MyBatis中Mapper的返回值类型

insert、update、delete语句的返回值类型

对数据库执行修改操作时，数据库会返回受影响的行数。

在MyBatis（使用版本3.4.6，早期版本不支持）中insert、update、delete语句的返回值可以是Integer、Long和Boolean。在定义Mapper接口时直接指定需要的类型即可，无需在对应的<insert><update><delete>标签中显示声明。

对应的代码在 org.apache.ibatis.binding.MapperMethod 类中，如下：

• 对于insert、update、delete语句，MyBatis都会使用 rowCountResult 方法对返回值进行转换；
• rowCountResult 方法会根据Mapper声明中方法的返回值类型来对参数进行转换；
• 对于返回类型为Boolean的情况，如果返回的值大于0，则返回True，否则返回False

 public Object execute(SqlSession sqlSession, Object[] args) {
     Object result;
     switch (command.getType()) {
       case INSERT: {
       Object param = method.convertArgsToSqlCommandParam(args);
         result = rowCountResult(sqlSession.insert(command.getName(), param));
         break;
       }
       case UPDATE: {
         Object param = method.convertArgsToSqlCommandParam(args);
         result = rowCountResult(sqlSession.update(command.getName(), param));
         break;
       }
       case DELETE: {
         Object param = method.convertArgsToSqlCommandParam(args);
         result = rowCountResult(sqlSession.delete(command.getName(), param));
         break;
       }
       case SELECT:
         if (method.returnsVoid() && method.hasResultHandler()) {
           executeWithResultHandler(sqlSession, args);
           result = null;
         } else if (method.returnsMany()) {
           result = executeForMany(sqlSession, args);
         } else if (method.returnsMap()) {
           result = executeForMap(sqlSession, args);
         } else if (method.returnsCursor()) {
           result = executeForCursor(sqlSession, args);
         } else {
           Object param = method.convertArgsToSqlCommandParam(args);
           result = sqlSession.selectOne(command.getName(), param);
         }
         break;
       case FLUSH:
         result = sqlSession.flushStatements();
         break;
       default:
         throw new BindingException("Unknown execution method for: " + command.getName());
     }
     if (result == null && method.getReturnType().isPrimitive() && !method.returnsVoid()) {
       throw new BindingException("Mapper method '" + command.getName()
           + " attempted to return null from a method with a primitive return type (" + method.getReturnType() + ").");
     }
     return result;
   }

   private Object rowCountResult(int rowCount) {
     final Object result;
     if (method.returnsVoid()) {
       result = null;
     } else if (Integer.class.equals(method.getReturnType()) || Integer.TYPE.equals(method.getReturnType())) {
       result = rowCount;
     } else if (Long.class.equals(method.getReturnType()) || Long.TYPE.equals(method.getReturnType())) {
       result = (long)rowCount;
     } else if (Boolean.class.equals(method.getReturnType()) || Boolean.TYPE.equals(method.getReturnType())) {
       result = rowCount > 0;
     } else {
       throw new BindingException("Mapper method '" + command.getName() + "' has an unsupported return type: " + method.getReturnType());
     }
     return result;
   }

select语句的返回值类型

对于select语句，必须在Mapper映射文件中显示声明返回值类型，否则会抛出异常，指出“A query was run and no Result Maps were found for the Mapped Statement”。

• 返回Integer或Long类型
• 与修改语句不同，select语句必须指定返回值类型，且mapper映射文件中的返回值类型需要与mapper接口中声明的返回值类型一致，都是int或long；
• • mapper接口中的返回值类型可以是Integer、Long,也可以是int、long

• 不能返回Boolean类型

　　　select语句不能返回Boolean类型

• 返回自定义类型的Bean

　　select语句返回的column值与Mapper方法返回值的属性的映射有两种方式（具体设置见下文）：

• • 通过名称实现自动映射
  • 通过resultMap标签指定映射方式

• 返回List、Set、Array类型

　　　直接指定select方法的返回值为List<T>、Set<T>、即可，注意此时mapper映射文件中select标签的 resultType类型一定要是自定义的Bean类型，而不是集合类型，可以理解为resultType指定的类型对应的是数据库中的一行数据

• 返回Map类型

　　　返回Map类型有两种含义：

• • resultType的类型为map，mapper接口的返回值也为Map，此时返回结果将Bean的属性作为key，值作为Value返回
  • resultType的类型为非map类型（自定义类型、数值类型、String等），mapper接口的返回值为Map，且指定了mapKey,此时的返回结果以指定的mapKey作为Key,以resultType类型对象作为value

通过column别名实现自定义类型的自动映射

只要保证数据库查询返回的column名称和Bean的属性名一致，Mybatis便能够实现自动映射。如：

    <select id="selectActorById"  resultType="canger.study.chapter04.bean.Actor">
        select actor_id as id, first_name as firstName ,last_name as lastName
        from actor
        where actor_id=#{id}
    </select>

public class Actor {
    Long id;
    String firstName;
    String lastName;
}

需要特别说明的有3个地方：

1. 返回值Bean无需为属性设置getter/setter方法，Mybatis依然能够为其赋值；
2. 如果column名称和Bean的属性名只有部分相同，那么只有名称相同的属性会被赋值，Bean依然会被创建；
3. 如果column名称与所有Bean的属性名都不相同，则select语句会返回null值，即使数据库中存在符合查询条件的记录；

设置mapUnderscoreToCamelCase属性实现自动映射

    <settings>
        <setting name="mapUnderscoreToCamelCase" value="true"/>
    </settings>

• 如果数据库采用了严格的under score命名规则，则通过设置mapUnderscoreToCamelCase属性并可实现column到实体类property的自动映射；
• 实际项目中，可将命名不规范的column通过resultMap进行显示声明，符合命名规范的进行自动映射

通过resultMap标签指定自定义类型的映射方式

需要在mapper映射文件中定义 resultMap标签

    <resultMap id="actorResultMap" type="canger.study.chapter04.bean.Actor">
        <id column="id" property="id" jdbcType="BIGINT" javaType="long" typeHandler="org.apache.ibatis.type.LongTypeHandler"/>
        <result column="firstName" property="firstName" jdbcType="VARCHAR" javaType="string"
                typeHandler="org.apache.ibatis.type.StringTypeHandler"/>
        <result column="lastName" property="lastName" jdbcType="VARCHAR" javaType="string"
                typeHandler="org.apache.ibatis.type.StringTypeHandler"/>
    </resultMap>

    <select id="selectActorById"  resultMap="actorResultMap">
        select actor_id as id, first_name as firstName ,last_name as lastName
        from actor
        where actor_id=#{id}
    </select>

需要注意的几点

• id或result标签中的column属性一定要使用SQL select 语句中最终返回到数据库客户端的列名，如对于select actor_id as id，在resultMap中需要使用 id而不是 actor_id；
• 无需将查询返回的所有column都进行显示声明，没有在resultMap中声明映射关系的column将进行自动映射
• typeHandler属性：如果使用MyBatis提供的typeHandler,则一定要是用全类名，MyBatis默认没有为typeHandler其注册别名；
• jdbcType 、 javaType和typeHandler的对照关系，以及jdbcType和数据库类型的对照关系参见 Mybatis中jdbcType和javaType、typeHandler的对照关系
• MyBatis注册的默认TypeHandler参见类 org.apache.ibatis.type.TypeHandlerRegistry，具体如下

public TypeHandlerRegistry() {
    register(Boolean.class, new BooleanTypeHandler());
    register(boolean.class, new BooleanTypeHandler());
    register(JdbcType.BOOLEAN, new BooleanTypeHandler());
    register(JdbcType.BIT, new BooleanTypeHandler());

    register(Byte.class, new ByteTypeHandler());
    register(byte.class, new ByteTypeHandler());
    register(JdbcType.TINYINT, new ByteTypeHandler());

    register(Short.class, new ShortTypeHandler());
    register(short.class, new ShortTypeHandler());
    register(JdbcType.SMALLINT, new ShortTypeHandler());

    register(Integer.class, new IntegerTypeHandler());
    register(int.class, new IntegerTypeHandler());
    register(JdbcType.INTEGER, new IntegerTypeHandler());

    register(Long.class, new LongTypeHandler());
    register(long.class, new LongTypeHandler());

    register(Float.class, new FloatTypeHandler());
    register(float.class, new FloatTypeHandler());
    register(JdbcType.FLOAT, new FloatTypeHandler());

    register(Double.class, new DoubleTypeHandler());
    register(double.class, new DoubleTypeHandler());
    register(JdbcType.DOUBLE, new DoubleTypeHandler());

    register(Reader.class, new ClobReaderTypeHandler());
    register(String.class, new StringTypeHandler());
    register(String.class, JdbcType.CHAR, new StringTypeHandler());
    register(String.class, JdbcType.CLOB, new ClobTypeHandler());
    register(String.class, JdbcType.VARCHAR, new StringTypeHandler());
    register(String.class, JdbcType.LONGVARCHAR, new ClobTypeHandler());
    register(String.class, JdbcType.NVARCHAR, new NStringTypeHandler());
    register(String.class, JdbcType.NCHAR, new NStringTypeHandler());
    register(String.class, JdbcType.NCLOB, new NClobTypeHandler());
    register(JdbcType.CHAR, new StringTypeHandler());
    register(JdbcType.VARCHAR, new StringTypeHandler());
    register(JdbcType.CLOB, new ClobTypeHandler());
    register(JdbcType.LONGVARCHAR, new ClobTypeHandler());
    register(JdbcType.NVARCHAR, new NStringTypeHandler());
    register(JdbcType.NCHAR, new NStringTypeHandler());
    register(JdbcType.NCLOB, new NClobTypeHandler());

    register(Object.class, JdbcType.ARRAY, new ArrayTypeHandler());
    register(JdbcType.ARRAY, new ArrayTypeHandler());

    register(BigInteger.class, new BigIntegerTypeHandler());
    register(JdbcType.BIGINT, new LongTypeHandler());

    register(BigDecimal.class, new BigDecimalTypeHandler());
    register(JdbcType.REAL, new BigDecimalTypeHandler());
    register(JdbcType.DECIMAL, new BigDecimalTypeHandler());
    register(JdbcType.NUMERIC, new BigDecimalTypeHandler());

    register(InputStream.class, new BlobInputStreamTypeHandler());
    register(Byte[].class, new ByteObjectArrayTypeHandler());
    register(Byte[].class, JdbcType.BLOB, new BlobByteObjectArrayTypeHandler());
    register(Byte[].class, JdbcType.LONGVARBINARY, new BlobByteObjectArrayTypeHandler());
    register(byte[].class, new ByteArrayTypeHandler());
    register(byte[].class, JdbcType.BLOB, new BlobTypeHandler());
    register(byte[].class, JdbcType.LONGVARBINARY, new BlobTypeHandler());
    register(JdbcType.LONGVARBINARY, new BlobTypeHandler());
    register(JdbcType.BLOB, new BlobTypeHandler());

    register(Object.class, UNKNOWN_TYPE_HANDLER);
    register(Object.class, JdbcType.OTHER, UNKNOWN_TYPE_HANDLER);
    register(JdbcType.OTHER, UNKNOWN_TYPE_HANDLER);

    register(Date.class, new DateTypeHandler());
    register(Date.class, JdbcType.DATE, new DateOnlyTypeHandler());
    register(Date.class, JdbcType.TIME, new TimeOnlyTypeHandler());
    register(JdbcType.TIMESTAMP, new DateTypeHandler());
    register(JdbcType.DATE, new DateOnlyTypeHandler());
    register(JdbcType.TIME, new TimeOnlyTypeHandler());

    register(java.sql.Date.class, new SqlDateTypeHandler());
    register(java.sql.Time.class, new SqlTimeTypeHandler());
    register(java.sql.Timestamp.class, new SqlTimestampTypeHandler());

    // mybatis-typehandlers-jsr310
    if (Jdk.dateAndTimeApiExists) {
      this.register(Instant.class, InstantTypeHandler.class);
      this.register(LocalDateTime.class, LocalDateTimeTypeHandler.class);
      this.register(LocalDate.class, LocalDateTypeHandler.class);
      this.register(LocalTime.class, LocalTimeTypeHandler.class);
      this.register(OffsetDateTime.class, OffsetDateTimeTypeHandler.class);
      this.register(OffsetTime.class, OffsetTimeTypeHandler.class);
      this.register(ZonedDateTime.class, ZonedDateTimeTypeHandler.class);
      this.register(Month.class, MonthTypeHandler.class);
      this.register(Year.class, YearTypeHandler.class);
      this.register(YearMonth.class, YearMonthTypeHandler.class);
      this.register(JapaneseDate.class, JapaneseDateTypeHandler.class);
    }