Spring FactoriesLoader机制实例详解

default V getOrDefault(Object key, V defaultValue) {
  V v;
  return (((v = get(key)) != null) || containsKey(key))
    ? v
    : defaultValue;
}

2.2 loadFactories 解析

public static <T> List<T> loadFactories(Class<T> factoryType, @Nullable ClassLoader classLoader) {
  Assert.notNull(factoryType, "'factoryType' must not be null");
  // 如果未指定类加载器，则使用默认的
  ClassLoader classLoaderToUse = classLoader;
  if (classLoaderToUse == null) {
    classLoaderToUse = SpringFactoriesLoader.class.getClassLoader();
  }
  // 获取指定工厂名称列表
  List<String> factoryImplementationNames = loadFactoryNames(factoryType, classLoaderToUse);
  // 如果记录器Trace跟踪激活的话,将工厂名称列表输出
  if (logger.isTraceEnabled()) {
    logger.trace("Loaded [" + factoryType.getName() + "] names: " + factoryImplementationNames);
  }
  // 创建结果集
  List<T> result = new ArrayList<>(factoryImplementationNames.size());
  for (String factoryImplementationName : factoryImplementationNames) {
    // 实例化工厂类,并添加到结果集中
    result.add(instantiateFactory(factoryImplementationName, factoryType, classLoaderToUse));
  }
  // 对结果集列表进行排序
  AnnotationAwareOrderComparator.sort(result);
  return result;
}

private static <T> T instantiateFactory(String factoryImplementationName, Class<T> factoryType, ClassLoader classLoader) {
  try {
    Class<?> factoryImplementationClass = ClassUtils.forName(factoryImplementationName, classLoader);
    if (!factoryType.isAssignableFrom(factoryImplementationClass)) {
      throw new IllegalArgumentException(
          "Class [" + factoryImplementationName + "] is not assignable to factory type [" + factoryType.getName() + "]");
    }
    return (T) ReflectionUtils.accessibleConstructor(factoryImplementationClass).newInstance();
  }
  catch (Throwable ex) {
    throw new IllegalArgumentException(
      "Unable to instantiate factory class [" + factoryImplementationName + "] for factory type [" + factoryType.getName() + "]",
      ex);
  }
}

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