Spring5源码，@Autowired

• 一、@Autowired所具有的功能
• 二、在Spring中如何使用@Autowired
• 三、@Autowired注解背后的工作原理

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一、@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。

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.migo.Comment">
    <property name="text" value="Content of the 1st comment" />
</bean>

<bean name="comment2" class="com.migo.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.migo.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;

二、在Spring中如何使用@Autowired

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)

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:

<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

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。

三、@Autowired注解背后的工作原理

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。

<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;
    }

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