spring中bean的创建流程

finishBeanFactoryInitialization(beanFactory)

在refresh方法中知道此方法就是初始化剩下的单实例

				// Instantiate all remaining (non-lazy-init) singletons.
				// 初始化剩下的单实例（非懒加载的）
				finishBeanFactoryInitialization(beanFactory);

protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
		// Initialize conversion service for this context.
		// 为上下文初始化类型转换器
		if (beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME) &&
				beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)) {
			beanFactory.setConversionService(
					beanFactory.getBean(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class));
		}

		// Register a default embedded value resolver if no bean post-processor
		// (such as a PropertyPlaceholderConfigurer bean) registered any before:
		// at this point, primarily for resolution in annotation attribute values.
		// 如果beanFactory之前没有注册嵌入值解析器，则注册默认的嵌入值解析器，主要用于注解属性值的解析
		if (!beanFactory.hasEmbeddedValueResolver()) {
			beanFactory.addEmbeddedValueResolver(strVal -> getEnvironment().resolvePlaceholders(strVal));
		}

		// Initialize LoadTimeWeaverAware beans early to allow for registering their transformers early.
		// 尽早初始化loadTimeWeaverAware bean,以便尽早注册它们的转换器
		String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
		for (String weaverAwareName : weaverAwareNames) {
			getBean(weaverAwareName);
		}

		// Stop using the temporary ClassLoader for type matching.
		// 禁止使用临时类加载器进行类型匹配
		beanFactory.setTempClassLoader(null);

		// Allow for caching all bean definition metadata, not expecting further changes.
		// 冻结所有的bean定义，说明注册的bean定义将不被修改或任何进一步的处理
		beanFactory.freezeConfiguration();

		// Instantiate all remaining (non-lazy-init) singletons.
		// 实例化剩下的单例对象
		beanFactory.preInstantiateSingletons();
	}

首先是初始化类型转换器，此转换器在spring mvc中较多，后续具体再看，这里的主要逻辑是获取名为conversionService类型为ConversionService.class的bean,使用此bean对对象做一个转换服务。
可以自定义实现这样的转换

public class Student {

    private Integer id;
    private String name;

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return "Student{" +
                "id=" + id +
                ", name='" + name + '\'' +
                '}';
    }
}

import org.springframework.core.convert.converter.Converter;

public class StudentConverter implements Converter<String,Student> {
    @Override
    public Student convert(String source) {
        System.out.println("-----");
        Student s  = new Student();
        String[] splits = source.split("_");
        s.setId(Integer.parseInt(splits[0]));
        s.setName(splits[1]);
        return s;
    }
}

beanFactory.preInstantiateSingletons();

此方法是实例化剩下的单例对象

public void preInstantiateSingletons() throws BeansException {
		if (logger.isTraceEnabled()) {
			logger.trace("Pre-instantiating singletons in " + this);
		}

		// Iterate over a copy to allow for init methods which in turn register new bean definitions.
		// While this may not be part of the regular factory bootstrap, it does otherwise work fine.
		// 将所有BeanDefinition的名字创建一个集合
		List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);

		// Trigger initialization of all non-lazy singleton beans...
		// 触发所有非延迟加载单例bean的初始化，遍历集合的对象
		for (String beanName : beanNames) {
			// 合并父类BeanDefinition
 			RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
			// 条件判断，抽象，单例，非懒加载
			if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
				// 判断是否实现了FactoryBean接口
				if (isFactoryBean(beanName)) {
					// 根据&+beanName来获取具体的对象
					Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
					// 进行类型转换
					if (bean instanceof FactoryBean) {
						FactoryBean<?> factory = (FactoryBean<?>) bean;
						// 判断这个FactoryBean是否希望立即初始化
						boolean isEagerInit;
						if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
							isEagerInit = AccessController.doPrivileged(
									(PrivilegedAction<Boolean>) ((SmartFactoryBean<?>) factory)::isEagerInit,
									getAccessControlContext());
						}
						else {
							isEagerInit = (factory instanceof SmartFactoryBean &&
									((SmartFactoryBean<?>) factory).isEagerInit());
						}
						//  如果希望急切的初始化，则通过beanName获取bean实例
						if (isEagerInit) {
							getBean(beanName);
						}
					}
				}
				else {
					// 如果beanName对应的bean不是FactoryBean，只是普通的bean，通过beanName获取bean实例
					getBean(beanName);
				}
			}
		}

		// Trigger post-initialization callback for all applicable beans...
		// 遍历beanNames，触发所有SmartInitializingSingleton的后初始化回调
		for (String beanName : beanNames) {
			// 获取beanName对应的bean实例
			Object singletonInstance = getSingleton(beanName);
			// 判断singletonInstance是否实现了SmartInitializingSingleton接口
			if (singletonInstance instanceof SmartInitializingSingleton) {
				// 类型转换
				SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
				// 触发SmartInitializingSingleton实现类的afterSingletonsInstantiated方法
				if (System.getSecurityManager() != null) {
					AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
						smartSingleton.afterSingletonsInstantiated();
						return null;
					}, getAccessControlContext());
				}
				else {
					smartSingleton.afterSingletonsInstantiated();
				}
			}
		}
	}

getMergedLocalBeanDefinition(beanName);流程

此方法是合并父类BeanDefinition，这里面使用了递归处理
流程如下

实现FactoryBean接口,里面的对象实例化过程

if (isFactoryBean(beanName)) {
					// 根据&+beanName来获取具体的对象
					Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
					// 进行类型转换
					if (bean instanceof FactoryBean) {
						FactoryBean<?> factory = (FactoryBean<?>) bean;
						// 判断这个FactoryBean是否希望立即初始化
						boolean isEagerInit;
						if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
							isEagerInit = AccessController.doPrivileged(
									(PrivilegedAction<Boolean>) ((SmartFactoryBean<?>) factory)::isEagerInit,
									getAccessControlContext());
						}
						else {
							isEagerInit = (factory instanceof SmartFactoryBean &&
									((SmartFactoryBean<?>) factory).isEagerInit());
						}
						//  如果希望急切的初始化，则通过beanName获取bean实例
						if (isEagerInit) {
							getBean(beanName);
						}
					}
				}

何为FactoryBean？他与BeanFactory有什么区别
先看FactoryBean的类的结构，此接口里面仅有getObject()、getObjectType()、isSingleton()这三个方法，使用这个类将实例注入到bean工厂中，不需要严格遵循BeanFactory里面bean的构建流程（初始化、实例化、aware接口、前置处理、后置处理），可以在FactoryBean中直接调用getObject()直接生成bean对象，并放入到factoryBeanObjectCache集合中，此集合是个Map。没有放到三级缓存中。后续如果需要则从factoryBeanObjectCache这个集合中获取。
特别：非单例情况下，实例始终不会往factoryBeanObjectCache中放，即非单例不会有缓存

如何实现FactoryBean

public class MyFactoryBean implements FactoryBean<User> {

    @Override
    public User getObject() throws Exception {
        return new User("zhangsan");
    }

    @Override
    public Class<?> getObjectType() {
        return User.class;
    }

    @Override
    public boolean isSingleton() {
        return false;
    }
}

public class User {

    private String username;

    public User() {
    }

    public User(String username) {
        this.username = username;
    }

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    @Override
    public String toString() {
        return "User{" +
                "username='" + username + '\'' +
                '}';
    }
}

当上面的逻辑处理完成时（这里粗略的看就是refresh方法完成），这时FactoryBean中已经有实现了FactoryBean的bean了，但是里面的对现象还没有放入到缓存中，如果这个类实现了SmartFactoryBean这个接口，并重写了isEagerInit方法，将其返回为true。那么在实例化FactoryBean这个对象的时候，里面的对象也会实例化并放入到factoryBeanObjectCache缓存中（非单例情况下，实例始终不会往factoryBeanObjectCache中放，即非单例不会有缓存）。这里没有实现SmartFactoryBean，只是实现了FactoryBean，所以没有直接实例化，而是在调用的时候进行的实例化。
实现流程