mysql/Java服务端对emoji(utf8mb4编码)的支持有关的问题

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mysql中保存4字节长度的UTF-8字符(例如emoji表情)，就需要使用 utf8mb4 字符集。

如果要写入emoji表情(utf8mb4)到mysql，需要应用客户端、客户端到MySQL的连接、以及MySQL实例内部这三者统一，统一使用支持utf8mb4字符集才行。

否则，mysql的jdbc链接驱动mysql-connetoer会抛异常：

java.sql.SQLException: Incorrect string value: '\xF0\x9F\x98\x84],...' for column 'name' at row 1

mysql server 5.5.3+ 版本才支持utf8mb4。

mysql> select version();

mysql-connector-5.1.13+版本才支持utf8mb4。

mysq配置文件修改，mysql修改字符集：
[client]
default-character-set = utf8mb4

[mysql]
default-character-set = utf8mb4

[mysqld]

character-set-server = utf8mb4

#collation-server = utf8mb4_unicode_ci #测试发现可能不用配置

#init_connect=’SET NAMES utf8mb4’ #测试发现可能不用配置

sql_mode='NO_ENGINE_SUBSTITUTION'

重启mysql

mysql> show variables like '%charact%';

+--------------------------+----------------------------+
| Variable_name | Value |
+--------------------------+----------------------------+
| character_set_client | utf8mb4 |
| character_set_connection | utf8mb4 |
| character_set_database | utf8mb4 |
| character_set_filesystem | binary |
| character_set_results | utf8mb4 |
| character_set_server | utf8mb4 |
| character_set_system | utf8 |
| character_sets_dir | /usr/share/mysql/charsets/ |
+--------------------------+----------------------------+
8 rows in set (0.01 sec)

必须保证下面几个正确：
character_set_client (客户端来源数据使用的字符集)
character_set_connection (连接层字符集)
character_set_database (当前选中数据库的默认字符集)
character_set_results (查询结果字符集)
character_set_server (默认的内部操作字符集)

客户端链接测试：

#1: 插入后为乱码：3F3F3F3F
set charset utf8;
insert into test_utf8mb4_src values(9,"test", '??'); #??为直接拷贝的表情符号
select hex(url) from test_utf8mb4_src where id=9;

#2: 插入OK, F09F9884
set charset utf8;
insert into test_utf8mb4_src values(9,"test", 0xF09F9884);
select hex(url) from test_utf8mb4_src where id=9;

#3: 插入OK, F09F9884
set charset utf8;
insert into test_utf8mb4_src values(9,"test", UNHEX('F09F9884'));
select hex(url) from test_utf8mb4_src where id=9;

#4: 插入OK, F09F9884
set charset utf8mb4;
insert into test_utf8mb4_src values(9,"test", '??'); #??为直接拷贝的表情符号
select hex(url) from test_utf8mb4_src where id=9;

#5: 插入OK, F09F9884
set charset utf8mb4;
insert into test_utf8mb4_src values(9,"test", 0xF09F9884);
select hex(url) from test_utf8mb4_src where id=9;

#6: 插入OK, F09F9884
set charset utf8mb4;
insert into test_utf8mb4_src values(9,"test", UNHEX('F09F9884'));
select hex(url) from test_utf8mb4_src where id=9;

----------

总结一下：

不同的配置，有不同测试结果。

1.mysql配置如下，写成功&查询hex值正确
#vim /etc/my.conf
[client]
default-character-set = utf8mb4
[mysql]
default-character-set = utf8mb4
[mysqld]
character-set-server = utf8mb4 #会修改@@character_set_server & @@character_set_database，该参数如果不配置成utf8mb4则无法存储

2.写入会失败
#vim /etc/my.conf

[client]
default-character-set = utf8
[mysql]
default-character-set = utf8
[mysqld]
character-set-server = utf8 #会修改@@character_set_server & @@character_set_database，该参数如果不配置成utf8mb4则无法存储

mysql> show variables like '%char%';
+--------------------------------------+----------------------------+
| Variable_name | Value |
+--------------------------------------+----------------------------+
| character_set_client | utf8 |
| character_set_connection | utf8 |
| character_set_database | utf8 |
| character_set_filesystem | binary |
| character_set_results | utf8 |
| character_set_server | utf8 |
| character_set_system | utf8 |
...
+--------------------------------------+----------------------------+

3.其他的配置，可能会导致写入虽然成功，但写入的值（查看hex）不正确。例如：

default-character-set = utf8mb4

character-set-server=utf8

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