# spring beans

# spring bean的生命周期

bean的声明周期分为如下几个阶段，在这些阶段的前后spring都预留了回调接口，比如各种postProcessor

Bean Definition扫描、注册
Bean实例化: 实例化就是创建bean对象的过程，比如通过构造函数或通过FactoryBean方法创建
Bean初始化: 对字段进行注入比如@Autowired的字段，执行@PostConstruct方法、实现InitializationBean的bean会调用afterPropertiesSet方法等
Bean销毁: bean销毁前，也可以做一些比如资源释放清理的工作，一般发生在jvm关闭的时候

# Bean Definition扫描、注册

BeanFactory默认的实现类是DefaultListableBeanFactory，提供了registerBeanDefinition的实现。在通过扫描包注册component的情况下，是通过ConfigurationClassPostProcessor在postProcessBeanRegistry时通过ClassPathBeanDefinitionScanner 扫描解析指定包下的BeanDefinition并注册到BeanFactory中。还有另一种方式是通过其他的BeanFactoryPostProcessor注册BeanDefinition，比如mybatis通过MapperScannerConfigurer扫描注册mybatis的Mapper bean。

bean definition 注册后，就可以通过getBean获取bean了，BeanFactory会负责创建bean、保存singleton等。

# bean实例化

bean创建的触发时机，是通过BeanFactory的getBean方法触发，如果是singleton（单例，也是默认的类型）类型的bean，并且BeanFactory的singleton map里没有这个bean，会触发创建过程。

如下面代码所示，doGetBean会先调用getSingleton，如果singleton缓存中有结果，并且bean的创建参数为空，则调用getObjectForBeanInstance 判断是否是factory bean如果不是factory bean直接返回，如果是factory bean也先去factory bean cache中查询，没有调用FactoryBean的getObject方法获取bean并缓存。

protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
			@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {

      final String beanName = transformedBeanName(name);
      Object bean;

      // Eagerly check singleton cache for manually registered singletons.
      Object sharedInstance = getSingleton(beanName);
      if (sharedInstance != null && args == null) {
          if (logger.isTraceEnabled()) {
              if (isSingletonCurrentlyInCreation(beanName)) {
                  logger.trace("Returning eagerly cached instance of singleton bean '" + beanName +
                          "' that is not fully initialized yet - a consequence of a circular reference");
              }
              else {
                  logger.trace("Returning cached instance of singleton bean '" + beanName + "'");
              }
          }
          bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
      }

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其他情况（singleton 缓存里没有或args不为null），先做个循环依赖检查（当前的bean正在创建过程中）

else {
      // Fail if we're already creating this bean instance:
      // We're assumably within a circular reference.
      if (isPrototypeCurrentlyInCreation(beanName)) {
          throw new BeanCurrentlyInCreationException(beanName);
      }

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然后判断下是singleton类型，调用getSingleton触发bean创建并保存singleton

if (mbd.isSingleton()) {
    sharedInstance = getSingleton(beanName, () -> {
        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);
}

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getSingleton方法，要加锁做好并发处理，避免并发创建bean。 beforeSingletonCreation会在singletonsCurrentlyInCreation 这个set中添加判断是否是循环依赖的情况。然后调用singletonFactory.getObject()，也就是createBean方法创建bean。

public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
      Assert.notNull(beanName, "Bean name 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 + "'");
              }
              beforeSingletonCreation(beanName);
              boolean newSingleton = false;
              boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
              if (recordSuppressedExceptions) {
                  this.suppressedExceptions = new LinkedHashSet<>();
              }
              try {
                  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;
                  }
                  afterSingletonCreation(beanName);
              }
              if (newSingleton) {
                  addSingleton(beanName, singletonObject);
              }
          }
          return singletonObject;
      }
  }

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createBean方法先调用resolveBeanClass解析加载bean的Class。

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

      if (logger.isTraceEnabled()) {
          logger.trace("Creating instance of bean '" + beanName + "'");
      }
      RootBeanDefinition mbdToUse = mbd;

      // Make sure bean class is actually resolved at this point, and
      // clone the bean definition in case of a dynamically resolved Class
      // which cannot be stored in the shared merged bean definition.
      Class<?> resolvedClass = resolveBeanClass(mbd, beanName);

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然后调用resolveBeforeInstantiation调用InstantiationAwareBeanPostProcessor的postProcessBeforeInstantiation方法。

try {
    // Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
    Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
    if (bean != null) {
        return bean;
    }
}

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有一些InstantiationAwareBeanPostProcessor可能会在postProcessBeforeInstantiation返回bean，比如aop有targetSource的情况，如果postProcessBeforeInstantiation返回了bean，就不用后面的bean创建流程了，直接调用BeanPostProcessor的postProcessAfterInitialization。

protected Object resolveBeforeInstantiation(String beanName, RootBeanDefinition mbd) {
    Object bean = null;
    if (!Boolean.FALSE.equals(mbd.beforeInstantiationResolved)) {
        // Make sure bean class is actually resolved at this point.
        if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
            Class<?> targetType = determineTargetType(beanName, mbd);
            if (targetType != null) {
                bean = applyBeanPostProcessorsBeforeInstantiation(targetType, beanName);
                if (bean != null) {
                    bean = applyBeanPostProcessorsAfterInitialization(bean, beanName);
                }
            }
        }
        mbd.beforeInstantiationResolved = (bean != null);
    }
    return bean;
}

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postProcessBeforeInstantiation没有创建bean的情况，调用doCreateBean

try {
    Object beanInstance = doCreateBean(beanName, mbdToUse, args);
    if (logger.isTraceEnabled()) {
        logger.trace("Finished creating instance of bean '" + beanName + "'");
    }
    return beanInstance;
}

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doCreateBean首先通过createBeanInstance创建bean对象，通过参数匹配构造函数，通过CglibSubclassingInstantiationStrategy创建bean实例。

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

      // Instantiate the bean.
      BeanWrapper instanceWrapper = null;
      if (mbd.isSingleton()) {
          instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
      }
      if (instanceWrapper == null) {
          instanceWrapper = createBeanInstance(beanName, mbd, args);
      }

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对应的bean没有MethodOverrides的情况下使用反射创建对象，否则使用cglib创建子类创建对象。

public Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner) {
    // Don't override the class with CGLIB if no overrides.
    if (!bd.hasMethodOverrides()) {
        Constructor<?> constructorToUse;
        synchronized (bd.constructorArgumentLock) {
            constructorToUse = (Constructor<?>) bd.resolvedConstructorOrFactoryMethod;
            if (constructorToUse == null) {
                final Class<?> clazz = bd.getBeanClass();
                if (clazz.isInterface()) {
                    throw new BeanInstantiationException(clazz, "Specified class is an interface");
                }
                try {
                    if (System.getSecurityManager() != null) {
                        constructorToUse = AccessController.doPrivileged(
                                (PrivilegedExceptionAction<Constructor<?>>) clazz::getDeclaredConstructor);
                    }
                    else {
                        constructorToUse = clazz.getDeclaredConstructor();
                    }
                    bd.resolvedConstructorOrFactoryMethod = constructorToUse;
                }
                catch (Throwable ex) {
                    throw new BeanInstantiationException(clazz, "No default constructor found", ex);
                }
            }
        }
        return BeanUtils.instantiateClass(constructorToUse);
    }
    else {
        // Must generate CGLIB subclass.
        return instantiateWithMethodInjection(bd, beanName, owner);
    }
}

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由此完成了bean实例对象的创建，然后会调用MergedBeanDefinitionPostProcessor的postProcessMergedBeanDefinition方法。

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;
    }
}

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AutowiredAnnotationBeanPostProcessor通过这个过程查找要@Autowired注入的方法、字段信息并保存下来

protected void applyMergedBeanDefinitionPostProcessors(RootBeanDefinition mbd, Class<?> beanType, String beanName) {
    for (MergedBeanDefinitionPostProcessor processor : getBeanPostProcessorCache().mergedDefinition) {
        processor.postProcessMergedBeanDefinition(mbd, beanType, beanName);
    }
}

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AutowiredAnnotationBeanPostProcessor的postProcessMergedBeanDefinition

@Override
public void postProcessMergedBeanDefinition(RootBeanDefinition beanDefinition, Class<?> beanType, String beanName) {
    InjectionMetadata metadata = findAutowiringMetadata(beanName, beanType, null);
    metadata.checkConfigMembers(beanDefinition);
}

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然后就进入到populateBean和initializeBean方法

// Initialize the bean instance.
Object exposedObject = bean;
try {
    populateBean(beanName, mbd, instanceWrapper);
    exposedObject = initializeBean(beanName, exposedObject, mbd);
}

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# 实例化后处理

populateBean方法中，会先调用InstantiationAwareBeanPostProcessor的postProcessAfterInstantiation

if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
    for (InstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().instantiationAware) {
        if (!bp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
            return;
        }
    }
}

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然后调用InstantiationAwareBeanPostProcessor的postProcessProperties方法，可以做一些字段、方法的赋值调用，

PropertyDescriptor[] filteredPds = null;
if (hasInstAwareBpps) {
    if (pvs == null) {
        pvs = mbd.getPropertyValues();
    }
    for (InstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().instantiationAware) {
        PropertyValues pvsToUse = bp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);
        if (pvsToUse == null) {
            if (filteredPds == null) {
                filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
            }
            pvsToUse = bp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
            if (pvsToUse == null) {
                return;
            }
        }
        pvs = pvsToUse;
    }
}

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@Autowired字段、方法参数的注入就是在这里执行的，在AutowiredAnnotationBeanPostProcessor中。CommonAnnotationBeanPostProcessor 会处理@Resource注入。

public PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName) {
    InjectionMetadata metadata = findAutowiringMetadata(beanName, bean.getClass(), pvs);
    try {
        metadata.inject(bean, beanName, pvs);
    }
    catch (BeanCreationException ex) {
        throw ex;
    }
    catch (Throwable ex) {
        throw new BeanCreationException(beanName, "Injection of autowired dependencies failed", ex);
    }
    return pvs;
}

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# 初始化阶段

经过前面的步骤，bean实例创建完成、属性（比如@Autowired也赋值好了），下面进入初始化阶段(initialize)

initializeBean方法会先调用invokeAwareMethods，判断是否实现了BeanNameAware、BeanClassLoaderAware、BeanFactoryAware等接口并赋值。

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 {
        invokeAwareMethods(beanName, bean);
    }

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

    try {
        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()) {
        wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
    }

    return wrappedBean;
}

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然后调用applyBeanPostProcessorsBeforeInitialization，也就是BeanPostProcessor的postProcessBeforeInitialization方法。

其中InitDestroyAnnotationBeanPostProcessor在这里调用@PostConstructor注解的方法。

@Override
public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
    LifecycleMetadata metadata = findLifecycleMetadata(bean.getClass());
    try {
        metadata.invokeInitMethods(bean, beanName);
    }
    catch (InvocationTargetException ex) {
        throw new BeanCreationException(beanName, "Invocation of init method failed", ex.getTargetException());
    }
    catch (Throwable ex) {
        throw new BeanCreationException(beanName, "Failed to invoke init method", ex);
    }
    return bean;
}

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然后调用invokeInitMethods，判断如果bean实现了InitializingBean接口，调用afterPropertiesSet方法。

try {
    invokeInitMethods(beanName, wrappedBean, mbd);
}

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protected void invokeInitMethods(String beanName, Object bean, @Nullable RootBeanDefinition mbd)
			throws Throwable {

    boolean isInitializingBean = (bean instanceof InitializingBean);
    if (isInitializingBean && (mbd == null || !mbd.hasAnyExternallyManagedInitMethod("afterPropertiesSet"))) {
        if (logger.isTraceEnabled()) {
            logger.trace("Invoking afterPropertiesSet() on bean with name '" + beanName + "'");
        }
        if (System.getSecurityManager() != null) {
            try {
                AccessController.doPrivileged((PrivilegedExceptionAction<Object>) () -> {
                    ((InitializingBean) bean).afterPropertiesSet();
                    return null;
                }, getAccessControlContext());
            }
            catch (PrivilegedActionException pae) {
                throw pae.getException();
            }
        }
        else {
            ((InitializingBean) bean).afterPropertiesSet();
        }
    }

    if (mbd != null && bean.getClass() != NullBean.class) {
        String initMethodName = mbd.getInitMethodName();
        if (StringUtils.hasLength(initMethodName) &&
                !(isInitializingBean && "afterPropertiesSet".equals(initMethodName)) &&
                !mbd.hasAnyExternallyManagedInitMethod(initMethodName)) {
            invokeCustomInitMethod(beanName, bean, mbd);
        }
    }
}

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最后调用applyBeanPostProcessorsAfterInitialization也就是BeanPostProcessor的postProcessAfterInitialization方法。

if (mbd == null || !mbd.isSynthetic()) {
    wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
}

return wrappedBean;

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

    Object result = existingBean;
    for (BeanPostProcessor processor : getBeanPostProcessors()) {
        Object current = processor.postProcessAfterInitialization(result, beanName);
        if (current == null) {
            return result;
        }
        result = current;
    }
    return result;
}

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spring-aop就是通过这里给bean创建代理的，在

public Object postProcessAfterInitialization(@Nullable Object bean, String beanName) {
      if (bean != null) {
          Object cacheKey = this.getCacheKey(bean.getClass(), beanName);
          if (this.earlyProxyReferences.remove(cacheKey) != bean) {
              return this.wrapIfNecessary(bean, beanName, cacheKey);
          }
      }

      return bean;
  }

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# QA

Q: spring bean创建完成字段的初始化是否保证happen before？比如@PostConstruct

A: @Autowired、@PostConstruct均执行在spring创建bean过程中，而spring创建singleton的bean时会加synchronized锁，从而实现@PostConstruct 执行happen before于锁释放后的代码，也就是bean创建完成后使用的代码，能够保证可见性，不需要volatile标识。

← spring boot BeanFactoryPostProcessor →