建立完物件之後, 接下來, 就應該想辦法對屬性進行注入了, 其中就包括 @Autowired 注入

但是在注入之前, 貌似 還沒有對 @Autowired 進行掃描和解析.

程式碼塊:

if (instanceWrapper == null) {
    /**
     * 建立 bean 例項，並將例項包裹在 BeanWrapper 實現類物件中返回。
     * createBeanInstance中包含三種建立 bean 例項的方式：
     *   1. 通過工廠方法建立 bean 例項
     *   2. 通過構造方法自動注入（autowire by constructor）的方式建立 bean 例項
     *   3. 通過無參構造方法方法建立 bean 例項
     *
     * 若 bean 的配置資訊中配置了 lookup-method 和 replace-method，則會使用 CGLIB 增強 bean 例項。
     
 
*/
    instanceWrapper = createBeanInstance(beanName, mbd, args);
}
final Object bean = instanceWrapper.getWrappedInstance();
Class<?> beanType = instanceWrapper.getWrappedClass();
if (beanType != NullBean.class) {
    mbd.resolvedTargetType = beanType;
}

// Allow post-processors to modify the merged bean definition.
 

synchronized (mbd.postProcessingLock) {
    if (!mbd.postProcessed) {
        try {
            //呼叫屬性合併後置處理器, 進行屬性合併
            //這裡會進行 一些註解 的掃描
            //CommonAnnotationBeanPostProcessor -> @PostConstruct @PreDestroy @Resource
            //AutowiredAnnotationBeanPostProcessor -> @Autowired @Value
 

            applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
        }
        catch (Throwable ex) {
            throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                    "Post-processing of merged bean definition failed", ex);
        }
        mbd.postProcessed = true;
    }
}

接著看裡面的內容

protected void applyMergedBeanDefinitionPostProcessors(RootBeanDefinition mbd, Class<?> beanType, String beanName) {
    for (BeanPostProcessor bp : getBeanPostProcessors()) {
        if (bp instanceof MergedBeanDefinitionPostProcessor) {
            MergedBeanDefinitionPostProcessor bdp = (MergedBeanDefinitionPostProcessor) bp;
            bdp.postProcessMergedBeanDefinition(mbd, beanType, beanName);
        }
    }
}

通過除錯, 可以獲取, 這裡滿足條件的, 有三個後置處理器, 按照呼叫先後順序為:

1.CommonAnnotationBeanPostProcessor

2.AutowiredAnnotationBeanPostProcessor

3.ApplicationListenerDetector

接下來看看裡面都幹了啥

CommonAnnotationBeanPostProcessor

這一步, 主要是掃描作用.

1. 掃描 @PostConstruct 和 @PreDestroy (這個很直觀的能看到, 其無參建構函式中, 出現了這兩個註解)

2. 掃描 @Resource , 這個需要到 findResourceMetadata 中, 才能看到

public CommonAnnotationBeanPostProcessor() {
    setOrder(Ordered.LOWEST_PRECEDENCE - 3);
    setInitAnnotationType(PostConstruct.class);
    setDestroyAnnotationType(PreDestroy.class);
    ignoreResourceType("javax.xml.ws.WebServiceContext");
}

/**
 * 1.遍歷掃描方法上標註了 @PostConstruct  和 @PreDestroy 註解的類
 * 2.遍歷掃描方法上標註了 @Resource 註解的類
 * @param beanDefinition
 * @param beanType
 * @param beanName
 */
@Override
public void postProcessMergedBeanDefinition(RootBeanDefinition beanDefinition, Class<?> beanType, String beanName) {
    super.postProcessMergedBeanDefinition(beanDefinition, beanType, beanName);
    InjectionMetadata metadata = findResourceMetadata(beanName, beanType, null);
    metadata.checkConfigMembers(beanDefinition);
}

1. 呼叫父類方法, 進行@PostConstruct 和 @PreDestroy 掃描

org.springframework.beans.factory.annotation.InitDestroyAnnotationBeanPostProcessor#postProcessMergedBeanDefinition

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

findLifecycleMetadata最終會呼叫org.springframework.beans.factory.annotation.InitDestroyAnnotationBeanPostProcessor#buildLifecycleMetadata 方法

private LifecycleMetadata buildLifecycleMetadata(final Class<?> clazz) {
    List<LifecycleElement> initMethods = new ArrayList<>();
    List<LifecycleElement> destroyMethods = new ArrayList<>();
    Class<?> targetClass = clazz;

    do {
        final List<LifecycleElement> currInitMethods = new ArrayList<>();
        final List<LifecycleElement> currDestroyMethods = new ArrayList<>();

        ReflectionUtils.doWithLocalMethods(targetClass, method -> {
            if (this.initAnnotationType != null && method.isAnnotationPresent(this.initAnnotationType)) {
                LifecycleElement element = new LifecycleElement(method);
                currInitMethods.add(element);
                if (logger.isTraceEnabled()) {
                    logger.trace("Found init method on class [" + clazz.getName() + "]: " + method);
                }
            }
            if (this.destroyAnnotationType != null && method.isAnnotationPresent(this.destroyAnnotationType)) {
                currDestroyMethods.add(new LifecycleElement(method));
                if (logger.isTraceEnabled()) {
                    logger.trace("Found destroy method on class [" + clazz.getName() + "]: " + method);
                }
            }
        });

        initMethods.addAll(0, currInitMethods);
        destroyMethods.addAll(currDestroyMethods);
        //當前類解析完後, 拿到父類繼續進行解析, 直到父類為 Object
        targetClass = targetClass.getSuperclass();
    }
    while (targetClass != null && targetClass != Object.class);

    return new LifecycleMetadata(clazz, initMethods, destroyMethods);
}

這裡就是遍歷類和其父類, 進行查詢的.

2.進行 @Resource 掃描

Resource 掃描, 是在本類中完成的.

org.springframework.context.annotation.CommonAnnotationBeanPostProcessor#findResourceMetadata

private InjectionMetadata findResourceMetadata(String beanName, final 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);
                }
                metadata = buildResourceMetadata(clazz);
                this.injectionMetadataCache.put(cacheKey, metadata);
            }
        }
    }
    return metadata;
}

這裡主要看 buildResourceMetadata( ) 方法.

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

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

        //掃描欄位
        ReflectionUtils.doWithLocalFields(targetClass, field -> {
            if (webServiceRefClass != null && field.isAnnotationPresent(webServiceRefClass)) {
                if (Modifier.isStatic(field.getModifiers())) {
                    throw new IllegalStateException("@WebServiceRef annotation is not supported on static fields");
                }
                currElements.add(new WebServiceRefElement(field, field, null));
            }
            else if (ejbRefClass != null && field.isAnnotationPresent(ejbRefClass)) {
                if (Modifier.isStatic(field.getModifiers())) {
                    throw new IllegalStateException("@EJB annotation is not supported on static fields");
                }
                currElements.add(new EjbRefElement(field, field, null));
            }
            else if (field.isAnnotationPresent(Resource.class)) {
                //靜態欄位上標註 @Resource , 會拋異常
                if (Modifier.isStatic(field.getModifiers())) {
                    throw new IllegalStateException("@Resource annotation is not supported on static fields");
                }
                if (!this.ignoredResourceTypes.contains(field.getType().getName())) {
                    currElements.add(new ResourceElement(field, field, null));
                }
            }
        });

        //掃描方法
        ReflectionUtils.doWithLocalMethods(targetClass, method -> {
            Method bridgedMethod = BridgeMethodResolver.findBridgedMethod(method);
            if (!BridgeMethodResolver.isVisibilityBridgeMethodPair(method, bridgedMethod)) {
                return;
            }
            if (method.equals(ClassUtils.getMostSpecificMethod(method, clazz))) {
                if (webServiceRefClass != null && bridgedMethod.isAnnotationPresent(webServiceRefClass)) {
                    if (Modifier.isStatic(method.getModifiers())) {
                        throw new IllegalStateException("@WebServiceRef annotation is not supported on static methods");
                    }
                    if (method.getParameterCount() != 1) {
                        throw new IllegalStateException("@WebServiceRef annotation requires a single-arg method: " + method);
                    }
                    PropertyDescriptor pd = BeanUtils.findPropertyForMethod(bridgedMethod, clazz);
                    currElements.add(new WebServiceRefElement(method, bridgedMethod, pd));
                }
                else if (ejbRefClass != null && bridgedMethod.isAnnotationPresent(ejbRefClass)) {
                    if (Modifier.isStatic(method.getModifiers())) {
                        throw new IllegalStateException("@EJB annotation is not supported on static methods");
                    }
                    if (method.getParameterCount() != 1) {
                        throw new IllegalStateException("@EJB annotation requires a single-arg method: " + method);
                    }
                    PropertyDescriptor pd = BeanUtils.findPropertyForMethod(bridgedMethod, clazz);
                    currElements.add(new EjbRefElement(method, bridgedMethod, pd));
                }
                else if (bridgedMethod.isAnnotationPresent(Resource.class)) {
                    if (Modifier.isStatic(method.getModifiers())) {
                        throw new IllegalStateException("@Resource annotation is not supported on static methods");
                    }
                    Class<?>[] paramTypes = method.getParameterTypes();
                    //如果目標方法, 沒有引數, 或者有多個引數, 則丟擲異常
                    if (paramTypes.length != 1) {
                        throw new IllegalStateException("@Resource annotation requires a single-arg method: " + method);
                    }
                    if (!this.ignoredResourceTypes.contains(paramTypes[0].getName())) {
                        //這裡拿到方法名, 會去跟類中的屬性進行匹配
                        //匹配的規則是: method.equals(pd.getReadMethod()) || method.equals(pd.getWriteMethod())
                        //這個 readMethod 就是 getXXX 或 isXXX, writeMethod 就是 setXXX
                        //如果匹配不上, pd = null
                        PropertyDescriptor pd = BeanUtils.findPropertyForMethod(bridgedMethod, clazz);
                        currElements.add(new ResourceElement(method, bridgedMethod, pd));
                    }
                }
            }
        });

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

    return new InjectionMetadata(clazz, elements);
}

注意到, 這裡只進行了掃描轉換工作, 並沒有進行屬性注入工作.

AutowiredAnnotationBeanPostProcessor

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.trace("JSR-330 'javax.inject.Inject' annotation found and supported for autowiring");
    }
    catch (ClassNotFoundException ex) {
        // JSR-330 API not available - simply skip.
    }
}

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

從這段程式碼, 可以猜測, 自動注入的型別, 是個集合,這裡有兩個

1. @Autowired

2. @Value

findAutowiringMetadata() 最終會呼叫本類方法:

org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor#buildAutowiringMetadata

private InjectionMetadata buildAutowiringMetadata(final Class<?> clazz) {
    List<InjectionMetadata.InjectedElement> elements = new ArrayList<>();
    //需要處理的目標類
    Class<?> targetClass = clazz;

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

        // 通過反射獲取該類所有的欄位，並遍歷每一個欄位，通過方法 findAutowiredAnnotation 遍歷每一個欄位的所用註解，
        // 如果用autowired修飾了，則返回 auotowired 相關屬性
        ReflectionUtils.doWithLocalFields(targetClass, field -> {
            AnnotationAttributes ann = findAutowiredAnnotation(field);
            if (ann != null) {
                // 校驗autowired註解是否用在了static方法上
                if (Modifier.isStatic(field.getModifiers())) {
                    if (logger.isInfoEnabled()) {
                        logger.info("Autowired annotation is not supported on static fields: " + field);
                    }
                    return;
                }
                boolean required = determineRequiredStatus(ann);
                currElements.add(new AutowiredFieldElement(field, required));
            }
        });

        //通過反射處理類的method
        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.isInfoEnabled()) {
                        logger.info("Autowired annotation is not supported on static methods: " + method);
                    }
                    return;
                }
                if (method.getParameterCount() == 0) {
                    if (logger.isInfoEnabled()) {
                        logger.info("Autowired annotation should only be used on methods with parameters: " +
                                method);
                    }
                }
                boolean required = determineRequiredStatus(ann);
                //這裡也會進行屬性的匹配, 按照規則 : method.equals(pd.getReadMethod()) || method.equals(pd.getWriteMethod())
                //如果匹配不上, pd = null
                PropertyDescriptor pd = BeanUtils.findPropertyForMethod(bridgedMethod, clazz);
                currElements.add(new AutowiredMethodElement(method, required, pd));
            }
        });

        //用@Autowired修飾的註解可能不止一個，因此都加在 elements 這個容器裡面，一起處理
        elements.addAll(0, currElements);
        targetClass = targetClass.getSuperclass();
    }
    while (targetClass != null && targetClass != Object.class);

    return new InjectionMetadata(clazz, elements);
}

這裡也只是進行了掃描, 沒有進行屬性注入工作.

ApplicationListenerDetector

@Override
public void postProcessMergedBeanDefinition(RootBeanDefinition beanDefinition, Class<?> beanType, String beanName) {
    this.singletonNames.put(beanName, beanDefinition.isSingleton());
}

這個類也是挺能湊熱鬧的. 這裡沒有進行任何掃描工作. 只是記錄了容器中的 bean 是否是單例模式.

這裡記錄這個標誌, 是為了後面用的. 對監聽器進行過濾用的.