java连接mysql设置UTF-8mb4

为了兼容emoji表情符及一些特殊字符（4字节utf保存的字符，若采用传统的uft8的3字节保存会导致保存报错），需要在项目中设置uft8mb4编码格式。

这里首先看一下mysql官网文档对于使用的mysql驱动的说明：https://dev.mysql.com/doc/connector-j/5.1/en/connector-j-reference-charsets.html 对于驱动版本有明确要求。

Notes
For Connector/J 5.1.46 and earlier: In order to use the utf8mb4 character set for the connection,
 the server MUST be configured with character_set_server=utf8mb4; if that is not the case, 
when UTF-8 is used for characterEncoding in the connection string, it will map to the MySQL character set name utf8, 
which is an alias for utf8mb3.
对于5.1.46及更早的版本，要使用utf8mb4字符集进行连接，服务器必须配置character_set_server=utf8mb4；如果不是这样，
则在连接字符串中使用UTF-8进行字符编码时，它将映射到MySQL字符集名称utf8，这是utf8mb3的别名。

For Connector/J 5.1.47 and later:

When UTF-8 is used for characterEncoding in the connection string, it maps to the MySQL character set name utf8mb4.
当UTF-8用于连接字符串中的字符编码时，它将映射到MySQL字符集名称utf8mb4。

If the connection option connectionCollation is also set alongside characterEncoding and is incompatible with it, 
characterEncoding will be overridden with the encoding corresponding to connectionCollation.

Because there is no Java-style character set name for utfmb3 that you can use with the connection option charaterEncoding, 
the only way to use utf8mb3 as your connection character set is to use a utf8mb3 collation (for example, utf8_general_ci) for the connection option connectionCollation, 
which forces a utf8mb3 character set to be used, as explained in the last bullet.
由于utfmb3没有可用于连接选项charatencoding的Java样式字符集名称，
因此使用utf8mb3作为连接字符集的唯一方法是对连接选项connection collation使用utf8mb3排序规则（例如，utf8_general_ci），
这将强制使用utf8mb3字符集。

另外：

• mysql的版本不能太低，低于5.5.3的版本不支持utf8mb4编码。
• 若出现仍未生效的情况，可考虑修改mysql数据库对于字符的配置，以及相应的表对于字符的配置。

大致需要修改的配置如下：

• mysql配置文件：

[client]
default-character-set=utf8mb4
[mysql]
default-character-set=utf8mb4
[mysqld]
character-set-server=utf8mb4
collation-server=utf8mb4_general_ci

• 建库时，指定字符集编码为 utf8mb4、字符集校对规则为 utf8mb4_general_ci 或 utf8mb4_unicode_ci。
• 建表时指定 utf8mb4 编码。