Spring源码之FactoryBean的实现

https://zhuanlan.zhihu.com/p/97005407
https://blog.csdn.net/qq_35634181/article/details/104507465

总结

1. FactoryBean的对象会先在createWebServer时，创建BeanWrapper，并放到factoryBeanInstanceCache缓存中
2. 在预实例化时，走FactoryBean逻辑，调用getBean去执行属性注入、初始化过程（此时对象实例化直接从factoryBeanInstanceCache缓存中取）
3. 完成FactoryBean的实现类初始化
4. 调用实现类的getObject方法，从而对工厂中类进行功能增强

FactoryBean（实现类）的初始化

初始化入口依然是AbstractApplicationContext#finishBeanFactoryInitialization() ，走到DefaultListableBeanFactory类preInstantiateSingletons方法时，和普通bean逻辑不同，执行以下代码：

@Override
public void preInstantiateSingletons() throws BeansException {


	// 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.
	List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);

	// Trigger initialization of all non-lazy singleton beans...
	for (String beanName : beanNames) {
		RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
		if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
		    //FactoryBean的初始化执行入口
			if (isFactoryBean(beanName)) {
				Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
				if (bean instanceof FactoryBean) {
					FactoryBean<?> factory = (FactoryBean<?>) bean;
					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());
					}
					if (isEagerInit) {
						getBean(beanName);
					}
				}
			}
			else {
			    //普通bean的执行入口
				getBean(beanName);
			}
		}
	}
}	

调用Object bean = getBean(FACTORY_BEAN_PREFIX + beanName)，后面基本和普通bean的初始化一致

其中不同有：instanceWrapper不为null，是createWebServer时放入factoryBeanInstanceCache中的

if (mbd.isSingleton()) {
	instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}

将FactoryBean实现类放入缓存调用链为：

SpringApplication#run() --> SpringApplication#refreshContext() --> SpringApplication#refresh() -->ServletWebServerApplicationContext#refresh() --> AbstractApplicationContext#refresh() --> AbstractApplicationContext#onRefresh() --> ServletWebServerApplicationContext#createWebServer() --> ServletWebServerApplicationContext#getWebServerFactory() --> DefaultListableBeanFactory#getBeanNamesForType() --> DefaultListableBeanFactory#doGetBeanNamesForType() --> AbstractBeanFactory#isTypeMatch() --> AbstractAutowireCapableBeanFactory#getTypeForFactoryBean() --> AbstractAutowireCapableBeanFactory#getSingletonFactoryBeanForTypeCheck()

mbd为：

// Generics potentially only match on the target class, not on the proxy...
RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);

bw为：

Object instance;
try {
	// Mark this bean as currently in creation, even if just partially.
	beforeSingletonCreation(beanName);
	// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
	instance = resolveBeforeInstantiation(beanName, mbd);
	if (instance == null) {
		bw = createBeanInstance(beanName, mbd, null);
		instance = bw.getWrappedInstance();
	}
}
catch (BeanCreationException ex) {
	......
}
finally {
	// Finished partial creation of this bean.
	afterSingletonCreation(beanName);
}

FactoryBean为：

FactoryBean<?> fb = getFactoryBean(beanName, instance);
if (bw != null) {
	this.factoryBeanInstanceCache.put(beanName, bw);
}

获取FactoryBean实现类（非工厂）的过程

如（此时beanName前面没有&）：

Object factoryBeanTest = context.getBean("factoryBeanTest");

会先在AbstractBeanFactory#doGetBean()中执行：

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


	// Eagerly check singleton cache for manually registered singletons.
	Object sharedInstance = getSingleton(beanName);
	if (sharedInstance != null && args == null) {
		......
		bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
	}
}

执行getObjectForBeanInstance方法：

protected Object getObjectForBeanInstance(
		Object beanInstance, String name, String beanName, @Nullable RootBeanDefinition mbd) {

    ......
    
	Object object = null;
	if (mbd != null) {
		mbd.isFactoryBean = true;
	}
	else {
		object = getCachedObjectForFactoryBean(beanName);
	}
	if (object == null) {
		// Return bean instance from factory.
		FactoryBean<?> factory = (FactoryBean<?>) beanInstance;
		// Caches object obtained from FactoryBean if it is a singleton.
		if (mbd == null && containsBeanDefinition(beanName)) {
			mbd = getMergedLocalBeanDefinition(beanName);
		}
		boolean synthetic = (mbd != null && mbd.isSynthetic());
		object = getObjectFromFactoryBean(factory, beanName, !synthetic);
	}
	return object;
}

调用getObjectFromFactoryBean，再调用doGetObjectFromFactoryBean方法：

protected Object getObjectFromFactoryBean(FactoryBean<?> factory, String beanName, boolean shouldPostProcess) {
	if (factory.isSingleton() && containsSingleton(beanName)) {
		synchronized (getSingletonMutex()) {
		    //object为null
			Object object = this.factoryBeanObjectCache.get(beanName);
			if (object == null) {
				object = doGetObjectFromFactoryBean(factory, beanName);
				
				......
			}
		}
	}
}	

最后调用实际FactoryBean实现类的getObject方法，完成工厂类的自定义功能增强

private Object doGetObjectFromFactoryBean(FactoryBean<?> factory, String beanName) throws BeanCreationException {
	Object object;
	try {
		if (System.getSecurityManager() != null) {
			......
		}
		else {
		    //去执行实际实现类的getObject方法
			object = factory.getObject();
		}
	}

	return object;
}

通过FactoryBean.getObject方式

如：

FactoryBean bean = (FactoryBean)context.getBean("&factoryBeanTest");
CustomTask customTask = (CustomTask)bean.getObject();

会通过AbstractBeanFactory#doGetBean()直接返回bean

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

	// Eagerly check singleton cache for manually registered singletons.
	//此时一级缓存singletonObjects中有数据
	Object sharedInstance = getSingleton(beanName);
	if (sharedInstance != null && args == null) {
		......
		bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
	}

调用getObjectForBeanInstance，返回beanInstance

protected Object getObjectForBeanInstance(
		Object beanInstance, String name, String beanName, @Nullable RootBeanDefinition mbd) {

	// Don't let calling code try to dereference the factory if the bean isn't a factory.
	//此时为true
	if (BeanFactoryUtils.isFactoryDereference(name)) {
		......
		if (mbd != null) {
			mbd.isFactoryBean = true;
		}
		return beanInstance;
	}
}

其后便是调用FactoryBean实现类的getObject方法，完成工厂类的自定义功能增强

FactoryBean实现示例：

@Component
public class FactoryBeanTest implements FactoryBean {
    @Override
    public Object getObject() throws Exception {
        //一些自定义功能
        ......
        return new CustomTask();
    }

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

springboot中调用例子：

@RestController
public class TestController {

    @Autowired
    private AnnotationConfigServletWebServerApplicationContext context;

    @RequestMapping("/test")
    public String test() throws Exception {

        Object factoryBeanTest = context.getBean("factoryBeanTest");
        System.out.println(factoryBeanTest.toString());

        FactoryBean bean = (FactoryBean)context.getBean("&factoryBeanTest");
        CustomTask customTask = (CustomTask)bean.getObject();
        customTask.execute();
        return bean.getObject().toString();
    }
}