"Incorrect string value" when trying to insert UTF-8 into MySQL via JDBC?

MySQL's utf8 permits only the Unicode characters that can be represented with 3 bytes in UTF-8. Here you have a character that needs 4 bytes: \xF0\x90\x8D\x83 (U+10343 GOTHIC LETTER SAUIL).

If you have MySQL 5.5 or later you can change the column encoding from utf8 to utf8mb4. This encoding allows storage of characters that occupy 4 bytes in UTF-8.

You may also have to set the server property character_set_server to utf8mb4 in the MySQL configuration file. It seems that Connector/J defaults to 3-byte Unicode otherwise:

For example, to use 4-byte UTF-8 character sets with Connector/J, configure the MySQL server with character_set_server=utf8mb4, and leave characterEncoding out of the Connector/J connection string. Connector/J will then autodetect the UTF-8 setting.

The strings that contain \xF0 are simply characters encoded as multiple bytes using UTF-8.

Although your collation is set to utf8_general_ci, I suspect that the character encoding of the database, table or even column may be different. They are independent settings. Try:

ALTER TABLE database.table MODIFY COLUMN col VARCHAR(255)  
    CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci NOT NULL;

Substitute whatever your actual data type is for VARCHAR(255)

Got the same problem, to save the data with utf8mb4 needs to make sure:

1. character_set_client, character_set_connection, character_set_results are utf8mb4: character_set_client and character_set_connection indicate the character set in which statements are sent by the client, character_set_results indicates the character set in which the server returns query results to the client.
   See charset-connection.

2. the table and column encoding is utf8mb4

For JDBC, there are two solutions:

Solution 1 (need to restart MySQL):

1. modify my.cnf like the following and restart MySQL:

   [mysql]
   default-character-set=utf8mb4
   
   [mysqld]
   character-set-server=utf8mb4
   collation-server=utf8mb4_unicode_ci
   

this can make sure the database and character_set_client, character_set_connection, character_set_results are utf8mb4 by default.

2. restart MySQL

3. change the table and column encoding to utf8mb4

4. STOP specifying characterEncoding=UTF-8 and characterSetResults=UTF-8 in the jdbc connector,cause this will override character_set_client, character_set_connection, character_set_results to utf8

Solution two (don't need to restart MySQL):

1. change the table and column encoding to utf8mb4

2. specifying characterEncoding=UTF-8 in the jdbc connector,cause the jdbc connector doesn't suport utf8mb4.

3. write your sql statment like this (need to add allowMultiQueries=true to jdbc connector):

   'SET NAMES utf8mb4;INSERT INTO Mytable ...';
   

this will make sure each connection to the server, character_set_client,character_set_connection,character_set_results are utf8mb4.
Also see charset-connection.