如何自己實(shí)現(xiàn)一個(gè)自動(dòng)注入的注解

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• 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屬性的位置是在以下代碼：populateBean?位置中
• 那么我們?cè)陧?xiàng)目中使用注解 產(chǎn)生一個(gè)bean的時(shí)候必定會(huì)經(jīng)過(guò)以下代碼進(jìn)行一個(gè)bean的創(chuàng)建流程

/**省略代碼**/ // 開始初始化 bean 實(shí)例對(duì)象 Object exposedObject = bean; try {// <5> 對(duì) bean 進(jìn)行填充，將各個(gè)屬性值注入，其中，可能存在依賴于其他 bean 的屬性populateBean(beanName, mbd, instanceWrapper);// <6> 調(diào)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);} } /**省略代碼**/

• 在生命周期中 populateBean 進(jìn)行填充bean數(shù)據(jù)。把其他依賴引入進(jìn)來(lái)
• BeanPostProcessor 是一個(gè)bean創(chuàng)建時(shí)候的一個(gè)鉤子。
• 以下代碼 是循環(huán)調(diào)用實(shí)現(xiàn)了 BeanPostProcessor 子類?InstantiationAwareBeanPostProcessor#postProcessProperties?方法
• Spring 在以下代碼中有自動(dòng)注入的拓展點(diǎn)。 關(guān)鍵就是實(shí)現(xiàn)?InstantiationAwareBeanPostProcessor#postProcessProperties

/**省略代碼**/ for (BeanPostProcessor bp : getBeanPostProcessors()) {if (bp instanceof InstantiationAwareBeanPostProcessor) {InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;// 對(duì)所有需要依賴檢查的屬性進(jìn)行后處理PropertyValues pvsToUse = ibp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);if (pvsToUse == null) {// 從 bw 對(duì)象中提取 PropertyDescriptor 結(jié)果集// PropertyDescriptor：可以通過(guò)一對(duì)存取方法提取一個(gè)屬性if (filteredPds == null) {filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);}pvsToUse = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);if (pvsToUse == null) {return;}}pvs = pvsToUse;} } /**省略代碼**/

• 我們展開來(lái)講一下 @Autowired 的實(shí)現(xiàn)是怎么樣的吧：
• 實(shí)現(xiàn)類為?AutowiredAnnotationBeanPostProcessor.java
• 從上面可以得知，填充bean的時(shí)候。時(shí)調(diào)用了方法?ibp.postProcessPropertyValues()
• 那么?AutowiredAnnotationBeanPostProcessor#postProcessPropertyValues()?則會(huì)被調(diào)用
• 調(diào)用?findAutowiringMetadata?獲取 class 以及父類 帶有?@Autowired?或者?@Value?的屬性或者方法：

/**省略代碼**/ public PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName) {// 獲取所有可以注入的元數(shù)據(jù)InjectionMetadata metadata = findAutowiringMetadata(beanName, bean.getClass(), pvs);try {// 注入數(shù)據(jù)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) {// 緩存名字獲取String cacheKey = (StringUtils.hasLength(beanName) ? beanName : clazz.getName());InjectionMetadata metadata = this.injectionMetadataCache.get(cacheKey);// 獲取是否已經(jīng)讀取過(guò)這個(gè) class 類的 InjectionMetadata 有的話直接從緩存中獲取出去if (InjectionMetadata.needsRefresh(metadata, clazz)) {synchronized (this.injectionMetadataCache) {// 雙重檢查metadata = this.injectionMetadataCache.get(cacheKey);if (InjectionMetadata.needsRefresh(metadata, clazz)) {if (metadata != null) {metadata.clear(pvs);}// 構(gòu)建自動(dòng)注入的元數(shù)據(jù)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<>();// 循環(huán) targetClass 的所有 field 并執(zhí) FieldCallback 邏輯 （函數(shù)式編程接口，傳入的是一個(gè)執(zhí)行函數(shù)）ReflectionUtils.doWithLocalFields(targetClass, field -> {// 獲得字段上面的 Annotation 注解MergedAnnotation<?> ann = findAutowiredAnnotation(field);if (ann != null) {// 判斷是否為靜態(tài)屬性 如果是，則不進(jìn)行注入if (Modifier.isStatic(field.getModifiers())) {if (logger.isInfoEnabled()) {logger.info("Autowired annotation is not supported on static fields: " + field);}return;}// 注解是否為必須依賴項(xiàng)boolean required = determineRequiredStatus(ann);currElements.add(new AutowiredFieldElement(field, required));}});// 循環(huán) targetClass 的所有 Method 并執(zhí) MethodCallback 邏輯 （函數(shù)式編程接口，傳入的是一個(gè)執(zhí)行函數(shù)）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))) {// 判斷是否為靜態(tài)方法 如果是，則不進(jìn)行注入if (Modifier.isStatic(method.getModifiers())) {if (logger.isInfoEnabled()) {logger.info("Autowired annotation is not supported on static methods: " + method);}return;}// 判斷靜態(tài)方法參數(shù)是否為0if (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));}});//數(shù)據(jù)加到數(shù)組最前方 父類的的注解都放在靠前的位置elements.addAll(0, currElements);// 如果有父類則設(shè)置 targetClass 為父類。 如此循環(huán)targetClass = targetClass.getSuperclass();}while (targetClass != null && targetClass != Object.class);return InjectionMetadata.forElements(elements, clazz); }/**省略代碼**/

• 真正注入數(shù)據(jù)的是?metadata.inject(bean, beanName, pvs);
• 調(diào)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;// 帶有注解的方法或者屬性列表Collection<InjectedElement> elementsToIterate =(checkedElements != null ? checkedElements : this.injectedElements);if (!elementsToIterate.isEmpty()) {for (InjectedElement element : elementsToIterate) {element.inject(target, beanName, pvs);}} }

• 循環(huán)調(diào)用之前加入的帶有注解的方法或者屬性構(gòu)建的對(duì)象?AutowiredFieldElement#inject，?AutowiredMethodElement#inject

/*** 屬性上有注解 構(gòu)建的處理對(duì)象*/private class AutowiredFieldElement extends InjectionMetadata.InjectedElement {private final boolean required;private volatile boolean cached;@Nullableprivate volatile Object cachedFieldValue;public AutowiredFieldElement(Field field, boolean required) {super(field, null);this.required = required;}@Overrideprotected void inject(Object bean, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {// 獲取屬性名Field field = (Field) this.member;Object value;// Bean 不是單例的話，會(huì)重復(fù)進(jìn)入注入的這個(gè)操作，if (this.cached) {try {value = resolvedCachedArgument(beanName, this.cachedFieldValue);}catch (NoSuchBeanDefinitionException ex) {// Unexpected removal of target bean for cached argument -> re-resolvevalue = resolveFieldValue(field, bean, beanName);}}else {// 首次創(chuàng)建的時(shí)候進(jìn)入該方法value = resolveFieldValue(field, bean, beanName);}if (value != null) {// 屬性如果不為public的話，則設(shè)置為可訪問(wèn)ReflectionUtils.makeAccessible(field);field.set(bean, value);}}@Nullableprivate Object resolveFieldValue(Field field, Object bean, @Nullable String beanName) {// 構(gòu)建DependencyDescriptor對(duì)象DependencyDescriptor desc = new DependencyDescriptor(field, this.required);desc.setContainingClass(bean.getClass());// 注入bean的數(shù)量。 有可能字段上是一個(gè)ListSet<String> autowiredBeanNames = new LinkedHashSet<>(1);Assert.state(beanFactory != null, "No BeanFactory available");// 獲得beanFactory類型轉(zhuǎn)換類TypeConverter typeConverter = beanFactory.getTypeConverter();Object value;try {// 查找依賴關(guān)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;// 填入依賴關(guān)系registerDependentBeans(beanName, autowiredBeanNames);// 判斷如果注入依賴是只有一個(gè)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 構(gòu)建的處理對(duì)象*/private class AutowiredMethodElement extends InjectionMetadata.InjectedElement {private final boolean required;private volatile boolean cached;@Nullableprivate volatile Object[] cachedMethodArguments;public AutowiredMethodElement(Method method, boolean required, @Nullable PropertyDescriptor pd) {super(method, pd);this.required = required;}@Overrideprotected void inject(Object bean, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {// 檢查屬性是不會(huì)在之前就已經(jīng)注入過(guò)了。如果主如果則不進(jìn)行二次覆蓋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-resolvearguments = resolveMethodArguments(method, bean, beanName);}}else {// 首次創(chuàng)建的時(shí)候進(jìn)入該方法arguments = resolveMethodArguments(method, bean, beanName);}if (arguments != null) {try {// 屬性如果不為public的話，則設(shè)置為可訪問(wèn)ReflectionUtils.makeAccessible(method);// 調(diào)用方法 并傳入?yún)?shù)method.invoke(bean, arguments);}catch (InvocationTargetException ex) {throw ex.getTargetException();}}}@Nullableprivate 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;}@Nullableprivate Object[] resolveMethodArguments(Method method, Object bean, @Nullable String beanName) {// 獲取方法上有幾個(gè)參數(shù)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++) {// 方法參數(shù)，從方法參數(shù)中取出 i 構(gòu)造 MethodParameter 對(duì)象MethodParameter methodParam = new MethodParameter(method, i);DependencyDescriptor currDesc = new DependencyDescriptor(methodParam, this.required);currDesc.setContainingClass(bean.getClass());descriptors[i] = currDesc;try {// 獲取方法中 i 參數(shù)的內(nèi)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?實(shí)現(xiàn)的完整流程。 可概括為：
  populateBean?->?AutowiredAnnotationBeanPostProcessor#postProcessPropertyValues
  -> 獲取帶有注解的屬性和方法構(gòu)建?AutowiredFieldElement，?AutowiredMethodElement?對(duì)象，然后循環(huán)調(diào)用了?inject?進(jìn)行屬性調(diào)用

自定義自動(dòng)注入注解。

• 編寫抽象類代碼：

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;/*** 抽象的自動(dòng)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());/*** 處理的 annotationType 對(duì)象* @return Annotation 自定義注解*/public abstract Class<? extends Annotation> annotationType();AbstractAnnotationBeanPostProcessor () {Class<? extends Annotation> annotation = this.annotationType();annotationTypes.add(annotation);}@Overridepublic Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName) throws BeansException {return InstantiationAwareBeanPostProcessor.super.postProcessBeforeInstantiation(beanClass, beanName);}@Overridepublic boolean postProcessAfterInstantiation(Object bean, String beanName) throws BeansException {return InstantiationAwareBeanPostProcessor.super.postProcessAfterInstantiation(bean, beanName);}@Overridepublic PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName) {InjectionMetadata metadata = findAbstractMetadata(beanName, bean.getClass(), pvs);try {// 注入數(shù)據(jù)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);// 獲取是否已經(jīng)讀取過(guò)這個(gè) class 類的 InjectionMetadata 有的話直接從緩存中獲取出去if (InjectionMetadata.needsRefresh(metadata, clazz)) {synchronized (this.injectionMetadataCache) {// 雙重檢查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<>();// 循環(huán) targetClass 的所有 field 并執(zhí) FieldCallback 邏輯 （函數(shù)式編程接口，傳入的是一個(gè)執(zhí)行函數(shù)）ReflectionUtils.doWithLocalFields(targetClass, field -> {// 獲得字段上面的 Annotation 注解MergedAnnotation<?> ann = findAbstractAnnotation(field);if (ann != null) {// 判斷是否為靜態(tài)屬性 如果是，則不進(jìn)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));}});//數(shù)據(jù)加到數(shù)組最前方 父類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);}@Nullableprivate MergedAnnotation<?> findAbstractAnnotation(AccessibleObject ao) {// 將指定方法上的注解合并成一個(gè)注解MergedAnnotations annotations = MergedAnnotations.from(ao);// 循環(huán)要掃描的注解 annotationTypes 那個(gè)在前面就認(rèn)哪個(gè)for (Class<? extends Annotation> type : this.annotationTypes) {MergedAnnotation<?> annotation = annotations.get(type);if (annotation.isPresent()) {return annotation;}}return null;}@Deprecated@Overridepublic PropertyValues postProcessPropertyValues(PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) {return postProcessProperties(pvs, bean, beanName);}@Overridepublic Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {return InstantiationAwareBeanPostProcessor.super.postProcessBeforeInitialization(bean, beanName);}@Overridepublic Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {return InstantiationAwareBeanPostProcessor.super.postProcessAfterInitialization(bean, beanName);}/*** 實(shí)現(xiàn)交給子類進(jìn)行實(shí)現(xiàn) 將注解透?jìng)鞒鋈? @param mergedAnnotation 屬性上的注解* @param bean bean實(shí)例* @param beanName bean的名字* @param field 字段* @param autowiredFieldElement 注解* @return 注入對(duì)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;}@Overrideprotected 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);}}}}

• 抽象類實(shí)現(xiàn)，實(shí)現(xiàn)一個(gè)?@RedisAutowired?自定義注入注解

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;@Overridepublic Class<? extends Annotation> annotationType() {return RedisAutowired.class;}@Overrideprotected Object getInjectedObject(MergedAnnotation<?> mergedAnnotation, Object bean, String beanName, Field field,AbstractAnnotationBeanPostProcessor.AbstractFieldElement autowiredFieldElement) throws Exception {// 從value中獲取bean的名字。 或者通過(guò)一些其他的邏輯獲取名字String s = mergedAnnotation.getString("value");return beanFactory.getBean(s);}@Overridepublic 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

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