spring（四）：DI流程

在IoC容器初始化过程中，一般不包含Bean依赖注入的实现。

依赖注入一般发生在应用第一次向容器获取Bean时（getBean），但是有一个例外。

在使用IoC容器时有一个预实例化的配置，即通过lazy-init属性控制Bean的实例化（依赖注入）时机。

一般情况

// <<AbstractBeanFactory>>    
	public Object getBean(String name) throws BeansException {
        return this.doGetBean(name, (Class)null, (Object[])null, false);
    }

    protected <T> T doGetBean(String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly) throws BeansException {
        String beanName = this.transformedBeanName(name);
        // 首先检查是否存在缓存的singleton bean，对于已经存在的不需要重复创建
        Object sharedInstance = this.getSingleton(beanName);
        Object bean;
        if (sharedInstance != null && args == null) {
            ...
            bean = this.getObjectForBeanInstance(sharedInstance, name, beanName, (RootBeanDefinition)null);
        } else {
            // 如果bean正在创建中则获取失败
            if (this.isPrototypeCurrentlyInCreation(beanName)) {
                throw new BeanCurrentlyInCreationException(beanName);
            }
			// 检查当前BeanFactory中是否存在以beanName为名的BeanDefinition
            // 如果当前BeanFactory中没有，则会顺着其父BeanFactory查找
            BeanFactory parentBeanFactory = this.getParentBeanFactory();
            // 这里的containsBeanDefinition是一个模板方法，由DefaultListableBeanFactory实现
            // 即在存储BeanDefinition的Map中查找是否存在
            if (parentBeanFactory != null && !this.containsBeanDefinition(beanName)) {
                String nameToLookup = this.originalBeanName(name);
                if (parentBeanFactory instanceof AbstractBeanFactory) {
                    return ((AbstractBeanFactory)parentBeanFactory).doGetBean(nameToLookup, requiredType, args, typeCheckOnly);
                }
                if (args != null) {
                    return parentBeanFactory.getBean(nameToLookup, args);
                }
                if (requiredType != null) {
                    return parentBeanFactory.getBean(nameToLookup, requiredType);
                }
                return parentBeanFactory.getBean(nameToLookup);
            }
			// 父BeanFactory为null，或者当前存在以beanName为名的BeanDefinition时
            ...
            try {
                // 根据beanName获取BeanFactory中的BeanDefinition
                // 这里的RootBeanDefinition代表着在继承关系下创建的BeanDefinition
                RootBeanDefinition mbd = this.getMergedLocalBeanDefinition(beanName);
                this.checkMergedBeanDefinition(mbd, beanName, args);
                // 获得当前Bean依赖的所有Bean，通过 depends-on="xxx" 属性指定
                // 表示初始化当前Bean需要先初始化其依赖的xxx
                String[] dependsOn = mbd.getDependsOn();
                String[] var11;
                if (dependsOn != null) {
                    var11 = dependsOn;
                    int var12 = dependsOn.length;

                    for(int var13 = 0; var13 < var12; ++var13) {
                        String dep = var11[var13];
                        if (this.isDependent(beanName, dep)) {
                            ...	// throws "Circular depends-on relationship "
                        }
                        this.registerDependentBean(dep, beanName);
                        try {
                            // 递归，直到取到没有depends-on的Bean
                            this.getBean(dep);
                        } catch (NoSuchBeanDefinitionException var24) {
                            ...
                        }
                    }
                }

                // 创建bean实例
                if (mbd.isSingleton()) {	// 创建singleton作用域Bean实例
                    sharedInstance = this.getSingleton(beanName, () -> {
                        try {
                            // 进行 doCreateBean 方法，★
                            return this.createBean(beanName, mbd, args);
                        } catch (BeansException var5) {
                            this.destroySingleton(beanName);
                            throw var5;
                        }
                    });
                    bean = this.getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
                } else if (mbd.isPrototype()) {	// 创建prototype作用域Bean实例
                    var11 = null;
                    Object prototypeInstance;
                    try {
                        this.beforePrototypeCreation(beanName);
                        prototypeInstance = this.createBean(beanName, mbd, args);
                    } finally {
                        this.afterPrototypeCreation(beanName);
                    }
                    bean = this.getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
                } else {	// 创建其他作用域Bean：request、session、global session
                    String scopeName = mbd.getScope();
                    Scope scope = (Scope)this.scopes.get(scopeName);
                    if (scope == null) {
                        ...
                    }
                    try {
                        Object scopedInstance = scope.get(beanName, () -> {
                            this.beforePrototypeCreation(beanName);
                            Object var4;
                            try {
                                var4 = this.createBean(beanName, mbd, args);
                            } finally {
                                this.afterPrototypeCreation(beanName);
                            }
                            return var4;
                        });
                        bean = this.getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
                    }...
                }
            }...
        }

        // 类型检查
        if (requiredType != null && !requiredType.isInstance(bean)) {
            try {
                // 类型转换
                T convertedBean = this.getTypeConverter().convertIfNecessary(bean, requiredType);
                if (convertedBean == null) {
                    ...
                } else {
                    // 返回类型转换后的bean实例
                    return convertedBean;
                }
            }...
        } else {
            return bean;
        }
    }

// <AbstractAutowireCapableBeanFactory>
	protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) throws BeanCreationException {
        ...logger
        // 根据class属性或className解析beanClass
        RootBeanDefinition mbdToUse = mbd;
        Class<?> resolvedClass = this.resolveBeanClass(mbd, beanName, new Class[0]);
        if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
            mbdToUse = new RootBeanDefinition(mbd);
            mbdToUse.setBeanClass(resolvedClass);
        }

        // 对override属性（lookup-method、replace-method）进行处理
        // 会检测当前beanDefinition的class中是否是否包含相应方法，
        // 并将override方法标记为覆盖，防止进行重载参数检查
        try {
            mbdToUse.prepareMethodOverrides();
        }...

        Object beanInstance;
        try {
            // 调用InstantiationAwareBeanPostProcessor
            // 允许在实例化前后进行处理
            // 如果返回值不为空，表示配置了InstantiationAwareBeanPostProcessor，并且修改了bean
            // 则返回一个代理，而不是bean实例
            beanInstance = this.resolveBeforeInstantiation(beanName, mbdToUse);
            if (beanInstance != null) {
                return beanInstance;
            }
        }...

        try {
            // ★
            beanInstance = this.doCreateBean(beanName, mbdToUse, args);
            ...
            return beanInstance;
        }...
    }

	// 真正开始创建bean
    protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) throws BeanCreationException {
        BeanWrapper instanceWrapper = null;
        // 如果是单例bean，则先清除缓存同名缓存
        if (mbd.isSingleton()) {
            instanceWrapper = (BeanWrapper)this.factoryBeanInstanceCache.remove(beanName);
        }
		// 创建bean实例★
        if (instanceWrapper == null) {
            instanceWrapper = this.createBeanInstance(beanName, mbd, args);
        }

        Object bean = instanceWrapper.getWrappedInstance();
        Class<?> beanType = instanceWrapper.getWrappedClass();
        if (beanType != NullBean.class) {
            mbd.resolvedTargetType = beanType;
        }
		// 通过post-processors合并继承关系下的beanDefinition
        synchronized(mbd.postProcessingLock) {
            if (!mbd.postProcessed) {
                try {
                    this.applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
                }...
                mbd.postProcessed = true;
            }
        }
		// 提前曝光bean（利用缓存），用于支持循环bean的循环依赖
        boolean earlySingletonExposure = mbd.isSingleton() && this.allowCircularReferences && this.isSingletonCurrentlyInCreation(beanName);
        if (earlySingletonExposure) {
            ...
            // 通过匿名内部类添加单例工厂到缓存
            this.addSingletonFactory(beanName, () -> {
                return this.getEarlyBeanReference(beanName, mbd, bean);
            });
        }
		// 初始化bean
        Object exposedObject = bean;

        try {
            // 这里开始进行依赖注入★
            this.populateBean(beanName, mbd, instanceWrapper);
            // 【初始化bean】：
            // applyBeanPostProcessorsBeforeInitialization【后置处理器】
            // invokeInitMethods：执行init-method
            // applyBeanPostProcessorsAfterInitialization【后置处理器】
            exposedObject = this.initializeBean(beanName, exposedObject, mbd);
        } catch (Throwable var18) {
            if (var18 instanceof BeanCreationException && beanName.equals(((BeanCreationException)var18).getBeanName())) {
                throw (BeanCreationException)var18;
            }...
		 // 检查在提前曝光时，注入的bean是否为最终版本
        if (earlySingletonExposure) {
            Object earlySingletonReference = this.getSingleton(beanName, false);
            if (earlySingletonReference != null) {
                if (exposedObject == bean) {
                    exposedObject = earlySingletonReference;
                } else if (!this.allowRawInjectionDespiteWrapping && this.hasDependentBean(beanName)) {
                    String[] dependentBeans = this.getDependentBeans(beanName);
                    Set<String> actualDependentBeans = new LinkedHashSet(dependentBeans.length);
                    String[] var12 = dependentBeans;
                    int var13 = dependentBeans.length;

                    for(int var14 = 0; var14 < var13; ++var14) {
                        String dependentBean = var12[var14];
                        if (!this.removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
                            actualDependentBeans.add(dependentBean);
                        }
                    }

                    if (!actualDependentBeans.isEmpty()) {
                        ...
                    }
                }
            }
        }
		// 给Bean注册一些必要的销毁操作，当容器shutdown时执行
        try {
            this.registerDisposableBeanIfNecessary(beanName, bean, mbd);
            // 返回Bean实例，这里已经完成了依赖注入
            return exposedObject;
        }...
    }
     
---开始bean的实例化---

    protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
        // 确保Bean类可以实例化
        Class<?> beanClass = this.resolveBeanClass(mbd, beanName, new Class[0]);
        if (beanClass != null && !Modifier.isPublic(beanClass.getModifiers()) && !mbd.isNonPublicAccessAllowed()) {
            ...
        } else {
            // 5.0的新特性
        	// doc注释：用作创建Bean的回调，设置该回调后会覆盖掉创建Bean的工厂方法和构造函数，
        	// 但不影响依赖注入过程
            Supplier<?> instanceSupplier = mbd.getInstanceSupplier();
            if (instanceSupplier != null) {
                return this.obtainFromSupplier(instanceSupplier, beanName);
            } else if (mbd.getFactoryMethodName() != null) {	// 使用工厂方法实例化Bean
                return this.instantiateUsingFactoryMethod(beanName, mbd, args);
            } else {
                // 创建Bean时需要根据beanClass解析构造方法，并缓存在RootBeanDefinition中
        	    // 这里通过缓存直接进行处理，在创建相同Bean时不需要重复解析构造方法
                boolean resolved = false;
                boolean autowireNecessary = false;
                if (args == null) {
                    synchronized(mbd.constructorArgumentLock) {
                        if (mbd.resolvedConstructorOrFactoryMethod != null) {
                            resolved = true;
                            autowireNecessary = mbd.constructorArgumentsResolved;
                        }
                    }
                }
			   // 已解析构造方法
                if (resolved) {
                    return autowireNecessary ? this.autowireConstructor(beanName, mbd, (Constructor[])null, (Object[])null) : this.instantiateBean(beanName, mbd);
                } else {
                    // 使用构造函数实例化Bean
                    Constructor<?>[] ctors = this.determineConstructorsFromBeanPostProcessors(beanClass, beanName);
                    if (ctors == null && mbd.getResolvedAutowireMode() != 3 && !mbd.hasConstructorArgumentValues() && ObjectUtils.isEmpty(args)) {
                        ctors = mbd.getPreferredConstructors();
                        // 1 有参构造函数
                        // 2 默认构造函数：instantiateBean()通过getInstantiationStrategy().instantiate(mbd, beanName, this)
                        // 这里的策略是SimpleInstantiationStrategy
                        return ctors != null ? this.autowireConstructor(beanName, mbd, ctors, (Object[])null) : this.instantiateBean(beanName, mbd);
                    } else {
                        return this.autowireConstructor(beanName, mbd, ctors, args);
                    }
                }
            }
        }
    }

// SimpleInstantiationStrategy
    // 这里共有两种方式创建Bean：
    // 其一，通过JVM反射；BeanUtils.instantiateClass(constructorToUse, new Object[0]);
    // 其二，通过cgLib的动态代理机制；this.instantiateWithMethodInjection(bd, beanName, owner);<<CglibSubclassingInstantiationStrategy>>

---结束bean的实例化---

    // 对bean属性包括依赖关系的处理，依据是BeanDefinition
    protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {
        if (bw == null) {
            if (mbd.hasPropertyValues()) {
                ...
            }
        } else {
            // 交由InstantiationAwareBeanPostProcessors处理是否对Bean实例进行依赖注入
            boolean continueWithPropertyPopulation = true;
            if (!mbd.isSynthetic() && this.hasInstantiationAwareBeanPostProcessors()) {
                Iterator var5 = this.getBeanPostProcessors().iterator();

                while(var5.hasNext()) {
                    BeanPostProcessor bp = (BeanPostProcessor)var5.next();
                    if (bp instanceof InstantiationAwareBeanPostProcessor) {
                        InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor)bp;
                        if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
                            continueWithPropertyPopulation = false;
                            break;
                        }
                    }
                }
            }

            if (continueWithPropertyPopulation) {
                PropertyValues pvs = mbd.hasPropertyValues() ? mbd.getPropertyValues() : null;
                // 首先处理autowire依赖注入，根据Bean的名字或类型进行注入
                // 我们可以在配置文件中指定注入方式：autowire="byName"
                if (mbd.getResolvedAutowireMode() == 1 || mbd.getResolvedAutowireMode() == 2) {
                    MutablePropertyValues newPvs = new MutablePropertyValues((PropertyValues)pvs);
                    if (mbd.getResolvedAutowireMode() == 1) {
                        // 按变量名注入
                        this.autowireByName(beanName, mbd, bw, newPvs);
                    }
                    if (mbd.getResolvedAutowireMode() == 2) {
                        // 按类型注入
                        this.autowireByType(beanName, mbd, bw, newPvs);
                    }
                    pvs = newPvs;
                }

                boolean hasInstAwareBpps = this.hasInstantiationAwareBeanPostProcessors();
                boolean needsDepCheck = mbd.getDependencyCheck() != 0;
                PropertyDescriptor[] filteredPds = null;
                // 在为Bean注入依赖前进行后置处理，比如检查@Required是否满足，添加或移除Property
                if (hasInstAwareBpps) {
                    if (pvs == null) {
                        pvs = mbd.getPropertyValues();
                    }

                    Iterator var9 = this.getBeanPostProcessors().iterator();

                    while(var9.hasNext()) {
                        BeanPostProcessor bp = (BeanPostProcessor)var9.next();
                        if (bp instanceof InstantiationAwareBeanPostProcessor) {
                            InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor)bp;
                            PropertyValues pvsToUse = ibp.postProcessProperties((PropertyValues)pvs, bw.getWrappedInstance(), beanName);
                            if (pvsToUse == null) {
                                if (filteredPds == null) {
                                    filteredPds = this.filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
                                }

                                pvsToUse = ibp.postProcessPropertyValues((PropertyValues)pvs, filteredPds, bw.getWrappedInstance(), beanName);
                                if (pvsToUse == null) {
                                    return;
                                }
                            }

                            pvs = pvsToUse;
                        }
                    }
                }
			    // 进行依赖检查
                if (needsDepCheck) {
                    if (filteredPds == null) {
                        filteredPds = this.filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
                    }

                    this.checkDependencies(beanName, mbd, filteredPds, (PropertyValues)pvs);
                }

                if (pvs != null) {
                    // 对属性进行注入★
                    this.applyPropertyValues(beanName, mbd, bw, (PropertyValues)pvs);
                }

            }
        }
    }
        
    protected void applyPropertyValues(String beanName, BeanDefinition mbd, BeanWrapper bw, PropertyValues pvs) {
        if (!pvs.isEmpty()) {
            if (System.getSecurityManager() != null && bw instanceof BeanWrapperImpl) {
                ((BeanWrapperImpl)bw).setSecurityContext(this.getAccessControlContext());
            }
		   // 注入的Property
            MutablePropertyValues mpvs = null;
            List original;
            // pvs是MutablePropertyValues实例，且可强制类型转换，则可以直接设置为注入的Property
            if (pvs instanceof MutablePropertyValues) {
                mpvs = (MutablePropertyValues)pvs;
                if (mpvs.isConverted()) {
                    try {
                        bw.setPropertyValues(mpvs);
                        return;
                    } catch (BeansException var18) {
                        throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Error setting property values", var18);
                    }
                }

                original = mpvs.getPropertyValueList();
            } else {
                original = Arrays.asList(pvs.getPropertyValues());
            }

            TypeConverter converter = this.getCustomTypeConverter();
            if (converter == null) {
                converter = bw;
            }
		   // BeanDefinitionValueResolver用于解析BeanDefinition中的Property
            BeanDefinitionValueResolver valueResolver = new BeanDefinitionValueResolver(this, beanName, mbd, (TypeConverter)converter);
            // 创建一个深拷贝副本，这个副本持有的Property会被注入到Bean中
            List<PropertyValue> deepCopy = new ArrayList(original.size());
            boolean resolveNecessary = false;
            Iterator var11 = original.iterator();

            while(true) {
                while(var11.hasNext()) {
                    PropertyValue pv = (PropertyValue)var11.next();
                    if (pv.isConverted()) {
                        deepCopy.add(pv);
                    } else {
                        String propertyName = pv.getName();
                        Object originalValue = pv.getValue();
                        // 利用valueResolver解析PropertyValue
                        // 对value的类型进行判断，然后解析，如果是依赖bean ref的话，会调用getBean方法
                        Object resolvedValue = valueResolver.resolveValueIfNecessary(pv, originalValue);
                        Object convertedValue = resolvedValue;
                        boolean convertible = bw.isWritableProperty(propertyName) && !PropertyAccessorUtils.isNestedOrIndexedProperty(propertyName);
                        if (convertible) {
                            convertedValue = this.convertForProperty(resolvedValue, propertyName, bw, (TypeConverter)converter);
                        }

                        if (resolvedValue == originalValue) {
                            if (convertible) {
                                pv.setConvertedValue(convertedValue);
                            }

                            deepCopy.add(pv);
                        } else if (convertible && originalValue instanceof TypedStringValue && !((TypedStringValue)originalValue).isDynamic() && !(convertedValue instanceof Collection) && !ObjectUtils.isArray(convertedValue)) {
                            pv.setConvertedValue(convertedValue);
                            deepCopy.add(pv);
                        } else {
                            resolveNecessary = true;
                            deepCopy.add(new PropertyValue(pv, convertedValue));
                        }
                    }
                }

                if (mpvs != null && !resolveNecessary) {
                    mpvs.setConverted();
                }

                try {
                    // BeanWrapper的setPropertyValues
                    // 将拷贝副本中的PropertyValues注入到Bean中<<PropertyAccessor>>
                    // 具体实现通过AbstractNestablePropertyAccessor.PropertyTokenHolder
                    // 然后根据tokens.keys处理本地值或键值
                    bw.setPropertyValues(new MutablePropertyValues(deepCopy));
                    return;
                } catch (BeansException var19) {
                    throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Error setting property values", var19);
                }
            }
        }
    }

当lazy-init=false时

会在容器初始化的过程中将所有的singleton bean提前进行实例化和依赖注入，因此singleton bean的依赖注入是容器初始化过程的一部分，这也是我们常用的ApplicationContext的默认配置。

对于非延迟单例bean的初始化在refresh()方法中的finishBeanFactoryInitialization(beanFactory)中完成。

    protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
        ...
        beanFactory.preInstantiateSingletons();	// <<ConfigurableListableBeanFactory>>
    }

// 具体实现：DefaultListableBeanFactory
    public void preInstantiateSingletons() throws BeansException {
        if (this.logger.isTraceEnabled()) {
            this.logger.trace("Pre-instantiating singletons in " + this);
        }
		// 这里的beanDefinitionNames上一章有提过~
        List<String> beanNames = new ArrayList(this.beanDefinitionNames);
        Iterator var2 = beanNames.iterator();

        while(true) {
            String beanName;
            Object bean;
            do {
                while(true) {
                    RootBeanDefinition bd;
                    do {
                        do {
                            do {
                                if (!var2.hasNext()) {
                                    var2 = beanNames.iterator();

                                    while(var2.hasNext()) {
                                        beanName = (String)var2.next();
                                        // ★
                                        Object singletonInstance = this.getSingleton(beanName);
                                        if (singletonInstance instanceof SmartInitializingSingleton) {
                                            SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton)singletonInstance;
                                            if (System.getSecurityManager() != null) {
                                                AccessController.doPrivileged(() -> {
                                                    smartSingleton.afterSingletonsInstantiated();
                                                    return null;
                                                }, this.getAccessControlContext());
                                            } else {
                                                smartSingleton.afterSingletonsInstantiated();
                                            }
                                        }
                                    }

                                    return;
                                }

                                beanName = (String)var2.next();
                                bd = this.getMergedLocalBeanDefinition(beanName);
                            } while(bd.isAbstract());
                        } while(!bd.isSingleton());
                    } while(bd.isLazyInit());	// 只会对非延迟单例bean进行初始化

                    if (this.isFactoryBean(beanName)) {
                        bean = this.getBean("&" + beanName);
                        break;
                    }

                    this.getBean(beanName);
                }
            } while(!(bean instanceof FactoryBean));

            FactoryBean<?> factory = (FactoryBean)bean;
            boolean isEagerInit;
            if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
                SmartFactoryBean var10000 = (SmartFactoryBean)factory;
                ((SmartFactoryBean)factory).getClass();
                isEagerInit = (Boolean)AccessController.doPrivileged(var10000::isEagerInit, this.getAccessControlContext());
            } else {
                isEagerInit = factory instanceof SmartFactoryBean && ((SmartFactoryBean)factory).isEagerInit();
            }

            if (isEagerInit) {
                // 触发依赖注入
                this.getBean(beanName);
            }
        }
    }

// DefaultSingletonBeanRegistry
    @Nullable
    protected Object getSingleton(String beanName, boolean allowEarlyReference) {
        Object singletonObject = this.singletonObjects.get(beanName);
        if (singletonObject == null && this.isSingletonCurrentlyInCreation(beanName)) {
            synchronized(this.singletonObjects) {
                singletonObject = this.earlySingletonObjects.get(beanName);
                if (singletonObject == null && allowEarlyReference) {
                    ObjectFactory<?> singletonFactory = (ObjectFactory)this.singletonFactories.get(beanName);
                    if (singletonFactory != null) {
                        singletonObject = singletonFactory.getObject();
                        this.earlySingletonObjects.put(beanName, singletonObject);
                        this.singletonFactories.remove(beanName);
                    }
                }
            }
        }

        return singletonObject;
    }