Spring中的AutowireCandidateResolver的具體使用詳解

更新時(shí)間：2020年04月23日 09:58:01 作者：程序員DMZ

這篇文章主要介紹了Spring中的AutowireCandidateResolver的具體使用詳解，文中通過(guò)示例代碼介紹的非常詳細(xì)，對(duì)大家的學(xué)習(xí)或者工作具有一定的參考學(xué)習(xí)價(jià)值，需要的朋友們下面隨著小編來(lái)一起學(xué)習(xí)學(xué)習(xí)吧

接口定義

用于推斷一個(gè)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(rèn)情況下直接根據(jù)bd中的定義返回，如果沒(méi)有進(jìn)行特殊配置的話為true
	default 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(shí)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(rèn)情況下返回false
	default boolean hasQualifier(DependencyDescriptor descriptor) {
		return false;
	}
	
  // QualifierAnnotationAutowireCandidateResolver做了實(shí)現(xiàn)
  // 獲取一個(gè)該依賴一個(gè)建議的值
	@Nullable
	default Object getSuggestedValue(DependencyDescriptor descriptor) {
		return null;
	}
	
  // 對(duì)某個(gè)依賴我們想要延遲注入，但是在創(chuàng)建Bean的過(guò)程中這個(gè)依賴又是必須的
  // 通過(guò)下面這個(gè)方法就能為延遲注入的依賴先生成一個(gè)代理注入到bean中
	@Nullable
	default 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(gè)一個(gè)往下看

SimpleAutowireCandidateResolver

相比于接口沒(méi)有什么區(qū)別，實(shí)現(xiàn)也就是父接口中的默認(rèn)實(shí)現(xiàn)，一般也不會(huì)使用這個(gè)類

public class SimpleAutowireCandidateResolver implements AutowireCandidateResolver {

	@Override
	public boolean isAutowireCandidate(BeanDefinitionHolder bdHolder, DependencyDescriptor descriptor) {
		return bdHolder.getBeanDefinition().isAutowireCandidate();
	}

	@Override
	public boolean isRequired(DependencyDescriptor descriptor) {
		return descriptor.isRequired();
	}

	@Override
	@Nullable
	public Object getSuggestedValue(DependencyDescriptor descriptor) {
		return null;
	}

	@Override
	@Nullable
	public Object getLazyResolutionProxyIfNecessary(DependencyDescriptor descriptor, @Nullable String beanName) {
		return null;
	}

}

GenericTypeAwareAutowireCandidateResolver

額外增加了對(duì)泛型的處理能力

public class GenericTypeAwareAutowireCandidateResolver extends SimpleAutowireCandidateResolver
		implements BeanFactoryAware {

	@Nullable
	private BeanFactory beanFactory;


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

	@Nullable
	protected final BeanFactory getBeanFactory() {
		return this.beanFactory;
	}


	@Override
	public boolean isAutowireCandidate(BeanDefinitionHolder bdHolder, DependencyDescriptor descriptor) {
		if (!super.isAutowireCandidate(bdHolder, descriptor)) {
			// 如果bd中已經(jīng)配置了這個(gè)bean不做為依賴進(jìn)行注入的話，直接返回false
			return false;
		}
    // 檢查泛型是否匹配
		return checkGenericTypeMatch(bdHolder, descriptor);
	}

}

QualifierAnnotationAutowireCandidateResolver

增加了對(duì)@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.
		}
	}

	// .......

	@Override
	public 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
	@Nullable
	public 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
	@Nullable
	protected 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(gè)類是最底層的子類，集成了所有的方法，并且額外提供了對(duì)依賴進(jìn)行延遲處理的能力

public class ContextAnnotationAutowireCandidateResolver extends QualifierAnnotationAutowireCandidateResolver {
	
  // 如果依賴需要進(jìn)行延遲處理，那么構(gòu)建一個(gè)代理對(duì)象先注入到bean中，不會(huì)直接去創(chuàng)建依賴對(duì)象
	@Override
	@Nullable
	public 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(duì)象
	protected Object buildLazyResolutionProxy(final DependencyDescriptor descriptor, final @Nullable String beanName) {
		final DefaultListableBeanFactory beanFactory = (DefaultListableBeanFactory) getBeanFactory();
    
    // 創(chuàng)建了一個(gè)TargetSource
		TargetSource ts = new TargetSource() {
			@Override
			public Class<?> getTargetClass() {
				return descriptor.getDependencyType();
			}
			@Override
			public boolean isStatic() {
				return false;
			}
      // target是我們實(shí)際想要使用的對(duì)象，如果不進(jìn)行延遲處理，那么注入到bean中的應(yīng)該就是這個(gè)對(duì)象
      // 但是因?yàn)橐M(jìn)行延遲注入依賴，所有會(huì)向外暴露一個(gè)TargetSource，這個(gè)TargetSource的目標(biāo)為實(shí)際想要使用的對(duì)象，生成代理時(shí)會(huì)基于TargetSource進(jìn)行生成。在運(yùn)行期間（完成注入后）我們使用這個(gè)延遲處理的依賴時(shí)實(shí)際調(diào)用的會(huì)是target中的方法。
			@Override
			public 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;
			}
			@Override
			public void releaseTarget(Object target) {
			}
		};
    
    // 使用ProxyFactory，給TargetSource生成一個(gè)代理
		ProxyFactory pf = new ProxyFactory();
		pf.setTargetSource(ts);
		Class<?> dependencyType = descriptor.getDependencyType();
    // 如果依賴的類型是一個(gè)接口，需要讓代理類也實(shí)現(xiàn)這個(gè)接口
		if (dependencyType.isInterface()) {
			pf.addInterface(dependencyType);
		}
    // 生成代理
		return pf.getProxy(beanFactory.getBeanClassLoader());
	}

}

總結(jié)

SimpleAutowireCandidateResolver：?jiǎn)渭兊膶⒔涌谧兂闪丝蓪?shí)例化的類，方法實(shí)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(jí)別的 @Lazy 注解，重寫了getLazyResolutionProxyIfNecessary 方法。

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