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> qualifierTypes = new LinkedHashSet<>(2);
 // @Value注解
 private Class valueAnnotationType = Value.class;

    // @Qualifier注解
 @SuppressWarnings("unchecked")
 public QualifierAnnotationAutowireCandidateResolver() {
  this.qualifierTypes.add(Qualifier.class);
  try {
   this.qualifierTypes.add((Class) 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 annotationType) {
  for (Class 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。

往期精选

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Spring杂谈
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