Bean后置处理器

创建完对象之后, 接下来, 就应该想办法对属性进行注入了, 其中就包括 @Autowired 注入

但是在注入之前, 貌似 还没有对 @Autowired 进行扫描和解析.

代码块:

if (instanceWrapper == null) {
    /**
     * 创建 bean 实例，并将实例包裹在 BeanWrapper 实现类对象中返回。
     * createBeanInstance中包含三种创建 bean 实例的方式：
     *   1. 通过工厂方法创建 bean 实例
     *   2. 通过构造方法自动注入（autowire by constructor）的方式创建 bean 实例
     *   3. 通过无参构造方法方法创建 bean 实例
     *
     * 若 bean 的配置信息中配置了 lookup-method 和 replace-method，则会使用 CGLIB 增强 bean 实例。
     */
    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 {
            //调用属性合并后置处理器, 进行属性合并
            //这里会进行 一些注解 的扫描
            //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;
    }
}

接着看里面的内容

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);
        }
    }
}

通过调试, 可以获取, 这里满足条件的, 有三个后置处理器, 按照调用先后顺序为: 

1.CommonAnnotationBeanPostProcessor

2.AutowiredAnnotationBeanPostProcessor

3.ApplicationListenerDetector

接下来看看里面都干了啥

CommonAnnotationBeanPostProcessor

这一步, 主要是扫描作用. 

1. 扫描 @PostConstruct 和 @PreDestroy (这个很直观的能看到, 其无参构造函数中, 出现了这两个注解)

2. 扫描 @Resource , 这个需要到 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.遍历扫描方法上标注了 @PostConstruct  和 @PreDestroy 注解的类
 * 2.遍历扫描方法上标注了 @Resource 注解的类
 * @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. 调用父类方法, 进行@PostConstruct 和 @PreDestroy 扫描

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最终会调用 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);
        //当前类解析完后, 拿到父类继续进行解析, 直到父类为 Object
        targetClass = targetClass.getSuperclass();
    }
    while (targetClass != null && targetClass != Object.class);

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

这里就是遍历类和其父类, 进行查找的.

2. 进行 @Resource 扫描

Resource 扫描, 是在本类中完成的.

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;
}

这里主要看 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<>();

        //扫描字段
        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)) {
                //静态字段上标注 @Resource , 会抛异常
                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));
                }
            }
        });

        //扫描方法
        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();
                    //如果目标方法, 没有参数, 或者有多个参数, 则抛出异常
                    if (paramTypes.length != 1) {
                        throw new IllegalStateException("@Resource annotation requires a single-arg method: " + method);
                    }
                    if (!this.ignoredResourceTypes.contains(paramTypes[0].getName())) {
                        //这里拿到方法名, 会去跟类中的属性进行匹配
                        //匹配的规则是: method.equals(pd.getReadMethod()) || method.equals(pd.getWriteMethod())
                        //这个 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, 这里只进行了扫描转换工作, 并没有进行属性注入工作.

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);
}

从这段代码, 可以猜测, 自动注入的类型, 是个集合, 这里有两个

1. @Autowired

2. @Value

findAutowiringMetadata() 最终会调用本类方法: 

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

private InjectionMetadata buildAutowiringMetadata(final Class<?> clazz) {
    List<InjectionMetadata.InjectedElement> elements = new ArrayList<>();
    //需要处理的目标类
    Class<?> targetClass = clazz;

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

        // 通过反射获取该类所有的字段，并遍历每一个字段，通过方法 findAutowiredAnnotation 遍历每一个字段的所用注解，
        // 如果用autowired修饰了，则返回 auotowired 相关属性
        ReflectionUtils.doWithLocalFields(targetClass, field -> {
            AnnotationAttributes ann = findAutowiredAnnotation(field);
            if (ann != null) {
                // 校验autowired注解是否用在了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));
            }
        });

        //通过反射处理类的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);
                //这里也会进行属性的匹配, 按照规则 : 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修饰的注解可能不止一个，因此都加在 elements 这个容器里面，一起处理
        elements.addAll(0, currElements);
        targetClass = targetClass.getSuperclass();
    }
    while (targetClass != null && targetClass != Object.class);

    return new InjectionMetadata(clazz, elements);
}

这里也只是进行了扫描, 没有进行属性注入工作.

ApplicationListenerDetector

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

这个类也是挺能凑热闹的. 这里没有进行任何扫描工作. 只是记录了容器中的 bean 是否是单例模式.

这里记录这个标志, 是为了后面用的. 对监听器进行过滤用的.