从底层源码深入分析Bean的实例化

生命周期的整体流程

Spring 容器可以管理 singleton 作用域 Bean 的生命周期，在此作用域下，Spring 能够精确地知道该 Bean 何时被创建，何时初始化完成，以及何时被销毁。

而对于 prototype 作用域的 Bean，Spring 只负责创建，当容器创建了 Bean 的实例后，Bean 的实例就交给客户端代码管理，Spring 容器将不再跟踪其生命周期。每次客户端请求 prototype 作用域的 Bean 时，Spring 容器都会创建一个新的实例，并且不会管那些被配置成 prototype 作用域的 Bean 的生命周期。

了解 Spring 生命周期的意义就在于，可以利用 Bean 在其存活期间的指定时刻完成一些相关操作 ，即扩展点。这种时刻可能有很多，但一般情况下，会在 Bean 被初始化后和被销毁前执行一些相关操作。具体扩展点的使用可以看这篇文章，可以这两篇文章结合着看。

在执行初始化方法之前和之后，还需要对Bean的后置处理器BeanPostProcessors进行处理：

在invokeInitMethods 的前后进行applyBeanPostProcessorsBeforeInitialization，applyBeanPostProcessorsAfterInitialization
在后置处理中处理了包括：AOP【AnnotationAwareAspectJAutoProxyCreator】，负责构造后@PostConstruct 和销毁前@PreDestroy 的 InitDestoryAnnotationBeanPostProcessor 等
以及通过实现BeanPostProcessor接口的自定义处理器

Bean生命周期整体流程如下：

加载Bean定义：通过 loadBeanDefinitions 扫描所有xml配置、注解将Bean记录在beanDefinitionMap中。即IOC容器的初始化过程
Bean实例化：遍历 beanDefinitionMap 创建bean，最终会使用getBean中的doGetBean方法调用 createBean来创建Bean对象
1. 构建对象：容器通过 createBeanInstance 进行对象构造
  1. 获取构造方法（大部分情况下只有一个构造方法）
    1. 如果只有一个构造方法，无论这个构造方法有没有入参，都用这个构造方法
    2. 有多个构造方法时
      1. 先拿带有@Autowired的构造方法，但是如果多个构造方法都有@Autowired就会报错
      2. 如果没有带有@Autowired的构造方法，那就找没有入参的；如果多个构造方法都是有入参的，那也会报错
  2. 准备参数
    1. 先根据类进行查找
    2. 如果这个类有多个实例，则再根据参数名匹配
    3. 如果没有找到则报错
  3. 构造对象：无参构造方法则直接实例化
2. 填充属性：通过populateBean方法为Bean内部所需的属性进行赋值，通常是 @Autowired 注解的变量；通过三级缓存机制进行填充，也就是依赖注入
3. 初始化Bean对象：通过initializeBean对填充后的实例进行初始化
  1. 执行Aware：检查是否有实现者三个Aware：BeanNameAware，BeanClassLoaderAware, BeanFactoryAware；让实例化后的对象能够感知自己在Spring容器里的存在的位置信息，创建信息
  2. 初始化前：BeanPostProcessor，也就是拿出所有的后置处理器对bean进行处理，当有一个处理器返回null，将不再调用后面的处理器处理。
  3. 初始化：afterPropertiesSet，init- method；
    1. 实现了InitializingBean接口的类执行其afterPropertiesSet()方法
    2. 从BeanDefinition中获取initMethod方法
  4. 初始化后：BeanPostProcessor,；获取所有的bean的后置处理器去执行。AOP也是在这里做的
4. 注册销毁：通过reigsterDisposableBean处理实现了DisposableBean接口的Bean的注册
  1. Bean是否有注册为DisposableBean的资格：
    1. 是否有destroyMethod。
    2. 是否有执行销毁方法的后置处理器。
  2. DisposableBeanAdapter：推断destoryMethod
  3. 完成注册
添加到单例池：通过 addSingleton 方法，将Bean 加入到单例池 singleObjects
销毁
1. 销毁前：如果有@PreDestory 注解的方法就执行
2. 如果有自定义的销毁后置处理器，通过 postProcessBeforeDestruction 方法调用destoryBean逐一销毁Bean
3. 销毁时：如果实现了destroyMethod就执行 destory方法
4. 执行客户自定义销毁：调用 invokeCustomDestoryMethod执行在Bean上自定义的destroyMethod方法
  1. 有这个自定义销毁就会执行
  2. 没有自定义destroyMethod方法就会去执行close方法
  3. 没有close方法就会去执行shutdown方法
  4. 都没有的话就都不执行，不影响

Bean的实例化

BeanFactory定义了Bean容器的规范，其中包含根据bean的名字, Class类型和参数等来得到bean实例。

java 复制代码

// 根据bean的名字和Class类型等来得到bean实例    
Object getBean(String name) throws BeansException;    
Object getBean(String name, Class requiredType) throws BeansException;    
Object getBean(String name, Object... args) throws BeansException;
<T> T getBean(Class<T> requiredType) throws BeansException;
<T> T getBean(Class<T> requiredType, Object... args) throws BeansException;

IoC初始化时，最终是将Bean的定义即BeanDefinition放到beanDefinitionMap中，本质上是一个ConcurrentHashMap；并且BeanDefinition接口中包含了这个类的Class信息以及是否是单例等；

当需要进行创建Bean对象时，就是通过遍历beanDefinitionMap来创建Bean

主体思路

BeanFactory实现getBean方法在AbstractBeanFactory中，这个方法重载都是调用doGetBean方法进行实现的：

java 复制代码

public Object getBean(String name) throws BeansException {
  return doGetBean(name, null, null, false);
}
public <T> T getBean(String name, Class<T> requiredType) throws BeansException {
  return doGetBean(name, requiredType, null, false);
}
public Object getBean(String name, Object... args) throws BeansException {
  return doGetBean(name, null, args, false);
}
public <T> T getBean(String name, @Nullable Class<T> requiredType, @Nullable Object... args)
    throws BeansException {
  return doGetBean(name, requiredType, args, false);
}

doGetBean

我们来看下doGetBean方法(这个方法很长，我们主要看它的整体思路和设计要点）：

java 复制代码

// 参数typeCheckOnly：bean实例是否包含一个类型检查
protected <T> T doGetBean(
			String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)
			throws BeansException {

  // 解析bean的真正name，如果bean是工厂类，name前缀会加&，需要去掉
  String beanName = transformedBeanName(name);
  Object beanInstance;

  // Eagerly check singleton cache for manually registered singletons.
  Object sharedInstance = getSingleton(beanName);
  if (sharedInstance != null && args == null) {
    // 无参单例从缓存中获取
    beanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, null);
  }

  else {
    // 如果bean实例还在创建中，则直接抛出异常
    if (isPrototypeCurrentlyInCreation(beanName)) {
      throw new BeanCurrentlyInCreationException(beanName);
    }

    // 如果 bean definition 存在于父的bean工厂中，委派给父Bean工厂获取
    BeanFactory parentBeanFactory = getParentBeanFactory();
    if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
      // Not found -> check parent.
      String nameToLookup = originalBeanName(name);
      if (parentBeanFactory instanceof AbstractBeanFactory) {
        return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
            nameToLookup, requiredType, args, typeCheckOnly);
      }
      else if (args != null) {
        // Delegation to parent with explicit args.
        return (T) parentBeanFactory.getBean(nameToLookup, args);
      }
      else if (requiredType != null) {
        // No args -> delegate to standard getBean method.
        return parentBeanFactory.getBean(nameToLookup, requiredType);
      }
      else {
        return (T) parentBeanFactory.getBean(nameToLookup);
      }
    }

    if (!typeCheckOnly) {
      // 将当前bean实例放入alreadyCreated集合里，标识这个bean准备创建了
      markBeanAsCreated(beanName);
    }

    StartupStep beanCreation = this.applicationStartup.start("spring.beans.instantiate")
        .tag("beanName", name);
    try {
      if (requiredType != null) {
        beanCreation.tag("beanType", requiredType::toString);
      }
      RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
      checkMergedBeanDefinition(mbd, beanName, args);

      // 确保它的依赖也被初始化了.
      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);
          try {
            getBean(dep); // 初始化它依赖的Bean
          }
          catch (NoSuchBeanDefinitionException ex) {
            throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                "'" + beanName + "' depends on missing bean '" + dep + "'", ex);
          }
        }
      }

      // 创建Bean实例：单例
      if (mbd.isSingleton()) {
        sharedInstance = getSingleton(beanName, () -> {
          try {
            // 真正创建bean的方法
            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;
          }
        });
        beanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
      }
      // 创建Bean实例：原型
      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);
        }
        beanInstance = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
      }
      // 创建Bean实例：根据bean的scope创建
      else {
        String scopeName = mbd.getScope();
        if (!StringUtils.hasLength(scopeName)) {
          throw new IllegalStateException("No scope name defined for bean ´" + beanName + "'");
        }
        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, () -> {
            beforePrototypeCreation(beanName);
            try {
              return createBean(beanName, mbd, args);
            }
            finally {
              afterPrototypeCreation(beanName);
            }
          });
          beanInstance = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
        }
        catch (IllegalStateException ex) {
          throw new ScopeNotActiveException(beanName, scopeName, ex);
        }
      }
    }
    catch (BeansException ex) {
      beanCreation.tag("exception", ex.getClass().toString());
      beanCreation.tag("message", String.valueOf(ex.getMessage()));
      cleanupAfterBeanCreationFailure(beanName);
      throw ex;
    }
    finally {
      beanCreation.end();
    }
  }

  return adaptBeanInstance(name, beanInstance, requiredType);
}

逻辑流程如下：

解析bean的真正name，如果bean是工厂类，name前缀会加&，需要去掉
无参单例先从缓存中尝试获取
如果bean实例还在创建中，则直接抛出异常
如果bean definition 存在于父的bean工厂中，委派给父Bean工厂获取
标记这个beanName的实例正在创建
确保它的依赖也被初始化
真正创建
- 单例时
- 原型时
- 根据bean的scope创建

接下来就是Bean真正创建的过程

createBean

这个方法整体流程图如下：

org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#createBean(java.lang.String, org.springframework.beans.factory.support.RootBeanDefinition, java.lang.Object[])

java 复制代码

protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) throws BeanCreationException {
    if (this.logger.isTraceEnabled()) {
        this.logger.trace("Creating instance of bean '" + beanName + "'");
    }

    RootBeanDefinition mbdToUse = mbd;
    //类加载
    Class<?> resolvedClass = this.resolveBeanClass(mbd, beanName, new Class[0]);
    if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
        mbdToUse = new RootBeanDefinition(mbd);
        mbdToUse.setBeanClass(resolvedClass);
    }

    try {
        //对通过XML定义的bean中的look-up方法进行预处理
        mbdToUse.prepareMethodOverrides();
    } catch (BeanDefinitionValidationException var9) {
        throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(), beanName, "Validation of method overrides failed", var9);
    }

    Object beanInstance;
    try {
        //实例化前，null
        beanInstance = this.resolveBeforeInstantiation(beanName, mbdToUse);
        if (beanInstance != null) {//这里就是实例化前去执行了 "初始化之前和之后" 的流程，那么就有可能返回一个不是null的Bean
            return beanInstance;//就直接返回这个Bean对象了，不会再往正常走后续的Spring正常流程了
        }
    } catch (Throwable var10) {
        throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName, "BeanPostProcessor before instantiation of bean failed", var10);
    }

    try {
        //这个就是走正常的Spring创建Bean的方法
        beanInstance = this.doCreateBean(beanName, mbdToUse, args);
        if (this.logger.isTraceEnabled()) {
            this.logger.trace("Finished creating instance of bean '" + beanName + "'");
        }

        return beanInstance;
    } catch (ImplicitlyAppearedSingletonException | BeanCreationException var7) {
        throw var7;
    } catch (Throwable var8) {
        throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", var8);
    }
}

实例化之前

会对Bean的后置处理器BeanPostProcessors进行处理

org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#resolveBeforeInstantiation

java 复制代码

protected Object resolveBeforeInstantiation(String beanName, RootBeanDefinition mbd) {
    Object bean = null;
    if (!Boolean.FALSE.equals(mbd.beforeInstantiationResolved)) {
        //检查有没有后置处理器
        if (!mbd.isSynthetic() && this.hasInstantiationAwareBeanPostProcessors()) {
            Class<?> targetType = this.determineTargetType(beanName, mbd);
            if (targetType != null) {
                //实例化前
                bean = this.applyBeanPostProcessorsBeforeInstantiation(targetType, beanName);
                if (bean != null) {//正常情况bean是null,如果不是Null
                    //那就去执行初始化之后的方法。但是这样的话就不会走正常spring创建Bean的流程了，会直接返回这个bean对象
                    bean = this.applyBeanPostProcessorsAfterInitialization(bean, beanName);
                }
            }
        }

        mbd.beforeInstantiationResolved = bean != null;
    }

    return bean;
}

java 复制代码

@Nullable
protected Object applyBeanPostProcessorsBeforeInstantiation(Class<?> beanClass, String beanName) {
    Iterator var3 = this.getBeanPostProcessors().iterator();
    //拿到所有的后置处理器去执行
    while(var3.hasNext()) {
        BeanPostProcessor bp = (BeanPostProcessor)var3.next();
        if (bp instanceof InstantiationAwareBeanPostProcessor) {
            InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor)bp;
            Object result = ibp.postProcessBeforeInstantiation(beanClass, beanName);
            if (result != null) {
                return result;
            }
        }
    }

    return null;
}

doCreateBean

org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#doCreateBean

java 复制代码

protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) throws BeanCreationException {
    BeanWrapper instanceWrapper = null;
    if (mbd.isSingleton()) {
        // factoryBeanObjectCache：存的是beanName对应的factoryBean.getObjetc返回的对象
        // factoryBeanInstanceCache：存的是beanName对应的FactoryBean实例对象
        instanceWrapper = (BeanWrapper)this.factoryBeanInstanceCache.remove(beanName);
    }
    
    //1.实例化
    if (instanceWrapper == null) {
        //这里就开始构造对象了
        instanceWrapper = this.createBeanInstance(beanName, mbd, args);
    }
    
    //原始对象
    Object bean = instanceWrapper.getWrappedInstance();
    Class<?> beanType = instanceWrapper.getWrappedClass();
    if (beanType != NullBean.class) {
        mbd.resolvedTargetType = beanType;
    }

    synchronized(mbd.postProcessingLock) {
        if (!mbd.postProcessed) {
            try {
                //运行修改合并娃了 "BeanDefinition
                //这里会查Autowired的注入点(InjectedELement),并把这些注入点添加到mbd的属性external
                this.applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
            } catch (Throwable var17) {
                throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Post-processing of merged bean definition failed", var17);
            }

            mbd.postProcessed = true;
        }
    }
    
    //如果当前创建的是单例bean，并且允许循环依赖，并且还在创建过程中，那么则提早暴露
    boolean earlySingletonExposure = mbd.isSingleton() && this.allowCircularReferences && this.isSingletonCurrentlyInCreation(beanName);
    if (earlySingletonExposure) {
        if (this.logger.isTraceEnabled()) {
            this.logger.trace("Eagerly caching bean '" + beanName + "' to allow for resolving potential circular references");
        }
        
        //此时的bean还没有完成属性注入，是一个非常简单的对象
        //构造一个对象工厂添加到singletonFactories中
        //第四次调用后置处理器
        this.addSingletonFactory(beanName, () -> {
            return this.getEarlyBeanReference(beanName, mbd, bean);
        });
    }
    
    //对象已经暴露出去了
    Object exposedObject = bean;

    try {
        //2. 填充属性
        this.populateBean(beanName, mbd, instanceWrapper);
        //3.初始化Bean对象
        exposedObject = this.initializeBean(beanName, exposedObject, mbd);
    } catch (Throwable var18) {
        if (var18 instanceof BeanCreationException && beanName.equals(((BeanCreationException)var18).getBeanName())) {
            throw (BeanCreationException)var18;
        }

        throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Initialization of bean failed", var18);
    }

    if (earlySingletonExposure) {
        // 在解决循环依赖时，当AService的属性注入完了之后，从getSingleton中得到AService AOP之后的代理对象
        Object earlySingletonReference = this.getSingleton(beanName, false);
       
        if (earlySingletonReference != null) {
             // 如果提前暴露的对象和经过了完整的生命周期后的对象相等，则把代理对象赋值给exposedObject
            // 最终会添加到singletonObjects中去
            if (exposedObject == bean) {
                exposedObject = earlySingletonReference;
            }
            // 如果提前暴露的对象和经过了完整的生命周期后的对象不相等
            // allowRawInjectionDespiteWrapping表示在循环依赖时，只能 
            else if (!this.allowRawInjectionDespiteWrapping && this.hasDependentBean(beanName)) {
                String[] dependentBeans = this.getDependentBeans(beanName);
                Set<String> actualDependentBeans = new LinkedHashSet(dependentBeans.length);
                String[] var12 = dependentBeans;
                int var13 = dependentBeans.length;

                for(int var14 = 0; var14 < var13; ++var14) {
                    String dependentBean = var12[var14];
                    if (!this.removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
                        actualDependentBeans.add(dependentBean);
                    }
                }

                if (!actualDependentBeans.isEmpty()) {
                    // 也就是说其他bean没有用到AService的最终版本
                    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 {
        //4.注册销毁流程
        this.registerDisposableBeanIfNecessary(beanName, bean, mbd);
        return exposedObject;
    } catch (BeanDefinitionValidationException var16) {
        throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Invalid destruction signature", var16);
    }
}

构建对象

createBeanInstance创建一个Bean实例，就是返回一个原始对象

org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#createBeanInstance

java 复制代码

protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
    //1.得到Bean的class,并验证class的访问权限是不是public
    Class<?> beanClass = this.resolveBeanClass(mbd, beanName, new Class[0]);
    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());
    } else {
        
        //2. Spring提供给开发者的扩展点
        //如果开发者要自己来实现创建对象的过程，那么，那么就可以提供一个Supplier的实现类
        //当一个BeanDefinition中存在一个Supplier的实现类，Spring就利用这个类的get方法获取实例
        //而不再走Spring的创建逻辑
        Supplier<?> instanceSupplier = mbd.getInstanceSupplier();
        if (instanceSupplier != null) {
            return this.obtainFromSupplier(instanceSupplier, beanName);
        } else if (mbd.getFactoryMethodName() != null) {
            //3. 通过FactoryMethod实例化这个Bean
            // FactoryMethod这个名称在xml中还是比较常见的，即通过工厂方法来创建Bean对象
            return this.instantiateUsingFactoryMethod(beanName, mbd, args);
        } else {
            boolean resolved = false;
            boolean autowireNecessary = false;
            //如果在创建bean时没有手动指定构造方法的参数。那么则看BeanDefinition是不是已经确定了要使用的构造方法
            //注意:如果没有手动指定参数，那么就肯定时自动推断出来的，所以一旦发现当BeanDefinition中已经确定了要使用
            //那么就要使autowireConstructor()方法来构造一个bean对象
            if (args == null) {
                synchronized(mbd.constructorArgumentLock) {
                    if (mbd.resolvedConstructorOrFactoryMethod != null) {
                        resolved = true;
                        autowireNecessary = mbd.constructorArgumentsResolved;
                    }
                }
            }

            if (resolved) {
                //resolved为true表示当前bean的构造方法已经确定了
                return autowireNecessary ? 
                this.autowireConstructor(beanName, mbd, (Constructor[])null, (Object[])null) 
                //如果构造方法已经确定了，但是没有确定构造方法参数，那就表示没有构造方法参数，用无参来实现构造方法
                : this.instantiateBean(beanName, mbd);
            } else {
                //推断构造法
                Constructor<?>[] ctors = this.determineConstructorsFromBeanPostProcessors(beanClass, beanName);
                //通过BeanPostProcessor找出了构造方法
                //或者BeanDefinition的autowire属性为AUTOWIRE_ CONSTRUCTOR
                //或者BeanDefinition中指定J构造方法参数值
                //或者在getBean()时指定了args
                if (ctors == null && mbd.getResolvedAutowireMode() != 3 && !mbd.hasConstructorArgumentValues() && ObjectUtils.isEmpty(args)) {
                    ctors = mbd.getPreferredConstructors();                    
                    return ctors != null ? this.autowireConstructor(beanName, mbd, ctors, (Object[])null) : this.instantiateBean(beanName, mbd);
                } else {
                    //进行构造方法推断并实例化
                    return this.autowireConstructor(beanName, mbd, ctors, args);
                }
            }
        }
    }
}

填充属性

Spring使用实现了InstantiationAwareBeanPostProcessor的后置处理器对实例化后的Bean进行处理

@Autowired注解的AutowiredAnnotationBeanPostProcessor
@Resource注解的CommonAnnotationBeanPostProcessor
byName和byType|

org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#populateBean

java 复制代码

protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {
    if (bw == null) {
        if (mbd.hasPropertyValues()) {
            throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Cannot apply property values to null instance");
        }
    } else {
        //可以提供InstantiationAwareBeanPostProcessor,控制对象的属性注入
        //可以自己写一个InstantiationAwareBeanPostProcessor,然后重写postProcessAfterInstantiation,
        if (!mbd.isSynthetic() && this.hasInstantiationAwareBeanPostProcessors()) {
            Iterator var4 = this.getBeanPostProcessors().iterator();

            while(var4.hasNext()) {
                BeanPostProcessor bp = (BeanPostProcessor)var4.next();
                if (bp instanceof InstantiationAwareBeanPostProcessor) {
                    InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor)bp;
                    if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
                        return;
                    }
                }
            }
        }
        
        //是否在BeanDefinition中设置了属性值
        PropertyValues pvs = mbd.hasPropertyValues() ? mbd.getPropertyValues() : null;
        int resolvedAutowireMode = mbd.getResolvedAutowireMode();
        // byname是根据根据属性名字找bean, 1就是byname
        // bytype足根据属性所对应的set方法的参数类型找bean，2就是bytype
        //找到bean之后都要调set方法进行注入
        if (resolvedAutowireMode == 1 || resolvedAutowireMode == 2) {
            MutablePropertyValues newPvs = new MutablePropertyValues((PropertyValues)pvs);
            if (resolvedAutowireMode == 1) {
                this.autowireByName(beanName, mbd, bw, newPvs);
            }

            if (resolvedAutowireMode == 2) {
                this.autowireByType(beanName, mbd, bw, newPvs);
            }
            
            //总结一下
            //其实就是Spring自动的根据某个类中的set方法来找bean, byName 就是根据基个set方法所对应的属性名去找Bean
            // byType, 就是根据菜个set方法的参数类型去找bean
            //注意，执行完这里的代码之后，这是把属性以及找到的值存在了pvs里面，并没有完成反射赋值
            pvs = newPvs;
        }
        
        //执行完了Spring的自动注入之后，就开始解Autowired,这里叫做实例化回调
        boolean hasInstAwareBpps = this.hasInstantiationAwareBeanPostProcessors();
        boolean needsDepCheck = mbd.getDependencyCheck() != 0;
        
        // @Autowired注解的AutowiredAnnotationBeanPostProcessor
        // @Resource注解的CommonAnnotationBeanPos tProcessor .
        PropertyDescriptor[] filteredPds = null;
        if (hasInstAwareBpps) {
            if (pvs == null) {
                pvs = mbd.getPropertyValues();
            }

            Iterator var9 = this.getBeanPostProcessors().iterator();

            while(var9.hasNext()) {
                BeanPostProcessor bp = (BeanPostProcessor)var9.next();
                if (bp instanceof InstantiationAwareBeanPostProcessor) {
                    InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor)bp;
                    //调BeanPostProcessor 分别解析 @Autowired. @Resource、 @Value， 得到属性值
                    PropertyValues pvsToUse = ibp.postProcessProperties((PropertyValues)pvs, bw.getWrappedInstance(), beanName);
                    if (pvsToUse == null) {
                        if (filteredPds == null) {
                            filteredPds = this.filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
                        }

                        pvsToUse = ibp.postProcessPropertyValues((PropertyValues)pvs, filteredPds, bw.getWrappedInstance(), beanName);
                        if (pvsToUse == null) {
                            return;
                        }
                    }

                    pvs = pvsToUse;
                }
            }
        }

        if (needsDepCheck) {
            if (filteredPds == null) {
                filteredPds = this.filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
            }

            this.checkDependencies(beanName, mbd, filteredPds, (PropertyValues)pvs);
        }

        if (pvs != null) {
            //pvs其实就是属性已经对应的值
            //这里面的逻辑就是通过反射获取到name和value来进行填充了
            this.applyPropertyValues(beanName, mbd, bw, (PropertyValues)pvs);
        }

    }
}

初始化Bean对象

通过initializeBean对填充属性后的实例进行初始化

执行Aware：检查是否有实现着三个Aware，BeanNameAware，BeanClassLoaderAware, BeanFactoryAware
初始化前：BeanPostProcessor
初始化：afterPropertiesSet，init- method
初始化后：BeanPostProcessor, AOP

如果是单列Bean,则加入单例池当中。以后使用单例，从单例池中获取

java 复制代码

protected Object initializeBean(String beanName, Object bean, @Nullable RootBeanDefinition mbd) {
    if (System.getSecurityManager() != null) {
        AccessController.doPrivileged(() -> {
            this.invokeAwareMethods(beanName, bean);
            return null;
        }, this.getAccessControlContext());
    } else {
        //1.执行Aware
        this.invokeAwareMethods(beanName, bean);
    }

    Object wrappedBean = bean;
    if (mbd == null || !mbd.isSynthetic()) {
        //2.初始化前：就是执行所有的后置处理器
        wrappedBean = this.applyBeanPostProcessorsBeforeInitialization(bean, beanName);
    }

    try {
        //3.初始化
        this.invokeInitMethods(beanName, wrappedBean, mbd);
    } catch (Throwable var6) {
        throw new BeanCreationException(mbd != null ? mbd.getResourceDescription() : null, beanName, "Invocation of init method failed", var6);
    }

    if (mbd == null || !mbd.isSynthetic()) {
        //4. 初始化后 AOP就是在这里做的
        wrappedBean = this.applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
    }

    return wrappedBean;
}

执行Aware

就是看看有没有实现这三个Aware，有的话就执行相关方法

org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#invokeAwareMethods

java 复制代码

private void invokeAwareMethods(String beanName, Object bean) {
    if (bean instanceof Aware) {
        if (bean instanceof BeanNameAware) {
            ((BeanNameAware)bean).setBeanName(beanName);
        }

        if (bean instanceof BeanClassLoaderAware) {
            ClassLoader bcl = this.getBeanClassLoader();
            if (bcl != null) {
                ((BeanClassLoaderAware)bean).setBeanClassLoader(bcl);
            }
        }

        if (bean instanceof BeanFactoryAware) {
            ((BeanFactoryAware)bean).setBeanFactory(this);
        }
    }

}

初始化前

拿出所有的后置处理器对bean进行处理，当有一个处理器返回null，将不再调用后面的处理器处理。

org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#applyBeanPostProcessorsBeforeInitialization

java 复制代码

public Object applyBeanPostProcessorsBeforeInitialization(Object existingBean, String beanName) throws BeansException {
    Object result = existingBean;

    Object current;
    for(Iterator var4 = this.getBeanPostProcessors().iterator(); var4.hasNext(); result = current) {
        BeanPostProcessor processor = (BeanPostProcessor)var4.next();
        current = processor.postProcessBeforeInitialization(result, beanName);
        if (current == null) {
            return result;
        }
    }

    return result;
}

完成初始化前的操作有2种方式：

TestBeanPostProcessor 实现了 BeanPostProcessor ，重写postProcessBeforeInitialization方法。
使用@PostConstruct。由CommonAnnotationBeanPostProcessor处理器处理。@PostConstruct是在实例化bean完成之后

初始化时

org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#invokeInitMethods

java 复制代码

protected void invokeInitMethods(String beanName, Object bean, @Nullable RootBeanDefinition mbd) throws Throwable {
    boolean isInitializingBean = bean instanceof InitializingBean;
    //如果Bean实现了InitializingBean，则进行处理
    if (isInitializingBean && (mbd == null || !mbd.isExternallyManagedInitMethod("afterPropertiesSet"))) {
        if (this.logger.isTraceEnabled()) {
            this.logger.trace("Invoking afterPropertiesSet() on bean with name '" + beanName + "'");
        }

        if (System.getSecurityManager() != null) {
            try {
                AccessController.doPrivileged(() -> {
                    //执行afterPropertiesSet
                    ((InitializingBean)bean).afterPropertiesSet();
                    return null;
                }, this.getAccessControlContext());
            } catch (PrivilegedActionException var6) {
                throw var6.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.isExternallyManagedInitMethod(initMethodName)) {
            //执行自定义的init-method
            this.invokeCustomInitMethod(beanName, bean, mbd);
        }
    }

}

实现了InitializingBean接口的类执行其afterPropertiesSet()方法
从BeanDefinition中获取initMethod方法。

可以是xml文件中配置的：

xml 复制代码

<bean id="userService" class="com.spring.service.UserService" init-method="initMethod"/>

或者是@Bean注解配置：

java 复制代码

@Bean(initMethod = "initMethod", destroyMethod = "destroyMethod")
public UserService userService() {
    return new UserService();
}

初始化后

getBeanPostProcessors()获取所有的后置处理器，包括自己定义的

org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#applyBeanPostProcessorsBeforeInitialization

java 复制代码

public Object applyBeanPostProcessorsBeforeInitialization(Object existingBean, String beanName) throws BeansException {
    Object result = existingBean;

    Object current;
    for(Iterator var4 = this.getBeanPostProcessors().iterator(); var4.hasNext(); result = current) {
        BeanPostProcessor processor = (BeanPostProcessor)var4.next();
        current = processor.postProcessBeforeInitialization(result, beanName);
        if (current == null) {
            return result;
        }
    }

    return result;
}

特别注意：.在实例化前如果获取到了bean那么将不执行spring正常创建bean的流程，而是直接调用初始化后的方法完成初始化后的操作。

注册销毁

Spring在容器关闭时，会remove容器里所有的Bean。如果需要某些Bean在被Spring删除前执行一些逻辑，Spring也可以做到，那么就需要在Bean完成创建时将这个Bean注册为DisposableBean。

Bean是否有注册为DisposableBean的资格，是否有destroyMethod。
是否有执行销毁方法的后置处理器。
DisposableBeanAdapter
1. 推断destoryMethod
2. 指定的destroyMethod, AutoCloseable(close方法)，没有shutdown方法
完成注册：就是将完成包装的DisposableBeanAdapter, put到销毁容器中。

是否有注册为DisposableBean的资格

java 复制代码

protected void registerDisposableBeanIfNecessary(String beanName, Object bean, RootBeanDefinition mbd) {
    AccessControlContext acc = System.getSecurityManager() != null ? this.getAccessControlContext() : null;
    //1. Bean是否有注册为DisposableBean的资格
    、、 不能是原型Bean（因为原型Bean不会放到Spring容器中），再通过requiresDestruction方法去判断一下是否需要销毁。
    if (!mbd.isPrototype() && this.requiresDestruction(bean, mbd)) {
        if (mbd.isSingleton()) {
            this.registerDisposableBean(beanName, new DisposableBeanAdapter(bean, beanName, mbd, this.getBeanPostProcessors(), acc));
        } else {
            Scope scope = (Scope)this.scopes.get(mbd.getScope());
            if (scope == null) {
                throw new IllegalStateException("No Scope registered for scope name '" + mbd.getScope() + "'");
            }

            scope.registerDestructionCallback(beanName, new DisposableBeanAdapter(bean, beanName, mbd, this.getBeanPostProcessors(), acc));
        }
    }

}

判断某个bean是否拥有destroyMethod方法

java 复制代码

protected boolean requiresDestruction(Object bean, RootBeanDefinition mbd) {
    // 1. 实现了DisposableBean接口或AutoCloseable按口
    // 2. BeanDefinition 中定义了destroyMethodName
    // 3. 类中是否存在@PreDestroy注解的方法
    // 4. 由DestructionAwareBeanPostProcessor判断是否需要销毁
    return bean.getClass() != NullBean.class 
    && (DisposableBeanAdapter.hasDestroyMethod(bean, mbd) 
    || this.hasDestructionAwareBeanPostProcessors() 
    && DisposableBeanAdapter.hasApplicableProcessors(bean, this.getBeanPostProcessors()));
}

是否有执行销毁方法的后置处理器

由DestructionAwareBeanPostProcessor去标识一个Bean是否需要销毁,并且交给这个后置处理器去处理销毁前的逻辑。

java 复制代码

    public static boolean hasApplicableProcessors(Object bean, List<BeanPostProcessor> postProcessors) {
        if (!CollectionUtils.isEmpty(postProcessors)) {
            for (BeanPostProcessor processor : postProcessors) {
                if (processor instanceof DestructionAwareBeanPostProcessor) {
                    DestructionAwareBeanPostProcessor dabpp = (DestructionAwareBeanPostProcessor) processor;
                    if (dabpp.requiresDestruction(bean)) {
                        return true;
                    }
                }
            }
        }
        return false;
    }

DisposableBeanAdapter

当一个bean有资格成为DisposableBean的时候，Spring不是将这个Bean直接放到需要销毁的容器当中，而是将其包装为DisposableBeanAdapter对象放入销毁容器中。

java 复制代码

    protected void registerDisposableBeanIfNecessary(String beanName, Object bean, RootBeanDefinition mbd) {
        AccessControlContext acc = (System.getSecurityManager() != null ? getAccessControlContext() : null);
        if (!mbd.isPrototype() && requiresDestruction(bean, mbd)) {
            if (mbd.isSingleton()) {
    
                registerDisposableBean(beanName,
                        new DisposableBeanAdapter(bean, beanName, mbd, getBeanPostProcessors(), acc));
            }
            else {
                
                Scope scope = this.scopes.get(mbd.getScope());
                if (scope == null) {
                    throw new IllegalStateException("No Scope registered for scope name '" + mbd.getScope() + "'");
                }
                scope.registerDestructionCallback(beanName,
                        new DisposableBeanAdapter(bean, beanName, mbd, getBeanPostProcessors(), acc));
            }
        }
    }

单例Bean销毁容器是一个以beanName为key，DisposableBean为value的Map。

java 复制代码

public class DefaultSingletonBeanRegistry extends SimpleAliasRegistry implements SingletonBeanRegistry {

    /** Disposable bean instances: bean name to disposable instance. */ 
    private final Map<String, Object> disposableBeans = new LinkedHashMap<>();

    public void registerDisposableBean(String beanName, DisposableBean bean) {
        synchronized (this.disposableBeans) {
            this.disposableBeans.put(beanName, bean);
        }
    }
}

推断destoryMethod

在将Bean包装为DisposableBeanAdapter时，要去推断destoryMethod。

java 复制代码

    //挑选到底执行哪一个销毁方法，仅仅返回方法名称。
    @Nullable
    private String inferDestroyMethodIfNecessary(Object bean, RootBeanDefinition beanDefinition) {
        //1.指定的destroyMethod就用
        String destroyMethodName = beanDefinition.getDestroyMethodName();

        //2.没有指定destroyMethodName，没有(inferred)，而且没有实现AutoCloseable(close方法)，返回null
        if (AbstractBeanDefinition.INFER_METHOD.equals(destroyMethodName) ||
                (destroyMethodName == null && bean instanceof AutoCloseable)) {

            // Only perform destroy method inference or Closeable detection
            // in case of the bean not explicitly implementing DisposableBean

            // 3.有(inferred)，或者实现了AutoCloseable(close方法)
            if (!(bean instanceof DisposableBean)) {
                try {
                    //没有实现DisposableBean(destroy方法)，并且有close方法就返回。
                    return bean.getClass().getMethod(CLOSE_METHOD_NAME).getName();
                }
                catch (NoSuchMethodException ex) {
                    try {
                        //没有close会报错，看有没有shutdown方法,有就返回
                        return bean.getClass().getMethod(SHUTDOWN_METHOD_NAME).getName();
                    }
                    catch (NoSuchMethodException ex2) {
                        //没有shutdown方法报错，但异常被吞了，不做任何处理，返回null
                        // no candidate destroy method found
                    }
                }
            }
            return null;
        }
        return (StringUtils.hasLength(destroyMethodName) ? destroyMethodName : null);
    }

推断方法名：
1. 如果指定了DestroyMethod 就返回指定的。
2. 有推断标记或者实现了AutoCloseable接口，再看是否实现DisposableBean接口的；如果实现了就直接返回null,如果没有实现就去推断是否有公有close方法，有就返回，没有再去推断是否有公有的shutdown方法，有就返回。
推断方法：

java 复制代码

    @Nullable
    private Method findDestroyMethod(String name) {
        return (this.nonPublicAccessAllowed ?
                BeanUtils.findMethodWithMinimalParameters(this.bean.getClass(), name) :
                BeanUtils.findMethodWithMinimalParameters(this.bean.getClass().getMethods(), name));
    }

根据方法名推断方法,获取最少参数的方法对象method。如果method的参数大于1会报错，如果等于1且参数类型不为boolean类型也会报错。

完成注册

就是将完成包装的DisposableBeanAdapter, put到销毁容器中。

添加到单例池

org.springframework.beans.factory.support.DefaultSingletonBeanRegistry#getSingleton(java.lang.String, org.springframework.beans.factory.ObjectFactory)

java 复制代码

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);
        //如果不存在实例，则创建单例bean
        if (singletonObject == null) {
            //当前bean正在销毁
            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 (this.logger.isDebugEnabled()) {
                this.logger.debug("Creating shared instance of singleton bean '" + beanName + "'");
            }

            this.beforeSingletonCreation(beanName);
            boolean newSingleton = false;
            boolean recordSuppressedExceptions = this.suppressedExceptions == null;
            if (recordSuppressedExceptions) {
                this.suppressedExceptions = new LinkedHashSet();
            }

            try {
                singletonObject = singletonFactory.getObject();
                newSingleton = true;
            } catch (IllegalStateException var16) {
                singletonObject = this.singletonObjects.get(beanName);
                if (singletonObject == null) {
                    throw var16;
                }
            } catch (BeanCreationException var17) {
                BeanCreationException ex = var17;
                if (recordSuppressedExceptions) {
                    Iterator var8 = this.suppressedExceptions.iterator();

                    while(var8.hasNext()) {
                        Exception suppressedException = (Exception)var8.next();
                        ex.addRelatedCause(suppressedException);
                    }
                }

                throw ex;
            } finally {
                if (recordSuppressedExceptions) {
                    this.suppressedExceptions = null;
                }
                //将刚刚正在创建的beanName从SingletonCreation中移除
                this.afterSingletonCreation(beanName);
            }

            if (newSingleton) {
                //将创建好的单例bean添加到单例池singletonObjects中
                this.addSingleton(beanName, singletonObject);
            }
        }

        return singletonObject;
    }
}

销毁

这里只关心单例Bean的销毁。Spring容器关闭时，会去销毁单例Bean。如果不去手动关闭容器，那么以上destroyMethod都不能执行。

总结销毁逻辑：

销毁前：如果有@PreDestory 注解的方法就执行
如果有自定义的销毁后置处理器，通过 postProcessBeforeDestruction 方法调用destoryBean逐一销毁Bean
销毁时：如果实现了destroyMethod就执行 destory方法
执行客户自定义销毁：调用 invokeCustomDestoryMethod执行在Bean上自定义的destroyMethod方法

手动执行close方法

org.springframework.context.support.AbstractApplicationContext#close

java 复制代码

public void close() {
    synchronized(this.startupShutdownMonitor) {
        this.doClose();
        if (this.shutdownHook != null) {
            try {
                Runtime.getRuntime().removeShutdownHook(this.shutdownHook);
            } catch (IllegalStateException var4) {
            }
        }

    }
}

protected void doClose() {
    if (this.active.get() && this.closed.compareAndSet(false, true)) {
        if (this.logger.isDebugEnabled()) {
            this.logger.debug("Closing " + this);
        }

        LiveBeansView.unregisterApplicationContext(this);

        try {
            this.publishEvent((ApplicationEvent)(new ContextClosedEvent(this)));
        } catch (Throwable var3) {
            this.logger.warn("Exception thrown from ApplicationListener handling ContextClosedEvent", var3);
        }

        if (this.lifecycleProcessor != null) {
            try {
                this.lifecycleProcessor.onClose();
            } catch (Throwable var2) {
                this.logger.warn("Exception thrown from LifecycleProcessor on context close", var2);
            }
        }

        this.destroyBeans();
        this.closeBeanFactory();
        this.onClose();
        if (this.earlyApplicationListeners != null) {
            this.applicationListeners.clear();
            this.applicationListeners.addAll(this.earlyApplicationListeners);
        }

        this.active.set(false);
    }

}

protected void destroyBeans() {
    this.getBeanFactory().destroySingletons();
}

org.springframework.beans.factory.support.DefaultSingletonBeanRegistry#destroySingletons

java 复制代码

public void destroySingletons() {
    if (this.logger.isTraceEnabled()) {
        this.logger.trace("Destroying singletons in " + this);
    }

    synchronized(this.singletonObjects) {
        this.singletonsCurrentlyInDestruction = true;
    }

    String[] disposableBeanNames;
    synchronized(this.disposableBeans) {//注册销毁的容器
        disposableBeanNames = StringUtils.toStringArray(this.disposableBeans.keySet());
    }
    
    //对注册销毁容器进行遍历
    for(int i = disposableBeanNames.length - 1; i >= 0; --i) {
        this.destroySingleton(disposableBeanNames[i]);
    }

    this.containedBeanMap.clear();
    this.dependentBeanMap.clear();
    this.dependenciesForBeanMap.clear();
    this.clearSingletonCache();
}

public void destroySingleton(String beanName) {
    this.removeSingleton(beanName);
    DisposableBean disposableBean;
    synchronized(this.disposableBeans) {
        disposableBean = (DisposableBean)this.disposableBeans.remove(beanName);
    }

    this.destroyBean(beanName, disposableBean);
}


protected void destroyBean(String beanName, @Nullable DisposableBean bean) {
    Set dependencies;
    //先处理依赖的bean
    synchronized(this.dependentBeanMap) {
        dependencies = (Set)this.dependentBeanMap.remove(beanName);
    }

    if (dependencies != null) {
        if (this.logger.isTraceEnabled()) {
            this.logger.trace("Retrieved dependent beans for bean '" + beanName + "': " + dependencies);
        }

        Iterator var4 = dependencies.iterator();

        while(var4.hasNext()) {
            String dependentBeanName = (String)var4.next();
            this.destroySingleton(dependentBeanName);
        }
    }

    if (bean != null) {
        try {
            //bean的销毁，这里就执行@PreDestroy注解的方法  和 自定义的销毁后置处理器
            bean.destroy();
        } catch (Throwable var13) {
            if (this.logger.isWarnEnabled()) {
                this.logger.warn("Destruction of bean with name '" + beanName + "' threw an exception", var13);
            }
        }
    }

    Set containedBeans;
    synchronized(this.containedBeanMap) {
        containedBeans = (Set)this.containedBeanMap.remove(beanName);
    }

    if (containedBeans != null) {
        Iterator var15 = containedBeans.iterator();

        while(var15.hasNext()) {
            String containedBeanName = (String)var15.next();
            this.destroySingleton(containedBeanName);
        }
    }

    synchronized(this.dependentBeanMap) {
        Iterator it = this.dependentBeanMap.entrySet().iterator();

        while(it.hasNext()) {
            Entry<String, Set<String>> entry = (Entry)it.next();
            Set<String> dependenciesToClean = (Set)entry.getValue();
            dependenciesToClean.remove(beanName);
            if (dependenciesToClean.isEmpty()) {
                it.remove();
            }
        }
    }

    this.dependenciesForBeanMap.remove(beanName);
}

面试题专栏

Java面试题专栏已上线，欢迎访问。

如果你不知道简历怎么写，简历项目不知道怎么包装；
如果简历中有些内容你不知道该不该写上去；
如果有些综合性问题你不知道怎么答；

那么可以私信我，我会尽我所能帮助你。