No suitable driver found when connected to mysql in groovy

    import com.mysql.jdbc.jdbc2.optional.MysqlDataSource
    import com.mysql.jdbc.*
    import groovy.sql.*

    /*  the commented code works fine

    MysqlDataSource ds = new MysqlDataSource()
    ds.user = 'root'
    ds.password = ""
    ds.url = 'jdbc:mysql://localhost:3306/test'

    Sql sql=Sql.newInstance(ds)

    sql.close()
    */
    d=Class.forName("com.mysql.jdbc.Driver").newInstance()
    println d.class // class com.mysql.jdbc.Driver


    Sql sql=Sql.newInstance(
    'jdbc:mysql://localhost:3306/test',
    'root',
     "",
    'com.mysql.jdbc.Driver'
    )

the code commented works fine and I can get the instance of Driver
But when I use the

     Sql sql=Sql.newInstance(
     'jdbc:mysql://localhost:3306/test',
     'root',
      "",
     'com.mysql.jdbc.Driver'
     )

it throws a exception: java.sql.SQLException: No suitable driver found for jdbc:mysql://localhost:3306/test
I can not fix it ,is there anyboy coming to help me?