Spring5源码解析-@Autowired

@Autowired所具有的功能

@Autowired是一个用来执行依赖注入的注解。每当一个Spring管理的bean发现有这个注解时候,它会直接注入相应的另一个Spring管理的bean。

该注解可以在不同的层次上应用:
  • 类字段:Spring将通过扫描自定义的packages(例如在我们所注解的controllers)或通过在配置文件中直接查找bean。
  • 方法:使用@Autowired注解的每个方法都要用到依赖注入。但要注意的是,方法签名中呈现的所有对象都必须是Spring所管理的bean。如果你有一个方法,比如setTest(Article article, NoSpringArticle noSpringArt) ,其中只有一个参数 (Article article)是由Spring管理的,那么就将抛出一个org.springframework.beans.factory.BeanCreationException异常。这是由于Spring容器里并没有指定的一个或多个参数所指向的bean,所以也就无法解析它们。完整的异常跟踪如下:
org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'testController': Injection of autowired dependencies failed; nested exception is org.springframework.beans.factory.BeanCreationException: Could not autowire method: public void org.krams.tutorial.controller.TestController.ix(com.mysite.controller.IndexController,com.mysite.nospring.data.Article); nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type [com.mysite.nospring.data.Article] found for dependency: expected at least 1 bean which qualifies as autowire candidate for this dependency. Dependency annotations: {}
  • 构造函数:@Autowired的工作方式和方法相同。
    对象注入需要遵循一些规则。一个bean可以按照下面的方式注入:

  • 名称:bean解析是通过bean名称(看后面的例子)。
    I * 类型:解析过程基于bean的类型。

原理剖析

启动注解的自动注入


首先看看 的定义


		
			
	tag for that purpose.

	See javadoc for org.springframework.context.annotation.AnnotationConfigApplicationContext
	for information on code-based alternatives to bootstrapping annotation-driven support.
			]]>
		
	

可以看出:类内部的注解,如@Autowired、@Value、@Required、@Resource以及EJB和WebService相关的注解,是容器对Bean对象实例化和依赖注入时,通过容器中注册的Bean后置处理器AnnotationConfigApplicationContext处理这些注解的。
所以配置了上面这个配置(context:component-scan(开启扫描包)假如有配置这个,那么就可以省略)后,将隐式地向Spring容器注册AutowiredAnnotationBeanPostProcessor、CommonAnnotationBeanPostProcessor、RequiredAnnotationBeanPostProcessor、PersistenceAnnotationBeanPostProcessor以及这4个专门用于处理注解的Bean后置处理器。
当 Spring 容器启动时,AutowiredAnnotationBeanPostProcessor 将扫描 Spring 容器中所有 Bean,当发现 Bean 中拥有@Autowired 注解时就找到和其匹配(默认按类型匹配)的 Bean,并注入到对应的地方中去。
通过org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor可以实现依赖自动注入
在AutowiredAnnotationBeanPostProcessor构造函数中定义要处理的注解:

public class AutowiredAnnotationBeanPostProcessor extends InstantiationAwareBeanPostProcessorAdapter
		implements MergedBeanDefinitionPostProcessor, PriorityOrdered, BeanFactoryAware {
		...
	/**
	 * Create a new AutowiredAnnotationBeanPostProcessor
	 * for Spring's standard {@link Autowired} annotation.
	 * 

Also supports JSR-330's {@link javax.inject.Inject} annotation, if available. */ @SuppressWarnings("unchecked") public AutowiredAnnotationBeanPostProcessor() { this.autowiredAnnotationTypes.add(Autowired.class); this.autowiredAnnotationTypes.add(Value.class); try { this.autowiredAnnotationTypes.add((Class) ClassUtils.forName("javax.inject.Inject", AutowiredAnnotationBeanPostProcessor.class.getClassLoader())); logger.info("JSR-330 'javax.inject.Inject' annotation found and supported for autowiring"); } catch (ClassNotFoundException ex) { // JSR-330 API not available - simply skip. } } ... }

之后,有几种方法来对@Autowired注解进行处理。

  1. private InjectionMetadata buildAutowiringMetadata(final Class clazz)解析等待自动注入类的所有属性。它通过分析所有字段和方法并初始化org.springframework.beans.factory.annotation.InjectionMetadata类的实例来实现。
private InjectionMetadata buildAutowiringMetadata(final Class clazz) {
		LinkedList elements = new LinkedList<>();
		Class targetClass = clazz;

		do {
			final LinkedList currElements = new LinkedList<>();
			//分析所有字段
			ReflectionUtils.doWithLocalFields(targetClass, field -> {
              //findAutowiredAnnotation(field)此方法后面会解释
				AnnotationAttributes ann = findAutowiredAnnotation(field);
				if (ann != null) {
					if (Modifier.isStatic(field.getModifiers())) {
						if (logger.isWarnEnabled()) {
							logger.warn("Autowired annotation is not supported on static fields: " + field);
						}
						return;
					}
					boolean required = determineRequiredStatus(ann);
					currElements.add(new AutowiredFieldElement(field, required));
				}
			});
			//分析所有方法
			ReflectionUtils.doWithLocalMethods(targetClass, method -> {
				Method bridgedMethod = BridgeMethodResolver.findBridgedMethod(method);
				if (!BridgeMethodResolver.isVisibilityBridgeMethodPair(method, bridgedMethod)) {
					return;
				}
				AnnotationAttributes ann = findAutowiredAnnotation(bridgedMethod);
				if (ann != null && method.equals(ClassUtils.getMostSpecificMethod(method, clazz))) {
					if (Modifier.isStatic(method.getModifiers())) {
						if (logger.isWarnEnabled()) {
							logger.warn("Autowired annotation is not supported on static methods: " + method);
						}
						return;
					}
					if (method.getParameterCount() == 0) {
						if (logger.isWarnEnabled()) {
							logger.warn("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);
		//返回一个InjectionMetadata初始化的对象实例
		return new InjectionMetadata(clazz, elements);
	}
...

  /**
	 * 'Native' processing method for direct calls with an arbitrary target instance,
	 * resolving all of its fields and methods which are annotated with {@code @Autowired}.
	 * @param bean the target instance to process
	 * @throws BeanCreationException if autowiring failed
	 */
	public void processInjection(Object bean) throws BeanCreationException {
		Class clazz = bean.getClass();
		InjectionMetadata metadata = findAutowiringMetadata(clazz.getName(), clazz, null);
		try {
			metadata.inject(bean, null, null);
		}
		catch (BeanCreationException ex) {
			throw ex;
		}
		catch (Throwable ex) {
			throw new BeanCreationException(
					"Injection of autowired dependencies failed for class [" + clazz + "]", ex);
		}
	}

InjectionMetadata类包含要注入的元素的列表。注入是通过Java的API Reflection (Field set(Object obj, Object value) 或Method invoke(Object obj,Object … args)方法完成的。此过程直接在AutowiredAnnotationBeanPostProcessor的方法中调用public void processInjection(Object bean) throws BeanCreationException。它将所有可注入的bean检索为InjectionMetadata实例,并调用它们的inject()方法。

public class InjectionMetadata {
  ...
public void inject(Object target, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
		Collection checkedElements = this.checkedElements;
		Collection elementsToIterate =
				(checkedElements != null ? checkedElements : this.injectedElements);
		if (!elementsToIterate.isEmpty()) {
			boolean debug = logger.isDebugEnabled();
			for (InjectedElement element : elementsToIterate) {
				if (debug) {
					logger.debug("Processing injected element of bean '" + beanName + "': " + element);
				}
              	//看下面静态内部类的方法
				element.inject(target, beanName, pvs);
			}
		}
	}
  ...
    public static abstract class InjectedElement {

		protected final Member member;

		protected final boolean isField;
      ...
        /**
		 * Either this or {@link #getResourceToInject} needs to be overridden.
		 */
		protected void inject(Object target, @Nullable String requestingBeanName, @Nullable PropertyValues pvs)
				throws Throwable {

			if (this.isField) {
				Field field = (Field) this.member;
				ReflectionUtils.makeAccessible(field);
				field.set(target, getResourceToInject(target, requestingBeanName));
			}
			else {
				if (checkPropertySkipping(pvs)) {
					return;
				}
				try {
                  	//具体的注入看此处咯
					Method method = (Method) this.member;
					ReflectionUtils.makeAccessible(method);
					method.invoke(target, getResourceToInject(target, requestingBeanName));
				}
				catch (InvocationTargetException ex) {
					throw ex.getTargetException();
				}
			}
		}
      ...
    }
}

AutowiredAnnotationBeanPostProcessor类中的另一个重要方法是private AnnotationAttributes findAutowiredAnnotation(AccessibleObject ao)。它通过分析属于一个字段或一个方法的所有注解来查找@Autowired注解。如果未找到@Autowired注解,则返回null,字段或方法也就视为不可注入。

@Nullable
	private AnnotationAttributes findAutowiredAnnotation(AccessibleObject ao) {
		if (ao.getAnnotations().length > 0) {
			for (Class type : this.autowiredAnnotationTypes) {
				AnnotationAttributes attributes = AnnotatedElementUtils.getMergedAnnotationAttributes(ao, type);
				if (attributes != null) {
					return attributes;
				}
			}
		}
		return null;
	}

在上面的文章中,我们看到了Spring中自动注入过程。通过整篇文章可以看到,这种依赖注入是一种便捷易操作方式(可以在字段以及方法上完成),也促使我们逐渐在抛弃XML配置文件。还增强了代码的可读性。

转自:
Spring5源码解析-@Autowired

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