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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()

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：

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

三个关注点：

createBeanInstance() 实例化
populateBean(); 属性装配
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！

本文由博客一文多发平台 OpenWrite 发布！

原文 https://segmentfault.com/a/1190000020713917

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