protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
    /**
     * ----------0，校驗(yàn)部分------------
     */
    Class<?> beanClass = resolveBeanClass(mbd, beanName);

    if (beanClass != null && !Modifier.isPublic(beanClass.getModifiers()) && !mbd.isNonPublicAccessAllowed()) {
        throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Bean class isn't public, and non-public access not allowed: " + beanClass.getName());
    }

    /**
     * ----------1，通過Supplier實(shí)例化部分------------
     */
    // BeanDefinition中添加了Supplier，則調(diào)用Supplier來得到對(duì)象
    Supplier<?> instanceSupplier = mbd.getInstanceSupplier();
    if (instanceSupplier != null) {
        return obtainFromSupplier(instanceSupplier, beanName);
    }

    /**
     * ----------2，通過工廠方法實(shí)例化部分------------
     */
    // @Bean對(duì)應(yīng)的BeanDefinition
    if (mbd.getFactoryMethodName() != null) {
        return instantiateUsingFactoryMethod(beanName, mbd, args);
    }


     /**
     * ----------3，用合適的構(gòu)造函數(shù)實(shí)例化部分------------
     *
     *  一個(gè)類可能有多個(gè)構(gòu)造器，所以Spring得根據(jù)參數(shù)個(gè)數(shù)、類型確定需要調(diào)用的構(gòu)造器。
     *  原型的BeanDefinition，會(huì)多次來創(chuàng)建Bean。故在使用構(gòu)造器創(chuàng)建實(shí)例后，Spring會(huì)將解析過后確定下來的構(gòu)造器或工廠方法保存在緩存中，避免再次創(chuàng)建相同bean時(shí)再次解析（節(jié)約時(shí)間）
     */
    boolean resolved = false; //對(duì)構(gòu)造器是否緩存了
    boolean autowireNecessary = false; //緩存的構(gòu)造器是否有參數(shù)
    if (args == null) {
        synchronized (mbd.constructorArgumentLock) {
            if (mbd.resolvedConstructorOrFactoryMethod != null) {
                resolved = true;
                // autowireNecessary表示有沒有必要要進(jìn)行注入，比如當(dāng)前BeanDefinition用的是無參構(gòu)造方法，那么autowireNecessary為false，否則為true，表示需要給構(gòu)造方法參數(shù)注入值
                autowireNecessary = mbd.constructorArgumentsResolved;
            }
        }
    }
    if (resolved) {
        // 如果確定了當(dāng)前BeanDefinition的構(gòu)造方法，那么看是否需要進(jìn)行對(duì)構(gòu)造方法進(jìn)行參數(shù)的依賴注入（構(gòu)造方法注入）
        if (autowireNecessary) {
            // 方法內(nèi)會(huì)拿到緩存好的構(gòu)造方法的入?yún)?
            return autowireConstructor(beanName, mbd, null, null);
        }
        else {
            // 構(gòu)造方法已經(jīng)找到了，但是沒有參數(shù)，那就表示是無參，直接進(jìn)行實(shí)例化
            return instantiateBean(beanName, mbd);
        }
    }

    //沒有緩存，從bean后置處理器中為自動(dòng)裝配尋找構(gòu)造方法
    Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
    if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||
            mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
        return autowireConstructor(beanName, mbd, ctors, args);
    }

    // 找出最合適的默認(rèn)構(gòu)造方法
    ctors = mbd.getPreferredConstructors();
    if (ctors != null) {
        // 構(gòu)造函數(shù)自動(dòng)注入
        return autowireConstructor(beanName, mbd, ctors, null);
    }

    /**
     * ----------4，使用默認(rèn)構(gòu)造函數(shù)構(gòu)造部分------------
     */
    // 不匹配以上情況，則直接使用無參構(gòu)造方法
    return instantiateBean(beanName, mbd);
}

// BeanDefinition中添加了Supplier，則調(diào)用Supplier來得到對(duì)象
Supplier<?> instanceSupplier = mbd.getInstanceSupplier();
if (instanceSupplier != null) {
    return obtainFromSupplier(instanceSupplier, beanName);
}

//在beanDefinition階段設(shè)置好
beanDefinition.setBeanClass(Student.class);
beanDefinition.setInstanceSupplier(new Supplier<Object>() {
    @Override
    public Object get() {
        return new UserServiceImpl1();
    }
});

//容器啟動(dòng)后嘗試獲取驗(yàn)證
applicationContext.getBean("student"); //返回new UserServiceImpl1()對(duì)象

// @Bean對(duì)應(yīng)的BeanDefinition
if (mbd.getFactoryMethodName() != null) {
    return instantiateUsingFactoryMethod(beanName, mbd, args);
}

protected BeanWrapper instantiateUsingFactoryMethod(String beanName, RootBeanDefinition mbd, @Nullable Object[] explicitArgs) {
    // 使用factoryBean來實(shí)例化對(duì)象
    return new ConstructorResolver(this).instantiateUsingFactoryMethod(beanName, mbd, explicitArgs);
}

//ConstructorResolver類#instantiateUsingFactoryMethod方法
//省略日志與異常
public BeanWrapper instantiateUsingFactoryMethod(String beanName, RootBeanDefinition mbd, @Nullable Object[] explicitArgs) {

    BeanWrapperImpl bw = new BeanWrapperImpl();
    this.beanFactory.initBeanWrapper(bw);

    Object factoryBean;
    Class<?> factoryClass;
    boolean isStatic;

    //通過beanDefinition獲取到factoryBeanName ，實(shí)際就是@Bean注解的方法所在的configuration類
    String factoryBeanName = mbd.getFactoryBeanName();
    //表示非靜態(tài)方法
    if (factoryBeanName != null) {
        if (factoryBeanName.equals(beanName)) {
            throw new BeanDefinitionStoreException(...);
        }
        factoryBean = this.beanFactory.getBean(factoryBeanName);

        // 該mbd已經(jīng)創(chuàng)建過了【代表這個(gè)邏輯已經(jīng)走過了】
        if (mbd.isSingleton() && this.beanFactory.containsSingleton(beanName)) {
            throw new ImplicitlyAppearedSingletonException();
        }
        factoryClass = factoryBean.getClass();
        isStatic = false;
    }
    else {//表示靜態(tài)方法
        // It's a static factory method on the bean class.
        if (!mbd.hasBeanClass()) {
            throw new BeanDefinitionStoreException(...);
        }
        factoryBean = null;
        factoryClass = mbd.getBeanClass();
        isStatic = true;
    }

    Method factoryMethodToUse = null;
    ArgumentsHolder argsHolderToUse = null;
    Object[] argsToUse = null;

    //如果在調(diào)用getBean方法時(shí)有傳參，那就用傳的參作為@Bean注解的方法（工廠方法）的參數(shù)， 一般懶加載的bean才會(huì)傳參，啟動(dòng)過程就實(shí)例化的實(shí)際上都沒有傳參
    if (explicitArgs != null) {
        argsToUse = explicitArgs;
    }
    else {
        Object[] argsToResolve = null;
        synchronized (mbd.constructorArgumentLock) {
            factoryMethodToUse = (Method) mbd.resolvedConstructorOrFactoryMethod;
            //不為空表示已經(jīng)使用過工廠方法，現(xiàn)在是再次使用工廠方法， 一般原型模式和Scope模式采用的上，直接使用該工廠方法和緩存的參數(shù)
            if (factoryMethodToUse != null && mbd.constructorArgumentsResolved) {
                // Found a cached factory method...
                argsToUse = mbd.resolvedConstructorArguments;
                if (argsToUse == null) {
                    argsToResolve = mbd.preparedConstructorArguments;
                }
            }
        }
        if (argsToResolve != null) {
            argsToUse = resolvePreparedArguments(beanName, mbd, bw, factoryMethodToUse, argsToResolve, true);
        }
    }

    // 調(diào)用getBean方法沒有傳參，同時(shí)也是第一次使用工廠方法
    if (factoryMethodToUse == null || argsToUse == null) {
        
        factoryClass = ClassUtils.getUserClass(factoryClass);

        // 獲取configuration類的所有候選方法
        Method[] rawCandidates = getCandidateMethods(factoryClass, mbd);
        List<Method> candidateList = new ArrayList<>();
        for (Method candidate : rawCandidates) {
            // 查找到與工廠方法同名的候選方法,即有@Bean的同名方法
            if (Modifier.isStatic(candidate.getModifiers()) == isStatic && mbd.isFactoryMethod(candidate)) {
                candidateList.add(candidate);
            }
        }

        //當(dāng)與工廠方法同名的候選方法只有一個(gè)，且調(diào)用getBean方法時(shí)沒有傳參，且沒有緩存過參數(shù)，直接通過調(diào)用實(shí)例化方法執(zhí)行該候選方法
        if (candidateList.size() == 1 && explicitArgs == null && !mbd.hasConstructorArgumentValues()) {
            Method uniqueCandidate = candidateList.get(0);
            if (uniqueCandidate.getParameterCount() == 0) {
                mbd.factoryMethodToIntrospect = uniqueCandidate;
                synchronized (mbd.constructorArgumentLock) {
                    mbd.resolvedConstructorOrFactoryMethod = uniqueCandidate;
                    mbd.constructorArgumentsResolved = true;
                    mbd.resolvedConstructorArguments = EMPTY_ARGS;
                }
                bw.setBeanInstance(instantiate(beanName, mbd, factoryBean, uniqueCandidate, EMPTY_ARGS));
                return bw;
            }
        }

        Method[] candidates = candidateList.toArray(new Method[0]);
        // 有多個(gè)與工廠方法同名的候選方法時(shí)，進(jìn)行排序。public的方法會(huì)往前排，然后參數(shù)個(gè)數(shù)多的方法往前排
        AutowireUtils.sortFactoryMethods(candidates);

        ConstructorArgumentValues resolvedValues = null;
        boolean autowiring = (mbd.getResolvedAutowireMode() == AutowireCapableBeanFactory.AUTOWIRE_CONSTRUCTOR);
        int minTypeDiffWeight = Integer.MAX_VALUE;
        Set<Method> ambiguousFactoryMethods = null;

        int minNrOfArgs;
        // 如果調(diào)用getBean方法時(shí)有傳參，那么工廠方法最少參數(shù)個(gè)數(shù)要等于傳參個(gè)數(shù)
        if (explicitArgs != null) {
            minNrOfArgs = explicitArgs.length;
        }
        else {
            //如果getBean的時(shí)候沒有傳參，如果BeanDefinition中有設(shè)置，則從BeanDefinition中獲取
            if (mbd.hasConstructorArgumentValues()) {
                ConstructorArgumentValues cargs = mbd.getConstructorArgumentValues();
                resolvedValues = new ConstructorArgumentValues();
                minNrOfArgs = resolveConstructorArguments(beanName, mbd, bw, cargs, resolvedValues);
            }
            else {
                minNrOfArgs = 0;
            }
        }

        LinkedList<UnsatisfiedDependencyException> causes = null;

        for (Method candidate : candidates) {
            Class<?>[] paramTypes = candidate.getParameterTypes();

            if (paramTypes.length >= minNrOfArgs) {
                ArgumentsHolder argsHolder;

                if (explicitArgs != null) {
                    // Explicit arguments given -> arguments length must match exactly.
                    if (paramTypes.length != explicitArgs.length) {
                        continue;
                    }
                    argsHolder = new ArgumentsHolder(explicitArgs);
                }
                else {
                    // Resolved constructor arguments: type conversion and/or autowiring necessary.
                    try {
                        String[] paramNames = null;
                        ParameterNameDiscoverer pnd = this.beanFactory.getParameterNameDiscoverer();
                        if (pnd != null) {
                            paramNames = pnd.getParameterNames(candidate);
                        }
                        //當(dāng)傳入的參數(shù)為空，需要根據(jù)工廠方法的參數(shù)類型注入相應(yīng)的bean
                        argsHolder = createArgumentArray(beanName, mbd, resolvedValues, bw,
                                paramTypes, paramNames, candidate, autowiring, candidates.length == 1);
                    }
                    catch (UnsatisfiedDependencyException ex) {
                        if (logger.isTraceEnabled()) {
                            logger.trace("Ignoring factory method [" + candidate + "] of bean '" + beanName + "': " + ex);
                        }
                        // Swallow and try next overloaded factory method.
                        if (causes == null) {
                            causes = new LinkedList<>();
                        }
                        causes.add(ex);
                        continue;
                    }
                }

                //計(jì)算工廠方法的權(quán)重
                int typeDiffWeight = (mbd.isLenientConstructorResolution() ?
                        argsHolder.getTypeDifferenceWeight(paramTypes) : argsHolder.getAssignabilityWeight(paramTypes));
                // Choose this factory method if it represents the closest match.
                if (typeDiffWeight < minTypeDiffWeight) {
                    /*
                    * 當(dāng)權(quán)重小時(shí)，重新設(shè)置factoryMethodToUse 和argsHolderToUse ，argsToUse ，
                    * 并把當(dāng)前權(quán)重值設(shè)置為最小權(quán)重值，等待遍歷的下一個(gè)候選工廠方法比對(duì)，
                    * 并且將ambiguousFactoryMethods （表示有含糊同樣權(quán)重的候選方法）設(shè)置為空
                    * */
                    factoryMethodToUse = candidate;
                    argsHolderToUse = argsHolder;
                    argsToUse = argsHolder.arguments;
                    minTypeDiffWeight = typeDiffWeight;
                    ambiguousFactoryMethods = null;
                }

                /*
                * 當(dāng)遍歷到下一個(gè)候選方法的時(shí)候，已經(jīng)設(shè)置了factoryMethodToUse 且權(quán)重值與上一次的最小權(quán)重值相等時(shí)，
                * ambiguousFactoryMethods填值，這個(gè)ambiguousFactoryMethods不為空表示有兩個(gè)候選方法的最小權(quán)重相等，
                * spring無法匹配出最適合的工廠方法，如果再繼續(xù)往下遍歷候選器，有更小的權(quán)重值，
                * 那ambiguousFactoryMethods會(huì)再次被設(shè)置為空
                * */
                else if (factoryMethodToUse != null && typeDiffWeight == minTypeDiffWeight &&
                        !mbd.isLenientConstructorResolution() &&
                        paramTypes.length == factoryMethodToUse.getParameterCount() &&
                        !Arrays.equals(paramTypes, factoryMethodToUse.getParameterTypes())) {
                    if (ambiguousFactoryMethods == null) {
                        ambiguousFactoryMethods = new LinkedHashSet<>();
                        ambiguousFactoryMethods.add(factoryMethodToUse);
                    }
                    ambiguousFactoryMethods.add(candidate);
                }
            }
        }

        if (factoryMethodToUse == null) {
            if (causes != null) {
                UnsatisfiedDependencyException ex = causes.removeLast();
                for (Exception cause : causes) {
                    this.beanFactory.onSuppressedException(cause);
                }
                throw ex;
            }
            List<String> argTypes = new ArrayList<>(minNrOfArgs);
            if (explicitArgs != null) {
                for (Object arg : explicitArgs) {
                    argTypes.add(arg != null ? arg.getClass().getSimpleName() : "null");
                }
            }
            else if (resolvedValues != null) {
                Set<ValueHolder> valueHolders = new LinkedHashSet<>(resolvedValues.getArgumentCount());
                valueHolders.addAll(resolvedValues.getIndexedArgumentValues().values());
                valueHolders.addAll(resolvedValues.getGenericArgumentValues());
                for (ValueHolder value : valueHolders) {
                    String argType = (value.getType() != null ? ClassUtils.getShortName(value.getType()) :
                            (value.getValue() != null ? value.getValue().getClass().getSimpleName() : "null"));
                    argTypes.add(argType);
                }
            }
            String argDesc = StringUtils.collectionToCommaDelimitedString(argTypes);
            throw new BeanCreationException(...);
        }
        //返回類型不能為void
        else if (void.class == factoryMethodToUse.getReturnType()) {
            throw new BeanCreationException(...);
        }
        //存在含糊的兩個(gè)工廠方法，不知選哪個(gè)
        else if (ambiguousFactoryMethods != null) {
            throw new BeanCreationException(...);
        }

        if (explicitArgs == null && argsHolderToUse != null) {
            mbd.factoryMethodToIntrospect = factoryMethodToUse;
            argsHolderToUse.storeCache(mbd, factoryMethodToUse);
        }
    }

    //去實(shí)例化
    bw.setBeanInstance(instantiate(beanName, mbd, factoryBean, factoryMethodToUse, argsToUse));
    return bw;
}

boolean resolved = false; //對(duì)構(gòu)造器是否緩存了
boolean autowireNecessary = false; //緩存的構(gòu)造器是否有參數(shù)
//判斷是否有傳參，因?yàn)間etBean方法可以通過傳參指定構(gòu)造方法參數(shù)類型來進(jìn)行匹配（故這種對(duì)于參數(shù)的不確定是不能緩存的）
if (args == null) {
    synchronized (mbd.constructorArgumentLock) {
        if (mbd.resolvedConstructorOrFactoryMethod != null) {
            resolved = true;
            // autowireNecessary表示有沒有必要要進(jìn)行注入，比如當(dāng)前BeanDefinition用的是無參構(gòu)造方法，那么autowireNecessary為false，否則為true，表示需要給構(gòu)造方法參數(shù)注入值
            autowireNecessary = mbd.constructorArgumentsResolved;
        }
    }
}
if (resolved) {
    // 如果確定了當(dāng)前BeanDefinition的構(gòu)造方法，那么看是否需要進(jìn)行對(duì)構(gòu)造方法進(jìn)行參數(shù)的依賴注入（構(gòu)造方法注入）
    if (autowireNecessary) {
        // 方法內(nèi)會(huì)拿到緩存好的構(gòu)造方法的入?yún)?
        return autowireConstructor(beanName, mbd, null, null);
    }
    else {
        // 構(gòu)造方法已經(jīng)找到了，但是沒有參數(shù)，那就表示是無參，直接進(jìn)行實(shí)例化
        return instantiateBean(beanName, mbd);
    }
}

//沒有緩存，從bean后置處理器中為自動(dòng)裝配尋找構(gòu)造方法
Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);

//存在推斷出來的構(gòu)造方法集合
//BeanDefinition被設(shè)置為AUTOWIRE_CONSTRUCTOR
//BeanDefinition指定了構(gòu)造方法參數(shù)值
//getBean的時(shí)候傳入了構(gòu)造方法參數(shù)值
if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||
        mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
    return autowireConstructor(beanName, mbd, ctors, args);
}

// 找出最合適的默認(rèn)構(gòu)造方法
ctors = mbd.getPreferredConstructors();
if (ctors != null) {
    // 構(gòu)造函數(shù)自動(dòng)注入
    return autowireConstructor(beanName, mbd, ctors, null);
}

protected Constructor<?>[] determineConstructorsFromBeanPostProcessors(@Nullable Class<?> beanClass, String beanName) throws BeansException {
    //如果類不為null，且存在InstantiationAwareBeanPostProcessor接口的處理器【其實(shí)用的不是它，而是他的子接口SmartInstantiationAwareBeanPostProcessor】
    if (beanClass != null && hasInstantiationAwareBeanPostProcessors()) {
        for (BeanPostProcessor bp : getBeanPostProcessors()) {
            if (bp instanceof SmartInstantiationAwareBeanPostProcessor) {
                SmartInstantiationAwareBeanPostProcessor ibp = (SmartInstantiationAwareBeanPostProcessor) bp;
                //真正存在實(shí)現(xiàn)的類是AutowiredAnnotationBeanPostProcessor類
                Constructor<?>[] ctors = ibp.determineCandidateConstructors(beanClass, beanName);
                if (ctors != null) {
                    return ctors;
                }
            }
        }
    }
    return null;
}
//AutowiredAnnotationBeanPostProcessor類#determineCandidateConstructors方法
//日志和異常的輸出會(huì)被省略
public Constructor<?>[] determineCandidateConstructors(Class<?> beanClass, final String beanName) throws BeanCreationException {
    // 判斷是否檢測(cè)過
    if (!this.lookupMethodsChecked.contains(beanName)) {
        try {
            // 遍歷目標(biāo)類中的method，查看是否寫了@Lookup方法
            ReflectionUtils.doWithMethods(beanClass, method -> {
                Lookup lookup = method.getAnnotation(Lookup.class);
                if (lookup != null) {
                    Assert.state(this.beanFactory != null, "No BeanFactory available");
                    // 將當(dāng)前method封裝成LookupOverride并設(shè)置到BeanDefinition的methodOverrides中
                    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,"...");
                    }
                }
            });
        }
        catch (IllegalStateException ex) {
            throw new BeanCreationException(beanName, "...", ex);
        }
        this.lookupMethodsChecked.add(beanName);
    }
    //先從緩存中獲取，看之前有沒有找過
    Constructor<?>[] candidateConstructors = this.candidateConstructorsCache.get(beanClass);
    //緩存中沒有
    if (candidateConstructors == null) {
        // 加鎖，開始尋找
        synchronized (this.candidateConstructorsCache) {
            //再次從緩存中獲取，看被阻塞的時(shí)候沒有有線程已經(jīng)找好了存在緩存中（加鎖并雙重判斷的常規(guī)做法）
            candidateConstructors = this.candidateConstructorsCache.get(beanClass);
            if (candidateConstructors == null) {
                Constructor<?>[] rawCandidates;
                try {
                    // 拿到所有的構(gòu)造方法
                    rawCandidates = beanClass.getDeclaredConstructors();
                }
                catch (Throwable ex) {
                    throw new BeanCreationException(beanName, "...", ex);
                }
                List<Constructor<?>> candidates = new ArrayList<>(rawCandidates.length);
                // 用來記錄required為true的構(gòu)造方法，一個(gè)類中只能有一個(gè)required為true的構(gòu)造方法
                Constructor<?> requiredConstructor = null;
                // 用來記錄默認(rèn)無參的構(gòu)造方法
                Constructor<?> defaultConstructor = null;
                // kotlin相關(guān)，不用管
                Constructor<?> primaryConstructor = BeanUtils.findPrimaryConstructor(beanClass);
                int nonSyntheticConstructors = 0;
                // 遍歷每個(gè)構(gòu)造方法
                for (Constructor<?> candidate : rawCandidates) {
                    if (!candidate.isSynthetic()) {
                        // 記錄一下普通的構(gòu)造方法
                        nonSyntheticConstructors++;
                    }
                    else if (primaryConstructor != null) {
                        continue;
                    }
                    // 當(dāng)前遍歷的構(gòu)造方法是否寫了@Autowired
                    AnnotationAttributes ann = findAutowiredAnnotation(candidate);
                    //沒有@Autowired注解的情況
                    if (ann == null) {
                        // 如果beanClass是代理類，則得到被代理的類的類型
                        Class<?> userClass = ClassUtils.getUserClass(beanClass);
                        if (userClass != beanClass) {
                            try {
                                //尋找被代理的類上的構(gòu)造方法有沒有@Autowired注解
                                Constructor<?> superCtor = userClass.getDeclaredConstructor(candidate.getParameterTypes());
                                ann = findAutowiredAnnotation(superCtor);
                            }
                            catch (NoSuchMethodException ex) {}
                        }
                    }
                    /**
                     * 構(gòu)造方法如果有@Autowired注解，最后的操作是candidates.add(candidate),但如果出現(xiàn)兩種情況會(huì)拋出異常
                     *      a、如果之前有遍歷過@Autowired的構(gòu)造器，即requiredConstructor不為空
                     *      b、如果當(dāng)前的@Autowired的required屬性為true，不管之前遍歷過的@Autowired屬性是true還是false
                     */
                    if (ann != null) {
                        // 整個(gè)類中如果有一個(gè)required為true的構(gòu)造方法，那就不能有其他的加了@Autowired的構(gòu)造方法
                        if (requiredConstructor != null) {
                            throw new BeanCreationException(...);
                        }
                        //獲取該方法上的@Autowired注解的required屬性
                        boolean required = determineRequiredStatus(ann);
                        if (required) {
                            if (!candidates.isEmpty()) {
                                throw new BeanCreationException(...);
                            }
                            requiredConstructor = candidate;
                        }
                        // 記錄所有加了@Autowired的構(gòu)造方法，不管required是true還是false，如果默認(rèn)無參的構(gòu)造方法上也加了@Autowired，那么也會(huì)加到candidates中
                        candidates.add(candidate);
                    }
                    else if (candidate.getParameterCount() == 0) { //沒有注解且參數(shù)個(gè)數(shù)為0【即無參】
                        // 記錄唯一一個(gè)無參的構(gòu)造方法
                        defaultConstructor = candidate;
                    }
                    //但是這里沒有處理：有可能存在有參、并且沒有添加@Autowired的構(gòu)造方法。
                }
                //存儲(chǔ)有@Autowired注解的構(gòu)造方法的集合不為空
                if (!candidates.isEmpty()) {
                    // 如果不存在一個(gè)required為true的構(gòu)造方法，則所有required為false的構(gòu)造方法和無參構(gòu)造方法都是合格的
                    if (requiredConstructor == null) {
                        if (defaultConstructor != null) {
                            //無參構(gòu)造方法也添加到集合中
                            candidates.add(defaultConstructor);
                        }
                        else if (candidates.size() == 1 && logger.isInfoEnabled()) {
                            logger.info(...);
                        }
                    }
                    // 轉(zhuǎn)為數(shù)組
                    candidateConstructors = candidates.toArray(new Constructor<?>[0]);
                }
                // 沒有添加了@Autowired注解的構(gòu)造方法，并且類中只有一個(gè)構(gòu)造方法，并且是有參的
                else if (rawCandidates.length == 1 && rawCandidates[0].getParameterCount() > 0) {
                    candidateConstructors = new Constructor<?>[] {rawCandidates[0]};
                }
                // primaryConstructor不用管
                else if (nonSyntheticConstructors == 2 && primaryConstructor != null && defaultConstructor != null && !primaryConstructor.equals(defaultConstructor)) { 
　　　　　　　　　　　　candidateConstructors = new Constructor<?>[] {primaryConstructor, defaultConstructor};
                }
                // primaryConstructor不用管
                else if (nonSyntheticConstructors == 1 && primaryConstructor != null) { candidateConstructors = new Constructor<?>[] {primaryConstructor};
                }
                else {
                    // 如果有多個(gè)有參、并且沒有添加@Autowired的構(gòu)造方法，是會(huì)返回空的
                    candidateConstructors = new Constructor<?>[0];
                }
                //將結(jié)果存入緩存
                this.candidateConstructorsCache.put(beanClass, candidateConstructors);
            }
        }
    }
    //有候選構(gòu)造器則返回，沒有返回null
    return (candidateConstructors.length > 0 ? candidateConstructors : null);
}
返回結(jié)果預(yù)測(cè)：
沒有加了@Autowired注解的構(gòu)造方法
    有多個(gè)構(gòu)造方法                             --》  返回null
    只有一個(gè)有參的構(gòu)造方法                     --》  返回此構(gòu)造方法
    只有一個(gè)無參的構(gòu)造方法                     --》  返回null
存在加了@Autowired注解的構(gòu)造方法
    只有一個(gè)required為true的構(gòu)造方法                      --》  返回此構(gòu)造方法
    有多個(gè)required為true的構(gòu)造方法                        --》  拋異常
    有一個(gè)required為true和其他的required為false的構(gòu)造方法  --》  視情況而定，可能會(huì)拋異常，可能會(huì)返回帶注解的構(gòu)造方法
    沒有required為true的構(gòu)造方法                          --》  返回所有required為false的構(gòu)造方法以及無參構(gòu)造方法

protected BeanWrapper autowireConstructor( String beanName, RootBeanDefinition mbd, @Nullable Constructor<?>[] ctors, @Nullable Object[] explicitArgs) {
    return new ConstructorResolver(this).autowireConstructor(beanName, mbd, ctors, explicitArgs);
}

public ConstructorResolver(AbstractAutowireCapableBeanFactory beanFactory) {
    this.beanFactory = beanFactory;
    this.logger = beanFactory.getLogger();
}

//ConstructorResolver類#autowireConstructor方法
//省略一些異常和日志
public BeanWrapper autowireConstructor(String beanName, RootBeanDefinition mbd, @Nullable Constructor<?>[] chosenCtors, @Nullable Object[] explicitArgs) {

    // 實(shí)例化BeanWrapper，是包裝bean的容器
    BeanWrapperImpl bw = new BeanWrapperImpl();
    this.beanFactory.initBeanWrapper(bw);

    Constructor<?> constructorToUse = null;
    ArgumentsHolder argsHolderToUse = null;
    Object[] argsToUse = null;

    // 如果getBean()傳入了args，那構(gòu)造方法要用的入?yún)⒕椭苯哟_定好了
    if (explicitArgs != null) {
        argsToUse = explicitArgs;
    }
    else {
         // 如果getBean()沒有傳入了參數(shù)
        Object[] argsToResolve = null;
        synchronized (mbd.constructorArgumentLock) {
            // 嘗試從緩存中獲取構(gòu)造方法
            constructorToUse = (Constructor<?>) mbd.resolvedConstructorOrFactoryMethod;
            if (constructorToUse != null && mbd.constructorArgumentsResolved) {
                // 從緩存中找到了構(gòu)造器，那么繼續(xù)從緩存中尋找緩存的構(gòu)造器參數(shù)
                argsToUse = mbd.resolvedConstructorArguments;
                if (argsToUse == null) {
                    // 緩存沒有的話，那就獲取BeanDefinition指定的那些參數(shù)【可以沒有指定】
                    argsToResolve = mbd.preparedConstructorArguments;
                }
            }
        }

        //argsToResolve不為空，即之前獲取到了參數(shù)【是從BeanDefinition指定的】要解析配置的參數(shù)
        if (argsToResolve != null) {
            // 解析參數(shù)類型，比如將配置的String類型轉(zhuǎn)換為list、boolean等類型
            argsToUse = resolvePreparedArguments(beanName, mbd, bw, constructorToUse, argsToResolve, true);
        }
    }

    // 如果沒有確定要使用的構(gòu)造方法，或者確定了構(gòu)造方法但是所要傳入的參數(shù)值沒有確定
    if (constructorToUse == null || argsToUse == null) {
        // 拿到傳入的構(gòu)造器數(shù)組
        Constructor<?>[] candidates = chosenCtors;
        //數(shù)組為空
        if (candidates == null) {
            Class<?> beanClass = mbd.getBeanClass();
            try {
                //使用public的構(gòu)造器（默認(rèn)）或者所有構(gòu)造器
                candidates = (mbd.isNonPublicAccessAllowed() ? beanClass.getDeclaredConstructors() : beanClass.getConstructors());
            }
            catch (Throwable ex) { throw new BeanCreationException(...); }
        }

        // 如果只有一個(gè)候選構(gòu)造方法，并且沒有指定所要使用的構(gòu)造方法參數(shù)值，并且該構(gòu)造方法是無參的，那就直接用這個(gè)無參構(gòu)造方法進(jìn)行實(shí)例化了
        if (candidates.length == 1 && explicitArgs == null && !mbd.hasConstructorArgumentValues()) {
            Constructor<?> uniqueCandidate = candidates[0];
            if (uniqueCandidate.getParameterCount() == 0) {
                synchronized (mbd.constructorArgumentLock) {
                    mbd.resolvedConstructorOrFactoryMethod = uniqueCandidate;
                    mbd.constructorArgumentsResolved = true;
                    mbd.resolvedConstructorArguments = EMPTY_ARGS;
                }
                bw.setBeanInstance(instantiate(beanName, mbd, uniqueCandidate, EMPTY_ARGS));
                return bw;
            }
        }

        //需要解析構(gòu)造函數(shù)標(biāo)志（當(dāng)構(gòu)造方法數(shù)組不為空，或是自動(dòng)裝配構(gòu)造函數(shù)時(shí)）
        boolean autowiring = (chosenCtors != null || mbd.getResolvedAutowireMode() == AutowireCapableBeanFactory.AUTOWIRE_CONSTRUCTOR);
        ConstructorArgumentValues resolvedValues = null;

        // 確定要選擇的構(gòu)造方法的參數(shù)個(gè)數(shù)的最小值，后續(xù)判斷候選構(gòu)造方法的參數(shù)個(gè)數(shù)如果小于minNrOfArgs，則直接pass掉
        int minNrOfArgs;
        if (explicitArgs != null) {
            //如果直接傳了構(gòu)造方法參數(shù)值，那么所用的構(gòu)造方法的參數(shù)個(gè)數(shù)肯定不能少于
            minNrOfArgs = explicitArgs.length;
        }
        else {
            // 提取BeanDefinition中的配置的構(gòu)造函數(shù)參數(shù)
            ConstructorArgumentValues cargs = mbd.getConstructorArgumentValues();
            // 用于承載解析后的構(gòu)造函數(shù)參數(shù)的值
            resolvedValues = new ConstructorArgumentValues();
            // 能解析到的參數(shù)個(gè)數(shù)
            minNrOfArgs = resolveConstructorArguments(beanName, mbd, bw, cargs, resolvedValues);
        }

        // 排序給定的構(gòu)造函數(shù)，public的構(gòu)造函數(shù)優(yōu)先，參數(shù)數(shù)量降序
        AutowireUtils.sortConstructors(candidates);
        int minTypeDiffWeight = Integer.MAX_VALUE;
        Set<Constructor<?>> ambiguousConstructors = null;
        LinkedList<UnsatisfiedDependencyException> causes = null;

        for (Constructor<?> candidate : candidates) {
            Class<?>[] paramTypes = candidate.getParameterTypes();

            // 如果已經(jīng)找到選用的構(gòu)造函數(shù)或者需要的參數(shù)個(gè)數(shù)小于當(dāng)前的構(gòu)造函數(shù)參數(shù)個(gè)數(shù)則終止，前面已經(jīng)經(jīng)過了排序操作
            if (constructorToUse != null && argsToUse != null && argsToUse.length > paramTypes.length) {
                break;
            }
            if (paramTypes.length < minNrOfArgs) {
                // 參數(shù)個(gè)數(shù)不相等
                continue;
            }

            ArgumentsHolder argsHolder;
            // 沒有通過getBean()指定構(gòu)造方法參數(shù)值
            if (resolvedValues != null) {
                try {
                    // 有參數(shù)則根據(jù)值構(gòu)造對(duì)應(yīng)參數(shù)類型的參數(shù)
                    String[] paramNames = ConstructorPropertiesChecker.evaluate(candidate, paramTypes.length);

                    if (paramNames == null) {
                        // 獲取參數(shù)名稱探索器
                        ParameterNameDiscoverer pnd = this.beanFactory.getParameterNameDiscoverer();
                        if (pnd != null) {
                            // 獲取指定構(gòu)造函數(shù)的參數(shù)名稱
                            paramNames = pnd.getParameterNames(candidate);
                        }
                    }
                    // 根據(jù)名稱和數(shù)據(jù)類型創(chuàng)建參數(shù)持有者
                    argsHolder = createArgumentArray(beanName, mbd, resolvedValues, bw, paramTypes, paramNames, getUserDeclaredConstructor(candidate), autowiring, candidates.length == 1);
                }
                catch (UnsatisfiedDependencyException ex) {
                    // 當(dāng)前正在遍歷的構(gòu)造方法找不到可用的入?yún)?duì)象，記錄一下
                    if (causes == null) {
                        causes = new LinkedList<>();
                    }
                    causes.add(ex);
                    continue;
                }
            }
            else {
                // 在調(diào)getBean方法是傳入了參數(shù)值，那就表示只能用對(duì)應(yīng)參數(shù)個(gè)數(shù)的構(gòu)造方法
                if (paramTypes.length != explicitArgs.length) {
                    continue;
                }
                // 構(gòu)造函數(shù)沒有參數(shù)的情況
                argsHolder = new ArgumentsHolder(explicitArgs);
            }

            // 當(dāng)前遍歷的構(gòu)造方法所需要的入?yún)?duì)象都找到了，根據(jù)參數(shù)類型和找到的參數(shù)對(duì)象計(jì)算出來一個(gè)匹配值，值越小越匹配（Lenient表示寬松模式）
            int typeDiffWeight = (mbd.isLenientConstructorResolution() ?
                    argsHolder.getTypeDifferenceWeight(paramTypes) : argsHolder.getAssignabilityWeight(paramTypes));
            // 值越小越匹配
            if (typeDiffWeight < minTypeDiffWeight) {
                constructorToUse = candidate;
                argsHolderToUse = argsHolder;
                argsToUse = argsHolder.arguments;
                minTypeDiffWeight = typeDiffWeight;
                ambiguousConstructors = null;
            }
            // 值相等的情況下，記錄一下匹配值相同的構(gòu)造方法
            else if (constructorToUse != null && typeDiffWeight == minTypeDiffWeight) {
                if (ambiguousConstructors == null) {
                    ambiguousConstructors = new LinkedHashSet<>();
                    ambiguousConstructors.add(constructorToUse);
                }
                ambiguousConstructors.add(candidate);
            }
        }
        // 遍歷結(jié)束

        // 如果沒有可用的構(gòu)造方法，就取記錄的最后一個(gè)異常并拋出
        if (constructorToUse == null) {
            if (causes != null) {
                UnsatisfiedDependencyException ex = causes.removeLast();
                for (Exception cause : causes) {
                    this.beanFactory.onSuppressedException(cause);
                }
                throw ex;
            }
            throw new BeanCreationException(...);
        }
        // 如果有可用的構(gòu)造方法，但是有多個(gè)
        else if (ambiguousConstructors != null && !mbd.isLenientConstructorResolution()) {
            throw new BeanCreationException(...);
        }

        if (explicitArgs == null && argsHolderToUse != null) {
            // 將解析的構(gòu)造函數(shù)加入緩存
            argsHolderToUse.storeCache(mbd, constructorToUse);
        }
    }

    // 將構(gòu)造的實(shí)例加入BeanWrapper中
    bw.setBeanInstance(instantiate(beanName, mbd, constructorToUse, argsToUse));
    return bw;
}

Object[] objects = new Object[]{new A()};
// 0
System.out.println(MethodInvoker.getTypeDifferenceWeight(new Class[]{A.class}, objects));
// 2
System.out.println(MethodInvoker.getTypeDifferenceWeight(new Class[]{B.class}, objects));
// 4
System.out.println(MethodInvoker.getTypeDifferenceWeight(new Class[]{C.class}, objects));
// 1
System.out.println(MethodInvoker.getTypeDifferenceWeight(new Class[]{D.class}, objects));

//AbstractAutowireCapableBeanFactory類#instantiateBean方法
protected BeanWrapper instantiateBean(String beanName, RootBeanDefinition mbd) {
    try {
        Object beanInstance;
        if (System.getSecurityManager() != null) {
            beanInstance = AccessController.doPrivileged( (PrivilegedAction<Object>) () -> getInstantiationStrategy().instantiate(mbd, beanName, this), getAccessControlContext());
        }
        else {
            //獲取實(shí)例化策略并且進(jìn)行實(shí)例化操作
            beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, this);
        }
        // 包裝成BeanWrapper
        BeanWrapper bw = new BeanWrapperImpl(beanInstance);
        initBeanWrapper(bw);
        return bw;
    }
    catch (Throwable ex) {
        throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Instantiation of bean failed", ex);
    }
}

//SimpleInstantiationStrategy類#instantiate方法
public Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner) {
    // bd對(duì)象定義中，是否包含MethodOverride列表，spring中有兩個(gè)標(biāo)簽參數(shù)會(huì)產(chǎn)生MethodOverrides,分別是lookup-method,replaced-method
    // 沒有MethodOverrides對(duì)象，可以直接實(shí)例化
    if (!bd.hasMethodOverrides()) {
        // 實(shí)例化對(duì)象的構(gòu)造方法
        Constructor<?> constructorToUse;
        // 鎖定對(duì)象，使獲得實(shí)例化構(gòu)造方法線程安全
        synchronized (bd.constructorArgumentLock) {
            // 查看bd對(duì)象里使用否含有構(gòu)造方法
            constructorToUse = (Constructor<?>) bd.resolvedConstructorOrFactoryMethod;
            // 如果沒有
            if (constructorToUse == null) {
                // 從bd中獲取beanClass
                final Class<?> clazz = bd.getBeanClass();
                // 如果要實(shí)例化的beanDefinition是一個(gè)接口，則直接拋出異常
                if (clazz.isInterface()) {
                    throw new BeanInstantiationException(clazz, "Specified class is an interface");
                }
                try {
                    // 獲取系統(tǒng)安全管理器
                    if (System.getSecurityManager() != null) {
                        constructorToUse = AccessController.doPrivileged((PrivilegedExceptionAction<Constructor<?>>) clazz::getDeclaredConstructor);
                    }
                    else {
                        // 獲取默認(rèn)的無參構(gòu)造器
                        constructorToUse = clazz.getDeclaredConstructor();
                    }
                    // 獲取到構(gòu)造器之后將構(gòu)造器賦值給bd中的屬性
                    bd.resolvedConstructorOrFactoryMethod = constructorToUse;
                }
                catch (Throwable ex) {
                    throw new BeanInstantiationException(clazz, "No default constructor found", ex);
                }
            }
        }
        // 通過反射生成具體的實(shí)例化對(duì)象
        return BeanUtils.instantiateClass(constructorToUse);
    }
    else {
        // 必須生成cglib子類
        return instantiateWithMethodInjection(bd, beanName, owner);
    }
}

public Constructor<T> getDeclaredConstructor(Class<?>... parameterTypes)
    throws NoSuchMethodException, SecurityException { checkMemberAccess(Member.DECLARED, Reflection.getCallerClass(), true);
    return getConstructor0(parameterTypes, Member.DECLARED);
}

private Constructor<T> getConstructor0(Class<?>[] parameterTypes, int which) throws NoSuchMethodException {
    //獲取所有的構(gòu)造器
    Constructor<T>[] constructors = privateGetDeclaredConstructors((which == Member.PUBLIC));
    //遍歷構(gòu)造器，匹配的返回
    for (Constructor<T> constructor : constructors) {
        if (arrayContentsEq(parameterTypes,
                            constructor.getParameterTypes())) {
            return getReflectionFactory().copyConstructor(constructor);
        }
    }
    throw new NoSuchMethodException(getName() + ".<init>" + argumentTypesToString(parameterTypes));
}

beanDefinition.getConstructorArgumentValues().addGenericArgumentValue(new CircularRefA());