myBatis中的实体类一旦给它构造方法，使用就会报错，为什么

Exception in thread "main" org.apache.ibatis.exceptions.PersistenceException:

Error querying database. Cause: org.apache.ibatis.reflection.ReflectionException: Error instantiating class cn.lyh.dto.Order with invalid types () or values (). Cause: java.lang.NoSuchMethodException: cn.lyh.dto.Order.()

The error may exist in cn/lyh/mapper/OrderMapper.xml

The error may involve cn.lyh.mapper.OrderMapper.getOrder-Inline

The error occurred while setting parameters

SQL: select *from orders where order_id=?

Cause: org.apache.ibatis.reflection.ReflectionException: Error instantiating class cn.lyh.dto.Order with invalid types () or values (). Cause: java.lang.NoSuchMethodException: cn.lyh.dto.Order.()

at org.apache.ibatis.exceptions.ExceptionFactory.wrapException(ExceptionFactory.java:23)
at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:107)
at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:98)
at org.apache.ibatis.session.defaults.DefaultSqlSession.selectOne(DefaultSqlSession.java:62)
at cn.lyh.test.test.main(test.java:14)

Caused by: org.apache.ibatis.reflection.ReflectionException: Error instantiating class cn.lyh.dto.Order with invalid types () or values (). Cause: java.lang.NoSuchMethodException: cn.lyh.dto.Order.()
at org.apache.ibatis.reflection.factory.DefaultObjectFactory.instantiateClass(DefaultObjectFactory.java:83)
at org.apache.ibatis.reflection.factory.DefaultObjectFactory.create(DefaultObjectFactory.java:45)
at org.apache.ibatis.reflection.factory.DefaultObjectFactory.create(DefaultObjectFactory.java:38)
at org.apache.ibatis.executor.resultset.DefaultResultSetHandler.createResultObject(DefaultResultSetHandler.java:530)
at org.apache.ibatis.executor.resultset.DefaultResultSetHandler.createResultObject(DefaultResultSetHandler.java:509)
at org.apache.ibatis.executor.resultset.DefaultResultSetHandler.getRowValue(DefaultResultSetHandler.java:329)
at org.apache.ibatis.executor.resultset.DefaultResultSetHandler.handleRowValuesForSimpleResultMap(DefaultResultSetHandler.java:289)
at org.apache.ibatis.executor.resultset.DefaultResultSetHandler.handleRowValues(DefaultResultSetHandler.java:264)
at org.apache.ibatis.executor.resultset.DefaultResultSetHandler.handleResultSet(DefaultResultSetHandler.java:234)
at org.apache.ibatis.executor.resultset.DefaultResultSetHandler.handleResultSets(DefaultResultSetHandler.java:152)
at org.apache.ibatis.executor.statement.PreparedStatementHandler.query(PreparedStatementHandler.java:57)
at org.apache.ibatis.executor.statement.RoutingStatementHandler.query(RoutingStatementHandler.java:70)
at org.apache.ibatis.executor.SimpleExecutor.doQuery(SimpleExecutor.java:57)
at org.apache.ibatis.executor.BaseExecutor.queryFromDatabase(BaseExecutor.java:259)
at org.apache.ibatis.executor.BaseExecutor.query(BaseExecutor.java:132)
at org.apache.ibatis.executor.CachingExecutor.query(CachingExecutor.java:105)
at org.apache.ibatis.executor.CachingExecutor.query(CachingExecutor.java:81)
at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:104)
... 3 more
Caused by: java.lang.NoSuchMethodException: cn.lyh.dto.Order.()
at java.lang.Class.getConstructor0(Unknown Source)
at java.lang.Class.getDeclaredConstructor(Unknown Source)
at org.apache.ibatis.reflection.factory.DefaultObjectFactory.instantiateClass(DefaultObjectFactory.java:57)
... 20 more

你的构造方法是有参的吗？ 实体类必须有一个无参构造方法，否则java通过反射找不到这个类 所以不能被实例化

刚刚创建出来的了类如果你不给他任何构造方法他会默认给你创建一个无参的构造方法,然后你如果手动创建了有产的构造方法,默认的无参方法就没了mybatis的返回值的封装方式是先构造一个无参的对象，然后通过set方法一个一个的往里面set值，所以如果不提供无参构造方法，就会报上述错误，同样的，针对需要的属性值也必须提供get／set方法。