Mysql 时间类型精度截取的bug

mysql-connector-java版本升级出现的一次问题。涉及到了时间精度的截取和四舍五入。

首先了解一点，timestamp,datetime如果不指定精度,默认的精度是秒。

当mysql-connector-java版本<=5.1.22时,db的客户端会将Datetime,Timestamp秒以下的精度丢弃。版本>5.1.22后，秒以下的值将不会截断

db的server端会对超出精度位数的数据进行四舍五入!!

举个例子：在db建表时没指定精度时，插入精确到毫秒级别的日期

如果使用mysql-connector-java版本<=5.1.22，在客户端用'2018-04-02 23:59:59.999'插入日期，精度会在客户端被截取到秒，插入db里是'2018-04-02 23:59:59'

如果升级版本，在db的客户端用'2018-04-02 23:59:59.999'插入日期，精度在客户端不会被截断，db的server端会对超出精度位数的数据进行四舍五入，即插入db里是'2018-04-03 00:00:00 '

所以说mysql-connector-java版本升级就带了时间与原本不一致的问题，结合具体业务逻辑上的使用，可能会造成不同大小的影响。

要想证实这个观点，可以分两步：

server端是否会四舍五入

客户端代码不同版本对精度是否有不同的处理方式

来实际测一下server会不会四舍五入：

CREATE TABLE `time_test` (  
  `id` int(11) NOT NULL AUTO_INCREMENT ,  
  `create_time` timestamp NOT NULL DEFAULT '1971-01-01 00:00:00' ,  
  `end_time` timestamp NOT NULL DEFAULT '1971-01-01 00:00:00' ,  
  PRIMARY KEY (`id`)  
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8mb4;  
    
    
insert into time_test (create_time,end_time) values('2018-04-02 23:59:59','2018-04-02 23:59:59.999');  
    
    
select * from time_test;

看一下记录：

+----+---------------------+---------------------+

| id | create_time | end_time |

+----+---------------------+---------------------+

| 2 | 2018-04-02 23:59:59 | 2018-04-03 00:00:00 |

+----+---------------------+---------------------+

可以看出db的server端果然会进行四舍五入。

再看一下mysql驱动里是怎么写的，是否真的是截断精度了。

Mysql对于时间精度的处理在com.mysql.jdbc.PreparedStatement#setTimestampInternal这个方法中

翻一下5.1.21的源码看一下：

private void setTimestampInternal(int parameterIndex,  
      Timestamp x, Calendar targetCalendar,  
      TimeZone tz, boolean rollForward) throws SQLException {  
   synchronized (checkClosed()) {  
      if (x == null) {  
         setNull(parameterIndex, java.sql.Types.TIMESTAMP);  
      } else {  
         checkClosed();  
             
         if (!this.useLegacyDatetimeCode) {  
            newSetTimestampInternal(parameterIndex, x, targetCalendar);  
         } else {  
            String timestampString = null;  
    
            Calendar sessionCalendar = this.connection.getUseJDBCCompliantTimezoneShift() ?  
                  this.connection.getUtcCalendar() :  
                     getCalendarInstanceForSessionOrNew();  
                   
            synchronized (sessionCalendar) {  
               x = TimeUtil.changeTimezone(this.connection,  
                     sessionCalendar,  
                     targetCalendar,  
                     x, tz, this.connection  
                  .getServerTimezoneTZ(), rollForward);  
            }  
       
            if (this.connection.getUseSSPSCompatibleTimezoneShift()) {  
               doSSPSCompatibleTimezoneShift(parameterIndex, x, sessionCalendar);  
            } else {  
               synchronized (this) {  
                  if (this.tsdf == null) {  
                     //这里，截断秒以下的精度  
                     this.tsdf = new SimpleDateFormat("''yyyy-MM-dd HH:mm:ss''", Locale.US); //$NON-NLS-1$  
                  }  
                      
                  timestampString = this.tsdf.format(x);  
                      
                  //这里永远不会执行添加秒以下的精度  
                  if (false) { // not so long as Bug#50774 is around  
                     StringBuffer buf = new StringBuffer();  
                     buf.append(timestampString);  
                     int nanos = x.getNanos();  
                         
                     if (nanos != 0) {  
                        buf.append('.');  
                        buf.append(formatNanos(nanos));  
                     }  
                         
                     buf.append('\'');  
                  }  
                      
                  setInternal(parameterIndex, timestampString); // SimpleDateFormat is not  
                                                     // thread-safe  
               }  
            }  
         }  
             
         this.parameterTypes[parameterIndex - 1 + getParameterIndexOffset()] = Types.TIMESTAMP;  
      }  
   }  
} 

再看下5.1.32的实现：

private void setTimestampInternal(int parameterIndex,  
      Timestamp x, Calendar targetCalendar,  
      TimeZone tz, boolean rollForward) throws SQLException {  
   synchronized (checkClosed().getConnectionMutex()) {  
      if (x == null) {  
         setNull(parameterIndex, java.sql.Types.TIMESTAMP);  
      } else {  
         checkClosed();  
             
         if (!this.useLegacyDatetimeCode) {  
            newSetTimestampInternal(parameterIndex, x, targetCalendar);  
         } else {  
            Calendar sessionCalendar = this.connection.getUseJDBCCompliantTimezoneShift() ?  
                  this.connection.getUtcCalendar() :  
                     getCalendarInstanceForSessionOrNew();  
                   
            synchronized (sessionCalendar) {  
               x = TimeUtil.changeTimezone(this.connection,  
                     sessionCalendar,  
                     targetCalendar,  
                     x, tz, this.connection  
                  .getServerTimezoneTZ(), rollForward);  
            }  
       
            if (this.connection.getUseSSPSCompatibleTimezoneShift()) {  
               doSSPSCompatibleTimezoneShift(parameterIndex, x, sessionCalendar);  
            } else {  
               synchronized (this) {  
                  //同样截断精度  
                  if (this.tsdf == null) {  
                     this.tsdf = new SimpleDateFormat("''yyyy-MM-dd HH:mm:ss", Locale.US); //$NON-NLS-1$  
                  }  
                      
                  StringBuffer buf = new StringBuffer();  
                  buf.append(this.tsdf.format(x));  
    
                  //这里，如果server支持fractional seconds的话，就加上毫秒的精度  
                  if (this.serverSupportsFracSecs) {  
                     int nanos = x.getNanos();  
                         
                     if (nanos != 0) {  
                        buf.append('.');  
                        buf.append(TimeUtil.formatNanos(nanos, this.serverSupportsFracSecs, true));  
                     }  
                  }  
    
                  buf.append('\'');  
    
                  setInternal(parameterIndex, buf.toString()); // SimpleDateFormat is not  
                                                     // thread-safe  
               }  
            }  
         }  
             
         this.parameterTypes[parameterIndex - 1 + getParameterIndexOffset()] = Types.TIMESTAMP;  
      }  
   }  
}  