梳理Spring获取Bean的流程

Spring获取Bean的方法在接口BeanFactory中定义

public interface BeanFactory {

	Object getBean(String name) throws BeansException;

	<T> T getBean(String name, Class<T> requiredType) throws BeansException;

	Object getBean(String name, Object... args) throws BeansException;

	<T> T getBean(Class<T> requiredType) throws BeansException;

	<T> T getBean(Class<T> requiredType, Object... args) throws BeansException;

}

可以根据Bean的名称或者类型获取。我们今天只看根据类型获取的逻辑，即下面的方法定义

<T> T getBean(Class<T> requiredType) throws BeansException;

我们首先写一段常规初始化工厂和获取Bean的代码

ApplicationContext context = new AnnotationConfigApplicationContext(AppConfig.class);
Smile smile = context.getBean(Smile.class);
smile.smile("hh");

AnnotationConfigApplicationContext extends GenericApplicationContext extends AbstractApplicationContext

在AbstractApplicationContext类中实现了BeanFactory中定义的getBean方法

@Override
public <T> T getBean(Class<T> requiredType) throws BeansException {
	assertBeanFactoryActive();
	return getBeanFactory().getBean(requiredType);
}

该类又将获取Bean的具体操作交给getBeanFactory()返回的类来实现。我们看下getBeanFactory()方法的定义

@Override
public abstract ConfigurableListableBeanFactory getBeanFactory() throws IllegalStateException;

该方法是抽象方法，需要子类去实现。在我们当前的示例中，子类为GenericApplicationContext。GenericApplicationContext实现了getBeanFactory()方法

@Override
public final ConfigurableListableBeanFactory getBeanFactory() {
	return this.beanFactory;
}

返回一个成员变量，这个成员变量的类型为

private final DefaultListableBeanFactory beanFactory;

所以我们获取Bean的逻辑转到了DefaultListableBeanFactory类中

@Override
public <T> T getBean(Class<T> requiredType) throws BeansException {
	return getBean(requiredType, (Object[]) null);
}

@SuppressWarnings("unchecked")
@Override
public <T> T getBean(Class<T> requiredType, @Nullable Object... args) throws BeansException {
	Assert.notNull(requiredType, "Required type must not be null");
	Object resolved = resolveBean(ResolvableType.forRawClass(requiredType), args, false);
	if (resolved == null) {
		throw new NoSuchBeanDefinitionException(requiredType);
	}
	return (T) resolved;
}

可以看到获取Bean的逻辑主要在resolveBean方法中，我们接着看resolveBean方法

@Nullable
private <T> T resolveBean(ResolvableType requiredType, @Nullable Object[] args, boolean nonUniqueAsNull) {
	NamedBeanHolder<T> namedBean = resolveNamedBean(requiredType, args, nonUniqueAsNull);
	if (namedBean != null) {
		return namedBean.getBeanInstance();
	}
	BeanFactory parent = getParentBeanFactory();
	if (parent instanceof DefaultListableBeanFactory) {
		return ((DefaultListableBeanFactory) parent).resolveBean(requiredType, args, nonUniqueAsNull);
	}
	else if (parent != null) {
		ObjectProvider<T> parentProvider = parent.getBeanProvider(requiredType);
		if (args != null) {
			return parentProvider.getObject(args);
		}
		else {
			return (nonUniqueAsNull ? parentProvider.getIfUnique() : parentProvider.getIfAvailable());
		}
	}
	return null;
}

我们只关注方法的前四行，resolveNamedBean方法根据Class信息获取Bean名称，然后将获取Bean的逻辑交给了下面的方法

public <T> T getBean(String name, @Nullable Class<T> requiredType, @Nullable Object... args)
			throws BeansException {

	return doGetBean(name, requiredType, args, false);
}

该方法在AbstractBeanFactory类中定义，真正的获取Bean的逻辑在doGetBean方法中，在doGetBean方法中真正获取Bean的逻辑在getSingleton方法中，该方法在DefaultSingletonBeanRegistry类中，定义如下

public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {}