Spring-IoC-依赖注入

前言

上一篇文章中，我们介绍了Spring IoC 的容器初始化过程 - IoC 容器初始化

本篇文章中，我们继续介绍Spring IoC 依赖注入的过程和源码解读。

还是如之前一样，为大家梳理一下步骤流程，以便于大家能在心里有个大概的脉络，更容易读懂源码，更容易抓住重点。

主要内容：

• beanName 解析转换
• 手动注册Bean检测
• 双亲容器检测
• 依赖初始化（递归）

• ★ 创建singleton 实例

  • 对象实例化
  • 属性装配
  • 处理Bean创建之后的各种回调事件
• ...

源码解析

上一章最后一节，容器初始化的倒数第二步，finishBeanFactoryInitialization(beanFactory)实例化所有单例，调用了getBean()方法来做singleton bean 的实例化操作。这就是Spring IoC 依赖注入的入口。

在开始之前，有一点需要提一下。前面我们是从容器初始化之后进来的，但实际操作中，我们有可能是在程序普通运行情况下，用ApplicationContext.getBean()去获取容器中bean。不要局限于刚刚的视角中。

现在让我们开始吧。

首先看看getBean()

源码位置：AbstractBeanFactory#getBean(String name)

    @Override    public Object getBean(String name) throws BeansException {        return doGetBean(name, null, null, false);    }

doGetBean()

deGetBean()

• beanName 解析转换
• 检测 手动注册Bean
• 双亲容器检测
• 依赖初始化（递归）
• 创建Bean createBean()

    protected <T> T doGetBean(            final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)            throws BeansException {        // 反正就是获取到真正beanName        // 处理两个情况，1. 将别名转化成真的beanName；2. 把FactoryBean的前缀"&"给去了        final String beanName = transformedBeanName(name);        Object bean;        // Eagerly check singleton cache for manually registered singletons.        // 检测已经注册的Bean，保证不重复创建        Object sharedInstance = getSingleton(beanName);        if (sharedInstance != null && args == null) {            if (logger.isDebugEnabled()) {                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 + "'");                }            }            // 这个方法还是有点逻辑的            // 如果目前获得的sharedInstance 不是FactoryBean，那bean就赋值成sharedInstance，直接返回            // 如果是FactoryBean就返回FactoryBean创建的实例，            // 这个也是FactoryBean的知识点，我在我的另一篇文章也讲过了，            bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);        }        else {            // Fail if we're already creating this bean instance:            // 直接翻译：创建过了此 beanName 的 prototype 类型的 bean，那么抛异常            // We're assumably within a circular reference.            // 往往是因为陷入了循环引用            if (isPrototypeCurrentlyInCreation(beanName)) {                throw new BeanCurrentlyInCreationException(beanName);            }            // Check if bean definition exists in this factory.            // 检查下这个BeanDefinition是否存在            BeanFactory parentBeanFactory = getParentBeanFactory();            if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {                // Not found -> check parent.                // 当前容器没有这个BeanDefinition，去parent 容器去找                String nameToLookup = originalBeanName(name);                if (args != null) {                    // Delegation to parent with explicit args.                    return (T) parentBeanFactory.getBean(nameToLookup, args);                }                else {                    // No args -> delegate to standard getBean method.                    return parentBeanFactory.getBean(nameToLookup, requiredType);                }            }            if (!typeCheckOnly) {                markBeanAsCreated(beanName);            }            try {                // 这个 getMergedLocalBeanDefinition 前面讲过哦                final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);                checkMergedBeanDefinition(mbd, beanName, args);                // 先初始化依赖的所有 Bean，这个很好理解。                // 注意，这里的依赖指的是 depends-on 中定义的依赖                String[] dependsOn = mbd.getDependsOn();                if (dependsOn != null) {                    for (String dep : dependsOn) {                        if (isDependent(beanName, dep)) {                            // 这里循环依赖概念不要紊乱了                            // 这里指的是通过 depends-on 定义造成的循环依赖，                            // 我们另外一种类成员式的循环引用Spring是支持的                            throw new BeanCreationException(mbd.getResourceDescription(), beanName,                                    "Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");                        }                        // 注册依赖关系                        // 这么做的原因是Spring在即将进行bean销毁的时候会【首先销毁被依赖的bean】。                        // 看SpringBean的初始化和销毁顺序就知道了,依赖关系的保存目的就是这个                        // 依赖关系的保存是通过一个ConcurrentHashMap<String, Set>完成的，key是bean的真实名字。                        registerDependentBean(dep, beanName);                        try {                            // 先去初始化被依赖项                            // 递归然后反递归回来                            getBean(dep);                        }                        catch (NoSuchBeanDefinitionException ex) {                            throw new BeanCreationException(mbd.getResourceDescription(), beanName,                                    "'" + beanName + "' depends on missing bean '" + dep + "'", ex);                        }                    }                }                // Create bean instance.                // 如果是 singleton scope 的，创建 singleton 的实例                if (mbd.isSingleton()) {                    sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {                        @Override                        public Object getObject() throws BeansException {                            try {                                // 创建Bean的详情，等下拉出来单独说                                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;                            }                        }                    });                    // 前面讲过了                    bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);                }                // 如果是 prototype scope 的，创建 prototype 的实例                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);                    }                    bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);                }                // 如果不是 singleton 和 prototype 的话，需要委托给相应的实现类来处理                // 这里非重点，我们的重点是singleton的创建                else {                    String scopeName = mbd.getScope();                    final 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, new ObjectFactory<Object>() {                            @Override                            public Object getObject() throws BeansException {                                beforePrototypeCreation(beanName);                                try {                                    return createBean(beanName, mbd, args);                                }                                finally {                                    afterPrototypeCreation(beanName);                                }                            }                        });                        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.        // 最后再检查下类型对不对，不对就抛异常了，对的话就返回        if (requiredType != null && bean != null && !requiredType.isInstance(bean)) {            try {                return getTypeConverter().convertIfNecessary(bean, requiredType);            }            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;    }

doCreateBean()

步骤概览：

1. 开始是单例的话要先清除缓存；
2. 实例化bean，将BeanDefinition转换为BeanWrapper；
3. 使用MergedBeanDefinitionPostProcessor，Autowired注解就是通过此方法实现类型的预解析；
4. 解决循环依赖问题；
5. 填充属性，将属性填充到bean实例中；
6. 注册DisposableBean；
7. 创建完成并返回

三个关注点：

1. createBeanInstance() 实例化
2. populateBean(); 属性装配
3. initializeBean() 处理Bean初始化之后的各种回调事件

    protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args)            throws BeanCreationException {        // Instantiate the bean.        // 这个BeanWrapper是创建出来持有对象的        BeanWrapper instanceWrapper = null;        if (mbd.isSingleton()) {            // 如果是singleton，先把缓存中的同名bean消除            instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);        }        if (instanceWrapper == null)             // 关键代码，后面拉出来单独讲            instanceWrapper = createBeanInstance(beanName, mbd, args);        }        final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);        Class<?> beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);        mbd.resolvedTargetType = beanType;        // Allow post-processors to modify the merged bean definition.        // 涉及接口：MergedBeanDefinitionPostProcessor        // 不是关键逻辑，不讲了        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.        // 这里是为了解决循环依赖的，先把初步实例化的Bean实例的引用缓存起来，暴露出去，        // 这个可以结合别的文章学习，面试题常考，我后面可能也会写        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");            }            addSingletonFactory(beanName, new ObjectFactory<Object>() {                @Override                public Object getObject() throws BeansException {                    return getEarlyBeanReference(beanName, mbd, bean);                }            });        }        // Initialize the bean instance.        Object exposedObject = bean;        try {            // 时序图中的一步，关键步骤，属性装配，前面的实例只是实例化，没有装配属性            // 和前面的createBeanInstance一样会拉出来讲，继续看下去吧            populateBean(beanName, mbd, instanceWrapper);            if (exposedObject != null) {                // 还记得 init-method 吗？还有 InitializingBean 接口？还有 BeanPostProcessor 接口？                 // 这里就是处理 bean 初始化完成后的各种回调                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);            }        }        // 这个逻辑分支我一次性说完吧。        // 如果该beanName对象已经注册单例模式，则从单例中获取，并判断获取到的bean实例(B)与BeanWrapper中的bean实例(A)是同一个实例，如果是，则返回A或者B，如果不是，则递归找出它的依赖bean。        if (earlySingletonExposure) {            Object earlySingletonReference = getSingleton(beanName, false);             // earlySingletonReference只有在检测到有循环依赖的情况下才会不为空            if (earlySingletonReference != null) {                if (exposedObject == bean) {                    // 两个是同一个引用，bean初始化完成                    exposedObject = earlySingletonReference;                }                else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {                    String[] dependentBeans = getDependentBeans(beanName);                    Set<String> actualDependentBeans = new LinkedHashSet<String>(dependentBeans.length);                    for (String dependentBean : dependentBeans) {                        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DisposableBean；        try {            registerDisposableBeanIfNecessary(beanName, bean, mbd);        }        catch (BeanDefinitionValidationException ex) {            throw new BeanCreationException(                    mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);        }        return exposedObject;    }

上一步的三个关注点，分开来讲。

1. createBeanInstance()

    protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, Object[] args) {        // Make sure bean class is actually resolved at this point.        // 解析出 Class        Class<?> beanClass = resolveBeanClass(mbd, beanName);        if (beanClass != null && !Modifier.isPublic(beanClass.getModifiers()) && !mbd.isNonPublicAccessAllowed()) {            throw new BeanCreationException(mbd.getResourceDescription(), beanName,                    "Bean class isn't public, and non-public access not allowed: " + beanClass.getName());        }        // 如果工厂方法不为空，则是用工厂方法初始化        if (mbd.getFactoryMethodName() != null)  {            // 相关知识点看另一篇文章关于FactoryBean的            return instantiateUsingFactoryMethod(beanName, mbd, args);        }        // Shortcut when re-creating the same bean...        // 如果不是第一次创建，比如第二次创建 prototype bean。           // 这种情况下，我们可以从第一次创建知道，采用无参构造函数，还是构造函数依赖注入 来完成实例化        // 所以注释说叫shortcut        boolean resolved = false;        boolean autowireNecessary = false;        if (args == null) {            synchronized (mbd.constructorArgumentLock) {                if (mbd.resolvedConstructorOrFactoryMethod != null) {                    // 有已经解析过的构造方法                    resolved = true;                    autowireNecessary = mbd.constructorArgumentsResolved;                }            }        }        // 如果已经解析过则使用解析好的构造方法不需要再次锁定        if (resolved) {            if (autowireNecessary) {                // 构造方法自动注入                return autowireConstructor(beanName, mbd, null, null);            }            else {                // 默认构造方法                return instantiateBean(beanName, mbd);            }        }        // Need to determine the constructor...        // 判断是否采用有参构造函数        // 构造器自动装配        Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);        if (ctors != null ||                mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR ||                mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args))  {            return autowireConstructor(beanName, mbd, ctors, args);        }        // No special handling: simply use no-arg constructor.        // 使用无参构造器        return instantiateBean(beanName, mbd);    }

2. populateBean(); 属性装配

入口方法: AbstractAutowireCapableBeanFactory#populateBean，

它的作用是: 根据autowire类型进行autowire by name，by type或者是直接进行设置

    protected void populateBean(String beanName, RootBeanDefinition mbd, BeanWrapper bw) {        PropertyValues pvs = mbd.getPropertyValues();        if (bw == null) {            if (!pvs.isEmpty()) {                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.        // 这里看注解是一个扩展点         // InstantiationAwareBeanPostProcessor 的实现类可以在这里对 bean 进行状态修改        // 不是个常用的扩展点，这里不讲了        boolean continueWithPropertyPopulation = true;        if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {            for (BeanPostProcessor bp : getBeanPostProcessors()) {                if (bp instanceof InstantiationAwareBeanPostProcessor) {                    InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;                    // 如果返回 false，代表不需要进行后续的属性设值，也不需要再经过其他的 BeanPostProcessor 的处理                    if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {                        continueWithPropertyPopulation = false;                        break;                    }                }            }        }        if (!continueWithPropertyPopulation) {            return;        }        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.            // 通过名字找到所有属性值，如果是 bean 依赖，先初始化依赖的 bean。记录依赖关系            if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME) {                autowireByName(beanName, mbd, bw, newPvs);            }            // Add property values based on autowire by type if applicable.            // 通过类型装配            if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {                autowireByType(beanName, mbd, bw, newPvs);            }            pvs = newPvs;        }        boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();        boolean needsDepCheck = (mbd.getDependencyCheck() != RootBeanDefinition.DEPENDENCY_CHECK_NONE);        if (hasInstAwareBpps || needsDepCheck) {            PropertyDescriptor[] filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);            if (hasInstAwareBpps) {                for (BeanPostProcessor bp : getBeanPostProcessors()) {                    if (bp instanceof InstantiationAwareBeanPostProcessor) {                        // InstantiationAwareBeanPostProcessor.postProcessPropertyValues方法                        // 代表能对属性值进行修改的能力                        // 其中一个很有用实现类提一下，AutowiredAnnotationBeanPostProcessor                        // 对采用@Autowired和@Value设值的就是这个BeanPostProcessor干的。                        // 不展开讲了，不然要讲不完了                        InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;                        pvs = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);                        if (pvs == null) {                            return;                        }                    }                }            }            if (needsDepCheck) {                checkDependencies(beanName, mbd, filteredPds, pvs);            }        }        // 这里才是设置bean实例的属性值        applyPropertyValues(beanName, mbd, bw, pvs);    }

3. initializeBean() 处理Bean初始化之后的各种回调事件

看这个方法的javadoc 描述

Initialize the given bean instance, applying factory callbacks as well as init methods and bean post processors.

    protected Object initializeBean(final String beanName, final Object bean, RootBeanDefinition mbd) {        if (System.getSecurityManager() != null) {            AccessController.doPrivileged(new PrivilegedAction<Object>() {                @Override                public Object run() {                    invokeAwareMethods(beanName, bean);                    return null;                }            }, getAccessControlContext());        }        else {            // 涉及到的回调接口点进去一目了然，代码都是自解释的            // BeanNameAware、BeanClassLoaderAware或BeanFactoryAware            invokeAwareMethods(beanName, bean);        }        Object wrappedBean = bean;        if (mbd == null || !mbd.isSynthetic()) {            // BeanPostProcessor 的 postProcessBeforeInitialization 回调            wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);        }        try {            // init-methods            // 或者是实现了InitializingBean接口，会调用afterPropertiesSet() 方法            invokeInitMethods(beanName, wrappedBean, mbd);        }        catch (Throwable ex) {            throw new BeanCreationException(                    (mbd != null ? mbd.getResourceDescription() : null),                    beanName, "Invocation of init method failed", ex);        }        if (mbd == null || !mbd.isSynthetic()) {            // BeanPostProcessor 的 postProcessAfterInitialization 回调            wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);        }        return wrappedBean;    }

大家发现没有，BeanPostProcessor 的两个回调都发生在这边，只不过中间处理了 init-method。这和我原来的认知有点不一样了？因为Spring的源码中代码的命名很多时候是自解释的，很多时候我看英文就知道这些方法的意图，但在这里，为什么BeanPostProcessor的前置处理方法postProcessBeforeInitialization()也是在属性设置完成后调用的？

后面查询了去看英文的解释，

BeanPostProcessor is used to interact with newly created bean instances before and/or after their initialization method is invoked by the Spring container. You can use BeanPostProcessor to execute custom logic before and/or after bean’s initialization method is invoked by the Spring container.

BeanPostProcessor的意图就是在初始化方法的前后做定制化操作。仔细想想好像这个命名也没毛病。如果真的是在SpringBean实例化之前调用应该叫BeforeInstantiation。源码中的initialization指的就是上面的invokeInitMethods操作。

结语

以上，就是关于Spring IoC 依赖注入的主要内容。

Spring IoC 容器这里还有一些其他的知识点，有关于SpringIoC 特性的。比如，Spring bean的生命周期、FactoryBean、BeanPostProcessor，都是在使用Spring IoC 容器经常遇到的特性。在了解了IoC容器的整体运行原理以后，你应该能够对这些特性进行一些分析，将你使用这些特性的方法和源码结合起来，融会贯通。

至此，Spring IoC关于依赖注入的源码“大致”解读完毕，还是那句话，上面的源码解析，肯定不会是完备的，只是提取了我认为重要的东西。

如有疏漏，敬请谅解和自己查阅相关资料学习。如果错误，敬请指正！

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