Spring5源码解析-@Autowired

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2017-08-25
原文链接:zhuanlan.zhihu.com
你有没有思考过Spring中的@Autowired注解?通常用于方便依赖注入,而隐藏在这个过程之后的机制到底是怎样,将在本篇中进行讲述。

@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)。
  • 类型:解析过程基于bean的类型。

在某些情况下,@Autowired应该通过@Qualifier注解协作注入。例如下面几个是相同类型的bean:

<bean name="comment1" class="com.waitingforcode.Comment">
    <property name="text" value="Content of the 1st comment" />
</bean>
 
<bean name="comment2" class="com.waitingforcode.Comment">
    <property name="text" value="Content of the 2nd comment" />
</bean>

上面这种情况,假如只是一个简单的@Autowired,Spring根本不知道你要注入哪个bean。这就是为什么我们要使用@Qualifier(value =“beanName”)这个注解。在我们的例子中,要从 com.waitingforcode.Comment这个类型的bean中区分comment1,comment2,我们可以写下面的代码:

@Qualifier(value="comment1")
@Autowired
private Comment firstComment;
 
@Qualifier(value="comment2")
@Autowired
private Comment secondComment;

在Spring中如何使用@Autowired

正如前面部分所看到的,我们知道了在Spring中实现@Autowired的不同方法。在这一部分中,我们将使用XML配置的方式激活@Autowired注解来自动注入。然后,我们将编写一个简单的类并配置一些bean。最后,我们将分别在另外两个类中使用它们:由@Controller注解的控件和不由Spring所管理的类。(为什么用XML配置来做例子,我觉得这样更直观,其实XML和使用注解没多少区别,都是往容器里添加一些bean和组织下彼此之间的依赖而已,不必要非要拘泥于一种形式,哪种顺手用哪种,不过Springboot自定义的这些还是推荐使用注解了)

我们从启动注解的自动注入开始:

<context:annotation-config />

你必须将上面这个放在应用程序上下文配置中。它可以使在遇到@Autowired注解时启用依赖注入。

现在,我们来编写和配置我们的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());
        }
    }
     
}

XML配置(在前面部分已经看到过):

<bean name="comment1" class="com.specimen.exchanger.Comment">
    <property name="content" value="Content of the 1st comment" />
</bean>
 
<bean name="comment2" class="com.specimen.exchanger.Comment">
    <property name="content" value="Content of the 2nd comment" />
</bean>

现在,我们打开http://localhost:8080/test来运行TestController。如预期的那样,TestController的注解字段正确地自动注入,而TestNoSpring的注解字段并没有注入进去:

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

哪里不对 ?TestNoSpring类不由Spring所管理。这就是为什么Spring不能注入Comment实例的依赖。我们将在下一部分中解释这个概念。

@Autowired注解背后的工作原理?

在讨论代码细节之前,我们再来了解下基础知识。Spring管理可用于整个应用程序的Java对象bean。他们所在的Spring容器,被称为应用程序上下文。这意味着我们不需要处理他们的生命周期(初始化,销毁)。该任务由此容器来完成。另外,该上下文具有入口点,在Web应用程序中,是dispatcher servlet。容器(也就是该上下文)会在它那里被启动并且所有的bean都会被注入。

说的再清楚点,请看<context:annotation-config />的定义:

<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>

可以看出 : 类内部的注解,如:@Autowired、@Value、@Required、@Resource以及EJB和WebSerivce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后置处理器处理这些注解的。

所以配置了上面这个配置(<context:component-scan>假如有配置这个,那么就可以省略<context:annotation-config />)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。 源码分析如下:

通过org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor可以实现依赖自动注入。通过这个类来处理@Autowired和@Value这俩Spring注解。它也可以管理JSR-303的@Inject注解(如果可用的话)。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.
	 * <p>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<? extends Annotation>)
					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。

第一个,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<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<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<? extends Annotation> 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。

原文: Spring5源码解析-@Autowired
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