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Spring Bean属性填充

​ 码农参上 ​ 人气:0

前言

上一篇文章中,我们分析了Spring中Bean的实例化过程,在结尾我们知道了虽然bean的实例化完成了,但是其中的属性还没有被注入,今天我们就接着来分析属性是如何被注入的。

属性填充

实例化完成后,回到上面第3条的doCreateBean方法中,看一下用BeanWrapper产生的原生对象,里面dao这个属性还是null值。

回归一下之前的代码,接下来要调用populateBean方法进行属性的填充:

Object exposedObject = bean;
try {
  populateBean(beanName, mbd, instanceWrapper);
  exposedObject = initializeBean(beanName, exposedObject, mbd);
}

看一下populateBean中的核心代码:

for (BeanPostProcessor bp : getBeanPostProcessors()) {
  if (bp instanceof InstantiationAwareBeanPostProcessor) {
    InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
    pvs = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
    if (pvs == null) {
      return;
    }
  }
}

这里通过getBeanPostProcessors方法获得当前注册的所有后置处理器,如果属于InstantiationAwareBeanPostProcessor类型,则调用它的postProcessPropertyValues方法。通过遍历,可以知道当前spring中存在7个后置处理器:

我们主要来看一下AutowiredAnnotationBeanPostProcessor,因为它负责对添加了 @Autowired@Value等注解的属性进行依赖的填充。进入它的postProcessPropertyValues方法:

public PropertyValues postProcessPropertyValues(
  PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) throws BeanCreationException {
  InjectionMetadata metadata = findAutowiringMetadata(beanName, bean.getClass(), pvs);
  try {
    metadata.inject(bean, beanName, pvs);
  }
 //异常处理代码省略...
  return pvs;
}

这里的InjectionMetadata可以理解为要注入的属性的元数据,在它里面维护了一个Collection,来存放所有需要注入的bean:

private final Collection<InjectedElement> injectedElements;

进入findAutowiringMetadata方法:

private InjectionMetadata findAutowiringMetadata(String beanName, Class<?> clazz, @Nullable PropertyValues pvs) {
 String cacheKey = (StringUtils.hasLength(beanName) ? beanName : clazz.getName());
 InjectionMetadata metadata = this.injectionMetadataCache.get(cacheKey);
  //省略非重要代码...
 return metadata;
}

在执行完这一步后,就把需要填充的属性放进了刚才提到的injectedElements中:

接下来,继续执行InjectionMetadatainject方法,在其中遍历所有需要注入的属性的列表,遍历调用AutowiredAnnotationBeanPostProcessor的inject方法:

protected void inject(Object bean, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
      Field field = (Field) this.member;
      Object value;
      if (this.cached) {
        value = resolvedCachedArgument(beanName, this.cachedFieldValue);
      }
      else {
        DependencyDescriptor desc = new DependencyDescriptor(field, this.required);
        desc.setContainingClass(bean.getClass());
        Set<String> autowiredBeanNames = new LinkedHashSet<>(1);
        Assert.state(beanFactory != null, "No BeanFactory available");
        TypeConverter typeConverter = beanFactory.getTypeConverter();
        try {//用beanFactory解决依赖
          value = beanFactory.resolveDependency(desc, beanName, autowiredBeanNames, typeConverter);
        }
   //后面代码省略...

这里创建了一个DependencyDescriptor,用来维护注入属性与它的容器类containingClass的关系,里面最重要的就是存放了注入属性的类型、名称,以及containingClass的类型等信息。

调用resolveDependency方法,其中没有做什么实质性的工作,继续调用了doResolveDependency方法:

public Object doResolveDependency(DependencyDescriptor descriptor, @Nullable String beanName,
    @Nullable Set<String> autowiredBeanNames, @Nullable TypeConverter typeConverter) throws BeansException {
  InjectionPoint previousInjectionPoint = ConstructorResolver.setCurrentInjectionPoint(descriptor);
  try {
    Object shortcut = descriptor.resolveShortcut(this);
    if (shortcut != null) {
      return shortcut;
    }
    //依赖的属性值的类型
    Class<?> type = descriptor.getDependencyType();
    Object value = getAutowireCandidateResolver().getSuggestedValue(descriptor);
    if (value != null) {
      if (value instanceof String) {
        String strVal = resolveEmbeddedValue((String) value);
        BeanDefinition bd = (beanName != null && containsBean(beanName) ? getMergedBeanDefinition(beanName) : null);
        value = evaluateBeanDefinitionString(strVal, bd);
      }
      TypeConverter converter = (typeConverter != null ? typeConverter : getTypeConverter());
      return (descriptor.getField() != null ?
          converter.convertIfNecessary(value, type, descriptor.getField()) :
          converter.convertIfNecessary(value, type, descriptor.getMethodParameter()));
    }

    Object multipleBeans = resolveMultipleBeans(descriptor, beanName, autowiredBeanNames, typeConverter);
    if (multipleBeans != null) {
      return multipleBeans;
    }
    //把匹配的值和类型拿出来,放到一个map中
    Map<String, Object> matchingBeans = findAutowireCandidates(beanName, type, descriptor);
    if (matchingBeans.isEmpty()) {
      if (isRequired(descriptor)) {
        raiseNoMatchingBeanFound(type, descriptor.getResolvableType(), descriptor);
      }
      return null;
    }

    String autowiredBeanName;
    Object instanceCandidate;
    //如果有超过一个匹配的,可能会有错误
    if (matchingBeans.size() > 1) {
      autowiredBeanName = determineAutowireCandidate(matchingBeans, descriptor);
      if (autowiredBeanName == null) {
        if (isRequired(descriptor) || !indicatesMultipleBeans(type)) {
          return descriptor.resolveNotUnique(type, matchingBeans);
        }
        else {
          return null;
        }
      }
      instanceCandidate = matchingBeans.get(autowiredBeanName);
    }
    else {
      Map.Entry<String, Object> entry = matchingBeans.entrySet().iterator().next();
      autowiredBeanName = entry.getKey();
      instanceCandidate = entry.getValue();
    }

    if (autowiredBeanNames != null) {
      //把找到的bean的名字放到set中
      autowiredBeanNames.add(autowiredBeanName);
    }
    if (instanceCandidate instanceof Class) {
      // 实际获取注入的bean
      instanceCandidate = descriptor.resolveCandidate(autowiredBeanName, type, this);
    }
    Object result = instanceCandidate;
    if (result instanceof NullBean) {
      if (isRequired(descriptor)) {
        raiseNoMatchingBeanFound(type, descriptor.getResolvableType(), descriptor);
      }
      result = null;
    }
    if (!ClassUtils.isAssignableValue(type, result)) {
      throw new BeanNotOfRequiredTypeException(autowiredBeanName, type, instanceCandidate.getClass());
    }
    return result;
  }
  finally {
    ConstructorResolver.setCurrentInjectionPoint(previousInjectionPoint);
  }
}

通过findAutowireCandidates方法,获取与注入属性匹配的值和类型,放到一个Map当中,再通过它的beanName,调用resolveCandidate方法,实际获取注入的bean实例。这一操作底层调用的也是BeanFactory的getBean方法。

回到inject方法,使用反射将注入的bean实例赋值给属性:

ReflectionUtils.makeAccessible(field);
field.set(bean, value);

在执行完populateBean方法后,依赖的属性已经被注入成功了。

执行回调方法及后置处理器

在bean实例化完成后,执行各种回调和后置管理器方法:

protected Object initializeBean(final String beanName, final Object bean, @Nullable RootBeanDefinition mbd) {
  if (System.getSecurityManager() != null) {
    AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
      invokeAwareMethods(beanName, bean);
      return null;
    }, getAccessControlContext());
  }
  else {
    //若bean实现了BeanNameAware、BeanClassLoaderAware、BeanFactoryAware接口,执行回调方法
    invokeAwareMethods(beanName, bean);
  }

  Object wrappedBean = bean;
  if (mbd == null || !mbd.isSynthetic()) {
    //执行所有后置处理器的before方法
    wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
  }

  try {
    //执行bean生命周期回调中的init-method
    //若bean实现了InitializingBean接口,执行afterPropertiesSet方法
    invokeInitMethods(beanName, wrappedBean, mbd);
  }
  catch (Throwable ex) {
    throw new BeanCreationException(
        (mbd != null ? mbd.getResourceDescription() : null),
        beanName, "Invocation of init method failed", ex);
  }
  if (mbd == null || !mbd.isSynthetic()) {
    //执行所有后置处理器的after方法
    wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
  }

  return wrappedBean;
}

具体执行内容:

在这一步完成后,bean的实例化过程全部结束。最后执行一下refresh方法中的finishRefresh方法,进行广播事件等操作。到这,一个完整的AnnotationConfigApplicationContext初始化就完成了。

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