mybatis底层组件介绍-ObjectFactory

ObjectFactory

mybatis使用ObjectFactory来创建所有需要的对象

public interface ObjectFactory {

  /**
   * Sets configuration properties.
   * @param properties configuration properties
   */
  default void setProperties(Properties properties) {
    // NOP
  }

  /**
   * Creates a new object with default constructor.
   *
   * @param <T>
   *          the generic type
   * @param type
   *          Object type
   * @return the t
   */
  <T> T create(Class<T> type);

  /**
   * Creates a new object with the specified constructor and params.
   *
   * @param <T>
   *          the generic type
   * @param type
   *          Object type
   * @param constructorArgTypes
   *          Constructor argument types
   * @param constructorArgs
   *          Constructor argument values
   * @return the t
   */
  <T> T create(Class<T> type, List<Class<?>> constructorArgTypes, List<Object> constructorArgs);

  /**
   * Returns true if this object can have a set of other objects.
   * It's main purpose is to support non-java.util.Collection objects like Scala collections.
   *
   * @param <T>
   *          the generic type
   * @param type
   *          Object type
   * @return whether it is a collection or not
   * @since 3.1.0
   */
  <T> boolean isCollection(Class<T> type);
}

看下默认实现类DefaultObjectFactory

public <T> T create(Class<T> type, List<Class<?>> constructorArgTypes, List<Object> constructorArgs) {
    Class<?> classToCreate = resolveInterface(type);
    // we know types are assignable
    return (T) instantiateClass(classToCreate, constructorArgTypes, constructorArgs);
  }

// 如果传入的类是集合接口，那么会返回实现类；否则直接返回
protected Class<?> resolveInterface(Class<?> type) {
    Class<?> classToCreate;
    if (type == List.class || type == Collection.class || type == Iterable.class) {
      classToCreate = ArrayList.class;
    } else if (type == Map.class) {
      classToCreate = HashMap.class;
    } else if (type == SortedSet.class) { // issue #510 Collections Support
      classToCreate = TreeSet.class;
    } else if (type == Set.class) {
      classToCreate = HashSet.class;
    } else {
      classToCreate = type;
    }
    return classToCreate;
  }

// 这里的逻辑也很简单
private  <T> T instantiateClass(Class<T> type, List<Class<?>> constructorArgTypes, List<Object> constructorArgs) {
    try {
      Constructor<T> constructor;
      // 如果构造参数类型或者构造参数有一个为null，那么就调用默认构造函数
      if (constructorArgTypes == null || constructorArgs == null) {
        constructor = type.getDeclaredConstructor();
        try {
          return constructor.newInstance();
        } catch (IllegalAccessException e) {
          if (Reflector.canControlMemberAccessible()) {
            constructor.setAccessible(true);
            return constructor.newInstance();
          } else {
            throw e;
          }
        }
      }
      // 根据传入的构造参数类型，调用对应的构造函数
      constructor = type.getDeclaredConstructor(constructorArgTypes.toArray(new Class[0]));
      try {
        return constructor.newInstance(constructorArgs.toArray(new Object[0]));
      } catch (IllegalAccessException e) {
        if (Reflector.canControlMemberAccessible()) {
          constructor.setAccessible(true);
          return constructor.newInstance(constructorArgs.toArray(new Object[0]));
        } else {
          throw e;
        }
      }
    } catch (Exception e) {
      String argTypes = Optional.ofNullable(constructorArgTypes).orElseGet(Collections::emptyList)
          .stream().map(Class::getSimpleName).collect(Collectors.joining(","));
      String argValues = Optional.ofNullable(constructorArgs).orElseGet(Collections::emptyList)
          .stream().map(String::valueOf).collect(Collectors.joining(","));
      throw new ReflectionException("Error instantiating " + type + " with invalid types (" + argTypes + ") or values (" + argValues + "). Cause: " + e, e);
    }
  }

只要我们实现了该接口，我们就可以自己定义一个用来创建对象的ObjectFactory，比如可以修改集合接口的默认实现类等
下面是官方给的一个自定义的例子，它这里主要做的就是修改集合接口的默认实现类

public class CustomObjectFactory implements ObjectFactory {

    @Override
    public <T> T create(Class<T> type) {
        return create(type, null, null);
    }

    @Override
    public <T> T create(Class<T> type, List<Class<?>> constructorArgTypes, List<Object> constructorArgs) {
        Class<?> classToCreate = resolveInterface(type);
        @SuppressWarnings("unchecked") // we know types are assignable
        T created = (T) instantiateClass(classToCreate, constructorArgTypes, constructorArgs);
        return created;
    }

    private <T> T instantiateClass(Class<T> type, List<Class<?>> constructorArgTypes, List<Object> constructorArgs) {
        try {
            Constructor<T> constructor;
            if (constructorArgTypes == null || constructorArgs == null) {
                constructor = type.getDeclaredConstructor();
                if (!constructor.isAccessible()) {
                    constructor.setAccessible(true);
                }
                return constructor.newInstance();
            }
            constructor = type.getDeclaredConstructor(constructorArgTypes.toArray(new Class[constructorArgTypes.size()]));
            if (!constructor.isAccessible()) {
                constructor.setAccessible(true);
            }
            return constructor.newInstance(constructorArgs.toArray(new Object[constructorArgs.size()]));
        } catch (Exception e) {
            StringBuilder argTypes = new StringBuilder();
            if (constructorArgTypes != null) {
                for (Class<?> argType : constructorArgTypes) {
                    argTypes.append(argType.getSimpleName());
                    argTypes.append(",");
                }
            }
            StringBuilder argValues = new StringBuilder();
            if (constructorArgs != null) {
                for (Object argValue : constructorArgs) {
                    argValues.append(String.valueOf(argValue));
                    argValues.append(",");
                }
            }
            throw new ReflectionException("Error instantiating " + type + " with invalid types (" + argTypes + ") or values (" + argValues + "). Cause: " + e, e);
        }
    }

    private Class<?> resolveInterface(Class<?> type) {
        Class<?> classToCreate;
        if (type == List.class || type == Collection.class) {
            classToCreate = LinkedList.class;
        } else if (type == Map.class) {
            classToCreate = LinkedHashMap.class;
        } else if (type == SortedSet.class) { // issue #510 Collections Support
            classToCreate = TreeSet.class;
        } else if (type == Set.class) {
            classToCreate = HashSet.class;
        } else {
            classToCreate = type;
        }
        return classToCreate;
    }

    @Override
    public <T> boolean isCollection(Class<T> type) {
      return CustomCollection.class.isAssignableFrom(type);
    }

    @SuppressWarnings("unchecked")
    public <T> T[] createArray(Class<T> type, int size) {
      return (T[]) Array.newInstance(type, size);
    }

}