Spring DI原理

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mob604756fb8908 2020-03-26 09:41:00 ©著作权

文章标签 spring框架实例化属性值依赖注入 ide 文章分类 其他其它

Spring DI原理
https://blog.51cto.com/u_15127629/4783109

DI（Dependency Injection）依赖注入：就是指对象是被动接受依赖类而不是自己主动去找，换句话说就是指对象不是从容器中查找它依赖的类，而是在容器实例化对象的时候主动将它依赖的类注入给它。

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，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<bean>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AbstractBeanFactory 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。

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

001002003004005006007008009010011012013014015016017018019020021022023024025026027028029030031032033034035036037038039040041042043044045046047048049050051052053054055056057058059060061062063064065066067068069070071072073074075076077078079080081082083084085086087088089090091092093094095096097098099100101102103104105106107108109110111112113114115116117118119120121122123124125126127128129130131132133134135136137138139140141142143144145146147148149150151152153154155156157158159160161162163164165166167168169170171172173174175176177178179180181182183184185186187188189190191192193194195196197198199200201202203204205206207208209210211212213214215216217218219220221222223224225	`//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``);` `}` `//获取IOC容器中指定名称和类型的Bean` `@Override` `public``<T> T getBean(String name,` `@Nullable``Class<T> requiredType)` `throws``BeansException {` `//doGetBean才是真正向IoC容器获取被管理Bean的过程` `return``doGetBean(name, requiredType,` `null``,` `false``);` `}` `//获取IOC容器中指定名称和参数的Bean` `@Override` `public``Object getBean(String name, Object... args)` `throws``BeansException {` `//doGetBean才是真正向IoC容器获取被管理Bean的过程` `return``doGetBean(name,` `null``, args,` `false``);` `}` `//获取IOC容器中指定名称、类型和参数的Bean` `public``<T> T getBean(String name,` `@Nullable``Class<T> requiredType,` `@Nullable``Object... args)` `throws``BeansException {` `//doGetBean才是真正向IoC容器获取被管理Bean的过程` `return``doGetBean(name, requiredType, args,` `false``);` `}` `@SuppressWarnings``(``"unchecked"``)` `//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(``final``String name,` `@Nullable``final``Class<T> requiredType,` `@Nullable``final``Object[] args,` `boolean``typeCheckOnly)` `throws``BeansException {` `//根据指定的名称获取被管理Bean的名称，剥离指定名称中对容器的相关依赖` `//如果指定的是别名，将别名转换为规范的Bean名称` `final``String beanName = transformedBeanName(name);` `Object bean;` `// Eagerly check singleton cache for manually registered singletons.` `//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 = getSingleton(beanName);` `//IOC容器创建单例模式Bean实例对象` `if``(sharedInstance !=` `null``&& args ==` `null``) {` `if``(logger.isDebugEnabled()) {` `//如果指定名称的Bean在容器中已有单例模式的Bean被创建` `//直接返回已经创建的Bean` `if``(isSingletonCurrentlyInCreation(beanName)) {` `logger.debug(``"Returning eagerly cached instance of singleton bean '"``+ beanName +` `"' that is not fully initialized yet - a consequence of a circular reference"``);` `}` `else``{` `logger.debug(``"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 = getObjectForBeanInstance(sharedInstance, name, beanName,` `null``);` `}` `else``{` `// Fail if we're already creating this bean instance:` `// We're assumably within a circular reference.` `//缓存没有正在创建的单例模式Bean` `//缓存中已经有已经创建的原型模式Bean` `//但是由于循环引用的问题导致实例化对象失败` `if``(isPrototypeCurrentlyInCreation(beanName)) {` `throw``new``BeanCurrentlyInCreationException(beanName);` `}` `// Check if bean definition exists in this factory.` `//对IOC容器中是否存在指定名称的BeanDefinition进行检查，首先检查是否` `//能在当前的BeanFactory中获取的所需要的Bean，如果不能则委托当前容器` `//的父级容器去查找，如果还是找不到则沿着容器的继承体系向父级容器查找` `BeanFactory parentBeanFactory = getParentBeanFactory();` `//当前容器的父级容器存在，且当前容器中不存在指定名称的Bean` `if``(parentBeanFactory !=` `null``&& !containsBeanDefinition(beanName)) {` `// Not found -> check parent.` `//解析指定Bean名称的原始名称` `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.` `//委派父级容器根据指定名称和显式的参数查找` `return``(T) parentBeanFactory.getBean(nameToLookup, args);` `}` `else``{` `// No args -> delegate to standard getBean method.` `//委派父级容器根据指定名称和类型查找` `return``parentBeanFactory.getBean(nameToLookup, requiredType);` `}` `}` `//创建的Bean是否需要进行类型验证，一般不需要` `if``(!typeCheckOnly) {` `//向容器标记指定的Bean已经被创建` `markBeanAsCreated(beanName);` `}` `try``{` `//根据指定Bean名称获取其父级的Bean定义` `//主要解决Bean继承时子类合并父类公共属性问题` `final``RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);` `checkMergedBeanDefinition(mbd, beanName, args);` `// Guarantee initialization of beans that the current bean depends on.` `//获取当前Bean所有依赖Bean的名称` `String[] dependsOn = mbd.getDependsOn();` `//如果当前Bean有依赖Bean` `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 +` `"'"``);` `}` `//递归调用getBean方法，获取当前Bean的依赖Bean` `registerDependentBean(dep, beanName);` `//把被依赖Bean注册给当前依赖的Bean` `getBean(dep);` `}` `}` `// Create bean instance.` `//创建单例模式Bean的实例对象` `if``(mbd.isSingleton()) {` `//这里使用了一个匿名内部类，创建Bean实例对象，并且注册给所依赖的对象` `sharedInstance = 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``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.` `//显式地从容器单例模式Bean缓存中清除实例对象` `destroySingleton(beanName);` `throw``ex;` `}` `});` `//获取给定Bean的实例对象` `bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);` `}` `//IOC容器创建原型模式Bean实例对象` `else``if``(mbd.isPrototype()) {` `// It's a prototype -> create a new instance.` `//原型模式(Prototype)是每次都会创建一个新的对象` `Object prototypeInstance =` `null``;` `try``{` `//回调beforePrototypeCreation方法，默认的功能是注册当前创建的原型对象` `beforePrototypeCreation(beanName);` `//创建指定Bean对象实例` `prototypeInstance = createBean(beanName, mbd, args);` `}` `finally``{` `//回调afterPrototypeCreation方法，默认的功能告诉IOC容器指定Bean的原型对象不再创建` `afterPrototypeCreation(beanName);` `}` `//获取给定Bean的实例对象` `bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);` `}` `//要创建的Bean既不是单例模式，也不是原型模式，则根据Bean定义资源中` `//配置的生命周期范围，选择实例化Bean的合适方法，这种在Web应用程序中` `//比较常用，如：request、session、application等生命周期` `else``{` `String scopeName = mbd.getScope();` `final``Scope scope =` `this``.scopes.get(scopeName);` `//Bean定义资源中没有配置生命周期范围，则Bean定义不合法` `if``(scope ==` `null``) {` `throw``new``IllegalStateException(``"No Scope registered for scope name '"``+ scopeName +` `"'"``);` `}` `try``{` `//这里又使用了一个匿名内部类，获取一个指定生命周期范围的实例` `Object scopedInstance = scope.get(beanName, () -> {` `beforePrototypeCreation(beanName);` `try``{` `return``createBean(beanName, mbd, args);` `}` `finally``{` `afterPrototypeCreation(beanName);` `}` `});` `//获取给定Bean的实例对象` `bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);` `}` `catch``(IllegalStateException ex) {` `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"``,` `ex);` `}` `}` `}` `catch``(BeansException ex) {` `cleanupAfterBeanCreationFailure(beanName);` `throw``ex;` `}` `}` `// Check if required type matches the type of the actual bean instance.` `//对创建的Bean实例对象进行类型检查` `if``(requiredType !=` `null``&& !requiredType.isInstance(bean)) {` `try``{` `T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);` `if``(convertedBean ==` `null``) {` `throw``new``BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());` `}` `return``convertedBean;` `}` `catch``(TypeMismatchException ex) {` `if``(logger.isDebugEnabled()) {` `logger.debug(``"Failed to convert bean '"``+ name +` `"' to required type '"``+` `ClassUtils.getQualifiedName(requiredType) +` `"'"``, ex);` `}` `throw``new``BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());` `}` `}` `return``(T) bean;` `}`

通过上面对向 IOC 容器获取 Bean 方法的分析，我们可以看到在 Spring 中，如果 Bean 定义的单例模式(Singleton)，则容器在创建之前先从缓存中查找，以确保整个容器中只存在一个实例对象。如果 Bean定义的是原型模式(Prototype)，则容器每次都会创建一个新的实例对象。除此之外，Bean 定义还可以通过其指定的生命周期范围来创建。

上面的源码只是定义了根据 Bean 定义的模式，采取的不同创建 Bean 实例对象的策略，具体的 Bean实例对象的创建过程由实现了AbstractBeanFactory接口的匿名内部类的createBean()方法完成，AbstractBeanFactory使用委派模式，具体的 Bean 实例创建过程交由其实现类AbstractAutowireCapableBeanFactory 完成，我们继续分析AbstractAutowireCapableBeanFactory的 createBean()方法的源码，理解其创建 Bean 实例的具体实现过程。

开始实例化AbstractAutowireCapableBeanFactory 类实现了 AbstractBeanFactory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

001002003004005006007008009010011012013014015016017018019020021022023024025026027028029030031032033034035036037038039040041042043044045046047048049050051052053054055056057058059060061062063064065066067068069070071072073074075076077078079080081082083084085086087088089090091092093094095096097098099100101102103104105106107108109110111112113114115116117118119120121122123124125126127128129130131132133134135136137138139140141142143144145146147148149150151152153154155156157158159160161162163164165166167168169170171172173174175176177178179	`//创建Bean实例对象` `@Override` `protected``Object createBean(String beanName, RootBeanDefinition mbd,` `@Nullable``Object[] args)` `throws``BeanCreationException {` `if``(logger.isDebugEnabled()) {` `logger.debug(``"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.` `//判断需要创建的Bean是否可以实例化，即是否可以通过当前的类加载器加载` `Class<?> resolvedClass = resolveBeanClass(mbd, beanName);` `if``(resolvedClass !=` `null``&& !mbd.hasBeanClass() && mbd.getBeanClassName() !=` `null``) {` `mbdToUse =` `new``RootBeanDefinition(mbd);` `mbdToUse.setBeanClass(resolvedClass);` `}` `// Prepare method overrides.` `//校验和准备Bean中的方法覆盖` `try``{` `mbdToUse.prepareMethodOverrides();` `}` `catch``(BeanDefinitionValidationException ex) {` `throw``new``BeanDefinitionStoreException(mbdToUse.getResourceDescription(),` `beanName,` `"Validation of method overrides failed"``, ex);` `}` `try``{` `// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.` `//如果Bean配置了初始化前和初始化后的处理器，则试图返回一个需要创建Bean的代理对象` `Object bean = resolveBeforeInstantiation(beanName, mbdToUse);` `if``(bean !=` `null``) {` `return``bean;` `}` `}` `catch``(Throwable ex) {` `throw``new``BeanCreationException(mbdToUse.getResourceDescription(), beanName,` `"BeanPostProcessor before instantiation of bean failed"``, ex);` `}` `try``{` `//创建Bean的入口` `Object beanInstance = doCreateBean(beanName, mbdToUse, args);` `if``(logger.isDebugEnabled()) {` `logger.debug(``"Finished creating instance of bean '"``+ beanName +` `"'"``);` `}` `return``beanInstance;` `}` `catch``(BeanCreationException ex) {` `// A previously detected exception with proper bean creation context already...` `throw``ex;` `}` `catch``(ImplicitlyAppearedSingletonException ex) {` `// An IllegalStateException to be communicated up to DefaultSingletonBeanRegistry...` `throw``ex;` `}` `catch``(Throwable ex) {` `throw``new``BeanCreationException(` `mbdToUse.getResourceDescription(), beanName,` `"Unexpected exception during bean creation"``, ex);` `}` `}` `//真正创建Bean的方法` `protected``Object doCreateBean(``final``String beanName,` `final``RootBeanDefinition mbd,` `final``@Nullable``Object[] args)` `throws``BeanCreationException {` `// Instantiate the bean.` `//封装被创建的Bean对象` `BeanWrapper instanceWrapper =` `null``;` `if``(mbd.isSingleton()) {` `instanceWrapper =` `this``.factoryBeanInstanceCache.remove(beanName);` `}` `if``(instanceWrapper ==` `null``) {` `instanceWrapper = createBeanInstance(beanName, mbd, args);` `}` `final``Object bean = instanceWrapper.getWrappedInstance();` `//获取实例化对象的类型` `Class<?> beanType = instanceWrapper.getWrappedClass();` `if``(beanType != NullBean.``class``) {` `mbd.resolvedTargetType = beanType;` `}` `// Allow post-processors to modify the merged bean definition.` `//调用PostProcessor后置处理器` `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``;` `}` `}` `// Eagerly cache singletons to be able to resolve circular references` `// even when triggered by lifecycle interfaces like BeanFactoryAware.` `//向容器中缓存单例模式的Bean对象，以防循环引用` `boolean``earlySingletonExposure = (mbd.isSingleton() &&` `this``.allowCircularReferences &&` `isSingletonCurrentlyInCreation(beanName));` `if``(earlySingletonExposure) {` `if``(logger.isDebugEnabled()) {` `logger.debug(``"Eagerly caching bean '"``+ beanName +` `"' to allow for resolving potential circular references"``);` `}` `//这里是一个匿名内部类，为了防止循环引用，尽早持有对象的引用` `addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));` `}` `// Initialize the bean instance.` `//Bean对象的初始化，依赖注入在此触发` `//这个exposedObject在初始化完成之后返回作为依赖注入完成后的Bean` `Object exposedObject = bean;` `try``{` `//将Bean实例对象封装，并且Bean定义中配置的属性值赋值给实例对象` `populateBean(beanName, mbd, instanceWrapper);` `//初始化Bean对象` `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) {` `//获取指定名称的已注册的单例模式Bean对象` `Object earlySingletonReference = getSingleton(beanName,` `false``);` `if``(earlySingletonReference !=` `null``) {` `//根据名称获取的已注册的Bean和正在实例化的Bean是同一个` `if``(exposedObject == bean) {` `//当前实例化的Bean初始化完成` `exposedObject = earlySingletonReference;` `}` `//当前Bean依赖其他Bean，并且当发生循环引用时不允许新创建实例对象` `else``if``(!``this``.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {` `String[] dependentBeans = getDependentBeans(beanName);` `Set<String> actualDependentBeans =` `new``LinkedHashSet<>(dependentBeans.length);` `//获取当前Bean所依赖的其他Bean` `for``(String dependentBean : dependentBeans) {` `//对依赖Bean进行类型检查` `if``(!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {` `actualDependentBeans.add(dependentBean);` `}` `}` `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 "``+` `"'getBeanNamesOfType' with the 'allowEagerInit' flag turned off, for example."``);` `}` `}` `}` `}` `// Register bean as disposable.` `//注册完成依赖注入的Bean` `try``{` `registerDisposableBeanIfNecessary(beanName, bean, mbd);` `}` `catch``(BeanDefinitionValidationException ex) {` `throw``new``BeanCreationException(` `mbd.getResourceDescription(), beanName,` `"Invalid destruction signature"``, ex);` `}` `return``exposedObject;``}`

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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。

下面继续分析这两个方法的代码实现。

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配特性生成 Java 实例对象，创建对象的源码如下：

0102030405060708091011121314151617181920212223242526272829303132333435363738394041424344454647484950515253545556575859606162636465666768697071727374757677787980818283848586	`//创建Bean的实例对象` `protected``BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd,` `@Nullable``Object[] args) {` `// Make sure bean class is actually resolved at this point.` `//检查确认Bean是可实例化的` `Class<?> beanClass = resolveBeanClass(mbd, beanName);` `//使用工厂方法对Bean进行实例化` `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);` `}` `if``(mbd.getFactoryMethodName() !=` `null``) {` `//调用工厂方法实例化` `return``instantiateUsingFactoryMethod(beanName, mbd, args);` `}` `// Shortcut when re-creating the same bean...` `//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``) {` `synchronized``(mbd.constructorArgumentLock) {` `if``(mbd.resolvedConstructorOrFactoryMethod !=` `null``) {` `resolved =` `true``;` `autowireNecessary = mbd.constructorArgumentsResolved;` `}` `}` `}` `if``(resolved) {` `if``(autowireNecessary) {` `//配置了自动装配属性，使用容器的自动装配实例化` `//容器的自动装配是根据参数类型匹配Bean的构造方法` `return``autowireConstructor(beanName, mbd,` `null``,` `null``);` `}` `else``{` `//使用默认的无参构造方法实例化` `return``instantiateBean(beanName, mbd);` `}` `}` `// Need to determine the constructor...` `//使用Bean的构造方法进行实例化` `Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);` `if``(ctors !=` `null``\|\|` `mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR \|\|` `mbd.hasConstructorArgumentValues() \|\| !ObjectUtils.isEmpty(args)) {` `//使用容器的自动装配特性，调用匹配的构造方法实例化` `return``autowireConstructor(beanName, mbd, ctors, args);` `}` `// No special handling: simply use no-arg constructor.` `//使用默认的无参构造方法实例化` `return``instantiateBean(beanName, mbd);` `}` `//使用默认的无参构造方法实例化Bean对象` `protected``BeanWrapper instantiateBean(``final``String beanName,` `final``RootBeanDefinition mbd) {` `try``{` `Object beanInstance;` `final``BeanFactory parent =` `this``;` `//获取系统的安全管理接口，JDK标准的安全管理API` `if``(System.getSecurityManager() !=` `null``) {` `//这里是一个匿名内置类，根据实例化策略创建实例对象` `beanInstance = AccessController.doPrivileged((PrivilegedAction<Object>) () ->` `getInstantiationStrategy().instantiate(mbd, beanName, parent),` `getAccessControlContext());` `}` `else``{` `//将实例化的对象封装起来` `beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, parent);` `}` `BeanWrapper bw =` `new``BeanWrapperImpl(beanInstance);` `initBeanWrapper(bw);` `return``bw;` `}` `catch``(Throwable ex) {` `throw``new``BeanCreationException(` `mbd.getResourceDescription(), beanName,` `"Instantiation of bean failed"``, ex);` `}``}`

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

010203040506070809101112131415161718192021222324252627282930313233343536373839404142	`//使用初始化策略实例化Bean对象` `@Override` `public``Object instantiate(RootBeanDefinition bd,` `@Nullable``String beanName, BeanFactory owner) {` `// Don't override the class with CGLIB if no overrides.` `//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()) {` `Constructor<?> constructorToUse;` `synchronized``(bd.constructorArgumentLock) {` `//获取对象的构造方法或工厂方法` `constructorToUse = (Constructor<?>) bd.resolvedConstructorOrFactoryMethod;` `//如果没有构造方法且没有工厂方法` `if``(constructorToUse ==` `null``) {` `//使用JDK的反射机制，判断要实例化的Bean是否是接口` `final``Class<?> clazz = bd.getBeanClass();` `if``(clazz.isInterface()) {` `throw``new``BeanInstantiationException(clazz,` `"Specified class is an interface"``);` `}` `try``{` `if``(System.getSecurityManager() !=` `null``) {` `//这里是一个匿名内置类，使用反射机制获取Bean的构造方法` `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);` `}` `}` `}` `//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);` `}` `else``{` `// Must generate CGLIB subclass.` `//使用CGLIB来实例化对象` `return``instantiateWithMethodInjection(bd, beanName, owner);` `}``}`

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通过上面的代码分析，我们看到了如果 Bean 有方法被覆盖了，则使用 JDK 的反射机制进行实例化，否则，使用 CGLib 进行实例化。instantiateWithMethodInjection() 方法调用SimpleInstantiationStrategy 的子类CGLibSubclassingInstantiationStrategy 使用 CGLib 来进行初始化，其源码如下：

01020304050607080910111213141516171819202122232425262728293031323334353637383940414243	`//使用CGLIB进行Bean对象实例化` `public``Object instantiate(``@Nullable``Constructor<?> ctor,` `@Nullable``Object... args) {` `//创建代理子类` `Class<?> subclass = createEnhancedSubclass(``this``.beanDefinition);` `Object instance;` `if``(ctor ==` `null``) {` `instance = BeanUtils.instantiateClass(subclass);` `}` `else``{` `try``{` `Constructor<?> enhancedSubclassConstructor = subclass.getConstructor(ctor.getParameterTypes());` `instance = enhancedSubclassConstructor.newInstance(args);` `}` `catch``(Exception ex) {` `throw``new``BeanInstantiationException(``this``.beanDefinition.getBeanClass(),` `"Failed to invoke constructor for CGLIB enhanced subclass ["``+ subclass.getName() +` `"]"``, ex);` `}` `}` `// SPR-10785: set callbacks directly on the instance instead of in the` `// enhanced class (via the Enhancer) in order to avoid memory leaks.` `Factory factory = (Factory) instance;` `factory.setCallbacks(``new``Callback[] {NoOp.INSTANCE,` `new``LookupOverrideMethodInterceptor(``this``.beanDefinition,` `this``.owner),` `new``ReplaceOverrideMethodInterceptor(``this``.beanDefinition,` `this``.owner)});` `return``instance;` `}` `private``Class<?> createEnhancedSubclass(RootBeanDefinition beanDefinition) {` `//CGLIB中的类` `Enhancer enhancer =` `new``Enhancer();` `//将Bean本身作为其基类` `enhancer.setSuperclass(beanDefinition.getBeanClass());` `enhancer.setNamingPolicy(SpringNamingPolicy.INSTANCE);` `if``(``this``.owner` `instanceof``ConfigurableBeanFactory) {` `ClassLoader cl = ((ConfigurableBeanFactory)` `this``.owner).getBeanClassLoader();` `enhancer.setStrategy(``new``ClassLoaderAwareGeneratorStrategy(cl));` `}` `enhancer.setCallbackFilter(``new``MethodOverrideCallbackFilter(beanDefinition));` `enhancer.setCallbackTypes(CALLBACK_TYPES);` `//使用CGLIB的createClass方法生成实例对象` `return``enhancer.createClass();` `}``}`

CGLib 是一个常用的字节码生成器的类库，它提供了一系列 API 实现 Java 字节码的生成和转换功能。JDK 的动态代理只能针对接口，如果一个类没有实现任何接口，要对其进行动态代理只能使用 CGLib。

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：

001002003004005006007008009010011012013014015016017018019020021022023024025026027028029030031032033034035036037038039040041042043044045046047048049050051052053054055056057058059060061062063064065066067068069070071072073074075076077078079080081082083084085086087088089090091092093094095096097098099100101102103104105106107108109110111112113114115116117118119120121122123124125126127128129130131132133134135136137138139140141142143144145146147148149150151152153154155156157158159160161162163164165166167168169170171172173174175176177178179180181182183184185186187188189190191192193194195196197198199200201	`//将Bean属性设置到生成的实例对象上` `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``{` `// Skip property population phase for null instance.` `return``;` `}` `}` `// Give any InstantiationAwareBeanPostProcessors the opportunity to modify the` `// state of the bean before properties are set. This can be used, for example,` `// to support styles of field injection.` `boolean``continueWithPropertyPopulation =` `true``;` `if``(!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {` `for``(BeanPostProcessor bp : getBeanPostProcessors()) {` `if``(bp` `instanceof``InstantiationAwareBeanPostProcessor) {` `InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;` `if``(!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {` `continueWithPropertyPopulation =` `false``;` `break``;` `}` `}` `}` `}` `if``(!continueWithPropertyPopulation) {` `return``;` `}` `//获取容器在解析Bean定义资源时为BeanDefiniton中设置的属性值` `PropertyValues pvs = (mbd.hasPropertyValues() ? mbd.getPropertyValues() :` `null``);` `//对依赖注入处理，首先处理autowiring自动装配的依赖注入` `if``(mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME \|\|` `mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {` `MutablePropertyValues newPvs =` `new``MutablePropertyValues(pvs);` `// Add property values based on autowire by name if applicable.` `//根据Bean名称进行autowiring自动装配处理` `if``(mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME) {` `autowireByName(beanName, mbd, bw, newPvs);` `}` `// Add property values based on autowire by type if applicable.` `//根据Bean类型进行autowiring自动装配处理` `if``(mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {` `autowireByType(beanName, mbd, bw, newPvs);` `}` `pvs = newPvs;` `}` `//对非autowiring的属性进行依赖注入处理` `boolean``hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();` `boolean``needsDepCheck = (mbd.getDependencyCheck() != RootBeanDefinition.DEPENDENCY_CHECK_NONE);` `if``(hasInstAwareBpps \|\| needsDepCheck) {` `if``(pvs ==` `null``) {` `pvs = mbd.getPropertyValues();` `}` `PropertyDescriptor[] filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);` `if``(hasInstAwareBpps) {` `for``(BeanPostProcessor bp : getBeanPostProcessors()) {` `if``(bp` `instanceof``InstantiationAwareBeanPostProcessor) {` `InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;` `pvs = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);` `if``(pvs ==` `null``) {` `return``;` `}` `}` `}` `}` `if``(needsDepCheck) {` `checkDependencies(beanName, mbd, filteredPds, pvs);` `}` `}` `if``(pvs !=` `null``) {` `//对属性进行注入` `applyPropertyValues(beanName, mbd, bw, pvs);` `}` `}` `//解析并注入依赖属性的过程` `protected``void``applyPropertyValues(String beanName, BeanDefinition mbd, BeanWrapper bw, PropertyValues pvs) {` `if``(pvs.isEmpty()) {` `return``;` `}` `//封装属性值` `MutablePropertyValues mpvs =` `null``;` `List<PropertyValue> original;` `if``(System.getSecurityManager() !=` `null``) {` `if``(bw` `instanceof``BeanWrapperImpl) {` `//设置安全上下文，JDK安全机制` `((BeanWrapperImpl) bw).setSecurityContext(getAccessControlContext());` `}` `}` `if``(pvs` `instanceof``MutablePropertyValues) {` `mpvs = (MutablePropertyValues) pvs;` `//属性值已经转换` `if``(mpvs.isConverted()) {` `// Shortcut: use the pre-converted values as-is.` `try``{` `//为实例化对象设置属性值` `bw.setPropertyValues(mpvs);` `return``;` `}` `catch``(BeansException ex) {` `throw``new``BeanCreationException(` `mbd.getResourceDescription(), beanName,` `"Error setting property values"``, ex);` `}` `}` `//获取属性值对象的原始类型值` `original = mpvs.getPropertyValueList();` `}` `else``{` `original = Arrays.asList(pvs.getPropertyValues());` `}` `//获取用户自定义的类型转换` `TypeConverter converter = getCustomTypeConverter();` `if``(converter ==` `null``) {` `converter = bw;` `}` `//创建一个Bean定义属性值解析器，将Bean定义中的属性值解析为Bean实例对象的实际值` `BeanDefinitionValueResolver valueResolver =` `new``BeanDefinitionValueResolver(``this``, beanName, mbd, converter);` `// Create a deep copy, resolving any references for values.` `//为属性的解析值创建一个拷贝，将拷贝的数据注入到实例对象中` `List<PropertyValue> deepCopy =` `new``ArrayList<>(original.size());` `boolean``resolveNecessary =` `false``;` `for``(PropertyValue pv : original) {` `//属性值不需要转换` `if``(pv.isConverted()) {` `deepCopy.add(pv);` `}` `//属性值需要转换` `else``{` `String propertyName = pv.getName();` `//原始的属性值，即转换之前的属性值` `Object originalValue = pv.getValue();` `//转换属性值，例如将引用转换为IOC容器中实例化对象引用` `Object resolvedValue = valueResolver.resolveValueIfNecessary(pv, originalValue);` `//转换之后的属性值` `Object convertedValue = resolvedValue;` `//属性值是否可以转换` `boolean``convertible = bw.isWritableProperty(propertyName) &&` `!PropertyAccessorUtils.isNestedOrIndexedProperty(propertyName);` `if``(convertible) {` `//使用用户自定义的类型转换器转换属性值` `convertedValue = convertForProperty(resolvedValue, propertyName, bw, converter);` `}` `// Possibly store converted value in merged bean definition,` `// in order to avoid re-conversion for every created bean instance.` `//存储转换后的属性值，避免每次属性注入时的转换工作` `if``(resolvedValue == originalValue) {` `if``(convertible) {` `//设置属性转换之后的值` `pv.setConvertedValue(convertedValue);` `}` `deepCopy.add(pv);` `}` `//属性是可转换的，且属性原始值是字符串类型，且属性的原始类型值不是` `//动态生成的字符串，且属性的原始值不是集合或者数组类型` `else``if``(convertible && originalValue` `instanceof``TypedStringValue &&` `!((TypedStringValue) originalValue).isDynamic() &&` `!(convertedValue` `instanceof``Collection \|\| ObjectUtils.isArray(convertedValue))) {` `pv.setConvertedValue(convertedValue);` `//重新封装属性的值` `deepCopy.add(pv);` `}` `else``{` `resolveNecessary =` `true``;` `deepCopy.add(``new``PropertyValue(pv, convertedValue));` `}` `}` `}` `if``(mpvs !=` `null``&& !resolveNecessary) {` `//标记属性值已经转换过` `mpvs.setConverted();` `}` `// Set our (possibly massaged) deep copy.` `//进行属性依赖注入` `try``{` `bw.setPropertyValues(``new``MutablePropertyValues(deepCopy));` `}` `catch``(BeansException ex) {` `throw``new``BeanCreationException(` `mbd.getResourceDescription(), beanName,` `"Error setting property values"``, ex);` `}``}`

分析上述代码，我们可以看出，对属性的注入过程分以下两种情况：

1)、属性值类型不需要强制转换时，不需要解析属性值，直接准备进行依赖注入。

2)、属性值需要进行类型强制转换时，如对其他对象的引用等，首先需要解析属性值，然后对解析后的属性值进行依赖注入。

对属性值的解析是在 BeanDefinitionValueResolver 类中的 resolveValueIfNecessary()方法中进行的，对属性值的依赖注入是通过 bw.setPropertyValues()方法实现的，在分析属性值的依赖注入之前，我们先分析一下对属性值的解析过程

解析属性注入规则当容器在对属性进行依赖注入时，如果发现属性值需要进行类型转换，如属性值是容器中另一个 Bean实例对象的引用，则容器首先需要根据属性值解析出所引用的对象，然后才能将该引用对象注入到目标实例对象的属性上去，对属性进行解析的由 resolveValueIfNecessary()方法实现。

Spring IOC 容器是如何将属性的值注入到 Bean 实例对象中去的：

1)、对于集合类型的属性，将其属性值解析为目标类型的集合后直接赋值给属性。

2)、对于非集合类型的属性，大量使用了 JDK 的反射机制，通过属性的 getter()方法获取指定属性注入以前的值，同时调用属性的 setter()方法为属性设置注入后的值。看到这里相信很多人都明白了 Spring的 setter()注入原理

Spring DI运行时序图

<ignore_js_op> Spring DI原理_ide