java.lang.ClassCastException: class com.mysql.cj.jdbc.ConnectionImpl cannot be cast to

Question

I am trying to set up micro service for my school project. I am using java spring boot framework. But when i compile and run the DB connnection it gave me the error: java.lang.ClassCastException: class com.mysql.cj.jdbc.ConnectionImpl cannot be cast to class com.hrms.employees.db.Connection (com.mysql.cj.jdbc.ConnectionImpl and com.hrms.employees.db.Connection are in unnamed module of loader 'app')

package com.hrms.employees.db;
import java.sql.DriverManager;
import java.sql.SQLException;

public class Connection {
    private static Connection conn;

    static{
        try {
            Class.forName("com.mysql.jdbc.Driver");
            conn = (Connection) DriverManager.getConnection("jdbc:mysql://itsatest2.c45g5gg8tx2m.ap-southeast-1.rds.amazonaws.com","admin","zx55774463");
            System.out.println("connected");
        }
        catch(ClassNotFoundException | SQLException e){
            System.out.println(e.getMessage());
        }
    }

    public static Connection getConnection(){
        return conn;
    }

}

dependency in grade.build

dependencies {
    implementation 'org.springframework.boot:spring-boot-starter-web'
    testImplementation 'org.springframework.boot:spring-boot-starter-test'
    compile group: 'mysql', name: 'mysql-connector-java', version: '8.0.15'
}

I expect the output string of "connected"

Answer 1

The problem is that your class is called Connection , while DriverManager.getConnection() returns a java.sql.Connection . When using MySQL Connector/J the actual returned implementation is com.mysql.cj.jdbc.ConnectionImpl , and this is clearly not an instance of your class com.hrms.employees.db.Connection , so the cast fails.

You either need to rename your class, or you need to define your variable as private static java.sql.Connection conn and remove the cast (so use conn = DriverManager.getConnection ).

As an aside, using a static variable for a database connection is almost always the wrong solution, in long running applications it can cause problems when the connection is closed or lost, and in highly concurrent applications this leads in sharing a single connection between multiple threads which can lead to race conditions or other hard to diagnose bugs.

The better solution would be to use a connection pooling data source like Apache DBCP, c3p0 or HikariCP, and get a connection only for a unit of work, and close it afterwards (which returns it to the pool for reuse).