之前已經(jīng)就spring中的BeanPostProcessor使用方法以及其實(shí)現(xiàn)細(xì)節(jié)談?wù)撨^(guò)，現(xiàn)在從更加宏觀、抽象的角度去理解spring的bpp具體是如何工作的，現(xiàn)在spring自身有多少bpp，如果我們有自定義的bpp需求，應(yīng)該如何實(shí)現(xiàn)。

其中如下demo的代碼位置 GitHub simple-spring

目錄

再談Spring BeanPostProcessor
1、BeanPostProcessor 種類
1.1、BeanPostProcessor 接口
1.2、InstantiationAwareBeanPostProcessor 接口
1.3、MergedBeanDefinitionPostProcessor 接口
2、源碼學(xué)習(xí)
2.1、實(shí)例化前的提前處理
2.2、實(shí)例化后的合并
2.3、實(shí)例化后的數(shù)據(jù)填充
2.4、初始化init方法
3、Demo

1、BeanPostProcessor 種類

image

如上圖，是在IDEA中使用control+H命令顯示出來(lái)的BeanPostProcessor的繼承實(shí)現(xiàn)類關(guān)系圖，重點(diǎn)介紹BeanPostProcessor，InstantiationAwareBeanPostProcessor、MergedBeanDefinitionPostProcessor三個(gè)接口，后面再繼續(xù)結(jié)合實(shí)際的實(shí)現(xiàn)類，分析spring是如何操作BeanPostProcessor對(duì)象的

閱讀這里面的源碼需要對(duì)Spring 源碼有一定的理解，具體的可看【目錄】Spring 源碼學(xué)習(xí)

1.1、BeanPostProcessor 接口

public interface BeanPostProcessor {
    Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException;
    // 在調(diào)用bean的init方法（非構(gòu)造方法）前調(diào)用
    // beanName 是在Spring IOC 容器的bean 名稱，返回的對(duì)象需要會(huì)被直接使用
    // 切記！！！默認(rèn)返回bean即可，不要無(wú)緣無(wú)故返回null，會(huì)出現(xiàn)各種NPE的情況
    Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException;
    // 在調(diào)用bean的init方法（非構(gòu)造方法）后調(diào)用
}

1.2、InstantiationAwareBeanPostProcessor 接口

public interface InstantiationAwareBeanPostProcessor extends BeanPostProcessor {
    Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName) throws BeansException;
    // 在對(duì)象實(shí)例化之前被調(diào)用，傳入的參數(shù)是類型以及bean的那么
    // 如果有返回非空的對(duì)象，則意味著不需要調(diào)用doCreate操作完成對(duì)象實(shí)例化等
    // 同時(shí)返回非空后會(huì)調(diào)用postProcessAfterInitialization方法
    // 注意看，不是postProcessAfterInstantiation而是postProcessAfterInitialization方法
    boolean postProcessAfterInstantiation(Object bean, String beanName) throws BeansException;
     // 返回的是boolean類型，并不是和postProcessBeforeInstantiation配套使用的，在正常的實(shí)例化之后
     // 主要功能是判斷是否需要完成對(duì)象填充
     // 如果返回false，則意味著不會(huì)完成對(duì)象屬性的填充，例如@Resource導(dǎo)入的對(duì)象還是為null
    PropertyValues postProcessPropertyValues(
            PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) throws BeansException;
    // 提供給已經(jīng)實(shí)例化的對(duì)象一種可以自定義完成對(duì)象pv屬性的修改操作
    // 注意不要隨意返回null，一旦返回null，不會(huì)進(jìn)行對(duì)象填充、對(duì)象依賴等操作
}

1.3、MergedBeanDefinitionPostProcessor 接口

public interface MergedBeanDefinitionPostProcessor extends BeanPostProcessor {
    void postProcessMergedBeanDefinition(RootBeanDefinition beanDefinition, Class<?> beanType, String beanName);
    // 在完成bean的實(shí)例化之后，填充數(shù)據(jù)（populateBean）之前，可自定義的修改beanDefinition內(nèi)容
    // 名字成為merge，合并BPP對(duì)象和對(duì)于的beanDefinition的內(nèi)容
    // 實(shí)際上可以完成任何想任何實(shí)現(xiàn)的功能
}

以上的三個(gè)接口基本上能夠覆蓋所有的BPP特定的功能點(diǎn)，從上面圖也可以看出來(lái)，spring、springboot很多模塊都有相應(yīng)的對(duì)象實(shí)現(xiàn)，完成特定的功能。

2、源碼學(xué)習(xí)

2.1、實(shí)例化前的提前處理

直接定位到AbstractAutowireCapableBeanFactory類的createBean方法，主要介紹的是InstantiationAwareBeanPostProcessor

try {
    Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
    if (bean != null) {
          // 如果bean有用，則會(huì)直接返回，不會(huì)再繼續(xù)執(zhí)行doCreateBean方法
        return bean;
    }
}
catch (Throwable ex) {
    throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName,
            "BeanPostProcessor before instantiation of bean failed", ex);
    // bean創(chuàng)建失敗，在初始化之前錯(cuò)誤了
}

protected Object resolveBeforeInstantiation(String beanName, RootBeanDefinition mbd) {
    Object bean = null;
    if (!Boolean.FALSE.equals(mbd.beforeInstantiationResolved)) {
        // Make sure bean class is actually resolved at this point.
        if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
              // 如果存在InstantiationAwareBeanPostProcessor對(duì)象
            Class<?> targetType = determineTargetType(beanName, mbd);
            if (targetType != null) {
                bean = applyBeanPostProcessorsBeforeInstantiation(targetType, beanName);
                // 調(diào)用的是postProcessBeforeInstantiation方法
                if (bean != null) {
                      // 如果返回的數(shù)據(jù)不為null，則調(diào)用的是postProcessAfterInitialization方法
                      // 注意別看錯(cuò)了，一個(gè)是實(shí)例化Instantiation，一個(gè)是初始化Initialization
                    bean = applyBeanPostProcessorsAfterInitialization(bean, beanName);
                }
            }
        }
        mbd.beforeInstantiationResolved = (bean != null);
        // 記錄下是否通過(guò)提前批次的處理（如果bean不為null，則肯定是）
    }
    // 最后返回結(jié)果
    return bean;
}

其意思就是利用在實(shí)例化之前檢查是否存在合適的InstantiationAwareBeanPostProcessor對(duì)象，去攔截某些需要被處理的bean提前完成bean的實(shí)例化過(guò)程，不會(huì)去調(diào)用init方法，也沒(méi)有數(shù)據(jù)的填充，@Resource對(duì)象的引入等操作。

2.2、實(shí)例化后的合并

實(shí)例化后，僅僅是完成了對(duì)象最基礎(chǔ)的實(shí)例化工作，還未涉及到填充數(shù)據(jù)，init方法執(zhí)行等操作，主要介紹的是MergedBeanDefinitionPostProcessor

synchronized (mbd.postProcessingLock) {
    if (!mbd.postProcessed) {
        try {
              // 合并管理bean
            applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
        }
        catch (Throwable ex) {
            throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                    "Post-processing of merged bean definition failed", ex);
        }
        mbd.postProcessed = true;
    }
}

protected void applyMergedBeanDefinitionPostProcessors(RootBeanDefinition mbd, Class<?> beanType, String beanName) {
    for (BeanPostProcessor bp : getBeanPostProcessors()) {
        if (bp instanceof MergedBeanDefinitionPostProcessor) {
              // 遍歷所有的BeanPostProcessor
              // 篩選出屬于MergedBeanDefinitionPostProcessor的對(duì)象，調(diào)用postProcessMergedBeanDefinition
            MergedBeanDefinitionPostProcessor bdp = (MergedBeanDefinitionPostProcessor) bp;
            bdp.postProcessMergedBeanDefinition(mbd, beanType, beanName);
        }
    }
}

2.3、實(shí)例化后的數(shù)據(jù)填充

主要的操作在populateBean方法中，涉及到的也是InstantiationAwareBeanPostProcessor

再次強(qiáng)調(diào)InstantiationAwareBeanPostProcessor接口不僅僅是用在實(shí)例化前，在實(shí)例化之后也同樣有用

boolean continueWithPropertyPopulation = true;
// 設(shè)置可以繼續(xù)設(shè)置pv值的boolean對(duì)象為true
if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
    for (BeanPostProcessor bp : getBeanPostProcessors()) {
        if (bp instanceof InstantiationAwareBeanPostProcessor) {
              // 如果是InstantiationAwareBeanPostProcessor對(duì)象
            InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
            if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
                  // 調(diào)用postProcessAfterInstantiation
                  // 如果返回false意味著不需要填充pv操作了，直接break
                continueWithPropertyPopulation = false;
                break;
            }
        }
    }
}

if (!continueWithPropertyPopulation) {
     // 上文設(shè)置為false后，直接返回，完全忽略后續(xù)的applyPropertyValues操作
    return;
}

if (hasInstAwareBpps || needsDepCheck) {
    PropertyDescriptor[] filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
    if (hasInstAwareBpps) {
        for (BeanPostProcessor bp : getBeanPostProcessors()) {
            if (bp instanceof InstantiationAwareBeanPostProcessor) {
                InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
                pvs = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
                // 對(duì)bean對(duì)象的pv值進(jìn)行處理操作
                if (pvs == null) {
                      // 如果返回為null，則直接返回
                      // 所以在自定義設(shè)置BPP的時(shí)候必須注意該方法的返回值
                      // 通過(guò)ide自動(dòng)生成的對(duì)象，其默認(rèn)返回值是null
                    return;
                }
            }
        }
    }
    if (needsDepCheck) {
         // 檢查對(duì)象的依賴問(wèn)題
        checkDependencies(beanName, mbd, filteredPds, pvs);
    }
}

2.4、初始化init方法

在spring中是可以設(shè)置對(duì)象的init方法，在對(duì)象實(shí)現(xiàn)之后，在initializeBean方法中,如下的代碼片段

Object wrappedBean = bean;
if (mbd == null || !mbd.isSynthetic()) {
    wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
    // 調(diào)用postProcessBeforeInitialization
}

try {
    invokeInitMethods(beanName, wrappedBean, mbd);
    // 動(dòng)態(tài)代理反射invoke調(diào)用初始化方法
    // 就是在這里獲取到bean的init方法調(diào)用信息，invoke調(diào)用
}
catch (Throwable ex) {
    throw new BeanCreationException(
            (mbd != null ? mbd.getResourceDescription() : null),
            beanName, "Invocation of init method failed", ex);
}

if (mbd == null || !mbd.isSynthetic()) {
    wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
    // 唯一一個(gè)被調(diào)用了2次的地方的方法postProcessAfterInitialization
}

public Object applyBeanPostProcessorsBeforeInitialization(Object existingBean, String beanName)
        throws BeansException {
    Object result = existingBean;
    for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) {
        result = beanProcessor.postProcessBeforeInitialization(result, beanName);
        if (result == null) {
            return result;
        }
    }
    return result;
}

public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName)
        throws BeansException {
    Object result = existingBean;
    for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) {
        result = beanProcessor.postProcessAfterInitialization(result, beanName);
        if (result == null) {
              // 都有個(gè)這樣的操作，返回為null，則退出
            return result;
        }
    }
    return result;
}

3、Demo

public class CustomInstantiationAwareBeanPostProcessor implements InstantiationAwareBeanPostProcessor {

    public Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName) throws BeansException {
        if (beanName.equals("student")) {
            Student student = new Student("bpp", 20);
            return student;
        }

        if (beanName.equals("superStudent")) {
            SuperStudent studentAndTeacher = new SuperStudent();
            studentAndTeacher.setName("zhangsan");
            return studentAndTeacher;
        }

        return null;
    }

    public boolean postProcessAfterInstantiation(Object bean, String beanName) throws BeansException {
        return true;
    }

    public PropertyValues postProcessPropertyValues(PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) throws BeansException {
        //return pvs;
        return null;
    }

    public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
        return bean;
    }

    public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
        return bean;
    }
}

public class SuperStudent {

    private String name;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Resource
    private Teacher teacher;

    public void doInit() {
        System.out.println("superstudent init");
    }

    public void doSet() {
        if (teacher != null) {
            teacher.setAge(19);
            teacher.setName("superStudent");
            System.out.println(teacher.toString());
        } else {
            System.out.println("teacher is null");
        }
    }

    @Override
    public String toString() {
        return "SuperStudent{" +
                "name='" + name + '\'' +
                ", teacher=" + teacher +
                '}';
    }
}

public static void runBPP() {
    ClassPathXmlApplicationContext applicationContext = new ClassPathXmlApplicationContext(
            new String[]{"context-bpp.xml"});
    SuperStudent student = (SuperStudent) applicationContext.getBean("superStudent");
    Teacher teacher = (Teacher) applicationContext.getBean("teacher");
    System.out.println(teacher.toString());
    System.out.println(student.toString());
    student.doSet();
    System.out.println(teacher.toString());
}

如下圖幾種樣例的代碼

• 返回pv為null
  
  image

很清楚在SuperStudent的對(duì)象是null，并沒(méi)有完成填充操作，但是調(diào)用了init方法

• 返回pv為原值，不作任何改變
  
  image

完成了填充操作，而且這個(gè)SuperStudent的teacher對(duì)象是和Spring IOC 容器內(nèi)的保持一致

• 設(shè)置postProcessBeforeInstantiation
  
  image

SuperStudent返回輸出的對(duì)象，teacher沒(méi)有值，而且也沒(méi)有調(diào)用其init的方法