接口定義

用于推斷一個特定的beanDefinition是否能作為指定依賴的候選者的策略接口

public interface AutowireCandidateResolver {// 默認情況下直接根據(jù)bd中的定義返回，如果沒有進行特殊配置的話為truedefault boolean isAutowireCandidate(BeanDefinitionHolder bdHolder, DependencyDescriptor descriptor) {return bdHolder.getBeanDefinition().isAutowireCandidate();} // 指定的依賴是否是必要的default boolean isRequired(DependencyDescriptor descriptor) {return descriptor.isRequired();} // QualifierAnnotationAutowireCandidateResolver做了實現(xiàn)，判斷是否有@Qualifier注解 // 一共有兩種注解： // 1.Spring內(nèi)置的@Qualifier注解，org.springframework.beans.factory.annotation.Qualifier // 2.添加了JSR-330相關(guān)依賴，javax.inject.Qualifier注解 // 默認情況下返回falsedefault boolean hasQualifier(DependencyDescriptor descriptor) {return false;} // QualifierAnnotationAutowireCandidateResolver做了實現(xiàn) // 獲取一個該依賴一個建議的值@Nullabledefault Object getSuggestedValue(DependencyDescriptor descriptor) {return null;} // 對某個依賴我們想要延遲注入，但是在創(chuàng)建Bean的過程中這個依賴又是必須的 // 通過下面這個方法就能為延遲注入的依賴先生成一個代理注入到bean中@Nullabledefault Object getLazyResolutionProxyIfNecessary(DependencyDescriptor descriptor, @Nullable String beanName) {return null;}}

繼承關(guān)系

可以看到繼承關(guān)系都是單層的，我們就一個一個往下看

SimpleAutowireCandidateResolver

相比于接口沒有什么區(qū)別，實現(xiàn)也就是父接口中的默認實現(xiàn)，一般也不會使用這個類

public class SimpleAutowireCandidateResolver implements AutowireCandidateResolver {@Overridepublic boolean isAutowireCandidate(BeanDefinitionHolder bdHolder, DependencyDescriptor descriptor) {return bdHolder.getBeanDefinition().isAutowireCandidate();}@Overridepublic boolean isRequired(DependencyDescriptor descriptor) {return descriptor.isRequired();}@Override@Nullablepublic Object getSuggestedValue(DependencyDescriptor descriptor) {return null;}@Override@Nullablepublic Object getLazyResolutionProxyIfNecessary(DependencyDescriptor descriptor, @Nullable String beanName) {return null;}}

GenericTypeAwareAutowireCandidateResolver

額外增加了對泛型的處理能力

public class GenericTypeAwareAutowireCandidateResolver extends SimpleAutowireCandidateResolverimplements BeanFactoryAware {@Nullableprivate BeanFactory beanFactory;@Overridepublic void setBeanFactory(BeanFactory beanFactory) {this.beanFactory = beanFactory;}@Nullableprotected final BeanFactory getBeanFactory() {return this.beanFactory;}@Overridepublic boolean isAutowireCandidate(BeanDefinitionHolder bdHolder, DependencyDescriptor descriptor) {if (!super.isAutowireCandidate(bdHolder, descriptor)) {// 如果bd中已經(jīng)配置了這個bean不做為依賴進行注入的話，直接返回falsereturn false;} // 檢查泛型是否匹配return checkGenericTypeMatch(bdHolder, descriptor);}}

QualifierAnnotationAutowireCandidateResolver

增加了對@Qualifier注解以及@Value注解的處理能力

public class QualifierAnnotationAutowireCandidateResolver extends GenericTypeAwareAutowireCandidateResolver {private final Set<Class<? extends Annotation>> qualifierTypes = new LinkedHashSet<>(2);// @Value注解private Class<? extends Annotation> valueAnnotationType = Value.class; // @Qualifier注解@SuppressWarnings('unchecked')public QualifierAnnotationAutowireCandidateResolver() {this.qualifierTypes.add(Qualifier.class);try {this.qualifierTypes.add((Class<? extends Annotation>) ClassUtils.forName('javax.inject.Qualifier',QualifierAnnotationAutowireCandidateResolver.class.getClassLoader()));}catch (ClassNotFoundException ex) {// JSR-330 API not available - simply skip.}}// .......@Overridepublic boolean isAutowireCandidate(BeanDefinitionHolder bdHolder, DependencyDescriptor descriptor) { // 類型上已經(jīng)匹配了boolean match = super.isAutowireCandidate(bdHolder, descriptor);if (match) { // 還需要判斷是否滿足@Qualifier注解的要求match = checkQualifiers(bdHolder, descriptor.getAnnotations());if (match) {MethodParameter methodParam = descriptor.getMethodParameter();if (methodParam != null) {Method method = methodParam.getMethod();if (method == null || void.class == method.getReturnType()) {match = checkQualifiers(bdHolder, methodParam.getMethodAnnotations());}}}}return match;}// ..... // 是否是@Qualifier注解protected boolean isQualifier(Class<? extends Annotation> annotationType) {for (Class<? extends Annotation> qualifierType : this.qualifierTypes) {if (annotationType.equals(qualifierType) || annotationType.isAnnotationPresent(qualifierType)) {return true;}}return false;}@Override@Nullablepublic Object getSuggestedValue(DependencyDescriptor descriptor) {Object value = findValue(descriptor.getAnnotations());if (value == null) {MethodParameter methodParam = descriptor.getMethodParameter();if (methodParam != null) {value = findValue(methodParam.getMethodAnnotations());}}return value;} // 查找@Value注解@Nullableprotected Object findValue(Annotation[] annotationsToSearch) {if (annotationsToSearch.length > 0) { AnnotationAttributes attr = AnnotatedElementUtils.getMergedAnnotationAttributes(AnnotatedElementUtils.forAnnotations(annotationsToSearch), this.valueAnnotationType);if (attr != null) {return extractValue(attr);}}return null;} // 獲取@Value注解中的值protected Object extractValue(AnnotationAttributes attr) {Object value = attr.get(AnnotationUtils.VALUE);if (value == null) {throw new IllegalStateException('Value annotation must have a value attribute');}return value;}}

ContextAnnotationAutowireCandidateResolver

這個類是最底層的子類，集成了所有的方法，并且額外提供了對依賴進行延遲處理的能力

public class ContextAnnotationAutowireCandidateResolver extends QualifierAnnotationAutowireCandidateResolver { // 如果依賴需要進行延遲處理，那么構(gòu)建一個代理對象先注入到bean中，不會直接去創(chuàng)建依賴對象@Override@Nullablepublic Object getLazyResolutionProxyIfNecessary(DependencyDescriptor descriptor, @Nullable String beanName) {return (isLazy(descriptor) ? buildLazyResolutionProxy(descriptor, beanName) : null);} // 依賴是否需要延遲處理protected boolean isLazy(DependencyDescriptor descriptor) {for (Annotation ann : descriptor.getAnnotations()) {Lazy lazy = AnnotationUtils.getAnnotation(ann, Lazy.class);if (lazy != null && lazy.value()) {return true;}}MethodParameter methodParam = descriptor.getMethodParameter();if (methodParam != null) {Method method = methodParam.getMethod();if (method == null || void.class == method.getReturnType()) {Lazy lazy = AnnotationUtils.getAnnotation(methodParam.getAnnotatedElement(), Lazy.class);if (lazy != null && lazy.value()) {return true;}}}return false;} // 構(gòu)建延遲處理的代理對象protected Object buildLazyResolutionProxy(final DependencyDescriptor descriptor, final @Nullable String beanName) {final DefaultListableBeanFactory beanFactory = (DefaultListableBeanFactory) getBeanFactory();// 創(chuàng)建了一個TargetSourceTargetSource ts = new TargetSource() {@Overridepublic Class<?> getTargetClass() {return descriptor.getDependencyType();}@Overridepublic boolean isStatic() {return false;} // target是我們實際想要使用的對象，如果不進行延遲處理，那么注入到bean中的應(yīng)該就是這個對象 // 但是因為要進行延遲注入依賴，所有會向外暴露一個TargetSource，這個TargetSource的目標(biāo)為實際想要使用的對象，生成代理時會基于TargetSource進行生成。在運行期間（完成注入后）我們使用這個延遲處理的依賴時實際調(diào)用的會是target中的方法。@Overridepublic Object getTarget() {Object target = beanFactory.doResolveDependency(descriptor, beanName, null, null);if (target == null) {Class<?> type = getTargetClass();if (Map.class == type) {return Collections.emptyMap();}else if (List.class == type) {return Collections.emptyList();}else if (Set.class == type || Collection.class == type) {return Collections.emptySet();}throw new NoSuchBeanDefinitionException(descriptor.getResolvableType(),'Optional dependency not present for lazy injection point');}return target;}@Overridepublic void releaseTarget(Object target) {}};// 使用ProxyFactory，給TargetSource生成一個代理ProxyFactory pf = new ProxyFactory();pf.setTargetSource(ts);Class<?> dependencyType = descriptor.getDependencyType(); // 如果依賴的類型是一個接口，需要讓代理類也實現(xiàn)這個接口if (dependencyType.isInterface()) {pf.addInterface(dependencyType);} // 生成代理return pf.getProxy(beanFactory.getBeanClassLoader());}}

總結(jié)

SimpleAutowireCandidateResolver：單純的將接口變成了可實例化的類，方法實現(xiàn)跟接口保持一致 GenericTypeAwareAutowireCandidateResolver： 判斷泛型是否匹配，支持泛型依賴注入（From Spring4.0） QualifierAnnotationAutowireCandidateResolver ：處理 @Qualifier 和 @Value 注解 ContextAnnotationAutowireCandidateResolver ：處理依賴級別的 @Lazy 注解，重寫了getLazyResolutionProxyIfNecessary 方法。

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