Spring源码解析 -- 读取bean元数据
spring源码解析 -- 构造bean
spring源码解析 -- 注入属性
spring源码解析 -- Spring Context
Spring源码解析 -- AOP原理(1)
Spring源码解析 -- AOP原理(2)
Spring源码解析 -- SpringMvc原理

源码分析基于spring 4.3.x

前面文章已经分析了spring如何加载xml配置中的bean元数据，现在来分析一下spring构造bean的过程。
关于阅读源码的思路，可参考 -- 如何阅读java源码

BeanFactory xmlBeanFactory = new XmlBeanFactory(new ClassPathResource("application.xml"));
Blog bean = (Blog)xmlBeanFactory.getBean("blog");

XmlBeanFactory的父类AbstractBeanFactory是spring中一个很重要的类，他为BeanFactory提供基础服务。构造bean就是由该类实现的。
AbstractBeanFactory#doGetBean

protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
        @Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {

    final String beanName = transformedBeanName(name);
    Object bean;

    Object sharedInstance = getSingleton(beanName); // #1
    if (sharedInstance != null && args == null) {
        ...
        bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);  // #2
    }

    else {
        if (isPrototypeCurrentlyInCreation(beanName)) { // #3
            throw new BeanCurrentlyInCreationException(beanName);
        }

        BeanFactory parentBeanFactory = getParentBeanFactory();
        if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {   // #4
            ...
        }

        if (!typeCheckOnly) {
            markBeanAsCreated(beanName);    // #5
        }

        try {
            final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);  // #6
            checkMergedBeanDefinition(mbd, beanName, args);

            String[] dependsOn = mbd.getDependsOn();
            if (dependsOn != null) {    
                for (String dep : dependsOn) {
                    if (isDependent(beanName, dep)) {
                        throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                                "Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
                    }
                    registerDependentBean(dep, beanName);   
                    try {
                        getBean(dep);   // #7
                    }
                    catch (NoSuchBeanDefinitionException ex) {
                        throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                                "'" + beanName + "' depends on missing bean '" + dep + "'", ex);
                    }
                }
            }

            // Create bean instance.
            if (mbd.isSingleton()) {
                sharedInstance = getSingleton(beanName, () -> {     // #8
                    try {
                        return createBean(beanName, mbd, args); // #9
                    }
                    catch (BeansException ex) {
                        destroySingleton(beanName);
                        throw ex;
                    }
                });
                bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
            }

            ... 
        }
        catch (BeansException ex) {
            cleanupAfterBeanCreationFailure(beanName);
            throw ex;
        }
    }

    // Check if required type matches the type of the actual bean instance.
    if (requiredType != null && !requiredType.isInstance(bean)) {   // #10
        try {
            T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
            if (convertedBean == null) {
                throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
            }
            return convertedBean;
        }
        catch (TypeMismatchException ex) {
            if (logger.isTraceEnabled()) {
                logger.trace("Failed to convert bean '" + name + "' to required type '" +
                        ClassUtils.getQualifiedName(requiredType) + "'", ex);
            }
            throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
        }
    }
    return (T) bean;
}

#1 单例的bean，先从缓冲中查询
getSingleton方法会查询
singletonObject，earlySingletonObjects，singletonFactories等缓存，它们都是简单的map，缓存了单例的bean，正在创建的bean和ObjectFactory对象。

#2 如果构造的bean是FactoryBean，进行对应的处理
getObjectForBeanInstance方法会根据参数beanInstance进行处理，如果beanInstance是FactoryBean，会调用其getObject()方法创建bean，如果不是，返回直接返回该beanInstance参数。
FactoryBean是spring提供的一个扩展接口，用户实现该接口可以自定义bean的创建。

#3 如果现在正在创建这个bean，则直接报错，这时很可能陷入循环引用了
#4 当前BeanFactory不存在对应的BeanDefinition，尝试通过父BeanFactory构造bean
#5 标记这个bean正在构造中
#6 获取BeanDefinition
前面解析spring加载bean数据的文章中说过，spring会将bean元数据转化为BeanDefinition，存入DefaultListableBeanFactory#beanDefinitionMap属性中。
getMergedLocalBeanDefinition方法会获取对应BeanDefinition，如果BeanDefinition存在ParentName，会获取父BeanDefinition，再合并元数据。

#7 先构造依赖的bean
#8 使用ObjectFactory构造并注册一个bean，getSingleton方法也要完成构造bean的准备和善后工作。
#9 匿名的ObjectFactory，调用AbstractAutowireCapableBeanFactory#createBean进行实际的构造bean工作
#10 bean类型转换

spring中bean有singleton，prototype等范围，这里只关注singleton类型的bean的构造过程。

#8步骤，DefaultSingletonBeanRegistry#getSingleton（注意getSingleton有重载方法）

    public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
        Assert.notNull(beanName, "Bean name must not be null");
        synchronized (this.singletonObjects) {
            Object singletonObject = this.singletonObjects.get(beanName);
            if (singletonObject == null) {
                ...
                beforeSingletonCreation(beanName);  // #1
                boolean newSingleton = false;
                boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
                if (recordSuppressedExceptions) {
                    this.suppressedExceptions = new LinkedHashSet<>();
                }
                try {
                    singletonObject = singletonFactory.getObject(); // #2
                    newSingleton = true;
                }
                ...
                finally {
                    if (recordSuppressedExceptions) {
                        this.suppressedExceptions = null;
                    }
                    afterSingletonCreation(beanName);
                }
                if (newSingleton) {
                    addSingleton(beanName, singletonObject);    // #3
                }
            }
            return singletonObject;
        }
    }

#1 仅做错误检查,也是提供给子类的扩展方法
#2 真正构造bean的方法，调用AbstractBeanFactory#doGetBean方法#9步骤的匿名类处理，实际调用AbstractAutowireCapableBeanFactory#createBean
#3 单例的bean，加入缓冲中

真正的构造bean方法
AbstractAutowireCapableBeanFactory#createBean

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

    if (logger.isTraceEnabled()) {
        logger.trace("Creating instance of bean '" + beanName + "'");
    }
    RootBeanDefinition mbdToUse = mbd;

    Class<?> resolvedClass = resolveBeanClass(mbd, beanName);   // #1
    if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
        mbdToUse = new RootBeanDefinition(mbd);
        mbdToUse.setBeanClass(resolvedClass);
    }

    try {
        mbdToUse.prepareMethodOverrides();  // #2
    }
    ...

    try {
        Object bean = resolveBeforeInstantiation(beanName, mbdToUse);   // #3
        if (bean != null) {
            return bean;
        }
    }
    ...

    try {
        Object beanInstance = doCreateBean(beanName, mbdToUse, args);   // #4
        if (logger.isTraceEnabled()) {
            logger.trace("Finished creating instance of bean '" + beanName + "'");
        }
        return beanInstance;
    }
    ...
}

#1 确保jdk已加载bean的class
#2 对bean的lookup-method和replace-method做检查工作
#3 spring的扩展机制，调用BeanPostProcessor#postProcessBeforeInstantiation方法，
注意，如果resolveBeforeInstantiation返回非null对象，这里将直接返回该对象作为bean，spring不再构造该bean。
#4 继续构造bean

AbstractAutowireCapableBeanFactory#doCreateBean

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

    BeanWrapper instanceWrapper = null;
    if (mbd.isSingleton()) {
        instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);   // #1
    }
    if (instanceWrapper == null) {
        instanceWrapper = createBeanInstance(beanName, mbd, args);  // #2
    }
    final Object bean = instanceWrapper.getWrappedInstance();
    Class<?> beanType = instanceWrapper.getWrappedClass();
    if (beanType != NullBean.class) {
        mbd.resolvedTargetType = beanType;
    }

    // Allow post-processors to modify the merged bean definition.
    synchronized (mbd.postProcessingLock) {
        if (!mbd.postProcessed) {
            try {
                applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);   // #3
            }
            catch (Throwable ex) {
                throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                        "Post-processing of merged bean definition failed", ex);
            }
            mbd.postProcessed = true;
        }
    }

    ...

    // Initialize the bean instance.
    Object exposedObject = bean;
    try {
        populateBean(beanName, mbd, instanceWrapper);   // #4
        exposedObject = initializeBean(beanName, exposedObject, mbd);   // #5
    }
    catch (Throwable ex) {
        if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {
            throw (BeanCreationException) ex;
        }
        else {
            throw new BeanCreationException(
                    mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);
        }
    }

    ...

    // Register bean as disposable.
    try {
        registerDisposableBeanIfNecessary(beanName, bean, mbd);     // #6
    }
    catch (BeanDefinitionValidationException ex) {
        throw new BeanCreationException(
                mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
    }

    return exposedObject;
}

#1 查询BeanWrapper缓冲
#2 构造一个空的（属性未注入）bean，生成BeanWrapper
#3 spring扩展机制，调用MergedBeanDefinitionPostProcessor#postProcessMergedBeanDefinition
#4 populateBean负责注入属性到bean中，后面有对应文章解析该步骤
#5 spring扩展机制，调用Aware方法和init方法，并调用BeanPostProcessor#postProcessAfterInitialization方法。
#6 如果bean存在Destroy方法，或存在对应的DestructionAwareBeanPostProcessor，注册该bean为disposable。（该bean销毁时要调用对应的销毁机制）

这里涉及到循环引用的处理，比较繁琐，所以省略了很多代码，只保留bean创建相关的代码。

BeanWrapper是对bean的包装类，提供了对bean的class，property进行操作的方法。

#2步骤，AbstractAutowireCapableBeanFactory#createBeanInstance

    protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, Object[] args) {
        Class<?> beanClass = resolveBeanClass(mbd, beanName);   // #1

        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());
        }

        if (mbd.getFactoryMethodName() != null)  {  // #2
            return instantiateUsingFactoryMethod(beanName, mbd, args);
        }

        boolean resolved = false;
        boolean autowireNecessary = false;
        if (args == null) {
            synchronized (mbd.constructorArgumentLock) {
                if (mbd.resolvedConstructorOrFactoryMethod != null) {   // #3
                    resolved = true;
                    autowireNecessary = mbd.constructorArgumentsResolved;
                }
            }
        }
        if (resolved) { 
            if (autowireNecessary) {
                return autowireConstructor(beanName, mbd, null, null);  // #4
            }
            else {
                return instantiateBean(beanName, mbd);  // #5
            }
        }

        Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);  // #6
        if (ctors != null ||
                mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR ||
                mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args))  {
            return autowireConstructor(beanName, mbd, ctors, args);
        }

        // No special handling: simply use no-arg constructor.
        return instantiateBean(beanName, mbd);
    }

#1 获取bean的class
#2 如果存在factoryMethod，通过factoryMethod构造bean
#3 判断该class已经构造过bean了
#4 使用之前选择好的构造函数构造bean
#5 使用无参构造函数构造bean
#6 spring根据构造函数的参数，自行选择构造函数，逻辑较复杂。

下面看看使用无参构造方法构造bean，AbstractAutowireCapableBeanFactory#instantiateBean -> SimpleInstantiationStrategy#instantiate

public Object instantiate(RootBeanDefinition bd, String beanName, BeanFactory owner) {
        if (bd.getMethodOverrides().isEmpty()) {
            Constructor<?> constructorToUse;
            synchronized (bd.constructorArgumentLock) {
                constructorToUse = (Constructor<?>) bd.resolvedConstructorOrFactoryMethod;
                if (constructorToUse == null) {
                    final Class<?> clazz = bd.getBeanClass();
                    if (clazz.isInterface()) {
                        throw new BeanInstantiationException(clazz, "Specified class is an interface");
                    }
                    try {
                        if (System.getSecurityManager() != null) {
                            ...
                        }
                        else {
                            constructorToUse =  clazz.getDeclaredConstructor((Class[]) null);   // #1
                        }
                        bd.resolvedConstructorOrFactoryMethod = constructorToUse;   // #2
                    }
                    catch (Throwable ex) {
                        throw new BeanInstantiationException(clazz, "No default constructor found", ex);
                    }
                }
            }
            return BeanUtils.instantiateClass(constructorToUse);    // #3
        }
        else {
            return instantiateWithMethodInjection(bd, beanName, owner);     // #4
        }
    }

#1 获取无参构造函数
#2 添加无参构造函数加入到BeanDefinition缓存中
#2 BeanUtils#instantiateClass通过构造函数构造bean
#3 bean中存在lookup-method或replace-method，必须使用CGLIB构造bean

到这里，bean已经构造完成，下一步就是注入属性了。

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