Spring启动过程中实例化部分代码的分析之Bean的推断构造方法

protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
    
    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());
    }

    
    // BeanDefinition中添加了Supplier，则调用Supplier来得到对象
    Supplier<?> instanceSupplier = mbd.getInstanceSupplier();
    if (instanceSupplier != null) {
        return obtainFromSupplier(instanceSupplier, beanName);
    }

    
    // @Bean对应的BeanDefinition
    if (mbd.getFactoryMethodName() != null) {
        return instantiateUsingFactoryMethod(beanName, mbd, args);
    }


     
    boolean resolved = false; //对构造器是否缓存了
    boolean autowireNecessary = false; //缓存的构造器是否有参数
    if (args == null) {
        synchronized (mbd.constructorArgumentLock) {
            if (mbd.resolvedConstructorOrFactoryMethod != null) {
                resolved = true;
                // autowireNecessary表示有没有必要要进行注入，比如当前BeanDefinition用的是无参构造方法，那么autowireNecessary为false，否则为true，表示需要给构造方法参数注入值
                autowireNecessary = mbd.constructorArgumentsResolved;
            }
        }
    }
    if (resolved) {
        // 如果确定了当前BeanDefinition的构造方法，那么看是否需要进行对构造方法进行参数的依赖注入（构造方法注入）
        if (autowireNecessary) {
            // 方法内会拿到缓存好的构造方法的入参
            return autowireConstructor(beanName, mbd, null, null);
        }
        else {
            // 构造方法已经找到了，但是没有参数，那就表示是无参，直接进行实例化
            return instantiateBean(beanName, mbd);
        }
    }

    //没有缓存，从bean后置处理器中为自动装配寻找构造方法
    Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
    if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||
            mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
        return autowireConstructor(beanName, mbd, ctors, args);
    }

    // 找出最合适的默认构造方法
    ctors = mbd.getPreferredConstructors();
    if (ctors != null) {
        // 构造函数自动注入
        return autowireConstructor(beanName, mbd, ctors, null);
    }

    
    // 不匹配以上情况，则直接使用无参构造方法
    return instantiateBean(beanName, mbd);
}

// BeanDefinition中添加了Supplier，则调用Supplier来得到对象
Supplier<?> instanceSupplier = mbd.getInstanceSupplier();
if (instanceSupplier != null) {
    return obtainFromSupplier(instanceSupplier, beanName);
}

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

//容器启动后尝试获取验证
applicationContext.getBean("student"); //返回new UserServiceImpl1()对象

// @Bean对应的BeanDefinition
if (mbd.getFactoryMethodName() != null) {
    return instantiateUsingFactoryMethod(beanName, mbd, args);
}

protected BeanWrapper instantiateUsingFactoryMethod(String beanName, RootBeanDefinition mbd, @Nullable Object[] explicitArgs) {
    // 使用factoryBean来实例化对象
    return new ConstructorResolver(this).instantiateUsingFactoryMethod(beanName, mbd, explicitArgs);
}

//ConstructorResolver类#instantiateUsingFactoryMethod方法
//省略日志与异常
public BeanWrapper instantiateUsingFactoryMethod(String beanName, RootBeanDefinition mbd, @Nullable Object[] explicitArgs) {

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

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

    //通过beanDefinition获取到factoryBeanName ，实际就是@Bean注解的方法所在的configuration类
    String factoryBeanName = mbd.getFactoryBeanName();
    //表示非静态方法
    if (factoryBeanName != null) {
        if (factoryBeanName.equals(beanName)) {
            throw new BeanDefinitionStoreException(...);
        }
        factoryBean = this.beanFactory.getBean(factoryBeanName);

        // 该mbd已经创建过了【代表这个逻辑已经走过了】
        if (mbd.isSingleton() && this.beanFactory.containsSingleton(beanName)) {
            throw new ImplicitlyAppearedSingletonException();
        }
        factoryClass = factoryBean.getClass();
        isStatic = false;
    }
    else {//表示静态方法
        // 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;

    //如果在调用getBean方法时有传参，那就用传的参作为@Bean注解的方法（工厂方法）的参数， 一般懒加载的bean才会传参，启动过程就实例化的实际上都没有传参
    if (explicitArgs != null) {
        argsToUse = explicitArgs;
    }
    else {
        Object[] argsToResolve = null;
        synchronized (mbd.constructorArgumentLock) {
            factoryMethodToUse = (Method) mbd.resolvedConstructorOrFactoryMethod;
            //不为空表示已经使用过工厂方法，现在是再次使用工厂方法， 一般原型模式和Scope模式采用的上，直接使用该工厂方法和缓存的参数
            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);
        }
    }

    // 调用getBean方法没有传参，同时也是第一次使用工厂方法
    if (factoryMethodToUse == null || argsToUse == null) {
        
        factoryClass = ClassUtils.getUserClass(factoryClass);

        // 获取configuration类的所有候选方法
        Method[] rawCandidates = getCandidateMethods(factoryClass, mbd);
        List<Method> candidateList = new ArrayList<>();
        for (Method candidate : rawCandidates) {
            // 查找到与工厂方法同名的候选方法,即有@Bean的同名方法
            if (Modifier.isStatic(candidate.getModifiers()) == isStatic && mbd.isFactoryMethod(candidate)) {
                candidateList.add(candidate);
            }
        }

        //当与工厂方法同名的候选方法只有一个，且调用getBean方法时没有传参，且没有缓存过参数，直接通过调用实例化方法执行该候选方法
        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]);
        // 有多个与工厂方法同名的候选方法时，进行排序。public的方法会往前排，然后参数个数多的方法往前排
        AutowireUtils.sortFactoryMethods(candidates);

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

        int minNrOfArgs;
        // 如果调用getBean方法时有传参，那么工厂方法最少参数个数要等于传参个数
        if (explicitArgs != null) {
            minNrOfArgs = explicitArgs.length;
        }
        else {
            //如果getBean的时候没有传参，如果BeanDefinition中有设置，则从BeanDefinition中获取
            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);
                        }
                        //当传入的参数为空，需要根据工厂方法的参数类型注入相应的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;
                    }
                }

                //计算工厂方法的权重
                int typeDiffWeight = (mbd.isLenientConstructorResolution() ?
                        argsHolder.getTypeDifferenceWeight(paramTypes) : argsHolder.getAssignabilityWeight(paramTypes));
                // Choose this factory method if it represents the closest match.
                if (typeDiffWeight < minTypeDiffWeight) {
                    
                    factoryMethodToUse = candidate;
                    argsHolderToUse = argsHolder;
                    argsToUse = argsHolder.arguments;
                    minTypeDiffWeight = typeDiffWeight;
                    ambiguousFactoryMethods = null;
                }

                
                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(...);
        }
        //返回类型不能为void
        else if (void.class == factoryMethodToUse.getReturnType()) {
            throw new BeanCreationException(...);
        }
        //存在含糊的两个工厂方法，不知选哪个
        else if (ambiguousFactoryMethods != null) {
            throw new BeanCreationException(...);
        }

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

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

boolean resolved = false; //对构造器是否缓存了
boolean autowireNecessary = false; //缓存的构造器是否有参数
//判断是否有传参，因为getBean方法可以通过传参指定构造方法参数类型来进行匹配（故这种对于参数的不确定是不能缓存的）
if (args == null) {
    synchronized (mbd.constructorArgumentLock) {
        if (mbd.resolvedConstructorOrFactoryMethod != null) {
            resolved = true;
            // autowireNecessary表示有没有必要要进行注入，比如当前BeanDefinition用的是无参构造方法，那么autowireNecessary为false，否则为true，表示需要给构造方法参数注入值
            autowireNecessary = mbd.constructorArgumentsResolved;
        }
    }
}
if (resolved) {
    // 如果确定了当前BeanDefinition的构造方法，那么看是否需要进行对构造方法进行参数的依赖注入（构造方法注入）
    if (autowireNecessary) {
        // 方法内会拿到缓存好的构造方法的入参
        return autowireConstructor(beanName, mbd, null, null);
    }
    else {
        // 构造方法已经找到了，但是没有参数，那就表示是无参，直接进行实例化
        return instantiateBean(beanName, mbd);
    }
}

//没有缓存，从bean后置处理器中为自动装配寻找构造方法
Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);

//存在推断出来的构造方法集合
//BeanDefinition被设置为AUTOWIRE_CONSTRUCTOR
//BeanDefinition指定了构造方法参数值
//getBean的时候传入了构造方法参数值
if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||
        mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
    return autowireConstructor(beanName, mbd, ctors, args);
}

// 找出最合适的默认构造方法
ctors = mbd.getPreferredConstructors();
if (ctors != null) {
    // 构造函数自动注入
    return autowireConstructor(beanName, mbd, ctors, null);
}

protected Constructor<?>[] determineConstructorsFromBeanPostProcessors(@Nullable Class<?> beanClass, String beanName) throws BeansException {
    //如果类不为null，且存在InstantiationAwareBeanPostProcessor接口的处理器【其实用的不是它，而是他的子接口SmartInstantiationAwareBeanPostProcessor】
    if (beanClass != null && hasInstantiationAwareBeanPostProcessors()) {
        for (BeanPostProcessor bp : getBeanPostProcessors()) {
            if (bp instanceof SmartInstantiationAwareBeanPostProcessor) {
                SmartInstantiationAwareBeanPostProcessor ibp = (SmartInstantiationAwareBeanPostProcessor) bp;
                //真正存在实现的类是AutowiredAnnotationBeanPostProcessor类
                Constructor<?>[] ctors = ibp.determineCandidateConstructors(beanClass, beanName);
                if (ctors != null) {
                    return ctors;
                }
            }
        }
    }
    return null;
}
//AutowiredAnnotationBeanPostProcessor类#determineCandidateConstructors方法
//日志和异常的输出会被省略
public Constructor<?>[] determineCandidateConstructors(Class<?> beanClass, final String beanName) throws BeanCreationException {
    // 判断是否检测过
    if (!this.lookupMethodsChecked.contains(beanName)) {
        try {
            // 遍历目标类中的method，查看是否写了@Lookup方法
            ReflectionUtils.doWithMethods(beanClass, method -> {
                Lookup lookup = method.getAnnotation(Lookup.class);
                if (lookup != null) {
                    Assert.state(this.beanFactory != null, "No BeanFactory available");
                    // 将当前method封装成LookupOverride并设置到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);
    }
    //先从缓存中获取，看之前有没有找过
    Constructor<?>[] candidateConstructors = this.candidateConstructorsCache.get(beanClass);
    //缓存中没有
    if (candidateConstructors == null) {
        // 加锁，开始寻找
        synchronized (this.candidateConstructorsCache) {
            //再次从缓存中获取，看被阻塞的时候没有有线程已经找好了存在缓存中（加锁并双重判断的常规做法）
            candidateConstructors = this.candidateConstructorsCache.get(beanClass);
            if (candidateConstructors == null) {
                Constructor<?>[] rawCandidates;
                try {
                    // 拿到所有的构造方法
                    rawCandidates = beanClass.getDeclaredConstructors();
                }
                catch (Throwable ex) {
                    throw new BeanCreationException(beanName, "...", ex);
                }
                List<Constructor<?>> candidates = new ArrayList<>(rawCandidates.length);
                // 用来记录required为true的构造方法，一个类中只能有一个required为true的构造方法
                Constructor<?> requiredConstructor = null;
                // 用来记录默认无参的构造方法
                Constructor<?> defaultConstructor = null;
                // Kotlin相关，不用管
                Constructor<?> primaryConstructor = BeanUtils.findPrimaryConstructor(beanClass);
                int nonSyntheticConstructors = 0;
                // 遍历每个构造方法
                for (Constructor<?> candidate : rawCandidates) {
                    if (!candidate.isSynthetic()) {
                        // 记录一下普通的构造方法
                        nonSyntheticConstructors++;
                    }
                    else if (primaryConstructor != null) {
                        continue;
                    }
                    // 当前遍历的构造方法是否写了@Autowired
                    AnnotationAttributes ann = findAutowiredAnnotation(candidate);
                    //没有@Autowired注解的情况
                    if (ann == null) {
                        // 如果beanClass是代理类，则得到被代理的类的类型
                        Class<?> userClass = ClassUtils.getUserClass(beanClass);
                        if (userClass != beanClass) {
                            try {
                                //寻找被代理的类上的构造方法有没有@Autowired注解
                                Constructor<?> superCtor = userClass.getDeclaredConstructor(candidate.getParameterTypes());
                                ann = findAutowiredAnnotation(superCtor);
                            }
                            catch (NoSuchMethodException ex) {}
                        }
                    }
                    
                    if (ann != null) {
                        // 整个类中如果有一个required为true的构造方法，那就不能有其他的加了@Autowired的构造方法
                        if (requiredConstructor != null) {
                            throw new BeanCreationException(...);
                        }
                        //获取该方法上的@Autowired注解的required属性
                        boolean required = determineRequiredStatus(ann);
                        if (required) {
                            if (!candidates.isEmpty()) {
                                throw new BeanCreationException(...);
                            }
                            requiredConstructor = candidate;
                        }
                        // 记录所有加了@Autowired的构造方法，不管required是true还是false，如果默认无参的构造方法上也加了@Autowired，那么也会加到candidates中
                        candidates.add(candidate);
                    }
                    else if (candidate.getParameterCount() == 0) { //没有注解且参数个数为0【即无参】
                        // 记录唯一一个无参的构造方法
                        defaultConstructor = candidate;
                    }
                    //但是这里没有处理：有可能存在有参、并且没有添加@Autowired的构造方法。
                }
                //存储有@Autowired注解的构造方法的集合不为空
                if (!candidates.isEmpty()) {
                    // 如果不存在一个required为true的构造方法，则所有required为false的构造方法和无参构造方法都是合格的
                    if (requiredConstructor == null) {
                        if (defaultConstructor != null) {
                            //无参构造方法也添加到集合中
                            candidates.add(defaultConstructor);
                        }
                        else if (candidates.size() == 1 && logger.isInfoEnabled()) {
                            logger.info(...);
                        }
                    }
                    // 转为数组
                    candidateConstructors = candidates.toArray(new Constructor<?>[0]);
                }
                // 没有添加了@Autowired注解的构造方法，并且类中只有一个构造方法，并且是有参的
                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 {
                    // 如果有多个有参、并且没有添加@Autowired的构造方法，是会返回空的
                    candidateConstructors = new Constructor<?>[0];
                }
                //将结果存入缓存
                this.candidateConstructorsCache.put(beanClass, candidateConstructors);
            }
        }
    }
    //有候选构造器则返回，没有返回null
    return (candidateConstructors.length > 0 ? candidateConstructors : null);
}
返回结果预测：
没有加了@Autowired注解的构造方法
    有多个构造方法                             --》  返回null
    只有一个有参的构造方法                     --》  返回此构造方法
    只有一个无参的构造方法                     --》  返回null
存在加了@Autowired注解的构造方法
    只有一个required为true的构造方法                      --》  返回此构造方法
    有多个required为true的构造方法                        --》  抛异常
    有一个required为true和其他的required为false的构造方法  --》  视情况而定，可能会抛异常，可能会返回带注解的构造方法
    没有required为true的构造方法                          --》  返回所有required为false的构造方法以及无参构造方法

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

    // 实例化BeanWrapper，是包装bean的容器
    BeanWrapperImpl bw = new BeanWrapperImpl();
    this.beanFactory.initBeanWrapper(bw);

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

    // 如果getBean()传入了args，那构造方法要用的入参就直接确定好了
    if (explicitArgs != null) {
        argsToUse = explicitArgs;
    }
    else {
         // 如果getBean()没有传入了参数
        Object[] argsToResolve = null;
        synchronized (mbd.constructorArgumentLock) {
            // 尝试从缓存中获取构造方法
            constructorToUse = (Constructor<?>) mbd.resolvedConstructorOrFactoryMethod;
            if (constructorToUse != null && mbd.constructorArgumentsResolved) {
                // 从缓存中找到了构造器，那么继续从缓存中寻找缓存的构造器参数
                argsToUse = mbd.resolvedConstructorArguments;
                if (argsToUse == null) {
                    // 缓存没有的话，那就获取BeanDefinition指定的那些参数【可以没有指定】
                    argsToResolve = mbd.preparedConstructorArguments;
                }
            }
        }

        //argsToResolve不为空，即之前获取到了参数【是从BeanDefinition指定的】要解析配置的参数
        if (argsToResolve != null) {
            // 解析参数类型，比如将配置的String类型转换为list、boolean等类型
            argsToUse = resolvePreparedArguments(beanName, mbd, bw, constructorToUse, argsToResolve, true);
        }
    }

    // 如果没有确定要使用的构造方法，或者确定了构造方法但是所要传入的参数值没有确定
    if (constructorToUse == null || argsToUse == null) {
        // 拿到传入的构造器数组
        Constructor<?>[] candidates = chosenCtors;
        //数组为空
        if (candidates == null) {
            Class<?> beanClass = mbd.getBeanClass();
            try {
                //使用public的构造器（默认）或者所有构造器
                candidates = (mbd.isNonPublicAccessAllowed() ? beanClass.getDeclaredConstructors() : beanClass.getConstructors());
            }
            catch (Throwable ex) { throw new BeanCreationException(...); }
        }

        // 如果只有一个候选构造方法，并且没有指定所要使用的构造方法参数值，并且该构造方法是无参的，那就直接用这个无参构造方法进行实例化了
        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;
            }
        }

        //需要解析构造函数标志（当构造方法数组不为空，或是自动装配构造函数时）
        boolean autowiring = (chosenCtors != null || mbd.getResolvedAutowireMode() == AutowireCapableBeanFactory.AUTOWIRE_CONSTRUCTOR);
        ConstructorArgumentValues resolvedValues = null;

        // 确定要选择的构造方法的参数个数的最小值，后续判断候选构造方法的参数个数如果小于minNrOfArgs，则直接pass掉
        int minNrOfArgs;
        if (explicitArgs != null) {
            //如果直接传了构造方法参数值，那么所用的构造方法的参数个数肯定不能少于
            minNrOfArgs = explicitArgs.length;
        }
        else {
            // 提取BeanDefinition中的配置的构造函数参数
            ConstructorArgumentValues cargs = mbd.getConstructorArgumentValues();
            // 用于承载解析后的构造函数参数的值
            resolvedValues = new ConstructorArgumentValues();
            // 能解析到的参数个数
            minNrOfArgs = resolveConstructorArguments(beanName, mbd, bw, cargs, resolvedValues);
        }

        // 排序给定的构造函数，public的构造函数优先，参数数量降序
        AutowireUtils.sortConstructors(candidates);
        int minTypeDiffWeight = Integer.MAX_VALUE;
        Set<Constructor<?>> ambiguousConstructors = null;
        LinkedList<UnsatisfiedDependencyException> causes = null;

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

            // 如果已经找到选用的构造函数或者需要的参数个数小于当前的构造函数参数个数则终止，前面已经经过了排序操作
            if (constructorToUse != null && argsToUse != null && argsToUse.length > paramTypes.length) {
                break;
            }
            if (paramTypes.length < minNrOfArgs) {
                // 参数个数不相等
                continue;
            }

            ArgumentsHolder argsHolder;
            // 没有通过getBean()指定构造方法参数值
            if (resolvedValues != null) {
                try {
                    // 有参数则根据值构造对应参数类型的参数
                    String[] paramNames = ConstructorPropertiesChecker.evaluate(candidate, paramTypes.length);

                    if (paramNames == null) {
                        // 获取参数名称探索器
                        ParameterNameDiscoverer pnd = this.beanFactory.getParameterNameDiscoverer();
                        if (pnd != null) {
                            // 获取指定构造函数的参数名称
                            paramNames = pnd.getParameterNames(candidate);
                        }
                    }
                    // 根据名称和数据类型创建参数持有者
                    argsHolder = createArgumentArray(beanName, mbd, resolvedValues, bw, paramTypes, paramNames, getUserDeclaredConstructor(candidate), autowiring, candidates.length == 1);
                }
                catch (UnsatisfiedDependencyException ex) {
                    // 当前正在遍历的构造方法找不到可用的入参对象，记录一下
                    if (causes == null) {
                        causes = new LinkedList<>();
                    }
                    causes.add(ex);
                    continue;
                }
            }
            else {
                // 在调getBean方法是传入了参数值，那就表示只能用对应参数个数的构造方法
                if (paramTypes.length != explicitArgs.length) {
                    continue;
                }
                // 构造函数没有参数的情况
                argsHolder = new ArgumentsHolder(explicitArgs);
            }

            // 当前遍历的构造方法所需要的入参对象都找到了，根据参数类型和找到的参数对象计算出来一个匹配值，值越小越匹配（Lenient表示宽松模式）
            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;
            }
            // 值相等的情况下，记录一下匹配值相同的构造方法
            else if (constructorToUse != null && typeDiffWeight == minTypeDiffWeight) {
                if (ambiguousConstructors == null) {
                    ambiguousConstructors = new LinkedHashSet<>();
                    ambiguousConstructors.add(constructorToUse);
                }
                ambiguousConstructors.add(candidate);
            }
        }
        // 遍历结束

        // 如果没有可用的构造方法，就取记录的最后一个异常并抛出
        if (constructorToUse == null) {
            if (causes != null) {
                UnsatisfiedDependencyException ex = causes.removeLast();
                for (Exception cause : causes) {
                    this.beanFactory.onSuppressedException(cause);
                }
                throw ex;
            }
            throw new BeanCreationException(...);
        }
        // 如果有可用的构造方法，但是有多个
        else if (ambiguousConstructors != null && !mbd.isLenientConstructorResolution()) {
            throw new BeanCreationException(...);
        }

        if (explicitArgs == null && argsHolderToUse != null) {
            // 将解析的构造函数加入缓存
            argsHolderToUse.storeCache(mbd, constructorToUse);
        }
    }

    // 将构造的实例加入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类#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 {
            //获取实例化策略并且进行实例化操作
            beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, this);
        }
        // 包装成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类#instantiate方法
public Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner) {
    // bd对象定义中，是否包含MethodOverride列表，spring中有两个标签参数会产生MethodOverrides,分别是lookup-method,replaced-method
    // 没有MethodOverrides对象，可以直接实例化
    if (!bd.hasMethodOverrides()) {
        // 实例化对象的构造方法
        Constructor<?> constructorToUse;
        // 锁定对象，使获得实例化构造方法线程安全
        synchronized (bd.constructorArgumentLock) {
            // 查看bd对象里使用否含有构造方法
            constructorToUse = (Constructor<?>) bd.resolvedConstructorOrFactoryMethod;
            // 如果没有
            if (constructorToUse == null) {
                // 从bd中获取beanClass
                final Class<?> clazz = bd.getBeanClass();
                // 如果要实例化的beanDefinition是一个接口，则直接抛出异常
                if (clazz.isInterface()) {
                    throw new BeanInstantiationException(clazz, "Specified class is an interface");
                }
                try {
                    // 获取系统安全管理器
                    if (System.getSecurityManager() != null) {
                        constructorToUse = AccessController.doPrivileged((PrivilegedExceptionAction<Constructor<?>>) clazz::getDeclaredConstructor);
                    }
                    else {
                        // 获取默认的无参构造器
                        constructorToUse = clazz.getDeclaredConstructor();
                    }
                    // 获取到构造器之后将构造器赋值给bd中的属性
                    bd.resolvedConstructorOrFactoryMethod = constructorToUse;
                }
                catch (Throwable ex) {
                    throw new BeanInstantiationException(clazz, "No default constructor found", ex);
                }
            }
        }
        // 通过反射生成具体的实例化对象
        return BeanUtils.instantiateClass(constructorToUse);
    }
    else {
        // 必须生成cglib子类
        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 {
    //获取所有的构造器
    Constructor<T>[] constructors = privateGetDeclaredConstructors((which == Member.PUBLIC));
    //遍历构造器，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());