如何自己实现一个自动注入的注解

• 首先，需要了解到的是。Spring Bean 的生命周期
• 在生命周期中。注入bean属性的位置是在以下代码：populateBean 位置中
• 那么我们在项目中使用注解 产生一个bean的时候必定会经过以下代码进行一个bean的创建流程

/**省略代码**/
// 开始初始化 bean 实例对象
Object exposedObject = bean;
try {
    // <5> 对 bean 进行填充，将各个属性值注入，其中，可能存在依赖于其他 bean 的属性
    populateBean(beanName, mbd, instanceWrapper);
    // <6> 调用初始化方法
    exposedObject = initializeBean(beanName, exposedObject, mbd);
}
catch (Throwable ex) {
    if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {
        throw (BeanCreationException) ex;
    }
    else {
        throw new BeanCreationException(
                mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);
    }
}
/**省略代码**/

• 在生命周期中 populateBean 进行填充bean数据。把其他依赖引入进来
• BeanPostProcessor 是一个bean创建时候的一个钩子。
• 以下代码 是循环调用实现了 BeanPostProcessor 子类 InstantiationAwareBeanPostProcessor#postProcessProperties 方法
• Spring 在以下代码中有自动注入的拓展点。 关键就是实现 InstantiationAwareBeanPostProcessor#postProcessProperties

/**省略代码**/
for (BeanPostProcessor bp : getBeanPostProcessors()) {
    if (bp instanceof InstantiationAwareBeanPostProcessor) {
        InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
        // 对所有需要依赖检查的属性进行后处理
        PropertyValues pvsToUse = ibp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);
        if (pvsToUse == null) {
            // 从 bw 对象中提取 PropertyDescriptor 结果集
            // PropertyDescriptor：可以通过一对存取方法提取一个属性
            if (filteredPds == null) {
                filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
            }
            pvsToUse = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
            if (pvsToUse == null) {
                return;
            }
        }
        pvs = pvsToUse;
    }
}
/**省略代码**/

• 我们展开来讲一下 @Autowired 的实现是怎么样的吧：
• 实现类为 AutowiredAnnotationBeanPostProcessor.java
• 从上面可以得知，填充bean的时候。时调用了方法 ibp.postProcessPropertyValues()
• 那么 AutowiredAnnotationBeanPostProcessor#postProcessPropertyValues() 则会被调用
• 调用 findAutowiringMetadata 获取 class 以及父类 带有 @Autowired 或者 @Value 的属性或者方法：

/**省略代码**/
public PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName) {
    // 获取所有可以注入的元数据
    InjectionMetadata metadata = findAutowiringMetadata(beanName, bean.getClass(), pvs);
    try {
        // 注入数据
        metadata.inject(bean, beanName, pvs);
    }
    catch (BeanCreationException ex) {
        throw ex;
    }
    catch (Throwable ex) {
        throw new BeanCreationException(beanName, "Injection of autowired dependencies failed", ex);
    }
    return pvs;
}
private InjectionMetadata findAutowiringMetadata(String beanName, Class<?> clazz, @Nullable PropertyValues pvs) {
    // 缓存名字获取
    String cacheKey = (StringUtils.hasLength(beanName) ? beanName : clazz.getName());
    InjectionMetadata metadata = this.injectionMetadataCache.get(cacheKey);
    // 获取是否已经读取过这个 class 类的 InjectionMetadata 有的话直接从缓存中获取出去
    if (InjectionMetadata.needsRefresh(metadata, clazz)) {
        synchronized (this.injectionMetadataCache) {
            // 双重检查
            metadata = this.injectionMetadataCache.get(cacheKey);
            if (InjectionMetadata.needsRefresh(metadata, clazz)) {
                if (metadata != null) {
                    metadata.clear(pvs);
                }
                // 构建自动注入的元数据
                metadata = buildAutowiringMetadata(clazz);
                this.injectionMetadataCache.put(cacheKey, metadata);
            }
        }
    }
    return metadata;
}
private InjectionMetadata buildAutowiringMetadata(final Class<?> clazz) {
    if (!AnnotationUtils.isCandidateClass(clazz, this.autowiredAnnotationTypes)) {
        return InjectionMetadata.EMPTY;
    }

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

    do {
        final List<InjectionMetadata.InjectedElement> currElements = new ArrayList<>();
        // 循环 targetClass 的所有 field 并执 FieldCallback 逻辑 （函数式编程接口，传入的是一个执行函数）
        ReflectionUtils.doWithLocalFields(targetClass, field -> {
            // 获得字段上面的 Annotation 注解
            MergedAnnotation<?> ann = findAutowiredAnnotation(field);
            if (ann != null) {
                // 判断是否为静态属性 如果是，则不进行注入
                if (Modifier.isStatic(field.getModifiers())) {
                    if (logger.isInfoEnabled()) {
                        logger.info("Autowired annotation is not supported on static fields: " + field);
                    }
                    return;
                }
                // 注解是否为必须依赖项
                boolean required = determineRequiredStatus(ann);
                currElements.add(new AutowiredFieldElement(field, required));
            }
        });

        // 循环 targetClass 的所有 Method 并执 MethodCallback 逻辑 （函数式编程接口，传入的是一个执行函数）
        ReflectionUtils.doWithLocalMethods(targetClass, method -> {
            Method bridgedMethod = BridgeMethodResolver.findBridgedMethod(method);
            if (!BridgeMethodResolver.isVisibilityBridgeMethodPair(method, bridgedMethod)) {
                return;
            }
            MergedAnnotation<?> ann = findAutowiredAnnotation(bridgedMethod);
            if (ann != null && method.equals(ClassUtils.getMostSpecificMethod(method, clazz))) {
                // 判断是否为静态方法 如果是，则不进行注入
                if (Modifier.isStatic(method.getModifiers())) {
                    if (logger.isInfoEnabled()) {
                        logger.info("Autowired annotation is not supported on static methods: " + method);
                    }
                    return;
                }
                // 判断静态方法参数是否为0
                if (method.getParameterCount() == 0) {
                    if (logger.isInfoEnabled()) {
                        logger.info("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));
            }
        });

        //数据加到数组最前方 父类的的注解都放在靠前的位置
        elements.addAll(0, currElements);
        // 如果有父类则设置 targetClass 为父类。 如此循环
        targetClass = targetClass.getSuperclass();
    }
    while (targetClass != null && targetClass != Object.class);

    return InjectionMetadata.forElements(elements, clazz);
}


/**省略代码**/

• 真正注入数据的是 metadata.inject(bean, beanName, pvs);
• 调用的是 InjectionMetadata#inject 方法

public void inject(Object target, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
    Collection<InjectedElement> checkedElements = this.checkedElements;
    // 带有注解的方法或者属性列表
    Collection<InjectedElement> elementsToIterate =
            (checkedElements != null ? checkedElements : this.injectedElements);
    if (!elementsToIterate.isEmpty()) {
        for (InjectedElement element : elementsToIterate) {
            element.inject(target, beanName, pvs);
        }
    }
}

• 循环调用之前加入的带有注解的方法或者属性构建的对象 AutowiredFieldElement#inject， AutowiredMethodElement#inject

    /**
	 * 属性上有注解 构建的处理对象
	 */
	private class AutowiredFieldElement extends InjectionMetadata.InjectedElement {

		private final boolean required;

		private volatile boolean cached;

		@Nullable
		private volatile Object cachedFieldValue;

		public AutowiredFieldElement(Field field, boolean required) {
			super(field, null);
			this.required = required;
		}

		@Override
		protected void inject(Object bean, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
			// 获取属性名
			Field field = (Field) this.member;
			Object value;
			// Bean 不是单例的话，会重复进入注入的这个操作，
			if (this.cached) {
				try {
					value = resolvedCachedArgument(beanName, this.cachedFieldValue);
				}
				catch (NoSuchBeanDefinitionException ex) {
					// Unexpected removal of target bean for cached argument -> re-resolve
					value = resolveFieldValue(field, bean, beanName);
				}
			}
			else {
				// 首次创建的时候进入该方法
				value = resolveFieldValue(field, bean, beanName);
			}
			if (value != null) {
				// 属性如果不为public的话，则设置为可访问
				ReflectionUtils.makeAccessible(field);
				field.set(bean, value);
			}
		}

		@Nullable
		private Object resolveFieldValue(Field field, Object bean, @Nullable String beanName) {
			// 构建DependencyDescriptor对象
			DependencyDescriptor desc = new DependencyDescriptor(field, this.required);
			desc.setContainingClass(bean.getClass());
			// 注入bean的数量。 有可能字段上是一个List
			Set<String> autowiredBeanNames = new LinkedHashSet<>(1);
			Assert.state(beanFactory != null, "No BeanFactory available");
			// 获得beanFactory类型转换类
			TypeConverter typeConverter = beanFactory.getTypeConverter();
			Object value;
			try {
				// 查找依赖关系
				value = beanFactory.resolveDependency(desc, beanName, autowiredBeanNames, typeConverter);
			}
			catch (BeansException ex) {
				throw new UnsatisfiedDependencyException(null, beanName, new InjectionPoint(field), ex);
			}
			synchronized (this) {
				if (!this.cached) {
					Object cachedFieldValue = null;
					if (value != null || this.required) {
						cachedFieldValue = desc;
						// 填入依赖关系
						registerDependentBeans(beanName, autowiredBeanNames);
						// 判断如果注入依赖是只有一个
						if (autowiredBeanNames.size() == 1) {
							String autowiredBeanName = autowiredBeanNames.iterator().next();
							if (beanFactory.containsBean(autowiredBeanName) &&
									beanFactory.isTypeMatch(autowiredBeanName, field.getType())) {
								cachedFieldValue = new ShortcutDependencyDescriptor(
										desc, autowiredBeanName, field.getType());
							}
						}
					}
					this.cachedFieldValue = cachedFieldValue;
					this.cached = true;
				}
			}
			return value;
		}
	}


     /**
	 * 方法上有注解 构建的处理对象
	 */
	private class AutowiredMethodElement extends InjectionMetadata.InjectedElement {

		private final boolean required;

		private volatile boolean cached;

		@Nullable
		private volatile Object[] cachedMethodArguments;

		public AutowiredMethodElement(Method method, boolean required, @Nullable PropertyDescriptor pd) {
			super(method, pd);
			this.required = required;
		}

		@Override
		protected void inject(Object bean, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
			// 检查属性是不会在之前就已经注入过了。如果主如果则不进行二次覆盖
			if (checkPropertySkipping(pvs)) {
				return;
			}
			Method method = (Method) this.member;
			Object[] arguments;
			if (this.cached) {
				try {
					arguments = resolveCachedArguments(beanName);
				}
				catch (NoSuchBeanDefinitionException ex) {
					// Unexpected removal of target bean for cached argument -> re-resolve
					arguments = resolveMethodArguments(method, bean, beanName);
				}
			}
			else {
				// 首次创建的时候进入该方法
				arguments = resolveMethodArguments(method, bean, beanName);
			}
			if (arguments != null) {
				try {
					// 属性如果不为public的话，则设置为可访问
					ReflectionUtils.makeAccessible(method);
					// 调用方法 并传入参数
					method.invoke(bean, arguments);
				}
				catch (InvocationTargetException ex) {
					throw ex.getTargetException();
				}
			}
		}

		@Nullable
		private Object[] resolveCachedArguments(@Nullable String beanName) {
			Object[] cachedMethodArguments = this.cachedMethodArguments;
			if (cachedMethodArguments == null) {
				return null;
			}
			Object[] arguments = new Object[cachedMethodArguments.length];
			for (int i = 0; i < arguments.length; i++) {
				arguments[i] = resolvedCachedArgument(beanName, cachedMethodArguments[i]);
			}
			return arguments;
		}

		@Nullable
		private Object[] resolveMethodArguments(Method method, Object bean, @Nullable String beanName) {
			// 获取方法上有几个参数
			int argumentCount = method.getParameterCount();
			Object[] arguments = new Object[argumentCount];
			DependencyDescriptor[] descriptors = new DependencyDescriptor[argumentCount];
			Set<String> autowiredBeans = new LinkedHashSet<>(argumentCount);
			Assert.state(beanFactory != null, "No BeanFactory available");
			TypeConverter typeConverter = beanFactory.getTypeConverter();
			for (int i = 0; i < arguments.length; i++) {
				// 方法参数，从方法参数中取出 i 构造 MethodParameter 对象
				MethodParameter methodParam = new MethodParameter(method, i);
				DependencyDescriptor currDesc = new DependencyDescriptor(methodParam, this.required);
				currDesc.setContainingClass(bean.getClass());
				descriptors[i] = currDesc;
				try {
					// 获取方法中 i 参数的内容
					Object arg = beanFactory.resolveDependency(currDesc, beanName, autowiredBeans, typeConverter);
					if (arg == null && !this.required) {
						arguments = null;
						break;
					}
					arguments[i] = arg;
				}
				catch (BeansException ex) {
					throw new UnsatisfiedDependencyException(null, beanName, new InjectionPoint(methodParam), ex);
				}
			}
			synchronized (this) {
				if (!this.cached) {
					if (arguments != null) {
						DependencyDescriptor[] cachedMethodArguments = Arrays.copyOf(descriptors, arguments.length);
						registerDependentBeans(beanName, autowiredBeans);
						if (autowiredBeans.size() == argumentCount) {
							Iterator<String> it = autowiredBeans.iterator();
							Class<?>[] paramTypes = method.getParameterTypes();
							for (int i = 0; i < paramTypes.length; i++) {
								String autowiredBeanName = it.next();
								if (beanFactory.containsBean(autowiredBeanName) &&
										beanFactory.isTypeMatch(autowiredBeanName, paramTypes[i])) {
									cachedMethodArguments[i] = new ShortcutDependencyDescriptor(
											descriptors[i], autowiredBeanName, paramTypes[i]);
								}
							}
						}
						this.cachedMethodArguments = cachedMethodArguments;
					}
					else {
						this.cachedMethodArguments = null;
					}
					this.cached = true;
				}
			}
			return arguments;
		}
	}

• 以上就是 @Autowired 实现的完整流程。 可概括为：
  populateBean -> AutowiredAnnotationBeanPostProcessor#postProcessPropertyValues
  -> 获取带有注解的属性和方法构建 AutowiredFieldElement， AutowiredMethodElement 对象，然后循环调用了 inject 进行属性调用

自定义自动注入注解。

• 编写抽象类代码：

package com.yunlongn.common.core.autowired;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.springframework.beans.BeansException;
import org.springframework.beans.PropertyValues;
import org.springframework.beans.factory.*;
import org.springframework.beans.factory.annotation.InjectionMetadata;
import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
import org.springframework.beans.factory.config.InstantiationAwareBeanPostProcessor;
import org.springframework.core.annotation.AnnotationUtils;
import org.springframework.core.annotation.MergedAnnotation;
import org.springframework.core.annotation.MergedAnnotations;
import org.springframework.lang.Nullable;
import org.springframework.util.ReflectionUtils;
import org.springframework.util.StringUtils;

import java.beans.PropertyDescriptor;
import java.lang.annotation.Annotation;
import java.lang.reflect.*;
import java.util.*;
import java.util.concurrent.ConcurrentHashMap;

/**
 * 抽象的自动注入方法
 * @author yunlgongn
 */
public abstract class AbstractAnnotationBeanPostProcessor implements InstantiationAwareBeanPostProcessor {

    private final Set<Class<? extends Annotation>> annotationTypes = new LinkedHashSet<>(4);

    private final Map<String, InjectionMetadata> injectionMetadataCache = new ConcurrentHashMap<>(256);

    protected final Log logger = LogFactory.getLog(getClass());

    /**
     * 处理的 annotationType 对象
     * @return Annotation 自定义注解
     */
    public abstract Class<? extends Annotation> annotationType();

    AbstractAnnotationBeanPostProcessor () {
        Class<? extends Annotation> annotation = this.annotationType();
        annotationTypes.add(annotation);
    }

    @Override
    public Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName) throws BeansException {
        return InstantiationAwareBeanPostProcessor.super.postProcessBeforeInstantiation(beanClass, beanName);
    }

    @Override
    public boolean postProcessAfterInstantiation(Object bean, String beanName) throws BeansException {
        return InstantiationAwareBeanPostProcessor.super.postProcessAfterInstantiation(bean, beanName);
    }

    @Override
    public PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName) {
        InjectionMetadata metadata = findAbstractMetadata(beanName, bean.getClass(), pvs);
        try {
            // 注入数据
            metadata.inject(bean, beanName, pvs);
        }
        catch (BeanCreationException ex) {
            throw ex;
        }
        catch (Throwable ex) {
            throw new BeanCreationException(beanName, "Injection of autowired dependencies failed", ex);
        }
        return pvs;
    }

    private InjectionMetadata findAbstractMetadata(String beanName, Class<?> clazz, @Nullable PropertyValues pvs) {
        String cacheKey = (StringUtils.hasLength(beanName) ? beanName : clazz.getName());
        InjectionMetadata metadata = this.injectionMetadataCache.get(cacheKey);
        // 获取是否已经读取过这个 class 类的 InjectionMetadata 有的话直接从缓存中获取出去
        if (InjectionMetadata.needsRefresh(metadata, clazz)) {
            synchronized (this.injectionMetadataCache) {
                // 双重检查
                metadata = this.injectionMetadataCache.get(cacheKey);
                if (InjectionMetadata.needsRefresh(metadata, clazz)) {
                    if (metadata != null) {
                        metadata.clear(pvs);
                    }
                    metadata = buildAbstractMetadata(clazz);
                    this.injectionMetadataCache.put(cacheKey, metadata);
                }
            }
        }
        return metadata;
    }

    private InjectionMetadata buildAbstractMetadata(final Class<?> clazz) {
        if (!AnnotationUtils.isCandidateClass(clazz, this.annotationTypes)) {
            return InjectionMetadata.EMPTY;
        }

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

        do {
            final List<InjectionMetadata.InjectedElement> currElements = new ArrayList<>();
            // 循环 targetClass 的所有 field 并执 FieldCallback 逻辑 （函数式编程接口，传入的是一个执行函数）
            ReflectionUtils.doWithLocalFields(targetClass, field -> {
                // 获得字段上面的 Annotation 注解
                MergedAnnotation<?> ann = findAbstractAnnotation(field);
                if (ann != null) {
                    // 判断是否为静态属性 如果是，则不进行注入
                    if (Modifier.isStatic(field.getModifiers())) {
                        if (logger.isInfoEnabled()) {
                            logger.info("Abstract annotation is not supported on static fields: " + field);
                        }
                        return;
                    }
                    currElements.add(new AbstractFieldElement(field, ann));
                }
            });
            //数据加到数组最前方 父类的的注解都放在靠前的位置
            elements.addAll(0, currElements);
            targetClass = targetClass.getSuperclass();
        }
        while (targetClass != null && targetClass != Object.class);

        return InjectionMetadata.forElements(elements, clazz);
    }

    @Nullable
    private MergedAnnotation<?> findAbstractAnnotation(AccessibleObject ao) {
        // 将指定方法上的注解合并成一个注解
        MergedAnnotations annotations = MergedAnnotations.from(ao);
        // 循环要扫描的注解 annotationTypes 那个在前面就认哪个
        for (Class<? extends Annotation> type : this.annotationTypes) {
            MergedAnnotation<?> annotation = annotations.get(type);
            if (annotation.isPresent()) {
                return annotation;
            }
        }
        return null;
    }

    @Deprecated
    @Override
    public PropertyValues postProcessPropertyValues(
            PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) {

        return postProcessProperties(pvs, bean, beanName);
    }


    @Override
    public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
        return InstantiationAwareBeanPostProcessor.super.postProcessBeforeInitialization(bean, beanName);
    }

    @Override
    public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
        return InstantiationAwareBeanPostProcessor.super.postProcessAfterInitialization(bean, beanName);
    }

    /**
     * 实现交给子类进行实现 将注解透传出去
     * @param mergedAnnotation 属性上的注解
     * @param bean bean实例
     * @param beanName bean的名字
     * @param field 字段
     * @param autowiredFieldElement 注解
     * @return 注入对象
     * @throws Exception
     */
    protected abstract Object getInjectedObject(MergedAnnotation<?> mergedAnnotation, Object bean, String beanName, Field field,
                                                AbstractFieldElement autowiredFieldElement) throws Exception;


    public class AbstractFieldElement extends InjectionMetadata.InjectedElement {

        private final MergedAnnotation<?> mergedAnnotation;

        public AbstractFieldElement(Field field, MergedAnnotation<?> mergedAnnotation) {
            super(field, null);
            this.mergedAnnotation = mergedAnnotation;
        }

        @Override
        protected void inject(Object bean, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
            Field field = (Field) this.member;
            Object value = getInjectedObject(this.mergedAnnotation, bean, beanName, field, this);
            if (value != null) {
                ReflectionUtils.makeAccessible(field);
                field.set(bean, value);
            }
        }

    }

}

• 抽象类实现，实现一个 @RedisAutowired 自定义注入注解

package com.yunlongn.common.core.autowired;

import org.springframework.beans.BeansException;
import org.springframework.beans.factory.BeanFactory;
import org.springframework.beans.factory.BeanFactoryAware;
import org.springframework.core.annotation.MergedAnnotation;

import java.lang.annotation.Annotation;
import java.lang.reflect.Field;

public class RedisAutowiredBeanPostProcessor extends AbstractAnnotationBeanPostProcessor implements BeanFactoryAware {

    private BeanFactory beanFactory = null;

    @Override
    public Class<? extends Annotation> annotationType() {
        return RedisAutowired.class;
    }

    @Override
    protected Object getInjectedObject(MergedAnnotation<?> mergedAnnotation, Object bean, String beanName, Field field,
                                       AbstractAnnotationBeanPostProcessor.AbstractFieldElement autowiredFieldElement) throws Exception {
        // 从value中获取bean的名字。 或者通过一些其他的逻辑获取名字
        String s = mergedAnnotation.getString("value");
        return beanFactory.getBean(s);
    }

    @Override
    public void setBeanFactory(BeanFactory beanFactory) throws BeansException {
        this.beanFactory = beanFactory;
    }
}

• 在 Application 引入以上 @Import(RedisAutowiredBeanPostProcessor.class) 那么 @RedisAutowired 注解就生效了

    @RedisAutowired("db102")
    private XXXService service;

• 使用注解如上