Spring-Bean实例是如何依赖注入？

// 如果myJndiObject时FactoryBean， 则 &myJndiObject 将返回工厂而不是返回实例。
String FACTORY_BEAN_PREFIX = "&";
// 获取bean实例
Object getBean(String name) throws BeansException;
// 判断一个bean是否时单例
boolean isSingleton(String name) throws NoSuchBeanDefinitionException;
// 判断一个bean是否是原型
boolean isPrototype(String name) throws NoSuchBeanDefinitionException;
// 检查bean的name和type是否匹配
boolean isTypeMatch(String name, Class targetType) throws NoSuchBeanDefinitionException;
// 获取bean类型
Class<?> getType(String name) throws NoSuchBeanDefinitionException;
// 获取bean别名
String[] getAliases(String name);

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

// 依赖注入 从这里开始发生
private <T> T doGetBean(
        final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
        throws BeansException {

    // 根据指定名字获取被管理Bean的名称
    // 如果是别名， 则转换为真正的bean名
    final String beanName = transformedBeanName(name);
    Object bean;

    // Eagerly check singleton cache for manually registered singletons.
    // 先从缓存中取单例 bean
    Object sharedInstance = getSingleton(beanName);
    if (sharedInstance != null && args == null) {
        if (logger.isDebugEnabled()) {
            // 如果有，则直接返回该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 bean 的实例对象
        bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
    }

    else {
        // Fail if we're already creating this bean instance:
        // We're assumably within a circular reference.
        // 如果不是单例对象， 而且 缓存中有原型模式bean， 就抛异常
        if (isPrototypeCurrentlyInCreation(beanName)) {
            throw new BeanCurrentlyInCreationException(beanName);
        }

        // 检查 BeanDefinition 是否再当前的factory中， 如果不在则委托父类容器取查找
        // Check if bean definition exists in this factory.
        BeanFactory parentBeanFactory = getParentBeanFactory();
        if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
            // Not found -> check parent.
            String nameToLookup = originalBeanName(name);
            if (args != null) {
                // Delegation to parent with explicit args.
                // 委托父类容器取找(名字+参数)
                return (T) parentBeanFactory.getBean(nameToLookup, args);
            }
            else {
                // 委托父类容器取找(名称+类型)
                // No args -> delegate to standard getBean method.
                return parentBeanFactory.getBean(nameToLookup, requiredType);
            }
        }

        if (!typeCheckOnly) {
            // 标记 bean 被创建
            markBeanAsCreated(beanName);
        }

        // 根据bean名称获取 父类的 beanDefinition， 合并继承公共属性
        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 的集合
        String[] dependsOn = mbd.getDependsOn();
        if (dependsOn != null) {
            for (String dependsOnBean : dependsOn) {
                // 递归调用， 获取当前Bean的依赖Bean
                getBean(dependsOnBean);
                // 把依赖Bean注册给当前的Bean
                registerDependentBean(dependsOnBean, beanName);
            }
        }

        // Create bean instance.
        // 创建bean 实例
        if (mbd.isSingleton()) {
            // 创建 bean 实例对象， 并且注册给所依赖的对象
            sharedInstance = getSingleton(beanName, new ObjectFactory() {
                public Object getObject() throws BeansException {
                    try {
                        // 创建一个指定bean 实例对象
                        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.
                        // 清除该单例
                        destroySingleton(beanName);
                        throw ex;
                    }
                }
            });
            bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
        }

        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);
        }

        // 如果创建的bean 不是单例也不是原型， 则根据声明周期选择实例化bean的方法
        // 如 request session 等不同范围的实例
        else {
            String scopeName = mbd.getScope();
            final Scope scope = this.scopes.get(scopeName);
            // 如果 scope 是空， 则抛异常
            if (scope == null) {
                throw new IllegalStateException("No Scope registered for scope '" + scopeName + "'");
            }
            // 否则
            try {
                // 获取一个指定了scope的bean实例
                Object scopedInstance = scope.get(beanName, new ObjectFactory() {
                    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);
            }
        }
    }

    // Check if required type matches the type of the actual bean instance.
    // 检查是否需要类型检测
    if (requiredType != null && bean != null && !requiredType.isAssignableFrom(bean.getClass())) {
        throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
    }
    return (T) bean;
}

3.7 检查是否需要类型检测

3.8 返回`3.1-3.7 `生成的实例。

protected abstract Object createBean(String beanName, RootBeanDefinition mbd, Object[] args)
            throws BeanCreationException;

// 创建bean 实例
@Override
protected Object createBean(final String beanName, final RootBeanDefinition mbd, final Object[] args)
        throws BeanCreationException {

    if (logger.isDebugEnabled()) {
        logger.debug("Creating instance of bean '" + beanName + "'");
    }
    // Make sure bean class is actually resolved at this point.
    // 解析和确定 bean 可以实例化
    resolveBeanClass(mbd, beanName);

    // Prepare method overrides.
    // 准备方法覆盖
    try {
        mbd.prepareMethodOverrides();
    }
    catch (BeanDefinitionValidationException ex) {
        throw new BeanDefinitionStoreException(mbd.getResourceDescription(),
                beanName, "Validation of method overrides failed", ex);
    }

    try {
        // Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
        // 给 Bean处理器 一个机会， 返回一个目标bean实例
        Object bean = resolveBeforeInstantiation(beanName, mbd);
        if (bean != null) {
            return bean;
        }
    }
    catch (Throwable ex) {
        throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                "BeanPostProcessor before instantiation of bean failed", ex);
    }

    Object beanInstance = doCreateBean(beanName, mbd, args);
    if (logger.isDebugEnabled()) {
        logger.debug("Finished creating instance of bean '" + beanName + "'");
    }
    return beanInstance;
}

// 真正创建bean实例
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args) {
    // Instantiat|e the bean.
    // 封装bean
    BeanWrapper instanceWrapper = null;
    if (mbd.isSingleton()) {
        // 如果是单例模式的bean，从容器中获取同名bean
        instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
    }
    // 如果没有同名bean， 则创建bean实例
    if (instanceWrapper == null) {
        instanceWrapper = createBeanInstance(beanName, mbd, args);
    }
    // 如果有同名bean， 则获取到封装实例
    final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
    // 获取实例化对象类型
    Class beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);

    // Allow post-processors to modify the merged bean definition.
    // 调用后置处理器
    synchronized (mbd.postProcessingLock) {
        if (!mbd.postProcessed) {
            applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
            mbd.postProcessed = true;
        }
    }

    // Eagerly cache singletons to be able to resolve circular references
    // even when triggered by lifecycle interfaces like BeanFactoryAware.
    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() {
            public Object getObject() throws BeansException {
                return getEarlyBeanReference(beanName, mbd, bean);
            }
        });
    }

    // Initialize the bean instance.
    // bean对象初始化， 依赖注入开始，exposedObject就是完成后的bean
    Object exposedObject = bean;
    try {
        // 将bean 实例封装， 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);
        }
    }

    // 如果指定名称bean已经注册单例模式
    if (earlySingletonExposure) {
        Object earlySingletonReference = getSingleton(beanName, false);
        if (earlySingletonReference != null) {
            if (exposedObject == bean) {
                // 如果两个对象相等， bean初始化完成
                exposedObject = earlySingletonReference;
            }
            // 如果不相等， 则找出当前bean的依赖bean
            else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
                String[] dependentBeans = getDependentBeans(beanName);
                Set<String> actualDependentBeans = new LinkedHashSet<String>(dependentBeans.length);
                for (String dependentBean : dependentBeans) {
                    // 检查依赖bean （是否继承接口，是否是父子关系。。）
                    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;
}

protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, Object[] args) {
    // Make sure bean class is actually resolved at this point.
    // 确保bean可实例化(不能是抽象类等)
    Class beanClass = resolveBeanClass(mbd, beanName);

    // 如果这个bean 不是public 修饰符或者不被允许公共访问， 抛出异常
    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());
    }

    if (mbd.getFactoryMethodName() != null)  {
        // 通过工厂方法实例化
        return instantiateUsingFactoryMethod(beanName, mbd, args);
    }

    // Shortcut when re-creating the same bean...
    // 是否有构造器
    if (mbd.resolvedConstructorOrFactoryMethod != null && args == null) {
        if (mbd.constructorArgumentsResolved) {
            return autowireConstructor(beanName, mbd, null, null);
        }
        else {
            return instantiateBean(beanName, mbd);
        }
    }

    // Need to determine the constructor...
    // 需要确认构造器
    Constructor[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
    if (ctors != null ||
            mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR ||
            mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args))  {
        // 自动装配，调用匹配的构造方法进行实例化
        return autowireConstructor(beanName, mbd, ctors, args);
    }

    // No special handling: simply use no-arg constructor.
    // 使用默认无参构造
    return instantiateBean(beanName, mbd);
}

// 使用默认无参构造方法实例化bean
protected BeanWrapper instantiateBean(final String beanName, final RootBeanDefinition mbd) {
    try {
        Object beanInstance;
        final BeanFactory parent = this;
        // 获取JDK安全管理
        if (System.getSecurityManager() != null) {
            // 根据实例化策略实例化对象
            beanInstance = AccessController.doPrivileged(new PrivilegedAction<Object>() {

                public Object run() {
                    return getInstantiationStrategy().instantiate(mbd, beanName, parent);
                }
            }, getAccessControlContext());
        }
        else {
            beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, parent);
        }
        // 对实例化对象进行封装
        BeanWrapper bw = new BeanWrapperImpl(beanInstance);
        initBeanWrapper(bw);
        return bw;
    }
    catch (Throwable ex) {
        throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Instantiation of bean failed", ex);
    }
}

// 使用初始化策略 实例化bean
public Object instantiate(
        RootBeanDefinition beanDefinition, String beanName, BeanFactory owner) {

    // Don't override the class with CGLIB if no overrides.
    // 如果beanDefinition 中没有方法覆盖， 就用jdk，否则用cglib
    if (beanDefinition.getMethodOverrides().isEmpty()) {
        // 获取对象的构造方法和工厂方法
        Constructor constructorToUse = (Constructor) beanDefinition.resolvedConstructorOrFactoryMethod;

        if (constructorToUse == null) {
            // 如果 没有构造方法和工厂方法， 使用JDK反射， 判断实例化的bean是不是接口
            final Class clazz = beanDefinition.getBeanClass();
            if (clazz.isInterface()) {
                throw new BeanInstantiationException(clazz, "Specified class is an interface");
            }
            try {
                if (System.getSecurityManager() != null) {
                    // 使用反射获取bean构造方法
                    constructorToUse = AccessController.doPrivileged(new PrivilegedExceptionAction<Constructor>() {
                        public Constructor run() throws Exception {
                            return clazz.getDeclaredConstructor((Class[]) null);
                        }
                    });
                } else {
                    constructorToUse =  clazz.getDeclaredConstructor((Class[]) null);
                }
                beanDefinition.resolvedConstructorOrFactoryMethod = constructorToUse;
            }
            catch (Exception ex) {
                throw new BeanInstantiationException(clazz, "No default constructor found", ex);
            }
        }
        // 使用beanUtils实例化   构造方法.newInstance(arg) 来实例化
        return BeanUtils.instantiateClass(constructorToUse);
    }
    else {
        //如果 有覆盖或者重写， 则用CGLIB来实例化对象
        // Must generate CGLIB subclass.
        return instantiateWithMethodInjection(beanDefinition, beanName, owner);
    }
}

@Override
protected Object instantiateWithMethodInjection(
        RootBeanDefinition beanDefinition, String beanName, BeanFactory owner) {

    // Must generate CGLIB subclass.
    return new CglibSubclassCreator(beanDefinition, owner).instantiate(null, null);
}

// 使用cglib 来进行bean实例化
public Object instantiate(Constructor ctor, Object[] args) {
    // cglib
    Enhancer enhancer = new Enhancer();
    // bean本身作为基类
    enhancer.setSuperclass(this.beanDefinition.getBeanClass());
    enhancer.setCallbackFilter(new CallbackFilterImpl());
    enhancer.setCallbacks(new Callback[] {
            NoOp.INSTANCE,
            new LookupOverrideMethodInterceptor(),
            new ReplaceOverrideMethodInterceptor()
    });

    // 生成实例对象
    return (ctor == null) ? 
            enhancer.create() : 
            enhancer.create(ctor.getParameterTypes(), args);
}