Spring杂谈 | Spring中的AutowireCandidateResolver

接口定义

用于推断一个特定的beanDefinition是否能作为指定依赖的候选者的策略接口

public interface AutowireCandidateResolver {
	// 默认情况下直接根据bd中的定义返回，如果没有进行特殊配置的话为true
	default boolean isAutowireCandidate(BeanDefinitionHolder bdHolder, DependencyDescriptor descriptor) {
		return bdHolder.getBeanDefinition().isAutowireCandidate();
	}
	
    // 指定的依赖是否是必要的
	default boolean isRequired(DependencyDescriptor descriptor) {
		return descriptor.isRequired();
	}
	
    // QualifierAnnotationAutowireCandidateResolver做了实现，判断是否有@Qualifier注解
    // 一共有两种注解：
    // 1.Spring内置的@Qualifier注解，org.springframework.beans.factory.annotation.Qualifier
    // 2.添加了JSR-330相关依赖，javax.inject.Qualifier注解
    // 默认情况下返回false
	default boolean hasQualifier(DependencyDescriptor descriptor) {
		return false;
	}
	
    // QualifierAnnotationAutowireCandidateResolver做了实现
    // 获取一个该依赖一个建议的值
	@Nullable
	default Object getSuggestedValue(DependencyDescriptor descriptor) {
		return null;
	}
	
    // 对某个依赖我们想要延迟注入，但是在创建Bean的过程中这个依赖又是必须的
    // 通过下面这个方法就能为延迟注入的依赖先生成一个代理注入到bean中
	@Nullable
	default Object getLazyResolutionProxyIfNecessary(DependencyDescriptor descriptor, @Nullable String beanName) {
		return null;
	}

}

继承关系

可以看到继承关系都是单层的，我们就一个一个往下看

SimpleAutowireCandidateResolver

相比于接口没有什么区别，实现也就是父接口中的默认实现，一般也不会使用这个类

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

额外增加了对泛型的处理能力

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中已经配置了这个bean不做为依赖进行注入的话，直接返回false
			return false;
		}
        // 检查泛型是否匹配
		return checkGenericTypeMatch(bdHolder, descriptor);
	}

}

QualifierAnnotationAutowireCandidateResolver

增加了对@Qualifier注解以及@Value注解的处理能力

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) {
        // 类型上已经匹配了
		boolean match = super.isAutowireCandidate(bdHolder, descriptor);
		if (match) {
            // 还需要判断是否满足@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;
	}
	
    // 获取@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

这个类是最底层的子类，集成了所有的方法，并且额外提供了对依赖进行延迟处理的能力

public class ContextAnnotationAutowireCandidateResolver extends QualifierAnnotationAutowireCandidateResolver {
	
    // 如果依赖需要进行延迟处理，那么构建一个代理对象先注入到bean中，不会直接去创建依赖对象
	@Override
	@Nullable
	public Object getLazyResolutionProxyIfNecessary(DependencyDescriptor descriptor, @Nullable String beanName) {
		return (isLazy(descriptor) ? buildLazyResolutionProxy(descriptor, beanName) : null);
	}
	
    // 依赖是否需要延迟处理
	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;
	}
	
    // 构建延迟处理的代理对象
	protected Object buildLazyResolutionProxy(final DependencyDescriptor descriptor, final @Nullable String beanName) {
		final DefaultListableBeanFactory beanFactory = (DefaultListableBeanFactory) getBeanFactory();
        
        // 创建了一个TargetSource
		TargetSource ts = new TargetSource() {
			@Override
			public Class<?> getTargetClass() {
				return descriptor.getDependencyType();
			}
			@Override
			public boolean isStatic() {
				return false;
			}
            // target是我们实际想要使用的对象，如果不进行延迟处理，那么注入到bean中的应该就是这个对象
            // 但是因为要进行延迟注入依赖，所有会向外暴露一个TargetSource，这个TargetSource的目标为实际想要使用的对象，生成代理时会基于TargetSource进行生成。在运行期间（完成注入后）我们使用这个延迟处理的依赖时实际调用的会是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，给TargetSource生成一个代理
		ProxyFactory pf = new ProxyFactory();
		pf.setTargetSource(ts);
		Class<?> dependencyType = descriptor.getDependencyType();
        // 如果依赖的类型是一个接口，需要让代理类也实现这个接口
		if (dependencyType.isInterface()) {
			pf.addInterface(dependencyType);
		}
        // 生成代理
		return pf.getProxy(beanFactory.getBeanClassLoader());
	}

}

总结

• SimpleAutowireCandidateResolver：单纯的将接口变成了可实例化的类，方法实现跟接口保持一致
• GenericTypeAwareAutowireCandidateResolver： 判断泛型是否匹配，支持泛型依赖注入（From Spring4.0）
• QualifierAnnotationAutowireCandidateResolver ：处理 @Qualifier 和 @Value 注解
• ContextAnnotationAutowireCandidateResolver ：处理依赖级别的 @Lazy 注解，重写了getLazyResolutionProxyIfNecessary 方法。