Mybatis配置之文件元素解析

MyBatis 每次创建结果对象的新实例时，它都会使用一个对象工厂（ObjectFactory）实例来完成。 默认的对象工厂需要做的仅仅是实例化目标类，要么通过默认构造方法，要么在参数映射存在的时候通过参数构造方法来实例化。 如果想覆盖对象工厂的默认行为，则可以通过创建自己的对象工厂来实现。ObjectFactory是一个接口，代码如下：

public interface ObjectFactory {

  /**
   * Sets configuration properties.
   * @param properties configuration properties
   */
  void setProperties(Properties properties);

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

  /**
   * Creates a new object with the specified constructor and params.
   * @param type Object type
   * @param constructorArgTypes Constructor argument types
   * @param constructorArgs Constructor argument values
   * @return
   */
  <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 type Object type
   * @return whether it is a collection or not
   * @since 3.1.0
   */
  <T> boolean isCollection(Class<T> type);

}

其默认的实现是org.apache.ibatis.reflection.factory.DefaultObjectFactory

public class DefaultObjectFactory implements ObjectFactory, Serializable {

  private static final long serialVersionUID = -8855120656740914948L;

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

  @SuppressWarnings("unchecked")
  @Override
  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);
  }

  @Override
  public void setProperties(Properties properties) {
    // no props for default
  }

  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 && !constructorArgTypes.isEmpty()) {
        for (Class<?> argType : constructorArgTypes) {
          argTypes.append(argType.getSimpleName());
          argTypes.append(",");
        }
        argTypes.deleteCharAt(argTypes.length() - 1); // remove trailing ,
      }
      StringBuilder argValues = new StringBuilder();
      if (constructorArgs != null && !constructorArgs.isEmpty()) {
        for (Object argValue : constructorArgs) {
          argValues.append(String.valueOf(argValue));
          argValues.append(",");
        }
        argValues.deleteCharAt(argValues.length() - 1); // remove trailing ,
      }
      throw new ReflectionException("Error instantiating " + type + " with invalid types (" + argTypes + ") or values (" + argValues + "). Cause: " + e, e);
    }
  }

  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;
  }

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

}

因此我们如果要自己实现ObjectFactory的话我们可以继承DefaultObjectFactory来简化编程。ObjectFactory 接口很简单，它包含两个创建用的方法，一个是处理默认构造方法的，另外一个是处理带参数的构造方法的。 最后，setProperties 方法可以被用来配置 ObjectFactory，在初始化你的 ObjectFactory 实例后， objectFactory 元素体中定义的属性会被传递给 setProperties 方法。