Spring 自动装配之依赖注入(DI)

it2025-09-12 5

依赖注入发生的时间

当 Spring IOC 容器完成了 Bean 定义资源的定位、载入和解析注册以后，IOC 容器中已经管理类 Bean 定义的相关数据(具体存放在DefaultListableBeanFactory Map<String, BeanDefinition> beanDefinitionMap = new ConcurrentHashMap(256))，但是此时 IOC 容器还没有对所管理的 Bean 进行依赖注入，依赖注入在以下两种情况发生：

1)、用户第一次调用 getBean()方法时，IOC 容器触发依赖注入。

2)、当用户在配置文件中将元素配置了 lazy-init=false 属性，即让容器在解析注册 Bean 定义时进行预实例化，触发依赖注入。

BeanFactory 接口定义了 Spring IOC 容器的基本功能规范，是 Spring IOC 容器所应遵守的最底层和最基本的编程规范。BeanFactory 接口中定义了几个 getBean()方法，就是用户向 IOC 容器索取管理的 Bean 的方法，我们通过分析其子类的具体实现，理解 Spring IOC 容器在用户索取 Bean 时如何完成依赖注入。来看看DefaultListableBeanFactory 的依赖图：

寻找获取 Bean 的入口

AbstractBeanFactory 的 getBean()相关方法的源码如下：

//获取 IOC 容器中指定名称的 Bean @Override public Object getBean(String name) throws BeansException { //doGetBean 才是真正向 IOC 容器获取被管理 Bean 的过程 return doGetBean(name, null, null, false); } //真正实现向 IOC 容器获取 Bean 的功能，也是触发依赖注入功能的地方 protected <T> T doGetBean(String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly) throws BeansException { String beanName = this.transformedBeanName(name); //先从缓存中取是否已经有被创建过的单态类型的 Bean //对于单例模式的 Bean 整个 IOC 容器中只创建一次，不需要重复创建 Object sharedInstance = this.getSingleton(beanName); Object bean; if (sharedInstance != null && args == null) { if (this.logger.isTraceEnabled()) { if (this.isSingletonCurrentlyInCreation(beanName)) { this.logger.trace("Returning eagerly cached instance of singleton bean '" + beanName + "' that is not fully initialized yet - a consequence of a circular reference"); } else { this.logger.trace("Returning cached instance of singleton bean '" + beanName + "'"); } } //获取给定 Bean 的实例对象，主要是完成 FactoryBean 的相关处理 //注意：BeanFactory 是管理容器中 Bean 的工厂，而 FactoryBean 是 //创建创建对象的工厂 Bean，两者之间有区别 bean = this.getObjectForBeanInstance(sharedInstance, name, beanName, (RootBeanDefinition)null); } else { if (this.isPrototypeCurrentlyInCreation(beanName)) { throw new BeanCurrentlyInCreationException(beanName); } BeanFactory parentBeanFactory = this.getParentBeanFactory(); if (parentBeanFactory != null && !this.containsBeanDefinition(beanName)) { String nameToLookup = this.originalBeanName(name); if (parentBeanFactory instanceof AbstractBeanFactory) { return ((AbstractBeanFactory)parentBeanFactory).doGetBean(nameToLookup, requiredType, args, typeCheckOnly); } if (args != null) { return parentBeanFactory.getBean(nameToLookup, args); } if (requiredType != null) { return parentBeanFactory.getBean(nameToLookup, requiredType); } return parentBeanFactory.getBean(nameToLookup); } if (!typeCheckOnly) { this.markBeanAsCreated(beanName); } try { RootBeanDefinition mbd = this.getMergedLocalBeanDefinition(beanName); this.checkMergedBeanDefinition(mbd, beanName, args); String[] dependsOn = mbd.getDependsOn(); String[] var11; if (dependsOn != null) { var11 = dependsOn; int var12 = dependsOn.length; for(int var13 = 0; var13 < var12; ++var13) { String dep = var11[var13]; if (this.isDependent(beanName, dep)) { throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Circular depends-on relationship between '" + beanName + "' and '" + dep + "'"); } this.registerDependentBean(dep, beanName); try { this.getBean(dep); } catch (NoSuchBeanDefinitionException var24) { throw new BeanCreationException(mbd.getResourceDescription(), beanName, "'" + beanName + "' depends on missing bean '" + dep + "'", var24); } } } if (mbd.isSingleton()) { //创建单例模式 Bean 的实例对象 sharedInstance = this.getSingleton(beanName, () -> { try { //创建一个指定 Bean 实例对象，如果有父级继承，则合并子类和父类的定义 return this.createBean(beanName, mbd, args); } catch (BeansException var5) { this.destroySingleton(beanName); throw var5; } }); bean = this.getObjectForBeanInstance(sharedInstance, name, beanName, mbd); } else if (mbd.isPrototype()) { //IOC 容器创建原型模式 Bean 实例对象 var11 = null; Object prototypeInstance; try { this.beforePrototypeCreation(beanName); prototypeInstance = this.createBean(beanName, mbd, args); } finally { this.afterPrototypeCreation(beanName); } bean = this.getObjectForBeanInstance(prototypeInstance, name, beanName, mbd); } else { String scopeName = mbd.getScope(); Scope 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, () -> { this.beforePrototypeCreation(beanName); Object var4; try { var4 = this.createBean(beanName, mbd, args); } finally { this.afterPrototypeCreation(beanName); } return var4; }); bean = this.getObjectForBeanInstance(scopedInstance, name, beanName, mbd); } catch (IllegalStateException var23) { throw new BeanCreationException(beanName, "Scope '" + scopeName + "' is not active for the current thread; consider defining a scoped proxy for this bean if you intend to refer to it from a singleton", var23); } } } catch (BeansException var26) { this.cleanupAfterBeanCreationFailure(beanName); throw var26; } } if (requiredType != null && !requiredType.isInstance(bean)) { try { T convertedBean = this.getTypeConverter().convertIfNecessary(bean, requiredType); if (convertedBean == null) { throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass()); } else { return convertedBean; } } catch (TypeMismatchException var25) { if (this.logger.isTraceEnabled()) { this.logger.trace("Failed to convert bean '" + name + "' to required type '" + ClassUtils.getQualifiedName(requiredType) + "'", var25); } throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass()); } } else { return bean; } }

开始实例化

AbstractAutowireCapableBeanFactory 类实现了 ObjectFactory 接口，创建容器指定的 Bean 实例对象，同时还对创建的 Bean 实例对象进行初始化处理。其创建 Bean 实例对象的方法源码如下：

protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) throws BeanCreationException { ######沈略部分代码######### Object beanInstance; try { //创建 Bean 的入口 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); } } //真正创建 Bean 的方法 protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) throws BeanCreationException { //封装被创建的 Bean 对象 BeanWrapper instanceWrapper = null; if (mbd.isSingleton()) { instanceWrapper = (BeanWrapper)this.factoryBeanInstanceCache.remove(beanName); } if (instanceWrapper == null) { instanceWrapper = this.createBeanInstance(beanName, mbd, args); } Object bean = instanceWrapper.getWrappedInstance(); Class<?> beanType = instanceWrapper.getWrappedClass(); if (beanType != NullBean.class) { mbd.resolvedTargetType = beanType; } ##########沈略部分代码############### Object exposedObject = bean; try { this.populateBean(beanName, mbd, instanceWrapper); 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); } ##########沈略部分代码############### try { this.registerDisposableBeanIfNecessary(beanName, bean, mbd); return exposedObject; } catch (BeanDefinitionValidationException var16) { throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Invalid destruction signature", var16); } }

通过上面的源码注释，我们看到具体的依赖注入实现其实就在以下两个方法中：

1)、createBeanInstance()方法，生成 Bean 所包含的 java 对象实例。

2)、populateBean()方法，对 Bean 属性的依赖注入进行处理。

选择 Bean 实例化策略

在 createBeanInstance()方法中，根据指定的初始化策略，使用简单工厂、工厂方法或者容器的自动装

protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) { 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 { Supplier<?> instanceSupplier = mbd.getInstanceSupplier(); if (instanceSupplier != null) { return this.obtainFromSupplier(instanceSupplier, beanName); } else if (mbd.getFactoryMethodName() != null) { return this.instantiateUsingFactoryMethod(beanName, mbd, args); } else { //使用容器的自动装配方法进行实例化 boolean resolved = false; boolean autowireNecessary = false; if (args == null) { Object var8 = mbd.constructorArgumentLock; synchronized(mbd.constructorArgumentLock) { if (mbd.resolvedConstructorOrFactoryMethod != null) { resolved = true; autowireNecessary = mbd.constructorArgumentsResolved; } } } if (resolved) { return autowireNecessary ? /使用容器的自动装配特性，调用匹配的构造方法实例化 this.autowireConstructor(beanName, mbd, (Constructor[])null, (Object[])null) : //使用默认的无参构造方法实例化 this.instantiateBean(beanName, mbd); } else { Constructor<?>[] ctors = this.determineConstructorsFromBeanPostProcessors(beanClass, beanName); 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 { /使用容器的自动装配特性，调用匹配的构造方法实例化 return this.autowireConstructor(beanName, mbd, ctors, args); } } } } } //使用默认的无参构造方法实例化 protected BeanWrapper instantiateBean(String beanName, RootBeanDefinition mbd) { try { Object beanInstance; if (System.getSecurityManager() != null) { beanInstance = AccessController.doPrivileged(() -> { return thisx.getInstantiationStrategy().instantiate(mbd, beanName, this); }, this.getAccessControlContext()); } else { beanInstance = this.getInstantiationStrategy().instantiate(mbd, beanName, this); } BeanWrapper bw = new BeanWrapperImpl(beanInstance); this.initBeanWrapper(bw); return bw; } catch (Throwable var6) { throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Instantiation of bean failed", var6); } }

经过对上面的代码分析，我们可以看出，对使用工厂方法和自动装配特性的 Bean 的实例化相当比较清楚，调用相应的工厂方法或者参数匹配的构造方法即可完成实例化对象的工作，但是对于我们最常使用的默认无参构造方法就需要使用相应的初始化策略(JDK 的反射机制或者 CGLib)来进行初始化了，在方法 getInstantiationStrategy().instantiate()中就具体实现类使用初始策略实例化对象

执行 Bean 实例化

在使用默认的无参构造方法创建 Bean 的实例化对象时，方法 getInstantiationStrategy().instantiate() 调用了 SimpleInstantiationStrategy 类中的实例化 Bean 的方法，其源码如下：

public Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner) { //如果 Bean 定义中没有方法覆盖，则就不需要 CGLib 父类类的方法 if (!bd.hasMethodOverrides()) { Object var5 = bd.constructorArgumentLock; Constructor constructorToUse; synchronized(bd.constructorArgumentLock) { constructorToUse = (Constructor)bd.resolvedConstructorOrFactoryMethod; if (constructorToUse == null) { Class<?> clazz = bd.getBeanClass(); if (clazz.isInterface()) { throw new BeanInstantiationException(clazz, "Specified class is an interface"); } try { //使用 JDK 的反射机制，判断要实例化的 Bean 是否是接口 if (System.getSecurityManager() != null) { clazz.getClass(); constructorToUse = (Constructor)AccessController.doPrivileged(() -> { return clazz.getDeclaredConstructor(); }); } else { constructorToUse = clazz.getDeclaredConstructor(); } bd.resolvedConstructorOrFactoryMethod = constructorToUse; } catch (Throwable var9) { throw new BeanInstantiationException(clazz, "No default constructor found", var9); } } } //使用 BeanUtils 实例化，通过反射机制调用”构造方法.newInstance(arg)”来进行实例化 return BeanUtils.instantiateClass(constructorToUse, new Object[0]); } else { return this.instantiateWithMethodInjection(bd, beanName, owner); } }

准备依赖注入

在前面的分析中我们已经了解到 Bean 的依赖注入主要分为两个步骤，首先调用 createBeanInstance() 方法生成 Bean 所包含的 Java 对象实例。然后，调用 populateBean()方法，对 Bean 属性的依赖注入进行处理。

上面我们已经分析了容器初始化生成 Bean 所包含的 Java 实例对象的过程，现在我们继续分析生成对象后，Spring IOC 容器是如何将 Bean 的属性依赖关系注入 Bean 实例对象中并设置好的，回到 AbstractAutowireCapableBeanFactory 的 populateBean()方法，对属性依赖注入的代码如下：

protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) { ##########沈略部分代码############### if (continueWithPropertyPopulation) { //获取容器在解析 Bean 定义资源时为 BeanDefiniton PropertyValues pvs = mbd.hasPropertyValues() ? mbd.getPropertyValues() : null; if (mbd.getResolvedAutowireMode() == 1 || mbd.getResolvedAutowireMode() == 2) { MutablePropertyValues newPvs = new MutablePropertyValues((PropertyValues)pvs); if (mbd.getResolvedAutowireMode() == 1) { this.autowireByName(beanName, mbd, bw, newPvs); } if (mbd.getResolvedAutowireMode() == 2) { this.autowireByType(beanName, mbd, bw, newPvs); } pvs = newPvs; } boolean hasInstAwareBpps = this.hasInstantiationAwareBeanPostProcessors(); boolean needsDepCheck = mbd.getDependencyCheck() != 0; 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; 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) { //对属性进行注入 this.applyPropertyValues(beanName, mbd, bw, (PropertyValues)pvs); } } } } //将 Bean 属性设置到生成的实例对象上 protected void applyPropertyValues(String beanName, BeanDefinition mbd, BeanWrapper bw, PropertyValues pvs) { if (!pvs.isEmpty()) { if (System.getSecurityManager() != null && bw instanceof BeanWrapperImpl) { ((BeanWrapperImpl)bw).setSecurityContext(this.getAccessControlContext()); } MutablePropertyValues mpvs = null; List original; if (pvs instanceof MutablePropertyValues) { mpvs = (MutablePropertyValues)pvs; if (mpvs.isConverted()) { try { bw.setPropertyValues(mpvs); return; } catch (BeansException var18) { throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Error setting property values", var18); } } original = mpvs.getPropertyValueList(); } else { original = Arrays.asList(pvs.getPropertyValues()); } TypeConverter converter = this.getCustomTypeConverter(); if (converter == null) { converter = bw; } BeanDefinitionValueResolver valueResolver = new BeanDefinitionValueResolver(this, beanName, mbd, (TypeConverter)converter); List<PropertyValue> deepCopy = new ArrayList(original.size()); boolean resolveNecessary = false; Iterator var11 = original.iterator(); while(true) { while(var11.hasNext()) { PropertyValue pv = (PropertyValue)var11.next(); if (pv.isConverted()) { deepCopy.add(pv); } else { String propertyName = pv.getName(); Object originalValue = pv.getValue(); Object resolvedValue = valueResolver.resolveValueIfNecessary(pv, originalValue); Object convertedValue = resolvedValue; boolean convertible = bw.isWritableProperty(propertyName) && !PropertyAccessorUtils.isNestedOrIndexedProperty(propertyName); if (convertible) { convertedValue = this.convertForProperty(resolvedValue, propertyName, bw, (TypeConverter)converter); } if (resolvedValue == originalValue) { if (convertible) { pv.setConvertedValue(convertedValue); } deepCopy.add(pv); } else if (convertible && originalValue instanceof TypedStringValue && !((TypedStringValue)originalValue).isDynamic() && !(convertedValue instanceof Collection) && !ObjectUtils.isArray(convertedValue)) { pv.setConvertedValue(convertedValue); deepCopy.add(pv); } else { resolveNecessary = true; deepCopy.add(new PropertyValue(pv, convertedValue)); } } } if (mpvs != null && !resolveNecessary) { mpvs.setConverted(); } //进行属性依赖注入 try { bw.setPropertyValues(new MutablePropertyValues(deepCopy)); return; } catch (BeansException var19) { throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Error setting property values", var19); } } } }

至此 Spring IOC 容器对 Bean 定义资源文件的定位，载入、解析和依赖注入已经全部分析完毕，现在 Spring IOC 容器中管理了一系列靠依赖关系联系起来的 Bean，程序不需要应用自己手动创建所需的对象，Spring IOC 容器会在我们使用的时候自动为我们创建，并且为我们注入好相关的依赖，这就是 Spring 核心功能的控制反转和依赖注入的相关功能。

DI时序图

关于 FactoryBean 和 BeanFactory

在 Spring 中，有两个很容易混淆的类：BeanFactory 和 FactoryBean。 BeanFactory：Bean 工厂，是一个工厂(Factory)，我们 Spring IOC 容器的最顶层接口就是这个 BeanFactory，它的作用是管理 Bean，即实例化、定位、配置应用程序中的对象及建立这些对象间的依赖。 FactoryBean：工厂 Bean，是一个 Bean，作用是产生其他 bean 实例。通常情况下，这种 Bean 没有什么特别的要求，仅需要提供一个工厂方法，该方法用来返回其他 Bean 实例。通常情况下，Bean 无须自己实现工厂模式，Spring 容器担任工厂角色；但少数情况下，容器中的 Bean 本身就是工厂，其作用是产生其它 Bean 实例。当用户使用容器本身时，可以使用转义字符”&”来得到 FactoryBean 本身，以区别通过 FactoryBean 产生的实例对象和 FactoryBean 对象本身。在 BeanFactory 中通过如下代码定义了该转义字符： String FACTORY_BEAN_PREFIX = "&"; 如果 myJndiObject 是一个 FactoryBean，则使用&myJndiObject 得到的是 myJndiObject 对象，而不是 myJndiObject 产生出来的对象

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