java.lang.RuntimeException: Class com.mysql.cj.jdbc.Driver not found

Question

I have this conection to MySql in python with JayDeBeApi, using JDBC:

def data_JDBC(db_name, table, db, user, pwd):
    
    db = db.lower()
    db_name = db_name.lower()
    print(db)
    if db == 'postgresql':
        sql_str = f"select * from {db}.{table}"
        host='ec2-18-191-149-107.us-east-2.compute.amazonaws.com'
        port='5432'
        user=user
        pwd = pwd
        driver_name = 'org.postgresql.Driver'
        driver_path = 'path/to/postgresql-42.2.12.jar'        
    elif db == 'mysql':
        sql_str = f"select * from {table}"
        host='localhost'
        port='3306'
        user=user
        pwd = ""
        driver_name = 'com.mysql.cj.jdbc.Driver'
        driver_path = 'path/to/mysql-connector-java-8.0.20.jar'
        print(driver_path)
    connection_string=f'jdbc:{db}://'+ host+':'+ port +'/'+ db_name+'?useSSL=false&&serverTimezone=UTC&useLegacyDatetimeCode=false'
    if jpype.isJVMStarted() and not jpype.isThreadAttachedToJVM():
        jpype.attachThreadToJVM()
        jpype.java.lang.Thread.currentThread().setContextClassLoader(jpype.java.lang.ClassLoader.getSystemClassLoader())
    conn = jaydebeapi.connect(driver_name, connection_string,[user, pwd], jars=driver_path)
    frame = pd.read_sql(sql_str, conn);
    pd.set_option('display.expand_frame_repr', False)
    conn.close()
    return frame

This function returns a Pandas DataFrame, i have the jar files in the same directory. PostgreSQL works fine, but when i select for MySQL i get this error:

java.lang.RuntimeExceptionPyRaisable      Traceback (most recent call last)
<ipython-input-426-311d4ecabc53> in <module>
----> 1 dfMysql=data_JDBC(db_name='fuentedatos', table='tests', db='MySql', user='root', pwd='')
      2 dfMysql

...

java.lang.RuntimeExceptionPyRaisable: java.lang.RuntimeException: Class com.mysql.cj.jdbc.Driver not found

This is how i execute the function:

data_JDBC(db_name='fuentedatos', table='tests', db='MySql', user='root', pwd='')

Answer 1

For that particular error case there are two potential error causes.

1- Usually in the Java world the java.lang.RuntimeException: Class driverClassName not found exception is raised when we hadn't add the jar file's path to the CLASSPATH environment variable and that's what was exactly attempted to be explained here

2- (the most likely one) It seems like Jaydebeapi doesn't support the establishment of multiple connections for different data sources at the same time ( github issue ).

Answer 2

I encountered this issue because I accidentally used 2 containers at the same time, plase make sure only one container is active to open the local directory.