4.1Spring源码解析——getBean方法的实现

 getBean方法在BeanFactory中实现，实现的类有5个

实现类图.png

在这5个类中AbstractBeanFactory完成了getBean()方法的具体实现

    public Object getBean(String name) throws BeansException {
        return doGetBean(name, null, null, false);
    }

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

    public Object getBean(String name, Object... args) throws BeansException {
        return doGetBean(name, null, args, false);
    }

    public  T getBean(String name, Class requiredType, Object... args) throws BeansException {
        return doGetBean(name, requiredType, args, false);
    }

 这几个方法最终调用的是doGetBean方法

protected  T doGetBean(
            final String name, final Class requiredType, final Object[] args, boolean typeCheckOnly)
            throws BeansException {
        //去除&符号，因为当配置文件中的class属性配置的实现类是FactoryBean时，通过getBean()方法返回的不是FactoryBean本身，
        //而是FactoryBean#getObject()方法所返回的对象，相当于FactoryBean#getObject()代理了getBean()方法，如果希望获取FactoryBean的实例，需要在beanName前加上“&”符号，即getBean("&beanName")
        //在这里要获取的是bean的实例，所以要去掉&符号
        final String beanName = transformedBeanName(name);
        Object bean;

        // Eagerly check singleton cache for manually registered singletons.
//-------------------------------------方法1------------------------------------------//
        //从缓存中获取单利的bean
        Object sharedInstance = getSingleton(beanName);
//-------------------------------------方法1------------------------------------------//
        if (sharedInstance != null && args == null) {
            if (logger.isDebugEnabled()) {
                if (isSingletonCurrentlyInCreation(beanName)) {
                    logger.debug("Returning eagerly cached instance of singleton bean '" + beanName +
                            "' that is not fully initialized yet - a consequence of a circular reference");
                }
                else {
                    logger.debug("Returning cached instance of singleton bean '" + beanName + "'");
                }
            }
                  //从已经加载的object中获取bean
            //-------------------------------------方法2------------------------------------------//
            bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
              //-------------------------------------方法2------------------------------------------//
        }
        else {
            // Fail if we're already creating this bean instance:
            // We're assumably within a circular reference.
            //检查要获取的bean是不是当前线程正在创建中的bean
            if (isPrototypeCurrentlyInCreation(beanName)) {
                throw new BeanCurrentlyInCreationException(beanName);
            }
            // Check if bean definition exists in this factory.
             //检查当前BeanFactory的父类BeanFactory是不是不为空，并且要获取的bean不包含在当前BeanFactory中
            BeanFactory parentBeanFactory = getParentBeanFactory();
            if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
                // Not found -> check parent.
                //还原要获取的bean的beanName
                String nameToLookup = originalBeanName(name);
                //调用parentBeanFactory获取bean  
                if (args != null) {
                    // Delegation to parent with explicit args.
                    return (T) parentBeanFactory.getBean(nameToLookup, args);
                }
                else {
                    // No args -> delegate to standard getBean method.
                    return parentBeanFactory.getBean(nameToLookup, requiredType);
                }
            }
            //typeCheckOnly默认为false
            if (!typeCheckOnly) {
                  //如果当前bean不在正在创建中的bean的缓存集合（alreadyCreated）中，就加进去
                markBeanAsCreated(beanName);
            }
            try {
                  //合并要获取的bean的属性并把属性赋值给一个RootBeanDefinition
                final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
          //检查合并之后的RootBeanDefinition的是不是抽象的，还会检查如果获取bean的参数不空的情况下，bean的类型是singleton的就会报错，因为单利情况下bean都是一样的，只有prototype情况下才能不一致
                checkMergedBeanDefinition(mbd, beanName, args);
                // Guarantee initialization of beans that the current bean depends on.
            //获取当前bean依赖的bean的名称的数组
                String[] dependsOn = mbd.getDependsOn();
            //如果依赖不为空就先加载依赖到的bean
                if (dependsOn != null) {
                    for (String dependsOnBean : dependsOn) {
                        getBean(dependsOnBean);
                //将要获取的bean的beanName和所依赖到的bean的beanName之间的关系缓存起来
                        registerDependentBean(dependsOnBean, beanName);
                    }
                }
                // Create bean instance.
                //开始创建单利bean实例
                if (mbd.isSingleton()) {
          //--------------------------------------方法4------------------------------//
                    sharedInstance = getSingleton(beanName, new ObjectFactory