Spring之依赖注入原理

<bean id="orderService" class="com.fandf.service.OrderService" autowire="byType"/>

@Override
public void postProcessMergedBeanDefinition(RootBeanDefinition beanDefinition, Class<?> beanType, String beanName) {
  InjectionMetadata metadata = findAutowiringMetadata(beanName, beanType, null);
  metadata.checkConfigMembers(beanDefinition);
}

@Override
private InjectionMetadata findAutowiringMetadata(String beanName, Class<?> clazz, @Nullable PropertyValues pvs) {
  // Fall back to class name as cache key, for backwards compatibility with custom callers.
  String cacheKey = (StringUtils.hasLength(beanName) ? beanName : clazz.getName());
  // Quick check on the concurrent map first, with minimal locking.
  InjectionMetadata metadata = this.injectionMetadataCache.get(cacheKey);
  if (InjectionMetadata.needsRefresh(metadata, clazz)) {
    synchronized (this.injectionMetadataCache) {
      metadata = this.injectionMetadataCache.get(cacheKey);
      if (InjectionMetadata.needsRefresh(metadata, clazz)) {
        if (metadata != null) {
          metadata.clear(pvs);
        }
        // 解析注入点并缓存
        metadata = buildAutowiringMetadata(clazz);
        this.injectionMetadataCache.put(cacheKey, metadata);
      }
    }
  }
  return metadata;
}

private InjectionMetadata buildAutowiringMetadata(final Class<?> clazz) {
  // 如果一个Bean的类型是String...，那么则根本不需要进行依赖注入
  if (!AnnotationUtils.isCandidateClass(clazz, this.autowiredAnnotationTypes)) {
    return InjectionMetadata.EMPTY;
  }

  List<InjectionMetadata.InjectedElement> elements = new ArrayList<>();
  Class<?> targetClass = clazz;

  do {
    final List<InjectionMetadata.InjectedElement> currElements = new ArrayList<>();

    // 遍历targetClass中的所有Field
    ReflectionUtils.doWithLocalFields(targetClass, field -> {
      // field上是否存在@Autowired、@Value、@Inject中的其中一个
      MergedAnnotation<?> ann = findAutowiredAnnotation(field);
      if (ann != null) {
        // static filed不是注入点，不会进行自动注入
        if (Modifier.isStatic(field.getModifiers())) {
          if (logger.isInfoEnabled()) {
            logger.info("Autowired annotation is not supported on static fields: " + field);
          }
          return;
        }

        // 构造注入点
        boolean required = determineRequiredStatus(ann);
        currElements.add(new AutowiredFieldElement(field, required));
      }
    });

    // 遍历targetClass中的所有Method
    ReflectionUtils.doWithLocalMethods(targetClass, method -> {

      Method bridgedMethod = BridgeMethodResolver.findBridgedMethod(method);
      if (!BridgeMethodResolver.isVisibilityBridgeMethodPair(method, bridgedMethod)) {
        return;
      }
      // method上是否存在@Autowired、@Value、@Inject中的其中一个
      MergedAnnotation<?> ann = findAutowiredAnnotation(bridgedMethod);
      if (ann != null && method.equals(ClassUtils.getMostSpecificMethod(method, clazz))) {
        // static method不是注入点，不会进行自动注入
        if (Modifier.isStatic(method.getModifiers())) {
          if (logger.isInfoEnabled()) {
            logger.info("Autowired annotation is not supported on static methods: " + method);
          }
          return;
        }
        // set方法最好有入参
        if (method.getParameterCount() == 0) {
          if (logger.isInfoEnabled()) {
            logger.info("Autowired annotation should only be used on methods with parameters: " +
                method);
          }
        }
        boolean required = determineRequiredStatus(ann);
        PropertyDescriptor pd = BeanUtils.findPropertyForMethod(bridgedMethod, clazz);
        currElements.add(new AutowiredMethodElement(method, required, pd));
      }
    });

    elements.addAll(0, currElements);
    targetClass = targetClass.getSuperclass();
  }
  while (targetClass != null && targetClass != Object.class);

  return InjectionMetadata.forElements(elements, clazz);
}

@Component
@Scope("prototype")
public class OrderService {


}

@Component
@Scope("prototype")
public class UserService  {

 @Autowired
 private static OrderService orderService;


}

public interface UserInterface<T> {
  void setOrderService(T t);
}

@Component
public class UserService implements UserInterface<OrderService> {

  private OrderService orderService;

  @Override
  @Autowired
  public void setOrderService(OrderService orderService) {
    this.orderService = orderService;
  }

}

@Override
public PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName) {
  // 找注入点（所有被@Autowired注解了的Field或Method）
  InjectionMetadata metadata = findAutowiringMetadata(beanName, bean.getClass(), pvs);
  try {
    metadata.inject(bean, beanName, pvs);
  }
  catch (BeanCreationException ex) {
    throw ex;
  }
  catch (Throwable ex) {
    throw new BeanCreationException(beanName, "Injection of autowired dependencies failed", ex);
  }
  return pvs;
}

public void inject(Object target, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
  Collection<InjectedElement> checkedElements = this.checkedElements;
  Collection<InjectedElement> elementsToIterate =
      (checkedElements != null ? checkedElements : this.injectedElements);
  if (!elementsToIterate.isEmpty()) {
    // 遍历每个注入点进行依赖注入
    for (InjectedElement element : elementsToIterate) {
      element.inject(target, beanName, pvs);
    }
  }
}

@Override
protected void inject(Object bean, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {

  Field field = (Field) this.member;
  Object value;
  if (this.cached) {
    // 对于原型Bean，第一次创建的时候，也找注入点，然后进行注入，此时cached为false，注入完了之后cached为true
    // 第二次创建的时候，先找注入点（此时会拿到缓存好的注入点），也就是AutowiredFieldElement对象，此时cache为true，也就进到此处了
    // 注入点内并没有缓存被注入的具体Bean对象，而是beanName，这样就能保证注入到不同的原型Bean对象
    try {
      value = resolvedCachedArgument(beanName, this.cachedFieldValue);
    }
    catch (NoSuchBeanDefinitionException ex) {
      // Unexpected removal of target bean for cached argument -> re-resolve
      value = resolveFieldValue(field, bean, beanName);
    }
  }
  else {
    // 根据filed从BeanFactory中查到的匹配的Bean对象
    value = resolveFieldValue(field, bean, beanName);
  }

  // 反射给filed赋值
  if (value != null) {
    ReflectionUtils.makeAccessible(field);
    field.set(bean, value);
  }
}

@Nullable
private Object resolvedCachedArgument(@Nullable String beanName, @Nullable Object cachedArgument) {
  if (cachedArgument instanceof DependencyDescriptor) {
    // ShortcutDependencyDescriptor
    DependencyDescriptor descriptor = (DependencyDescriptor) cachedArgument;
    Assert.state(this.beanFactory != null, "No BeanFactory available");
    //调用DefaultListableBeanFactory的resolveDependency
    return this.beanFactory.resolveDependency(descriptor, beanName, null, null);
  }
  else {
    return cachedArgument;
  }
}

@Override
@Nullable
public Object resolveDependency(DependencyDescriptor descriptor, @Nullable String requestingBeanName,
    @Nullable Set<String> autowiredBeanNames, @Nullable TypeConverter typeConverter) throws BeansException {
  // 用来获取方法入参名字的
  descriptor.initParameterNameDiscovery(getParameterNameDiscoverer());

  // 所需要的类型是Optional
  if (Optional.class == descriptor.getDependencyType()) {
    return createOptionalDependency(descriptor, requestingBeanName);
  }
  // 所需要的的类型是ObjectFactory，或ObjectProvider
  else if (ObjectFactory.class == descriptor.getDependencyType() ||
      ObjectProvider.class == descriptor.getDependencyType()) {
    return new DependencyObjectProvider(descriptor, requestingBeanName);
  }
  else if (javaxInjectProviderClass == descriptor.getDependencyType()) {
    return new Jsr330Factory().createDependencyProvider(descriptor, requestingBeanName);
  }
  else {
    // 在属性或set方法上使用了@Lazy注解，那么则构造一个代理对象并返回，真正使用该代理对象时才进行类型筛选Bean
    Object result = getAutowireCandidateResolver().getLazyResolutionProxyIfNecessary(
        descriptor, requestingBeanName);

    if (result == null) {
      // descriptor表示某个属性或某个set方法
      // requestingBeanName表示正在进行依赖注入的Bean
      result = doResolveDependency(descriptor, requestingBeanName, autowiredBeanNames, typeConverter);
    }
    return result;
  }
}

package com.fandf.service;

import java.lang.reflect.Field;
import java.lang.reflect.GenericArrayType;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.TypeVariable;
import java.util.List;

public class TypeTest<T> {

	private int i;
	private Integer it;
	private int[] iarray;
	private List list;
	private List<String> slist;
	private List<T> tlist;
	private T t;
	private T[] tarray;

	public static void main(String[] args) throws NoSuchFieldException {

		test(TypeTest.class.getDeclaredField("i"));
		System.out.println("=======");
		test(TypeTest.class.getDeclaredField("it"));
		System.out.println("=======");
		test(TypeTest.class.getDeclaredField("iarray"));
		System.out.println("=======");
		test(TypeTest.class.getDeclaredField("list"));
		System.out.println("=======");
		test(TypeTest.class.getDeclaredField("slist"));
		System.out.println("=======");
		test(TypeTest.class.getDeclaredField("tlist"));
		System.out.println("=======");
		test(TypeTest.class.getDeclaredField("t"));
		System.out.println("=======");
		test(TypeTest.class.getDeclaredField("tarray"));

	}

	public static void test(Field field) {

		if (field.getType().isPrimitive()) {
			System.out.println(field.getName() + "是基本数据类型");
		} else {
			System.out.println(field.getName() + "不是基本数据类型");
		}

		if (field.getGenericType() instanceof ParameterizedType) {
			System.out.println(field.getName() + "是泛型类型");
		} else {
			System.out.println(field.getName() + "不是泛型类型");
		}

		if (field.getType().isArray()) {
			System.out.println(field.getName() + "是普通数组");
		} else {
			System.out.println(field.getName() + "不是普通数组");
		}

		if (field.getGenericType() instanceof GenericArrayType) {
			System.out.println(field.getName() + "是泛型数组");
		} else {
			System.out.println(field.getName() + "不是泛型数组");
		}

		if (field.getGenericType() instanceof TypeVariable) {
			System.out.println(field.getName() + "是泛型变量");
		} else {
			System.out.println(field.getName() + "不是泛型变量");
		}

	}

}

i是基本数据类型
i不是泛型类型
i不是普通数组
i不是泛型数组
i不是泛型变量
=======
it不是基本数据类型
it不是泛型类型
it不是普通数组
it不是泛型数组
it不是泛型变量
=======
iarray不是基本数据类型
iarray不是泛型类型
iarray是普通数组
iarray不是泛型数组
iarray不是泛型变量
=======
list不是基本数据类型
list不是泛型类型
list不是普通数组
list不是泛型数组
list不是泛型变量
=======
slist不是基本数据类型
slist是泛型类型
slist不是普通数组
slist不是泛型数组
slist不是泛型变量
=======
tlist不是基本数据类型
tlist是泛型类型
tlist不是普通数组
tlist不是泛型数组
tlist不是泛型变量
=======
t不是基本数据类型
t不是泛型类型
t不是普通数组
t不是泛型数组
t是泛型变量
=======
tarray不是基本数据类型
tarray不是泛型类型
tarray是普通数组
tarray是泛型数组
tarray不是泛型变量

@Target({ElementType.TYPE, ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
@Qualifier("random")
public @interface Random {
}

@Target({ElementType.TYPE, ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
@Qualifier("roundRobin")
public @interface RoundRobin {
}

public interface LoadBalance {
    String select();
}

@Component
@Random
public class RandomStrategy implements LoadBalance {

    @Override
    public String select() {
        return null;
    }
}

@Component
@RoundRobin
public class RoundRobinStrategy implements LoadBalance {

    @Override
    public String select() {
        return null;
    }
}

@Component
public class UserService {

    @Autowired
    @RoundRobin
    private LoadBalance loadBalance;

    public void test() {
        System.out.println(loadBalance);
    }

}