一、bean的实例化过程源码分析

上一章节讲了bean定义的加载过程，现在我们来讲一下bean的实例化过程，bean的实例化，主要是finishBeanFactoryInitialization(beanFactory);这个方法

protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
    //bean工厂创建转化器
    if (beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME) &&
            beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)) {
        beanFactory.setConversionService(
                beanFactory.getBean(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class));
    }
    if (!beanFactory.hasEmbeddedValueResolver()) {
        beanFactory.addEmbeddedValueResolver(strVal -> getEnvironment().resolvePlaceholders(strVal));
    }
    //实例化AOP相关的组件
    String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
    for (String weaverAwareName : weaverAwareNames) {
        //调用工厂实例化方法，这个方法我们后面来分析
        getBean(weaverAwareName);
    }
    beanFactory.setTempClassLoader(null);
    //冻结所有bean定义，在实例化时，不允许再修改bean定义
    beanFactory.freezeConfiguration();
    //实例化所有bean
    beanFactory.preInstantiateSingletons();
}
//实例化所有bean
public void preInstantiateSingletons() throws BeansException {
    if (logger.isTraceEnabled()) {
        logger.trace("Pre-instantiating singletons in " + this);
    }
    //获取到容器中所有bean定义
    List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);
    //循环去实例化
    for (String beanName : beanNames) {
        //把各种不同类型的bean定义转换成RootBeanDefinition类型
        RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
        //只有不是抽象类，必须是单例类，并且不是懒加载才会去实例化
        if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
            //判断是不是工厂类
            if (isFactoryBean(beanName)) {
                //工厂类会先生存实际的bean，用 &+bean名称
                Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
                if (bean instanceof FactoryBean) {
                    FactoryBean<?> factory = (FactoryBean<?>) bean;
                    boolean isEagerInit;
                    if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
                        isEagerInit = AccessController.doPrivileged(
                                (PrivilegedAction<Boolean>) ((SmartFactoryBean<?>) factory)::isEagerInit,
                                getAccessControlContext());
                    }
                    else {
                        isEagerInit = (factory instanceof SmartFactoryBean &&
                                ((SmartFactoryBean<?>) factory).isEagerInit());
                    }
                    //实际去实例化bean
                    if (isEagerInit) {
                        getBean(beanName);
                    }
                }
            }
            else {
                //调用实例化bean
                getBean(beanName);
            }
        }
    }
    //遍历所有的bean，如果实现了SmartInitializingSingleton接口，调用afterSingletonsInstantiated方法
    for (String beanName : beanNames) {
        Object singletonInstance = getSingleton(beanName);
        if (singletonInstance instanceof SmartInitializingSingleton) {
            SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
            if (System.getSecurityManager() != null) {
                AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
                    smartSingleton.afterSingletonsInstantiated();
                    return null;
                }, getAccessControlContext());
            }
            else {
                smartSingleton.afterSingletonsInstantiated();
            }
        }
    }
}
public Object getBean(String name) throws BeansException {
    //真实去实例化bean
    return doGetBean(name, null, null, false);
}
//这个方法就是最重要的实例化过程，最主要是关注spring是怎么解决循环依赖的
protected <T> T doGetBean(
        String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)
        throws BeansException {
    String beanName = transformedBeanName(name);
    Object bean;
    //先从一级缓存中去获取bean，可以看下getSingleton方法
    Object sharedInstance = getSingleton(beanName);
    if (sharedInstance != null && args == null) {
        if (logger.isTraceEnabled()) {
            if (isSingletonCurrentlyInCreation(beanName)) {
                logger.trace("Returning eagerly cached instance of singleton bean '" + beanName +
                        "' that is not fully initialized yet - a consequence of a circular reference");
            }
            else {
                logger.trace("Returning cached instance of singleton bean '" + beanName + "'");
            }
        }
        //如果是普通单例bean则会直接返回，如果是工厂bean。则会调用getObject获取真正的bean实例
        bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
    }
    else {
        //Spring只解决单例bean的循环依赖，如果非单例，循环依赖则会直接报错
        if (isPrototypeCurrentlyInCreation(beanName)) {
            throw new BeanCurrentlyInCreationException(beanName);
        }
        BeanFactory parentBeanFactory = getParentBeanFactory();
        //获取到父工厂，如果当前工厂中没有当前bean，则委托给父工厂实例化
        if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
            String nameToLookup = originalBeanName(name);
            if (parentBeanFactory instanceof AbstractBeanFactory) {
                return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
                        nameToLookup, requiredType, args, typeCheckOnly);
            }
            else if (args != null) {
                return (T) parentBeanFactory.getBean(nameToLookup, args);
            }
            else if (requiredType != null) {
                return parentBeanFactory.getBean(nameToLookup, requiredType);
            }
            else {
                return (T) parentBeanFactory.getBean(nameToLookup);
            }
        }
        //如果不仅仅做类型检查，而是创建bean，则标记这个bean
        if (!typeCheckOnly) {
            markBeanAsCreated(beanName);
        }

        try {
            //把不同类型的bean定义转换成RootBeanDefinition
            RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
            //如果是抽象类，则会报错
            checkMergedBeanDefinition(mbd, beanName, args);
            //处理dependsOn依赖
            String[] dependsOn = mbd.getDependsOn();
            if (dependsOn != null) {
                for (String dep : dependsOn) {
                    //如果依赖的bean是正在创建的bean，就是循环依赖问题，这个直接抛异常，这种是@Bean依赖，不给解决循环依赖
                    if (isDependent(beanName, dep)) {
                        throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                                "Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
                    }
                    registerDependentBean(dep, beanName);
                    try {
                        getBean(dep);
                    }
                    catch (NoSuchBeanDefinitionException ex) {
                        throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                                "'" + beanName + "' depends on missing bean '" + dep + "'", ex);
                    }
                }
            }
            //如果是单例bean放进三级缓存，并且获取依赖注入的bean
            if (mbd.isSingleton()) {
                sharedInstance = getSingleton(beanName, () -> {
                    try {
                        //这段逻辑可以看下面createBean的详解
                        return createBean(beanName, mbd, args);
                    }
                    catch (BeansException ex) {
                        //创建bean的逻辑报异常，则销毁当前bean相关的所有信息
                        destroySingleton(beanName);
                        throw ex;
                    }
                });
                bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
            }

            else if (mbd.isPrototype()) {
                Object prototypeInstance = null;
                try {
                    beforePrototypeCreation(beanName);
                    prototypeInstance = createBean(beanName, mbd, args);
                }
                finally {
                    afterPrototypeCreation(beanName);
                }
                bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
            }

            else {
                String scopeName = mbd.getScope();
                if (!StringUtils.hasLength(scopeName)) {
                    throw new IllegalStateException("No scope name defined for bean ´" + beanName + "'");
                }
                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, () -> {
                        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;
        }
    }
    if (requiredType != null && !requiredType.isInstance(bean)) {
        try {
            T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
            if (convertedBean == null) {
                throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
            }
            return convertedBean;
        }
        catch (TypeMismatchException ex) {
            if (logger.isTraceEnabled()) {
                logger.trace("Failed to convert bean '" + name + "' to required type '" +
                        ClassUtils.getQualifiedName(requiredType) + "'", ex);
            }
            throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
        }
    }
    return (T) bean;
}

protected Object getSingleton(String beanName, boolean allowEarlyReference) {
    //第一步先从缓存中获取bean
    Object singletonObject = this.singletonObjects.get(beanName);
    //如果没有获取到bean，并且这个bean正在实例化过程中
    if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
        //从二级缓存中获取bean
        singletonObject = this.earlySingletonObjects.get(beanName);
        //如果未获取到
        if (singletonObject == null && allowEarlyReference) {
            //先对一级缓存加锁，保证获取到的是一个实例化完成的bean
            synchronized (this.singletonObjects) {
                //因为获取到锁，可能这个bean已经放入到一级缓存，如果不去获取一次，可能出现多个bean
                singletonObject = this.singletonObjects.get(beanName);
                if (singletonObject == null) {
                    //然后再去从二级缓存中获取
                    singletonObject = this.earlySingletonObjects.get(beanName);
                    if (singletonObject == null) {
                        //然后从三级缓存中获取生成bean的方法，这是解决循环依赖的关键，生成代理类，也是这个方法调用后才生成的
                        ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
                        if (singletonFactory != null) {
                            //调用这个钩子方法，我们在上一个方法doGetBean中可以找到这个钩子方法
                            singletonObject = singletonFactory.getObject();
                            //然后把这个类放到二级缓存中
                            this.earlySingletonObjects.put(beanName, singletonObject);
                            //再从三级缓存中移除这个bean
                            this.singletonFactories.remove(beanName);
                        }
                    }
                }
            }
        }
    }
}
//获取单例bean
public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
    Assert.notNull(beanName, "Bean name must not be null");
    //加锁，如果同时有多线程在获取bean，保证不会获取到不完成的bean
    synchronized (this.singletonObjects) {
        //获取到锁后先从一级缓存中获取
        Object singletonObject = this.singletonObjects.get(beanName);
        if (singletonObject == null) {
            if (this.singletonsCurrentlyInDestruction) {
                throw new BeanCreationNotAllowedException(beanName,
                        "Singleton bean creation not allowed while singletons of this factory are in destruction " +
                        "(Do not request a bean from a BeanFactory in a destroy method implementation!)");
            }
            if (logger.isDebugEnabled()) {
                logger.debug("Creating shared instance of singleton bean '" + beanName + "'");
            }
            //标记当前bean正在被创建，用于判断是否是循环依赖，就是加入到set集合中
            beforeSingletonCreation(beanName);
            boolean newSingleton = false;
            boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
            if (recordSuppressedExceptions) {
                this.suppressedExceptions = new LinkedHashSet<>();
            }
            try {
                //调用外面传入进来的createBean钩子方法
                singletonObject = singletonFactory.getObject();
                newSingleton = true;
            }
            catch (IllegalStateException ex) {
                singletonObject = this.singletonObjects.get(beanName);
                if (singletonObject == null) {
                    throw ex;
                }
            }
            catch (BeanCreationException ex) {
                if (recordSuppressedExceptions) {
                    for (Exception suppressedException : this.suppressedExceptions) {
                        ex.addRelatedCause(suppressedException);
                    }
                }
                throw ex;
            }
            finally {
                if (recordSuppressedExceptions) {
                    this.suppressedExceptions = null;
                }
                afterSingletonCreation(beanName);
            }
            if (newSingleton) {
                addSingleton(beanName, singletonObject);
            }
        }
        //返回实例化后的bean
        return singletonObject;
    }
}
//实例化bean
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
        throws BeanCreationException {
    if (logger.isTraceEnabled()) {
        logger.trace("Creating instance of bean '" + beanName + "'");
    }
    RootBeanDefinition mbdToUse = mbd;
    if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
        mbdToUse = new RootBeanDefinition(mbd);
        mbdToUse.setBeanClass(resolvedClass);
    }
    try {
        mbdToUse.prepareMethodOverrides();
    }
    catch (BeanDefinitionValidationException ex) {
        throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
                beanName, "Validation of method overrides failed", ex);
    }
    try {
        //调用第一个bean的后置处理器，生成代理对象，我们一般的AOP是不会在这里生成代理的，因为还没实例化出对象
        //这里这里是AOP和事务的关键，会生成切面信息进行缓存
        Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
        if (bean != null) {
            return bean;
        }
    }
    catch (Throwable ex) {
        throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName,
                "BeanPostProcessor before instantiation of bean failed", ex);
    }
    try {
        //真正去创建bean，进行实例化
        Object beanInstance = doCreateBean(beanName, mbdToUse, args);
        if (logger.isTraceEnabled()) {
            logger.trace("Finished creating instance of bean '" + beanName + "'");
        }
        return beanInstance;
    }
    catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) {
        throw ex;
    }
    catch (Throwable ex) {
        throw new BeanCreationException(
                mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
    }
}

protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
        throws BeanCreationException {
    BeanWrapper instanceWrapper = null;
    if (mbd.isSingleton()) {
        instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
    }
    if (instanceWrapper == null) {
        instanceWrapper = createBeanInstance(beanName, mbd, args);
    }
    //获取早期对象
    Object bean = instanceWrapper.getWrappedInstance();
    Class<?> beanType = instanceWrapper.getWrappedClass();
    if (beanType != NullBean.class) {
        mbd.resolvedTargetType = beanType;
    }
    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;
        }
    }
    /**
     * 这一段就是循环依赖最关键的点，会先放一个早期对象到钩子方法（getEarlyBeanReference）中
     * 这个钩子方法如果这个对象不用创建动态代理，则会直接返回当前对象
     * 把这个钩子方法放入三级缓存，等真正来获取bean时，执行
     * 过了这段逻辑，才开始去注入各种属性，如果有需要循环获取的属性，则会从三级缓存中获取
     **/
    boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
            isSingletonCurrentlyInCreation(beanName));
    if (earlySingletonExposure) {
        if (logger.isTraceEnabled()) {
            logger.trace("Eagerly caching bean '" + beanName +
                    "' to allow for resolving potential circular references");
        }
        addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
    }

    Object exposedObject = bean;
    try {
        //去注入属性，比如有依赖的属性，则是从这里去注入，会循环调用getBean
        populateBean(beanName, mbd, instanceWrapper);
        /**
         * 到这一步，对象已经实例化完成
         * 如果有经过循环依赖的话，需要进行动态代理的类，已经生成了动态代理
         * 但是，未经过循环依赖的类，需要我们实例化完成后，进行动态代理
         * 动态代理时，还需判断是否是接口，需要用JDK的动态代理，还是CGLIB的动态代理
         **/
        exposedObject = initializeBean(beanName, exposedObject, mbd);
    }
    catch (Throwable ex) {
        if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {
            throw (BeanCreationException) ex;
        }
        else {
            throw new BeanCreationException(
                    mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);
        }
    }
    if (earlySingletonExposure) {
        //如果是正常bean，一般只会从一级或者二级缓存中获取，不会把bean存入三级缓存
        Object earlySingletonReference = getSingleton(beanName, false);
        if (earlySingletonReference != null) {
            if (exposedObject == bean) {
                exposedObject = earlySingletonReference;
            }
            else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
                String[] dependentBeans = getDependentBeans(beanName);
                Set<String> actualDependentBeans = new LinkedHashSet<>(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 " +
                            "'getBeanNamesForType' with the 'allowEagerInit' flag turned off, for example.");
                }
            }
        }
    }
    try {
        //注册销毁的bean的销毁接口
        registerDisposableBeanIfNecessary(beanName, bean, mbd);
    }
    catch (BeanDefinitionValidationException ex) {
        throw new BeanCreationException(
                mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
    }
    return exposedObject;
}

Spring IOC容器bean的实例化，到这基本就完成了，这次所有源代码的分析把重要的地方，大致流程都讲了一遍，但是有很多细节没有讲到，比如说怎么创建动态代理，怎么具体怎么实例化，这些代码根据我的注释找到相应的地方，可以去自己看看源码，我基本上都标注了每一步的步骤，只是如果要深挖的话，层级太深，绕也会绕晕，最好是自己脑子里有个框架，然后再根据框架去找细节，这样就会事半功倍。

二、最后总结

这次Spring只分析了源代码，这次就分析到这里，其实我们分析源代码不仅仅只是知道它内部逻辑是怎么运行的，关键是需要学习这个框架的思想，以及各种设计模式的运用，这样才算学到了东西；其实Spring的代码，你过了一段时间，基本上就只会记得大概，主要是确实层级太深，所以我们需要了解到精髓，例如里面生成bean用到的工厂模式，去解析@ComponentScan，里面的这个type用到的策略模式，还有AOP切面调用advisor使用的责任链模式，等等，都是我们学习的点；