Spring在正常創(chuàng)建Bean的時(shí)候，實(shí)際上是調(diào)用了createBean方法中的doCreateBean方法。

createBean方法關(guān)鍵源碼如下：

@Override
    protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
            throws BeanCreationException {
            。。。

        try {
            //讓InstantiationAwareBeanPostProcessor有機(jī)會(huì)返回代理而不是目標(biāo)bean實(shí)例
            //實(shí)際上就是調(diào)用:
            // applyBeanPostProcessorsBeforeInstantiation 內(nèi)部for循環(huán)調(diào)用postProcessBeforeInstantiation 
            // applyBeanPostProcessorsAfterInitialization 內(nèi)部for循環(huán)調(diào)用postProcessAfterInitialization
            // 上述兩個(gè)具體的方法主要就是進(jìn)行代理，實(shí)例化前和初始化后作一些增強(qiáng)性的操作，影響bean的創(chuàng)建流程
            Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
            if (bean != null) { //如果不為空，說(shuō)明有了代理類(lèi)
                return bean;
            }
        }
        catch (Throwable ex) {
            throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName,
                    "BeanPostProcessor before instantiation of bean failed", ex);
        }

        try {
            //spring正常利用構(gòu)造器創(chuàng)建對(duì)象的流程
            Object beanInstance = doCreateBean(beanName, mbdToUse, args);
            if (logger.isTraceEnabled()) {
                logger.trace("Finished creating instance of bean '" + beanName + "'");
            }
            return beanInstance;
        }
        。。。
    }

InstantiationAwareBeanPostProcessor使用場(chǎng)景

當(dāng)不想使用spring默認(rèn)創(chuàng)建bean的方法通過(guò)構(gòu)造器創(chuàng)建bean，比如Mybatis的Mapper，它只有接口，沒(méi)有實(shí)現(xiàn)類(lèi)的構(gòu)造方法，postProcessBeforeInstantiation就是在spring使用構(gòu)造器幫你創(chuàng)建bean之前創(chuàng)建對(duì)象的，你可以創(chuàng)建一個(gè)代理對(duì)象返回，那么就可以實(shí)現(xiàn)自定義實(shí)例化對(duì)象了。

doCreateBean方法關(guān)鍵源碼如下：

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

        。。。
        //這里開(kāi)始對(duì)bean進(jìn)行初始化
        Object exposedObject = bean;
        try {
            populateBean(beanName, mbd, instanceWrapper);// 對(duì)屬性賦值
            exposedObject = initializeBean(beanName, exposedObject, mbd); // 對(duì)bean進(jìn)行初始化
        }

        。。。

initializeBean方法關(guān)鍵源碼如下：

protected Object initializeBean(String beanName, Object bean, @Nullable RootBeanDefinition mbd) {
        。。。
        Object wrappedBean = bean;
        if (mbd == null || !mbd.isSynthetic()) {
          //  執(zhí)行后置處理器 before
            wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
        }

        try {
            //執(zhí)行用戶自定義的 init 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()) {
           //  執(zhí)行后置處理器 after 
            wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
        }

        return wrappedBean;
    }

invokeInitMethods方法關(guān)鍵源碼如下：

protected void invokeInitMethods(String beanName, final Object bean, @Nullable RootBeanDefinition mbd)
        throws Throwable {

    // 是否實(shí)現(xiàn)了 InitializingBean 接口
    boolean isInitializingBean = (bean instanceof InitializingBean);
    if (isInitializingBean && (mbd == null || !mbd.isExternallyManagedInitMethod("afterPropertiesSet"))) {
        if (logger.isTraceEnabled()) {
            logger.trace("Invoking afterPropertiesSet() on bean with name '" + beanName + "'");
        }
        if (System.getSecurityManager() != null) {
            try {
                AccessController.doPrivileged((PrivilegedExceptionAction<Object>) () -> {
                    ((InitializingBean) bean).afterPropertiesSet();
                    return null;
                }, getAccessControlContext());
            }
            catch (PrivilegedActionException pae) {
                throw pae.getException();
            }
        }
        else {
            // 直接調(diào)用 afterPropertiesSet()
            ((InitializingBean) bean).afterPropertiesSet();
        }
    }

    if (mbd != null && bean.getClass() != NullBean.class) {
        // 判斷是否指定了 init-method()，
        // 如果指定了 init-method()，則再調(diào)用制定的init-method
        String initMethodName = mbd.getInitMethodName();
        if (StringUtils.hasLength(initMethodName) &&
                !(isInitializingBean && "afterPropertiesSet".equals(initMethodName)) &&
                !mbd.isExternallyManagedInitMethod(initMethodName)) {
            invokeCustomInitMethod(beanName, bean, mbd);
        }
    }
}