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

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;
    }

}

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);
    }

}

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;
    }

}

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。