一文了解mybatis的延迟加载

  private Object createResultObject(ResultSetWrapper rsw, ResultMap resultMap, ResultLoaderMap lazyLoader, String columnPrefix) throws SQLException {
    this.useConstructorMappings = false; // reset previous mapping result
    final List<Class<?>> constructorArgTypes = new ArrayList<>();
    final List<Object> constructorArgs = new ArrayList<>();
    // 创建result接收对象
    Object resultObject = createResultObject(rsw, resultMap, constructorArgTypes, constructorArgs, columnPrefix);
    if (resultObject != null && !hasTypeHandlerForResultObject(rsw, resultMap.getType())) {
      // 处理其他属性properties
      final List<ResultMapping> propertyMappings = resultMap.getPropertyResultMappings();
      for (ResultMapping propertyMapping : propertyMappings) {
        // issue gcode #109 && issue #149 创建代理
        if (propertyMapping.getNestedQueryId() != null && propertyMapping.isLazy()) {
          resultObject = configuration.getProxyFactory().createProxy(resultObject, lazyLoader, configuration, objectFactory, constructorArgTypes, constructorArgs);
          break;
        }
      }
    }
    // 使用有参构造函数创建了对象
    this.useConstructorMappings = resultObject != null && !constructorArgTypes.isEmpty(); // set current mapping result
    return resultObject;
  }

public class HelloMethodHandler implements MethodHandler {

  private Object target;

  public HelloMethodHandler(Object o) {
    this.target = o;
  }

  @Override
  public Object invoke(Object self, Method thisMethod, Method proceed, Object[] args) throws Throwable {
    String methodName = thisMethod.getName();
    if (methodName.startsWith("get")) {
      System.out.println("select database....");
      // 进行sql查询到结果并set设置值
      ((Student)self).setName("monian");
    }
    return proceed.invoke(self, args);
  }

  public static void main(String[] args) throws Exception {
    Student student = new Student();
    ProxyFactory proxyFactory = new ProxyFactory();
    proxyFactory.setSuperclass(Student.class);

    Constructor<Student> declaredConstructor = Student.class.getDeclaredConstructor();
    Object o = proxyFactory.create(declaredConstructor.getParameterTypes(), new Object[]{});
    ((Proxy)o).setHandler(new HelloMethodHandler(student));

    Student proxy = (Student)o;
    System.out.println(proxy.getName());
  }
}

public class UserDO {

  private Integer userId;

  private String username;

  private String password;

  private String nickname;

  private List<PermitDO> permitDOList;

  public UserDO() {}
}

<resultMap id="BaseMap" type="org.apache.ibatis.study.entity.UserDO">
    <id column="user_id" jdbcType="INTEGER" property="userId" />
    <result column="username" jdbcType="VARCHAR" property="username" />
    <result column="password" jdbcType="VARCHAR" property="password" />
    <result column="nickname" jdbcType="VARCHAR" property="nickname"/>

    <collection property="permitDOList" column="user_id" select="getPermitsByUserId"
     fetchType="lazy">

    </collection>
</resultMap>

  <resultMap id="PermitBaseMap" type="org.apache.ibatis.study.entity.PermitDO">
    <id column="id" jdbcType="INTEGER" property="id"/>
    <result column="code" jdbcType="VARCHAR" property="code"/>
    <result column="name" jdbcType="VARCHAR" property="name"/>
    <result column="type" jdbcType="TINYINT" property="type"/>
    <result column="pid" jdbcType="INTEGER" property="pid"/>
  </resultMap>
  
      
   <select id="getByUserId2" resultMap="BaseMap">
    select * from user
    where user_id = #{userId}
  </select>

  <select id="getPermitsByUserId" resultMap="PermitBaseMap">
    select p.*
    from user_permit up
    inner join permit p on up.permit_id = p.id
    where up.user_id = #{userId}
  </select>

public class Test {

  public static void main(String[] args) throws IOException {

    try (InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml")) {
      // 构建session工厂 DefaultSqlSessionFactory
      SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
      SqlSession sqlSession = sqlSessionFactory.openSession();
      UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
      UserDO userDO = userMapper.getByUserId2(1);
      System.out.println(userDO);
    }
  }

}