Spring Bean 的创建过程

一.Bean创建核心流程图

Bean的创建流程图

二.Bean 创建的源码分析

@Override
    public  T getBean(String name, Class requiredType) throws BeansException {
        return doGetBean(name, requiredType, null, false);
    }

protected  T doGetBean(
            String name, @Nullable Class requiredType, @Nullable Object[] args, boolean typeCheckOnly)
            throws BeansException {
        // 处理别名 和工厂的BeanName 看2.1
        String beanName = transformedBeanName(name);
        Object beanInstance;

        // Eagerly check singleton cache for manually registered singletons.// 缓存中获取Bean的实例（三级缓存）
        Object sharedInstance = getSingleton(beanName);
        if (sharedInstance != null && args == null) {
            if (logger.isTraceEnabled()) {
                if (isSingletonCurrentlyInCreation(beanName)) {
                    logger.trace("Returning eagerly cached instance of singleton bean '" + beanName +
                            "' that is not fully initialized yet - a consequence of a circular reference");
                }
                else {
                    logger.trace("Returning cached instance of singleton bean '" + beanName + "'");
                }
            }
            // 这里如果是普通Bean 的话，直接返回，如果是 FactoryBean 的话，返回它创建的那个实例对象 ，看2.2
            beanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, null);
        }

        else {
            // Fail if we're already creating this bean instance:
            // We're assumably within a circular reference. // 循环依赖有三种，setter注入、多实例和构造函数，Spring 只能解决 setter 注入，所以这里是 Prototype 则会抛出异常
            if (isPrototypeCurrentlyInCreation(beanName)) {
                throw new BeanCurrentlyInCreationException(beanName);
            }

            // Check if bean definition exists in this factory.// 1. 父 bean 工厂存在   // 2. 当前 bean 不存在于当前bean工厂，则到父工厂查找 bean 实例
            BeanFactory parentBeanFactory = getParentBeanFactory();
            if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
                // Not found -> check parent. 
                // 获取 name 对应的 beanName，如果 name 是以 & 开头，则返回 & + beanName 看2.3
                String nameToLookup = originalBeanName(name);
                if (parentBeanFactory instanceof AbstractBeanFactory) {
                    return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
                            nameToLookup, requiredType, args, typeCheckOnly);
                }
                else if (args != null) {
                    // Delegation to parent with explicit args.
                    return (T) parentBeanFactory.getBean(nameToLookup, args);
                }
                else if (requiredType != null) {
                    // No args -> delegate to standard getBean method.
                    return parentBeanFactory.getBean(nameToLookup, requiredType);
                }
                else {
                    return (T) parentBeanFactory.getBean(nameToLookup);
                }
            }

            if (!typeCheckOnly) {
                markBeanAsCreated(beanName);
            }

            StartupStep beanCreation = this.applicationStartup.start("spring.beans.instantiate")
                    .tag("beanName", name);
            try {
                if (requiredType != null) {
                    beanCreation.tag("beanType", requiredType::toString);
                }
                RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
                checkMergedBeanDefinition(mbd, beanName, args);

                // Guarantee initialization of beans that the current bean depends on.
                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 + "'");
                        }
                        // 注册依赖记录
                        registerDependentBean(dep, beanName);
                        try {
                            getBean(dep);
                        }
                        catch (NoSuchBeanDefinitionException ex) {
                            throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                                    "'" + beanName + "' depends on missing bean '" + dep + "'", ex);
                        }
                    }
                }

                // Create bean instance.
                if (mbd.isSingleton()) {
                    // 创建单例Bean 看2.666
                    sharedInstance = getSingleton(beanName, () -> {
                        try {
                            return createBean(beanName, mbd, args);
                        }
                        catch (BeansException ex) {
                            // Explicitly remove instance from singleton cache: It might have been put there
                            // eagerly by the creation process, to allow for circular reference resolution.
                            // Also remove any beans that received a temporary reference to the bean.
                            destroySingleton(beanName);
                            throw ex;
                        }
                    });
                    beanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
                }

                else if (mbd.isPrototype()) {
                    // It's a prototype -> create a new instance.
                    Object prototypeInstance = null;
                    try {
                        beforePrototypeCreation(beanName);
                        prototypeInstance = createBean(beanName, mbd, args);
                    }
                    finally {
                        afterPrototypeCreation(beanName);
                    }
                    beanInstance = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
                }

                else {
                    String scopeName = mbd.getScope();
                    if (!StringUtils.hasLength(scopeName)) {
                        throw new IllegalStateException("No scope name defined for bean ´" + beanName + "'");
                    }
                    Scope scope = this.scopes.get(scopeName);
                    if (scope == null) {
                        throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
                    }
                    try {
                        Object scopedInstance = scope.get(beanName, () -> {
                            beforePrototypeCreation(beanName);
                            try {
                                return createBean(beanName, mbd, args);
                            }
                            finally {
                                afterPrototypeCreation(beanName);
                            }
                        });
                        beanInstance = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
                    }
                    catch (IllegalStateException ex) {
                        throw new ScopeNotActiveException(beanName, scopeName, ex);
                    }
                }
            }
            catch (BeansException ex) {
                beanCreation.tag("exception", ex.getClass().toString());
                beanCreation.tag("message", String.valueOf(ex.getMessage()));
                cleanupAfterBeanCreationFailure(beanName);
                throw ex;
            }
            finally {
                beanCreation.end();
            }
        }
        // 转化Bean为真正需要的类型，有点像适配器模式
        return adaptBeanInstance(name, beanInstance, requiredType);
    }

2.1 transformedBeanName，处理别名BeanName、处理带&符的工厂BeanName

    // 处理Bean的别名
    protected String transformedBeanName(String name) {
        return canonicalName(BeanFactoryUtils.transformedBeanName(name));
    }
    public String canonicalName(String name) {
        String canonicalName = name;
        // Handle aliasing...
        String resolvedName;
        do {
            resolvedName = this.aliasMap.get(canonicalName);
            if (resolvedName != null) {
                canonicalName = resolvedName;
            }
        }
        while (resolvedName != null);
        return canonicalName;
    }
    // 非FactoryBean 直接返回,是的话去除&返回
    public static String transformedBeanName(String name) {
        Assert.notNull(name, "'name' must not be null");
        if (!name.startsWith(BeanFactory.FACTORY_BEAN_PREFIX)) {
            return name;
        }
        return transformedBeanNameCache.computeIfAbsent(name, beanName -> {
            do {
                beanName = beanName.substring(BeanFactory.FACTORY_BEAN_PREFIX.length());
            }
            while (beanName.startsWith(BeanFactory.FACTORY_BEAN_PREFIX));
            return beanName;
        });
    }

2.2 getObjectForBeanInstance，如果 sharedInstance 是普通的 Bean 实例直接返回。另外 sharedInstance 是工厂Bean类型，则需要获取 getObject 方法。

//  返回FactoryBean 核心方法
private Object doGetObjectFromFactoryBean(FactoryBean factory, String beanName) throws BeanCreationException {
        Object object;
        try {
            if (System.getSecurityManager() != null) {
                AccessControlContext acc = getAccessControlContext();
                try {
                    object = AccessController.doPrivileged((PrivilegedExceptionAction