上一節(jié),研究了下獲取bean的源碼,邏輯還是比較簡單的,這次,我們一起來研究下創(chuàng)建bean的源碼,話不多說,我們直接看源碼
發(fā)現(xiàn)bean的創(chuàng)建的代碼都是在AbstractAutowireCapableBeanFactory這個類中,之前分析@Autowired注解的時候,也是在這個類中
createBean的其他方法我在這里就不分析了,咱們具體就分析doCreateBean

AbstractAutowireCapableBeanFactory

這里將成三個步驟,創(chuàng)建bean實例,填充bean的屬性,執(zhí)行后置處理器,
···
@Override
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreationException {

    RootBeanDefinition mbdToUse = mbd;

    //確保當(dāng)前的bean類已經(jīng)被解析
    Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
    if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
        mbdToUse = new RootBeanDefinition(mbd);
        mbdToUse.setBeanClass(resolvedClass);
    }
        mbdToUse.prepareMethodOverrides();

    try {
        //1:在獲取bean之前獲取一下嘗試獲取下代理對象
        //2:判斷是不是基本類型的對象
        //3:是否需要跳過(獲取增強(qiáng)對象,判斷存不存在@Aspect注解,存在,證明是AOP類,判斷切點,根據(jù)@Before等注解獲取一個完整的增強(qiáng)對象)
        //4:將獲取到的增強(qiáng)對象集合與緩存緩存中的advisor.getName執(zhí)行比較,如果是同一個bean,直接返回
        //判斷是否有自定義的targetSource,存在的話,targetSource以自定義的方式處理目標(biāo)類,然后返回,否則返回null
        Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
        if (bean != null) {
            return bean;
        }
    }

    try {
        //真正創(chuàng)建bean實例的方法,
        Object beanInstance = doCreateBean(beanName, mbdToUse, args);
        return beanInstance;
    }

}

···
這個方法很長,咱們先簡單看一下這個方法到底做了什么操作,為了看得清楚,我會刪除部分拋異常代碼
1:從緩存中獲取bean的包裝對象并刪除緩存
2:緩存中獲取不到bean,創(chuàng)建bean實例
3:解決循環(huán)依賴
4:填充bean中的屬性
5:重新注冊一次bean實例
···

AbstractAutowireCapableBeanFactory

doCreateBean

protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
throws BeanCreationException {

    // 1:創(chuàng)建bean包裝對象,
    BeanWrapper instanceWrapper = null;
    if (mbd.isSingleton()) {
        //如果是單例,從緩存中獲取bean包裝對象,并刪除緩存
        instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
    }
    if (instanceWrapper == null) {
        //創(chuàng)建bean實例,并返回bean包裝對象
        //這里創(chuàng)建bean實例并返回bean包裝對象
        //createBeanInstance方法中會通過三種方式創(chuàng)建bean實例
        //1:通過工廠方法創(chuàng)建
        //2:通過構(gòu)造方法注入,比如autowire或者constructor的方式創(chuàng)建bean實例
        //3:通過無參構(gòu)造的方式創(chuàng)建bean實例
        //4:如果配置文件中配置了lookup-method 和 replace-method,則會利用Cglib代理的方式增強(qiáng)bean實例
        instanceWrapper = createBeanInstance(beanName, mbd, args);
    }
    final Object bean = instanceWrapper.getWrappedInstance();
    Class<?> beanType = instanceWrapper.getWrappedClass();
    if (beanType != NullBean.class) {
        mbd.resolvedTargetType = beanType;
    }

    // 后置處理器處理流程,就不展開說了
    synchronized (mbd.postProcessingLock) {
        if (!mbd.postProcessed) {
            try {
                applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
            }
            mbd.postProcessed = true;
        }
    }
    //===========================================================================================
    //2.1:判斷是否存在循環(huán)引用,
    //這個earlySingletonExposure的意思是:單例bean是否提前暴露
    //同時滿足三個條件,才會變?yōu)閠rue
    //1:mbd.isSingleton():是否是單例
    //2:this.allowCircularReferences:是否循環(huán)引用
    //3:isSingletonCurrentlyInCreation:該單例bean是否處于創(chuàng)建中
    boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
            isSingletonCurrentlyInCreation(beanName));
    if (earlySingletonExposure) {
        //添加單例工廠對象,用于處理循環(huán)引用,這個地方是利用了函數(shù)式接口,我們把它展開看,就好看一點
        addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));

        addSingletonFactory(beanName, new ObjectFactory<Object>() {
                    @Override
                    public Object getObject() throws BeansException {
                        // 獲取早期bean引用
                        return getEarlyBeanReference(beanName, mbd, bean);
                    }
                });
    }

    //=================================================================================================
    //3:填充屬性
    Object exposedObject = bean;
        populateBean(beanName, mbd, instanceWrapper);
        exposedObject = initializeBean(beanName, exposedObject, mbd);

    //2.2解決循環(huán)引用
    if (earlySingletonExposure) {
        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 " +
                            "'getBeanNamesOfType' with the 'allowEagerInit' flag turned off, for example.");
                }
            }
        }
    }

    // 必要時再注冊一次bean
    registerDisposableBeanIfNecessary(beanName, bean, mbd);

    return exposedObject;
}

···