看完讓你吊打面試官-@Autowired註解到底怎麼實現的?

1 @Autowired 幹嘛的?

用來執行依賴注入.每當一個Spring管理的bean發現有該註解時,會直接注入相應的另一個Spring管理的bean.

1.1 不同地放置有不同作用

• 屬性
  Spring將通過掃描自定義的package或通過在配置文件中直接查找bean
• 方法
  使用@Autowired註解的每個方法都要用到依賴注入
  但要注意的是,簽名中呈現的所有對象都必須是Spring所管理的bean
  如果你有一個方法，比如setTest(Article article, NoSpringArticle noSpringArt) ,其中只有一個參數 (Article article)是由Spring管理的，那麼就將拋出一個org.springframework.beans.factory.BeanCreationException異常
  這是由於Spring容器裏並沒有指定的一個或多個參數所指向的bean，所以也就無法解析它們

1.2 bean的注入方式

• 名稱
  bean解析是通過bean名稱
• 類型
  解析基於bean的類型

1.3 @Qualifier 協作

如下相同類型的bean

<bean name="article1" class="com.sss.Article">
    <property name="text" value="Content of the 1st Article" />
</bean>

<bean name="article2" class="com.sss.Article">
    <property name="text" value="Content of the 2nd Article" />
</bean>

假如只是一個簡單的@Autowired，Spring根本不知道你要注入哪個bean。這就需要@Qualifier(value =“beanName”)協作.

譬如,要從 com.javaedge.Article類型的bean中區分article1,article2:

@Qualifier(value="article1")
@Autowired
private Article firstArticle;

@Qualifier(value="article2")
@Autowired
private Article secondArticle;

2 優雅地使用@Autowired

啓動自動注入

<context:annotation-config />

放在應用程序上下文配置。可以使在遇到@Autowired註解時啓用依賴注入

• bean

// beans first
public class Comment {

    private String content;

    public void setContent(String content) {
        this.content = content;
    }

    public String getContent() {
        return this.content;
    }

}

// sample controller
@Controller
public class TestController {

    @Qualifier(value="comment1")
    @Autowired
    private Comment firstComment;

    @Qualifier(value="comment2")
    @Autowired
    private Comment secondComment;

    @RequestMapping(value = "/test", method = RequestMethod.GET)
    public String test() {
        System.out.println("1st comment text: "+firstComment.getText());
        System.out.println("2nd comment text: "+secondComment.getText());
        return "test";
    }

}

// no-Spring managed class
public class TestNoSpring {

    @Autowired
    private Comment comment;

    public void testComment(String content) {
        if (comment == null) {
            System.out.println("Comment's instance wasn't autowired because this class is not Spring-managed bean");
        } else {
            comment.setContent(content);
            System.out.println("Comment's content: "+comment.getContent());
        }
    }

}

• 配置

<bean name="comment1" class="com.sss.exchanger.Comment">
    <property name="content" value="Content of the 1st comment" />
</bean>

<bean name="comment2" class="com.sss.exchanger.Comment">
    <property name="content" value="Content of the 2nd comment" />
</bean>

打開http://localhost:8080/test來運行TestController
TestController的註解字段正確地自動注入，而TestNoSpring的註解字段並沒有注入進去

1st comment text: Content of the 1st comment
2nd comment text: Content of the 2nd comment
Comment's instance wasn't autowired because this class is not Spring-managed bean

哪裏不對 ？TestNoSpring類不由Spring所管理
這就是爲什麼Spring不能注入Comment實例的依賴

3 隱藏在@Autowired註解背後的祕密

Spring管理可用於整個應用程序的Java對象bean,我們不需要處理他們的生命週期(初始化，銷燬)。該任務由此容器來完成。
該上下文具有入口點，在Web應用程序中，是dispatcherservlet。
容器(也就是該上下文)會在它那裏被啓動並且所有的bean都會被注入

看<context:annotation-config />的定義

<xsd:element name="annotation-config">
		<xsd:annotation>
			<xsd:documentation><![CDATA[
	Activates various annotations to be detected in bean classes: Spring's @Required and
	@Autowired, as well as JSR 250's @PostConstruct, @PreDestroy and @Resource (if available),
	JAX-WS's @WebServiceRef (if available), EJB 3's @EJB (if available), and JPA's
	@PersistenceContext and @PersistenceUnit (if available). Alternatively, you may
	choose to activate the individual BeanPostProcessors for those annotations.
	Note: This tag does not activate processing of Spring's @Transactional or EJB 3's
	@TransactionAttribute annotation. Consider the use of the <tx:annotation-driven>
	tag for that purpose.
	See javadoc for org.springframework.context.annotation.AnnotationConfigApplicationContext
	for information on code-based alternatives to bootstrapping annotation-driven support.
			]]></xsd:documentation>
		</xsd:annotation>
	</xsd:element>

類內部的註解，如@Autowired、@Value、@Required、@Resource以及Web Serivce相關的註解，是容器對Bean對象實例化和依賴注入時，通過容器中註冊的Bean後置處理器處理這些註解的

所以配置了上面這個配置(<context:component-scan>假如有配置這個，那麼就可以省略<context:annotation-config />)後，將隱式地向Spring容器註冊AutowiredAnnotationBeanPostProcessor、CommonAnnotationBeanPostProcessor、RequiredAnnotationBeanPostProcessor、PersistenceAnnotationBeanPostProcessor以及這4個專門用於處理註解的Bean後置處理器。

當 Spring 容器啓動時，AutowiredAnnotationBeanPostProcessor 將掃描 Spring 容器中所有 Bean
當發現 Bean 中擁有@Autowired 註解時就找到和其匹配（默認按類型匹配）的 Bean
並注入到對應的地方中去。

4 源碼分析

通過org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor可以實現依賴自動注入
通過這個類來處理@Autowired @Value Spring
它也可以管理JSR-303的@Inject

• 在AutowiredAnnotationBeanPostProcessor構造函數中定義要處理的註解
  
  

之後,有幾種方法對@Autowired處理

第一個，private InjectionMetadata buildAutowiringMetadata(final Class<?> clazz)解析等待自動注入類的所有屬性。它通過分析所有字段和方法並初始化org.springframework.beans.factory.annotation.InjectionMetadata類的實例來實現。

private InjectionMetadata buildAutowiringMetadata(final Class<?> clazz) {
		LinkedList<InjectionMetadata.InjectedElement> elements = new LinkedList<>();
		Class<?> targetClass = clazz;
		do {
			final LinkedList<InjectionMetadata.InjectedElement> currElements = new LinkedList<>();
			//分析所有字段
			ReflectionUtils.doWithLocalFields(targetClass, field -> {
              //findAutowiredAnnotation(field)此方法後面會解釋
				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);
		//返回一個InjectionMetadata初始化的對象實例
		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類包含要注入的元素的列表

注入是通過Java的API Reflection (Field set(Object obj, Object value) 或Method invoke(Object obj，Object … args)方法完成的
此過程直接在AutowiredAnnotationBeanPostProcessor的方法中調用
public void processInjection(Object bean) throws BeanCreationException
它將所有可注入的bean檢索爲InjectionMetadata實例，並調用它們的inject()方法

public class InjectionMetadata {
  ...
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()) {
			boolean debug = logger.isDebugEnabled();
			for (InjectedElement element : elementsToIterate) {
				if (debug) {
					logger.debug("Processing injected element of bean '" + beanName + "': " + element);
				}
              	//看下面靜態內部類的方法
				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 {
                  	//具體的注入看此處咯
					Method method = (Method) this.member;
					ReflectionUtils.makeAccessible(method);
					method.invoke(target, getResourceToInject(target, requestingBeanName));
				}
				catch (InvocationTargetException ex) {
					throw ex.getTargetException();
				}
			}
		}
      ...
    }
}

findAutowiredAnnotation(AccessibleObject ao)

分析屬於一個字段或一個方法的所有註解來查找@Autowired註解。如果未找到@Autowired註解，則返回null，字段或方法也就視爲不可注入。