Bean后置处理器

spring在创建对象(org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#createBeanInstance)的时候, 使用了这个 构造函数后置处理器, 用来选择使用哪个构造函数的.

所以这个后置处理器的执行时机是: 对象实例化之前执行

org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#determineConstructorsFromBeanPostProcessors

@Nullable
protected Constructor<?>[] determineConstructorsFromBeanPostProcessors(@Nullable Class<?> beanClass, String beanName)
        throws BeansException {

    if (beanClass != null && hasInstantiationAwareBeanPostProcessors()) {
        for (BeanPostProcessor bp : getBeanPostProcessors()) {
            if (bp instanceof SmartInstantiationAwareBeanPostProcessor) {
                SmartInstantiationAwareBeanPostProcessor ibp = (SmartInstantiationAwareBeanPostProcessor) bp;
                Constructor<?>[] ctors = ibp.determineCandidateConstructors(beanClass, beanName);
                if (ctors != null) {
                    return ctors;
                }
            }
        }
    }
    return null;
}

这里满足条件的后置处理器, 就两个:

1.ConfigurationClassPostProcessor$ImportAwareBeanPostProcessor

2. AutowiredAnnotationBeanPostProcessor

ImportAwareBeanPostProcessor

@Override
@Nullable
public Constructor<?>[] determineCandidateConstructors(Class<?> beanClass, String beanName) throws BeansException {
    return null;
}

这个类并没有实现这个接口, 而是父类去实现的:

org.springframework.beans.factory.config.InstantiationAwareBeanPostProcessorAdapter#determineCandidateConstructors

然而, 他也是 啥也没干.   

怎么感觉这个后置处理器, 哪哪都有他, 却哪哪都不干活.

AutowiredAnnotationBeanPostProcessor

@Override
@Nullable
public Constructor<?>[] determineCandidateConstructors(Class<?> beanClass, final String beanName)
        throws BeanCreationException {

    // Let's check for lookup methods here...
    if (!this.lookupMethodsChecked.contains(beanName)) {
        try {
            ReflectionUtils.doWithMethods(beanClass, method -> {
                Lookup lookup = method.getAnnotation(Lookup.class);
                if (lookup != null) {
                    Assert.state(this.beanFactory != null, "No BeanFactory available");
                    LookupOverride override = new LookupOverride(method, lookup.value());
                    try {
                        RootBeanDefinition mbd = (RootBeanDefinition)
                                this.beanFactory.getMergedBeanDefinition(beanName);
                        mbd.getMethodOverrides().addOverride(override);
                    }
                    catch (NoSuchBeanDefinitionException ex) {
                        throw new BeanCreationException(beanName,
                                "Cannot apply @Lookup to beans without corresponding bean definition");
                    }
                }
            });
        }
        catch (IllegalStateException ex) {
            throw new BeanCreationException(beanName, "Lookup method resolution failed", ex);
        }
        this.lookupMethodsChecked.add(beanName);
    }

    // Quick check on the concurrent map first, with minimal locking.
    Constructor<?>[] candidateConstructors = this.candidateConstructorsCache.get(beanClass);
    if (candidateConstructors == null) {
        // Fully synchronized resolution now...
        synchronized (this.candidateConstructorsCache) {
            candidateConstructors = this.candidateConstructorsCache.get(beanClass);
            if (candidateConstructors == null) {
                Constructor<?>[] rawCandidates;
                try {
                    //反射获取所有构造函数
                    rawCandidates = beanClass.getDeclaredConstructors();
                }
                catch (Throwable ex) {
                    throw new BeanCreationException(beanName,
                            "Resolution of declared constructors on bean Class [" + beanClass.getName() +
                            "] from ClassLoader [" + beanClass.getClassLoader() + "] failed", ex);
                }
                //候选构造方法
                List<Constructor<?>> candidates = new ArrayList<>(rawCandidates.length);
                Constructor<?> requiredConstructor = null;
                Constructor<?> defaultConstructor = null;
                //这个貌似是 Kotlin 上用的, 不用管它
                Constructor<?> primaryConstructor = BeanUtils.findPrimaryConstructor(beanClass);
                int nonSyntheticConstructors = 0;
                //遍历这些构造函数
                for (Constructor<?> candidate : rawCandidates) {
                    //判断构造方法是否是合成的
                    if (!candidate.isSynthetic()) {
                        nonSyntheticConstructors++;
                    }
                    else if (primaryConstructor != null) {
                        continue;
                    }
                    //查看是否有 @Autowired 注解
                    //如果有多个构造方法, 可以通过标注 @Autowired 的方式来指定使用哪个构造方法
                    AnnotationAttributes ann = findAutowiredAnnotation(candidate);
                    if (ann == null) {
                        Class<?> userClass = ClassUtils.getUserClass(beanClass);
                        if (userClass != beanClass) {
                            try {
                                Constructor<?> superCtor =
                                        userClass.getDeclaredConstructor(candidate.getParameterTypes());
                                ann = findAutowiredAnnotation(superCtor);
                            }
                            catch (NoSuchMethodException ex) {
                                // Simply proceed, no equivalent superclass constructor found...
                            }
                        }
                    }
                    //有 @Autowired 的情况
                    if (ann != null) {
                        if (requiredConstructor != null) {
                            throw new BeanCreationException(beanName,
                                    "Invalid autowire-marked constructor: " + candidate +
                                    ". Found constructor with 'required' Autowired annotation already: " +
                                    requiredConstructor);
                        }
                        boolean required = determineRequiredStatus(ann);
                        if (required) {
                            if (!candidates.isEmpty()) {
                                throw new BeanCreationException(beanName,
                                        "Invalid autowire-marked constructors: " + candidates +
                                        ". Found constructor with 'required' Autowired annotation: " +
                                        candidate);
                            }
                            requiredConstructor = candidate;
                        }
                        candidates.add(candidate);
                    }
                    //无参构造函数的情况
                    else if (candidate.getParameterCount() == 0) {
                        //构造函数没有参数, 则设置为默认的构造函数
                        defaultConstructor = candidate;
                    }
                }
                //到这里, 已经循环完了所有的构造方法

                //候选者不为空时
                if (!candidates.isEmpty()) {
                    // Add default constructor to list of optional constructors, as fallback.
                    if (requiredConstructor == null) {
                        if (defaultConstructor != null) {
                            candidates.add(defaultConstructor);
                        }
                        else if (candidates.size() == 1 && logger.isInfoEnabled()) {
                            logger.info("Inconsistent constructor declaration on bean with name '" + beanName +
                                    "': single autowire-marked constructor flagged as optional - " +
                                    "this constructor is effectively required since there is no " +
                                    "default constructor to fall back to: " + candidates.get(0));
                        }
                    }
                    candidateConstructors = candidates.toArray(new Constructor<?>[0]);
                }
                //类的构造方法只有1个, 且该构造方法有多个参数
                else if (rawCandidates.length == 1 && rawCandidates[0].getParameterCount() > 0) {
                    candidateConstructors = new Constructor<?>[] {rawCandidates[0]};
                }
                //这里不会进, 因为 primaryConstructor = null
                else if (nonSyntheticConstructors == 2 && primaryConstructor != null &&
                        defaultConstructor != null && !primaryConstructor.equals(defaultConstructor)) {
                    candidateConstructors = new Constructor<?>[] {primaryConstructor, defaultConstructor};
                }
                //这里也不会进, 因为 primaryConstructor = null
                else if (nonSyntheticConstructors == 1 && primaryConstructor != null) {
                    candidateConstructors = new Constructor<?>[] {primaryConstructor};
                }
                else {
                    //如果方法进了这里, 就是没找到合适的构造方法
                    //1. 类定义了多个构造方法, 且没有 @Autowired , 则有可能会进这里
                    candidateConstructors = new Constructor<?>[0];
                }
                this.candidateConstructorsCache.put(beanClass, candidateConstructors);
            }
        }
    }
　　 //这里如果没找到, 则会返回 null, 而不会返回空数组
    return (candidateConstructors.length > 0 ? candidateConstructors : null);
}

这个方法比较长, 但是仔细看, 是能看懂的. 大致可以划分为几个步骤:

1. 获取类的所有构造方法

2. 遍历构造方法

　　|-> 只有一个无参构造方法, 则返回null

　　|-> 只有一个有参构造方法, 则返回这个构造方法

　　|-> 有多个构造方法且没有@Autowired, 此时spring则会蒙圈了, 不知道使用哪一个了. 这里的后置处理器, 翻译过来, 叫智能选择构造方法后置处理器.

　　　  当选择不了的时候, 干脆返回 null

　　|-> 有多个构造方法, 且在其中一个方法上标注了 @Autowired , 则会返回这个标注的构造方法

　　|-> 有多个构造方法, 且在多个方法上标注了@Autowired, 则spring会抛出异常, spring会认为, 你指定了几个给我, 是不是你弄错了

注:

这地方有个问题需要注意一下, 如果你写了多个构造方法, 且没有写 无参构造方法, 那么此处返回null, 

在回到 createBeanInstance 方法中, 如果不能走 autowireConstructor(), 而走到 instantiateBean() 中去的话, 会报错的.

因为类已经没有无参构造函数了