Spring源码 21 Bean生命周期

参考源

https://www.bilibili.com/video/BV1tR4y1F75R?spm_id_from=333.337.search-card.all.click

https://www.bilibili.com/video/BV12Z4y197MU?spm_id_from=333.999.0.0

《Spring源码深度解析（第2版）》

版本

本文章基于 Spring 5.3.15

---

Bean 的生命周期示意图（存在循环依赖和 AOP）

执行创建 Bean

protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) throws BeanCreationException

AbstractAutowireCapableBeanFactory

protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) throws BeanCreationException {
    // 实例化 Bean
    BeanWrapper instanceWrapper = null;
    // 是否为单例
    if (mbd.isSingleton()) {
        // 有可能在本 Bean 创建之前
        // 就有其他 Bean 把当前 Bean 给创建出来了（比如依赖注入过程中）
        // 所以这里先将其删除
        instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
    }
    if (instanceWrapper == null) {
        // 创建 Bean 实例
        // 根据指定 bean 使用对应的策略创建新的实例
        // 如：工厂方法、构造函数自动注入、简单初始化
        instanceWrapper = createBeanInstance(beanName, mbd, args);
    }
    // 获得 Bean 对象
    Object bean = instanceWrapper.getWrappedInstance();
    Class<?> beanType = instanceWrapper.getWrappedClass();
    if (beanType != NullBean.class) {
        mbd.resolvedTargetType = beanType;
    }

    // 后置处理合并的 BeanDefinition
    synchronized (mbd.postProcessingLock) {
        if (!mbd.postProcessed) {
            try {
                // 应用 MergedBeanDefinitionPostProcessors
                applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
            } catch (Throwable ex) {
                throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Post-processing of merged bean definition failed", ex);
            }
            mbd.postProcessed = true;
        }
    }

    // 是否提前暴露
    // 为了解决循环依赖提早缓存单例创建工厂
    // 是否需要提早曝光：单例 & 允许循环依赖 & 当前 bean 正在创建中
    // 用于检测循环依赖
    boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences && isSingletonCurrentlyInCreation(beanName));
    if (earlySingletonExposure) {
        if (logger.isTraceEnabled()) {
            logger.trace("Eagerly caching bean '" + beanName + "' to allow for resolving potential circular references");
        }
        // 添加到三级缓存
        // 为避免后期循环依赖，可以在 bean 初始化完成前将创建实例的 ObjectFactory 加入工厂
        addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
    }

    Object exposedObject = bean;
    try {
        // 属性填充
        // 对 bean 进行填充，将各个属性值注入，其中，可能存在依赖于其他 bean 的属性，则会递归初始依赖 bean
        populateBean(beanName, mbd, instanceWrapper);
        // 初始化
        // 调用初始化方法，比如 init-method
        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) {
        // 获取单例对象
        Object earlySingletonReference = getSingleton(beanName, false);
        // 在检测到有循环依赖的情况下才会不为空
        if (earlySingletonReference != null) {
            // 如果 exposedObject 没有在初始化方法中被改变，也就是没有被增强
            if (exposedObject == bean) {
                exposedObject = earlySingletonReference;
            } else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
                String[] dependentBeans = getDependentBeans(beanName);
                Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
                for (String dependentBean : dependentBeans) {
                    // 检测依赖
                    if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
                        actualDependentBeans.add(dependentBean);
                    }
                }
                // 因为 bean 创建后其所依赖的 bean 一定是已经创建的
                // actualDependentBeans 不为空则表示当前bean 创建后其依赖的 bean 却没有全部创建完，也就是说存在循环依赖
                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 " +
                                                               "'getBeanNamesForType' with the 'allowEagerInit' flag turned off, for example.");
                }
            }
        }
    }

    try {
        // 根据 scope 注册 bean
        registerDisposableBeanIfNecessary(beanName, bean, mbd);
    } catch (BeanDefinitionValidationException ex) {
        throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
    }

    return exposedObject;
}

实例化

创建 Bean 实例

根据指定 bean 使用对应的策略创建新的实例

如：工厂方法、构造函数自动注入、简单初始化

createBeanInstance(beanName, mbd, args)

AbstractAutowireCapableBeanFactory

protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
   // 解析 class
   Class<?> beanClass = resolveBeanClass(mbd, beanName);

   if (beanClass != null && !Modifier.isPublic(beanClass.getModifiers()) && !mbd.isNonPublicAccessAllowed()) {
      throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Bean class isn't public, and non-public access not allowed: " + beanClass.getName());
   }

   Supplier<?> instanceSupplier = mbd.getInstanceSupplier();
   if (instanceSupplier != null) {
      return obtainFromSupplier(instanceSupplier, beanName);
   }
   // 如果工厂方法不为空则使用工厂方法初始化策略
   if (mbd.getFactoryMethodName() != null) {
      return instantiateUsingFactoryMethod(beanName, mbd, args);
   }

   boolean resolved = false;
   boolean autowireNecessary = false;
   if (args == null) {
      synchronized (mbd.constructorArgumentLock) {
         // 一个类有多个构造函数，每个构造函数都有不同的参数， 所以调用前需要先根据参数锁定构造函数或对应的工厂方法
         if (mbd.resolvedConstructorOrFactoryMethod != null) {
            resolved = true;
            autowireNecessary = mbd.constructorArgumentsResolved;
         }
      }
   }
   // 如果已经解析过则使用解析好的构造函数方法不需要再次锁定
   if (resolved) {
      // 构造函数自动注入
      if (autowireNecessary) {
         return autowireConstructor(beanName, mbd, null, null);
      }
      // 使用默认构造函数构造
      else {
         return instantiateBean(beanName, mbd);
      }
   }

   // 需要根据参数解析构造函数
   Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
   if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR || mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
      // 构造函数自动注入
      return autowireConstructor(beanName, mbd, ctors, args);
   }

   ctors = mbd.getPreferredConstructors();
   if (ctors != null) {
      return autowireConstructor(beanName, mbd, ctors, null);
   }
   // 使用默认构造函数构造
   return instantiateBean(beanName, mbd);
}

是否提早暴露

是否提前暴露

为了解决循环依赖提早缓存单例创建工厂

是否需要提早曝光：单例 & 允许循环依赖 & 当前 bean 正在创建中

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

DefaultSingletonBeanRegistry

/**
 * 单例是否正在创建中
 */
private final Set<String> singletonsCurrentlyInCreation = Collections.newSetFromMap(new ConcurrentHashMap<>(16));

public boolean isSingletonCurrentlyInCreation(String beanName) {
   return this.singletonsCurrentlyInCreation.contains(beanName);
}

添加到三级缓存

添加到三级缓存

为避免后期循环依赖，可以在 bean 初始化完成前将创建实例的 ObjectFactory 加入工厂

addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean))

DefaultSingletonBeanRegistry

// 一级缓存：也叫单例池。
// 用于保存 BeanName 和创建 bean 实例之间的关系，bean name -> bean instance
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);

// 二级缓存：存放提前AOP生成的代理对象，保持代理对象的单例。
private final Map<String, Object> earlySingletonObjects = new ConcurrentHashMap<>(16);

// 三级缓存：存放用于生成代理对象的Lambda，用于循环依赖中生成代理对象，以打破循环依赖。
// 用于保存 BeanName 和创建 bean 的工厂之间的关系，bean name -> ObjectFactory
private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);

// 用来保存当前所有已注册的 bean
private final Set<String> registeredSingletons = new LinkedHashSet<>(256);

protected void addSingletonFactory(String beanName, ObjectFactory<?> singletonFactory) {
   Assert.notNull(singletonFactory, "Singleton factory must not be null");
   // 为了保持操作的原子性，加锁
   synchronized (this.singletonObjects) {
      // 单例池中是否存在
      if (!this.singletonObjects.containsKey(beanName)) {
         // 对应记录放人三级缓存
         this.singletonFactories.put(beanName, singletonFactory);
         // 二级缓存移除对应记录
         this.earlySingletonObjects.remove(beanName);
         // 添加到已注册的 Bean
         this.registeredSingletons.add(beanName);
      }
   }
}

获取单例对象

获取单例对象

getSingleton(beanName, false)

DefaultSingletonBeanRegistry

// 一级缓存：也叫单例池。
// 用于保存 BeanName 和创建 bean 实例之间的关系，bean name -> bean instance
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);

// 二级缓存：存放提前AOP生成的代理对象，保持代理对象的单例。
private final Map<String, Object> earlySingletonObjects = new ConcurrentHashMap<>(16);

// 三级缓存：存放用于生成代理对象的Lambda，用于循环依赖中生成代理对象，以打破循环依赖。
// 用于保存 BeanName 和创建 bean 的工厂之间的关系，bean name -> ObjectFactory
private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);

protected Object getSingleton(String beanName, boolean allowEarlyReference) {
   // 检查单例池中是否存在实例
   Object singletonObject = this.singletonObjects.get(beanName);
   // 想要获取的 Bean 正在创建中，说明出现了循环依赖
   if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
      // 从二级缓存中获取该 Bean 的代理对象
      singletonObject = this.earlySingletonObjects.get(beanName);
      if (singletonObject == null && allowEarlyReference) {
         // 如果二级缓存没有，则锁定单例池并进行处理
         synchronized (this.singletonObjects) {
            // 检查单例池中是否存在实例
            singletonObject = this.singletonObjects.get(beanName);
            if (singletonObject == null) {
               // 从二级缓存中获取该 Bean 的代理对象
               singletonObject = this.earlySingletonObjects.get(beanName);
               if (singletonObject == null) {
                  // 当某些方法需要提前初始化的时候则会调用 addSingletonFactory 方法将对应的 ObjectFactory 初始化策略存储在三级缓存中
                  ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
                  // 从三级缓存中找到了
                  if (singletonFactory != null) {
                     // 调用预先设定的 getObject 方法
                     // 这里会执行 Lambda 表达式
                     singletonObject = singletonFactory.getObject();
                     // 记录在二级缓存中，删除三级缓存对应记录
                     // 二级缓存和三级缓存互斥
                     this.earlySingletonObjects.put(beanName, singletonObject);
                     this.singletonFactories.remove(beanName);
                  }
               }
            }
         }
      }
   }
   return singletonObject;
}

提前 AOP

提前 AOP 就是三级缓存中存放的 Lambda 表达式执行的方法

addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean))

AbstractAutowireCapableBeanFactory

protected Object getEarlyBeanReference(String beanName, RootBeanDefinition mbd, Object bean) {
   Object exposedObject = bean;
   if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
      for (SmartInstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().smartInstantiationAware) {
          // 提前 AOP
          exposedObject = bp.getEarlyBeanReference(exposedObject, beanName);
      }
   }
   return exposedObject;
}

提前 AOP

getEarlyBeanReference(exposedObject, beanName)

AbstractAutoProxyCreator

public Object getEarlyBeanReference(Object bean, String beanName) {
   Object cacheKey = getCacheKey(bean.getClass(), beanName);
   // 早期进行 AOP，则将标识传入
   // 供后面再进行 AOP 时进行判断
   this.earlyProxyReferences.put(cacheKey, bean);
   // 必要时填充
   return wrapIfNecessary(bean, beanName, cacheKey);
}

正常 AOP

初始化

initializeBean(beanName, exposedObject, mbd)

AbstractAutowireCapableBeanFactory

protected Object initializeBean(String beanName, Object bean, @Nullable RootBeanDefinition mbd) {
   if (System.getSecurityManager() != null) {
      AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
         invokeAwareMethods(beanName, bean);
         return null;
      }, getAccessControlContext());
   } else {
      // 对特殊的 bean 处理：Aware、BeanClassLoaderAware、BeanFactoryAware
      invokeAwareMethods(beanName, bean);
   }

   Object wrappedBean = bean;
   if (mbd == null || !mbd.isSynthetic()) {
      // 初始化前
      wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
   }

   try {
      // 激活用户自定义的 init 方法
      invokeInitMethods(beanName, wrappedBean, mbd);
   } catch (Throwable ex) {
      throw new BeanCreationException(
            (mbd != null ? mbd.getResourceDescription() : null),
            beanName, "Invocation of init method failed", ex);
   }
   if (mbd == null || !mbd.isSynthetic()) {
      // 初始化后
      wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
   }

   return wrappedBean;
}

初始化后

applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName)

AbstractAutowireCapableBeanFactory

public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName) throws BeansException {

   Object result = existingBean;
   for (BeanPostProcessor processor : getBeanPostProcessors()) {
      // 初始化后的后处理
      Object current = processor.postProcessAfterInitialization(result, beanName);
      if (current == null) {
         return result;
      }
      result = current;
   }
   return result;
}

初始化后的后处理

postProcessAfterInitialization(result, beanName)

AbstractAutoProxyCreator

public Object postProcessAfterInitialization(@Nullable Object bean, String beanName) {
   if (bean != null) {
      Object cacheKey = getCacheKey(bean.getClass(), beanName);
      // 如果前面没有进行过 AOP，这里进行 AOP
      // 如果前面进行过 AOP，删除对应记录，这里不进行 AOP
      if (this.earlyProxyReferences.remove(cacheKey) != bean) {
         return wrapIfNecessary(bean, beanName, cacheKey);
      }
   }
   return bean;
}