abstract class AbstractApplicationContext extends DefaultResourceLoader
    4. 		implements ConfigurableApplicationContext {
    5. ...
    6. 	@Override
    7. 	public void refresh() throws BeansException, IllegalStateException {
    8. 		synchronized (this.startupShutdownMonitor) {
    9. 			// Prepare this context for refreshing.
    10. 			prepareRefresh();
    11.  
    12. 			// Tell the subclass to refresh the internal bean factory.
    13. 			ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
    14.  
    15. 			// Prepare the bean factory for use in this context.
    16. 			prepareBeanFactory(beanFactory);
    17.  
    18. 			try {
    19. 				// Allows post-processing of the bean factory in context subclasses.
    20. 				postProcessBeanFactory(beanFactory);
    21.  
    22. 				// Invoke factory processors registered as beans in the context.
    23. 				invokeBeanFactoryPostProcessors(beanFactory);
    24.  
    25. 				// Register bean processors that intercept bean creation.
    26. 				registerBeanPostProcessors(beanFactory);
    27.  
    28. 				// Initialize message source for this context.
    29. 				initMessageSource();
    30.  
    31. 				// Initialize event multicaster for this context.
    32. 				initApplicationEventMulticaster();
    33.  
    34. 				// Initialize other special beans in specific context subclasses.
    35. 				onRefresh();
    36.  
    37. 				// Check for listener beans and register them.
    38. 				registerListeners();
    39.  
    40. 				// Instantiate all remaining (non-lazy-init) singletons.
    41. 				finishBeanFactoryInitialization(beanFactory);  //F5
    42.  
    43. 				// Last step: publish corresponding event.
    44. 				finishRefresh();
    45. 			}
    46.  
    47. 			catch (BeansException ex) {
    48. 				if (logger.isWarnEnabled()) {
    49. 					logger.warn("Exception encountered during context initialization - " +
    50. 							"cancelling refresh attempt: " + ex);
    51. 				}
    52.  
    53. 				// Destroy already created singletons to avoid dangling resources.
    54. 				destroyBeans();
    55.  
    56. 				// Reset 'active' flag.
    57. 				cancelRefresh(ex);
    58.  
    59. 				// Propagate exception to caller.
    60. 				throw ex;
    61. 			}
    62.  
    63. 			finally {
    64. 				// Reset common introspection caches in Spring's core, since we
    65. 				// might not ever need metadata for singleton beans anymore...
    66. 				resetCommonCaches();
    67. 			}
    68. 		}
    69. 	}

    1. 	protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
    2. 		// Initialize conversion service for this context.
    3. 		if (beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME) &&
    4. 				beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)) {
    5. 			beanFactory.setConversionService(
    6. 					beanFactory.getBean(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class));
    7. 		}
    8.  
    9. 		// Register a default embedded value resolver if no bean post-processor
    10. 		// (such as a PropertyPlaceholderConfigurer bean) registered any before:
    11. 		// at this point, primarily for resolution in annotation attribute values.
    12. 		if (!beanFactory.hasEmbeddedValueResolver()) {
    13. 			beanFactory.addEmbeddedValueResolver(strVal -> getEnvironment().resolvePlaceholders(strVal));
    14. 		}
    15.  
    16. 		// Initialize LoadTimeWeaverAware beans early to allow for registering their transformers early.
    17. 		String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
    18. 		for (String weaverAwareName : weaverAwareNames) {
    19. 			getBean(weaverAwareName);
    20. 		}
    21.  
    22. 		// Stop using the temporary ClassLoader for type matching.
    23. 		beanFactory.setTempClassLoader(null);
    24.  
    25. 		// Allow for caching all bean definition metadata, not expecting further changes.
    26. 		beanFactory.freezeConfiguration();
    27.  
    28. 		// Instantiate all remaining (non-lazy-init) singletons.
    29. 		beanFactory.preInstantiateSingletons();  //F5 实例化所有剩余的（非惰性init）singleton。
    30. 	}

    1. 	@Override
    2. 	public void preInstantiateSingletons() throws BeansException {
    3. 		if (logger.isTraceEnabled()) {
    4. 			logger.trace("Pre-instantiating singletons in " + this);
    5. 		}
    6.  
    7. 		// Iterate over a copy to allow for init methods which in turn register new bean definitions.
    8. 		// While this may not be part of the regular factory bootstrap, it does otherwise work fine.
    9. 		List beanNames = new ArrayList<>(this.beanDefinitionNames);
    10.  
    11. 		// Trigger initialization of all non-lazy singleton beans...
    12. 		for (String beanName : beanNames) {
    13. 			RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
    14. 			if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
    15. 				if (isFactoryBean(beanName)) {
    16. 					Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
    17. 					if (bean instanceof FactoryBean) {
    18. 						final FactoryBean factory = (FactoryBean) bean;
    19. 						boolean isEagerInit;
    20. 						if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
    21. 							isEagerInit = AccessController.doPrivileged((PrivilegedAction)
    22. 											((SmartFactoryBean) factory)::isEagerInit,
    23. 									getAccessControlContext());
    24. 						}
    25. 						else {
    26. 							isEagerInit = (factory instanceof SmartFactoryBean &&
    27. 									((SmartFactoryBean) factory).isEagerInit());
    28. 						}
    29. 						if (isEagerInit) {
    30. 							getBean(beanName);
    31. 						}
    32. 					}
    33. 				}
    34. 				else {
    35. 					getBean(beanName);//F5
    36. 				}
    37. 			}
    38. 		}
    39.  
    40. 		// Trigger post-initialization callback for all applicable beans...
    41. 		for (String beanName : beanNames) {
    42. 			Object singletonInstance = getSingleton(beanName);
    43. 			if (singletonInstance instanceof SmartInitializingSingleton) {
    44. 				final SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
    45. 				if (System.getSecurityManager() != null) {
    46. 					AccessController.doPrivileged((PrivilegedAction) () -> {
    47. 						smartSingleton.afterSingletonsInstantiated();
    48. 						return null;
    49. 					}, getAccessControlContext());
    50. 				}
    51. 				else {
    52. 					smartSingleton.afterSingletonsInstantiated();
    53. 				}
    54. 			}
    55. 		}
    56. 	}
      57.  

      Spring容器以do开头的方法是真正写业务逻辑的方法。
       

      1. 	@Override
      2. 	public Object getBean(String name) throws BeansException {
      3. 		return doGetBean(name, null, null, false);
      4. 	}
      5.  
      6. 	protected  T doGetBean(final String name, @Nullable final Class requiredType,
      7. 			@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
      8.  
      9. 		final String beanName = transformedBeanName(name);
      10. 		Object bean;
      11.  
      12. 		// Eagerly check singleton cache for manually registered singletons.
      13. 		Object sharedInstance = getSingleton(beanName);//F5 获取Bean
      14. 		if (sharedInstance != null && args == null) {
      15. 			if (logger.isTraceEnabled()) {
      16. 				if (isSingletonCurrentlyInCreation(beanName)) {
      17. 					logger.trace("Returning eagerly cached instance of singleton bean '" + beanName +
      18. 							"' that is not fully initialized yet - a consequence of a circular reference");
      19. 				}
      20. 				else {
      21. 					logger.trace("Returning cached instance of singleton bean '" + beanName + "'");
      22. 				}
      23. 			}
      24. 			bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
      25. 		}
      26.  
      27. 		else {
      28. 			// Fail if we're already creating this bean instance:
      29. 			// We're assumably within a circular reference.
      30. 			if (isPrototypeCurrentlyInCreation(beanName)) {
      31. 				throw new BeanCurrentlyInCreationException(beanName);
      32. 			}
      33.  
      34. 			// Check if bean definition exists in this factory.
      35. 			BeanFactory parentBeanFactory = getParentBeanFactory();
      36. 			if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
      37. 				// Not found -> check parent.
      38. 				String nameToLookup = originalBeanName(name);
      39. 				if (parentBeanFactory instanceof AbstractBeanFactory) {
      40. 					return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
      41. 							nameToLookup, requiredType, args, typeCheckOnly);
      42. 				}
      43. 				else if (args != null) {
      44. 					// Delegation to parent with explicit args.
      45. 					return (T) parentBeanFactory.getBean(nameToLookup, args);
      46. 				}
      47. 				else if (requiredType != null) {
      48. 					// No args -> delegate to standard getBean method.
      49. 					return parentBeanFactory.getBean(nameToLookup, requiredType);
      50. 				}
      51. 				else {
      52. 					return (T) parentBeanFactory.getBean(nameToLookup);
      53. 				}
      54. 			}
      55.  
      56. 			if (!typeCheckOnly) {
      57. 				markBeanAsCreated(beanName);
      58. 			}
      59.  
      60. 			try {
      61. 				final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
      62. 				checkMergedBeanDefinition(mbd, beanName, args);
      63.  
      64. 				// Guarantee initialization of beans that the current bean depends on.
      65. 				String[] dependsOn = mbd.getDependsOn();
      66. 				if (dependsOn != null) {
      67. 					for (String dep : dependsOn) {
      68. 						if (isDependent(beanName, dep)) {
      69. 							throw new BeanCreationException(mbd.getResourceDescription(), beanName,
      70. 									"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
      71. 						}
      72. 						registerDependentBean(dep, beanName);
      73. 						try {
      74. 							getBean(dep);
      75. 						}
      76. 						catch (NoSuchBeanDefinitionException ex) {
      77. 							throw new BeanCreationException(mbd.getResourceDescription(), beanName,
      78. 									"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
      79. 						}
      80. 					}
      81. 				}
      82.  
      83. 				// Create bean instance. 创建Bean实例
      84. 				if (mbd.isSingleton()) {//F5
      85. 					sharedInstance = getSingleton(beanName, () -> {
      86. 						try {
      87. 							return createBean(beanName, mbd, args);
      88. 						}
      89. 						catch (BeansException ex) {
      90. 							// Explicitly remove instance from singleton cache: It might have been put there
      91. 							// eagerly by the creation process, to allow for circular reference resolution.
      92. 							// Also remove any beans that received a temporary reference to the bean.
      93. 							destroySingleton(beanName);
      94. 							throw ex;
      95. 						}
      96. 					});
      97. 					bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
      98. 				}
      99.  
      100. 				else if (mbd.isPrototype()) {
      101. 					// It's a prototype -> create a new instance.
      102. 					Object prototypeInstance = null;
      103. 					try {
      104. 						beforePrototypeCreation(beanName);
      105. 						prototypeInstance = createBean(beanName, mbd, args);
      106. 					}
      107. 					finally {
      108. 						afterPrototypeCreation(beanName);
      109. 					}
      110. 					bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
      111. 				}
      112.  
      113. 				else {
      114. 					String scopeName = mbd.getScope();
      115. 					final Scope scope = this.scopes.get(scopeName);
      116. 					if (scope == null) {
      117. 						throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
      118. 					}
      119. 					try {
      120. 						Object scopedInstance = scope.get(beanName, () -> {
      121. 							beforePrototypeCreation(beanName);
      122. 							try {
      123. 								return createBean(beanName, mbd, args);
      124. 							}
      125. 							finally {
      126. 								afterPrototypeCreation(beanName);
      127. 							}
      128. 						});
      129. 						bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
      130. 					}
      131. 					catch (IllegalStateException ex) {
      132. 						throw new BeanCreationException(beanName,
      133. 								"Scope '" + scopeName + "' is not active for the current thread; consider " +
      134. 								"defining a scoped proxy for this bean if you intend to refer to it from a singleton",
      135. 								ex);
      136. 					}
      137. 				}
      138. 			}
      139. 			catch (BeansException ex) {
      140. 				cleanupAfterBeanCreationFailure(beanName);
      141. 				throw ex;
      142. 			}
      143. 		}
      144.  
      145. 		// Check if required type matches the type of the actual bean instance.
      146. 		if (requiredType != null && !requiredType.isInstance(bean)) {
      147. 			try {
      148. 				T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
      149. 				if (convertedBean == null) {
      150. 					throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
      151. 				}
      152. 				return convertedBean;
      153. 			}
      154. 			catch (TypeMismatchException ex) {
      155. 				if (logger.isTraceEnabled()) {
      156. 					logger.trace("Failed to convert bean '" + name + "' to required type '" +
      157. 							ClassUtils.getQualifiedName(requiredType) + "'", ex);
      158. 				}
      159. 				throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
      160. 			}
      161. 		}
      162. 		return (T) bean;
      163. 	}
       

      getSingleton() ：获取Bean、createBean()-->doCreateBean()
       

      1. 	@Nullable
      2. 	protected Object getSingleton(String beanName, boolean allowEarlyReference) {
      3.         //从singletonObjects获取实例，singletonObjects中的实例都是准备好的bean实例，可以直接使用
      4. 		Object singletonObject = this.singletonObjects.get(beanName);
      5.         //isSingletonCurrentlyInCreation(beanName)：判断当前单例bean是否正在创建中
      6. 		if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
      7. 			synchronized (this.singletonObjects) {
      8.                 //一级缓存没有就去二级缓存找
      9. 				singletonObject = this.earlySingletonObjects.get(beanName);
      10. 				if (singletonObject == null && allowEarlyReference) {
      11.                     //二级缓存没有就去三级缓存找
      12. 					ObjectFactory singletonFactory = this.singletonFactories.get(beanName);
      13. 					if (singletonFactory != null) {
      14.                         //三级缓存有的话，就把它移动到二级缓存，并在三级缓存中删除
      15. 						singletonObject = singletonFactory.getObject();
      16. 						this.earlySingletonObjects.put(beanName, singletonObject);
      17. 						this.singletonFactories.remove(beanName);
      18. 					}
      19. 				}
      20. 			}
      21. 		}
      22. 		return singletonObject;
      23. 	}
      24.  
      25. 	@Override
      26. 	protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
      27. 			throws BeanCreationException {
      28.  
      29. 		if (logger.isTraceEnabled()) {
      30. 			logger.trace("Creating instance of bean '" + beanName + "'");
      31. 		}
      32. 		RootBeanDefinition mbdToUse = mbd;
      33.  
      34. 		// Make sure bean class is actually resolved at this point, and
      35. 		// clone the bean definition in case of a dynamically resolved Class
      36. 		// which cannot be stored in the shared merged bean definition.
      37. 		Class resolvedClass = resolveBeanClass(mbd, beanName);
      38. 		if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
      39. 			mbdToUse = new RootBeanDefinition(mbd);
      40. 			mbdToUse.setBeanClass(resolvedClass);
      41. 		}
      42.  
      43. 		// Prepare method overrides.
      44. 		try {
      45. 			mbdToUse.prepareMethodOverrides();
      46. 		}
      47. 		catch (BeanDefinitionValidationException ex) {
      48. 			throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
      49. 					beanName, "Validation of method overrides failed", ex);
      50. 		}
      51.  
      52. 		try {
      53. 			// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
      54. 			Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
      55. 			if (bean != null) {
      56. 				return bean;
      57. 			}
      58. 		}
      59. 		catch (Throwable ex) {
      60. 			throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName,
      61. 					"BeanPostProcessor before instantiation of bean failed", ex);
      62. 		}
      63.  
      64. 		try {
      65. 			Object beanInstance = doCreateBean(beanName, mbdToUse, args);//F5
      66. 			if (logger.isTraceEnabled()) {
      67. 				logger.trace("Finished creating instance of bean '" + beanName + "'");
      68. 			}
      69. 			return beanInstance;
      70. 		}
      71. 		catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) {
      72. 			// A previously detected exception with proper bean creation context already,
      73. 			// or illegal singleton state to be communicated up to DefaultSingletonBeanRegistry.
      74. 			throw ex;
      75. 		}
      76. 		catch (Throwable ex) {
      77. 			throw new BeanCreationException(
      78. 					mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
      79. 		}
      80. 	}
       

      doCreateBean()-->populateBean()属性填充
       

      1. 	protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
      2. 			throws BeanCreationException {
      3.  
      4. 		// Instantiate the bean.
      5. 		BeanWrapper instanceWrapper = null;
      6. 		if (mbd.isSingleton()) {
      7. 			instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
      8. 		}
      9. 		if (instanceWrapper == null) {
      10. 			instanceWrapper = createBeanInstance(beanName, mbd, args);//F5创建bean的实例
      11. 		}
      12. 		final Object bean = instanceWrapper.getWrappedInstance();
      13. 		Class beanType = instanceWrapper.getWrappedClass();
      14. 		if (beanType != NullBean.class) {
      15. 			mbd.resolvedTargetType = beanType;
      16. 		}
      17.  
      18. 		// Allow post-processors to modify the merged bean definition.
      19. 		synchronized (mbd.postProcessingLock) {
      20. 			if (!mbd.postProcessed) {
      21. 				try {
      22. 					applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
      23. 				}
      24. 				catch (Throwable ex) {
      25. 					throw new BeanCreationException(mbd.getResourceDescription(), beanName,
      26. 							"Post-processing of merged bean definition failed", ex);
      27. 				}
      28. 				mbd.postProcessed = true;
      29. 			}
      30. 		}
      31.  
      32. 		// Eagerly cache singletons to be able to resolve circular references
      33. 		// even when triggered by lifecycle interfaces like BeanFactoryAware.
      34. 		boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
      35. 				isSingletonCurrentlyInCreation(beanName));
      36. 		if (earlySingletonExposure) {
      37. 			if (logger.isTraceEnabled()) {
      38. 				logger.trace("Eagerly caching bean '" + beanName +
      39. 						"' to allow for resolving potential circular references");
      40. 			}
      41.             //F5
      42. 			addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
      43. 		}
      44.  
      45. 		// Initialize the bean instance.
      46. 		Object exposedObject = bean;
      47. 		try {
      48. 			populateBean(beanName, mbd, instanceWrapper);
      49. 			exposedObject = initializeBean(beanName, exposedObject, mbd);
      50. 		}
      51. 		catch (Throwable ex) {
      52. 			if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {
      53. 				throw (BeanCreationException) ex;
      54. 			}
      55. 			else {
      56. 				throw new BeanCreationException(
      57. 						mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);
      58. 			}
      59. 		}
      60.  
      61. 		if (earlySingletonExposure) {
      62. 			Object earlySingletonReference = getSingleton(beanName, false);
      63. 			if (earlySingletonReference != null) {
      64. 				if (exposedObject == bean) {
      65. 					exposedObject = earlySingletonReference;
      66. 				}
      67. 				else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
      68. 					String[] dependentBeans = getDependentBeans(beanName);
      69. 					Set actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
      70. 					for (String dependentBean : dependentBeans) {
      71. 						if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
      72. 							actualDependentBeans.add(dependentBean);
      73. 						}
      74. 					}
      75. 					if (!actualDependentBeans.isEmpty()) {
      76. 						throw new BeanCurrentlyInCreationException(beanName,
      77. 								"Bean with name '" + beanName + "' has been injected into other beans [" +
      78. 								StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +
      79. 								"] in its raw version as part of a circular reference, but has eventually been " +
      80. 								"wrapped. This means that said other beans do not use the final version of the " +
      81. 								"bean. This is often the result of over-eager type matching - consider using " +
      82. 								"'getBeanNamesOfType' with the 'allowEagerInit' flag turned off, for example.");
      83. 					}
      84. 				}
      85. 			}
      86. 		}
      87.  
      88. 		// Register bean as disposable.
      89. 		try {
      90. 			registerDisposableBeanIfNecessary(beanName, bean, mbd);
      91. 		}
      92. 		catch (BeanDefinitionValidationException ex) {
      93. 			throw new BeanCreationException(
      94. 					mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
      95. 		}
      96.  
      97. 		return exposedObject;
      98. 	}
       

      属性填充populateBean：a中有属性b，需要再去创建b,重复上面步骤
       

      1. 	protected void addSingletonFactory(String beanName, ObjectFactory singletonFactory) {
      2. 		Assert.notNull(singletonFactory, "Singleton factory must not be null");
      3. 		synchronized (this.singletonObjects) {
      4. 			if (!this.singletonObjects.containsKey(beanName)) {
      5. 				this.singletonFactories.put(beanName, singletonFactory);
      6. 				this.earlySingletonObjects.remove(beanName);
      7. 				this.registeredSingletons.add(beanName);
      8. 			}
      9. 		}
      10. 	}
      11.  
      12. 	protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {
      13. 		if (bw == null) {
      14. 			if (mbd.hasPropertyValues()) {
      15. 				throw new BeanCreationException(
      16. 						mbd.getResourceDescription(), beanName, "Cannot apply property values to null instance");
      17. 			}
      18. 			else {
      19. 				// Skip property population phase for null instance.
      20. 				return;
      21. 			}
      22. 		}
      23.  
      24. 		// Give any InstantiationAwareBeanPostProcessors the opportunity to modify the
      25. 		// state of the bean before properties are set. This can be used, for example,
      26. 		// to support styles of field injection.
      27. 		boolean continueWithPropertyPopulation = true;
      28.  
      29. 		if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
      30. 			for (BeanPostProcessor bp : getBeanPostProcessors()) {
      31. 				if (bp instanceof InstantiationAwareBeanPostProcessor) {
      32. 					InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
      33. 					if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
      34. 						continueWithPropertyPopulation = false;
      35. 						break;
      36. 					}
      37. 				}
      38. 			}
      39. 		}
      40.  
      41. 		if (!continueWithPropertyPopulation) {
      42. 			return;
      43. 		}
      44.  
      45. 		PropertyValues pvs = (mbd.hasPropertyValues() ? mbd.getPropertyValues() : null);
      46.  
      47. 		if (mbd.getResolvedAutowireMode() == AUTOWIRE_BY_NAME || mbd.getResolvedAutowireMode() == AUTOWIRE_BY_TYPE) {
      48. 			MutablePropertyValues newPvs = new MutablePropertyValues(pvs);
      49. 			// Add property values based on autowire by name if applicable.
      50. 			if (mbd.getResolvedAutowireMode() == AUTOWIRE_BY_NAME) {
      51. 				autowireByName(beanName, mbd, bw, newPvs);
      52. 			}
      53. 			// Add property values based on autowire by type if applicable.
      54. 			if (mbd.getResolvedAutowireMode() == AUTOWIRE_BY_TYPE) {
      55. 				autowireByType(beanName, mbd, bw, newPvs);
      56. 			}
      57. 			pvs = newPvs;
      58. 		}
      59.  
      60. 		boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();
      61. 		boolean needsDepCheck = (mbd.getDependencyCheck() != AbstractBeanDefinition.DEPENDENCY_CHECK_NONE);
      62.  
      63. 		PropertyDescriptor[] filteredPds = null;
      64. 		if (hasInstAwareBpps) {
      65. 			if (pvs == null) {
      66. 				pvs = mbd.getPropertyValues();
      67. 			}
      68. 			for (BeanPostProcessor bp : getBeanPostProcessors()) {
      69. 				if (bp instanceof InstantiationAwareBeanPostProcessor) {
      70. 					InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
      71. 					PropertyValues pvsToUse = ibp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);
      72. 					if (pvsToUse == null) {
      73. 						if (filteredPds == null) {
      74. 							filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
      75. 						}
      76. 						pvsToUse = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
      77. 						if (pvsToUse == null) {
      78. 							return;
      79. 						}
      80. 					}
      81. 					pvs = pvsToUse;
      82. 				}
      83. 			}
      84. 		}
      85. 		if (needsDepCheck) {
      86. 			if (filteredPds == null) {
      87. 				filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
      88. 			}
      89. 			checkDependencies(beanName, mbd, filteredPds, pvs);
      90. 		}
      91.  
      92. 		if (pvs != null) {
      93. 			applyPropertyValues(beanName, mbd, bw, pvs);//F5 应用属性
      94. 		}
      95. 	}
       

      addSingleton()：添加Bean
       

      1. 	public Object getSingleton(String beanName, ObjectFactory singletonFactory) {
      2. 		Assert.notNull(beanName, "Bean name must not be null");
      3. 		synchronized (this.singletonObjects) {
      4. 			Object singletonObject = this.singletonObjects.get(beanName);
      5. 			if (singletonObject == null) {
      6. 				if (this.singletonsCurrentlyInDestruction) {
      7. 					throw new BeanCreationNotAllowedException(beanName,
      8. 							"Singleton bean creation not allowed while singletons of this factory are in destruction " +
      9. 							"(Do not request a bean from a BeanFactory in a destroy method implementation!)");
      10. 				}
      11. 				if (logger.isDebugEnabled()) {
      12. 					logger.debug("Creating shared instance of singleton bean '" + beanName + "'");
      13. 				}
      14. 				beforeSingletonCreation(beanName);
      15. 				boolean newSingleton = false;
      16. 				boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
      17. 				if (recordSuppressedExceptions) {
      18. 					this.suppressedExceptions = new LinkedHashSet<>();
      19. 				}
      20. 				try {
      21. 					singletonObject = singletonFactory.getObject();
      22. 					newSingleton = true;
      23. 				}
      24. 				catch (IllegalStateException ex) {
      25. 					// Has the singleton object implicitly appeared in the meantime ->
      26. 					// if yes, proceed with it since the exception indicates that state.
      27. 					singletonObject = this.singletonObjects.get(beanName);
      28. 					if (singletonObject == null) {
      29. 						throw ex;
      30. 					}
      31. 				}
      32. 				catch (BeanCreationException ex) {
      33. 					if (recordSuppressedExceptions) {
      34. 						for (Exception suppressedException : this.suppressedExceptions) {
      35. 							ex.addRelatedCause(suppressedException);
      36. 						}
      37. 					}
      38. 					throw ex;
      39. 				}
      40. 				finally {
      41. 					if (recordSuppressedExceptions) {
      42. 						this.suppressedExceptions = null;
      43. 					}
      44. 					afterSingletonCreation(beanName);
      45. 				}
      46. 				if (newSingleton) {
      47. 					addSingleton(beanName, singletonObject); //F5 
      48. 				}
      49. 			}
      50. 			return singletonObject;
      51. 		}
      52. 	}
       

      addSingleton()：添加Bean
       

      把创建的b放入一级缓存，删除三级和二级缓存中的bean、b创建完继续创建a
       

      1. 	protected void addSingleton(String beanName, Object singletonObject) {
      2. 		synchronized (this.singletonObjects) {
      3. 			this.singletonObjects.put(beanName, singletonObject);
      4. 			this.singletonFactories.remove(beanName);
      5. 			this.earlySingletonObjects.remove(beanName);
      6. 			this.registeredSingletons.add(beanName);
      7. 		}
      8. 	}
       

      总结：
       

              Spring创建bean主要分为两个步骤，创建原始bean对象，接着去填充对象属性和初始化。
       

      每次创建bean之前，我们都会从缓存中查下有没有该bean，因为是单例只能有一个。
       

              当我们创建beanA的原始对象后，并把它放到三级缓存中，接下来就该填充对象属性了，这时候发现了beanB，接着就又去创建beanB，同样的流程，创建完beanB填充属性时又发现它依赖beanA又是同样的流程。
       

      不同的是：
       

              这时候可以在三级缓存中查到刚放进去的原始对象baenA，所以不需要继续创建，用它注入beanB，完成beanB的创建，既然beanB创建好了，所以beanA就可以完成填充属性的步骤了，接着执行剩下的逻辑，闭环完成。
       

      1. 	@Nullable
      2. 	protected Object getSingleton(String beanName, boolean allowEarlyReference) {
      3.         //从singletonObjects获取实例，singletonObjects中的实例都是准备好的bean实例，可以直接使用
      4. 		Object singletonObject = this.singletonObjects.get(beanName);
      5.         //isSingletonCurrentlyInCreation(beanName)：判断当前单例bean是否正在创建中
      6. 		if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
      7. 			synchronized (this.singletonObjects) {
      8.                 //一级缓存没有就去二级缓存找
      9. 				singletonObject = this.earlySingletonObjects.get(beanName);
      10. 				if (singletonObject == null && allowEarlyReference) {
      11.                     //二级缓存没有就去三级缓存找
      12. 					ObjectFactory singletonFactory = this.singletonFactories.get(beanName);
      13. 					if (singletonFactory != null) {
      14.                         //三级缓存有的话，就把它移动到二级缓存，并在三级缓存中删除
      15. 						singletonObject = singletonFactory.getObject();
      16. 						this.earlySingletonObjects.put(beanName, singletonObject);
      17. 						this.singletonFactories.remove(beanName);
      18. 					}
      19. 				}
      20. 			}
      21. 		}
      22. 		return singletonObject;
      23. 	}
       

      Spring解决循环依赖依靠的是Bean的“中间态”这个概念，而这个中间态指的是已经实例化但还没初始化的状态----->半成品。
       

      实例化的过程又是通过构造器创建的，如果A还没创建好出来怎么可能提前曝光，所以构造器的循环依赖无法解决。
       

      Spring为了解决单例的循环依赖问题，使用了三级缓存。
       

      其中一级缓存为单例池(singletonObjects)
       

      二级缓存为提前曝光对象(earlySingletonObjects)
       

      三级缓存为提前曝光对象工厂(singletonFactories)
       

      假设A、B循环引用，实例化A的时候就将其放入三级缓存中，接着填充属性的时候，发现依赖了B，同样的流程也是实例化后放入三级缓存，接着去填充属性时又发现自己依赖A，这时候从缓存中查找到早期暴露的A，没有AOP代理的话，直接将A的原始对象注入B，完成B的初始化后，进行属性填充和初始化，这时候B完成后，就去完成剩下的A的步骤，如果有AOP代理，就进行AOP处理获取代理后的对象A，注入B，走剩下的流程。
       

      Spring解决循环依赖过程
       

      1、调用doGetBean()方法，想要获取beanA，于是调用getSingleton()方法从缓存中查找beanA
       

      2、在getSingleton()方法中，从一级缓存中查找，没有，返回null
       

      3、doGetBean()方法中获取到的beanA为null，于是走对应的处理逻辑，调用getSingleton()的重载方法(参数为ObjectFactory的)
       

      4、在getSingleton()方法中，先将beanA_name添加到一个集合中，用于标记该bean正在创建中。然后回调匿名内部类的creatBean方法
       

      5、进入AbstractAutowireCapableBeanFactory#doCreateBean，先反射调用构造器创建出beanA的实例，然后判断：是否为单例、是否允许提前暴露引用(对于单例一般为true)、是否正在创建中(即是否在第四步的集合中)。判断为true则将beanA添加到[三级缓存]中
       

      6、对beanA进行属性填充，此时检测到beanA依赖于beanB，于是开始查找beanB
       

      7、调用doGetBean()方法，和上面beanA的过程一样，到缓存中查找beanB，没有则创建，然后给beanB填充属性
       

      8、此时beanB依赖于beanA，调用getSingleton()获取beanA，依次从一级、二级、三级缓存中找，此时从三级缓存中获取到beanA的创建工厂，通过创建工厂获取到singletonObject，此时这个singletonObject指向的就是上面在doCreateBean()方法中实例化的beanA
       

      9、这样beanB就获取到了beanA的依赖，于是beanB顺利完成实例化，并将beanA从三级缓存移动到二级缓存中
       

      10、随后beanA继续它的属性填充工作，此时也获取到了beanB，beanA也随之完成了创建，回到getSingleton()方法中继续向下执行，将beanA从二级缓存移动到一级缓存中
       

       

       

      Spring Aop
       

       

      一个程序员最重要的能力是：写出高质量的代码！！
       有道无术，术尚可求也，有术无道，止于术。
       无论你是年轻还是年长，所有程序员都需要记住：时刻努力学习新技术，否则就会被时代抛弃！
      
                
                    
                

                
    57. 

                    相关阅读:
      
                    
设计模式简介                            
      
Redis核心结构以及渐进式扩容                            
      
[Linux]下制作简易myshell                            
      
Android Surface 是如何跨进程传递的？                            
      
SpringCloud集成Nacos配置管理                            
      
MySQL：温备份和恢复-mysqldump （4）                            
      
从源码分析vue3组件的生命周期                            
      
JavaFx之Hello, World!                            
      
【设计模式】聊聊职责链模式                            
      
保险行业采购管理痛点及解决方案（数智化采购系统）                            
      
                          
                
      58. 
                
    58. 
                    原文地址：https://blog.csdn.net/weixin_42472027/article/details/133758607
                
      59.