Add new Datasource (mysql) wildfly

Did you add a driver definition? Your datasources subsystem should look something like this:

    <subsystem xmlns="urn:jboss:domain:datasources:2.0">
        <datasources>
            <datasource jndi-name="java:/jdbc/myds" pool-name="myds" enabled="true" use-java-context="true">
                <connection-url>jdbc:mysql://localhost/mydb</connection-url>
                <driver>mysql</driver>
                <security>
                    <user-name>foo</user-name>
                    <password>bar</password>
                </security>
            </datasource>
            <drivers>
                <driver name="h2" module="com.h2database.h2">
                    <xa-datasource-class>org.h2.jdbcx.JdbcDataSource</xa-datasource-class>
                </driver>
                <driver name="mysql" module="com.mysql">
                    <xa-datasource-class>com.mysql.jdbc.jdbc2.optional.MysqlXADataSource</xa-datasource-class>
                </driver>
            </drivers>
        </datasources>
    </subsystem>

The driver element in the data source definition must reference a driver element by name. The module attribute must match the name of your MySQL driver module.

Actually I meet the same problem (I could add the datasources and test connection successfully before) So I'm just confused and I find a way works for me:)

See my services and I find it stopped,and I started it then tried again,It works well again! Even though your service does not stop, maybe just restart it. I have to say it may not work for you if you never success to connect before,good luck~