How to Resolve com.mysql.jdbc.exceptions.jdbc4.CommunicationsException in JDBC?

What does the com.mysql.jdbc.exceptions.jdbc4.CommunicationsException mean, and how can I resolve it?

import com.mysql.jdbc.*;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;

public class SqlTest {
    public static void main(String [] args) {
        try {
            Connection conn = DriverManager.getConnection( 
                "jdbc:mysql://localhost:3306/projects?user=user1&password=123");
            System.out.println("Connected?");
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }
}

The 'Communications link failure' error indicates that the JDBC driver cannot communicate with the MySQL server. This might be due to incorrect configuration, server issues, or network problems.

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;

public class SqlTest {
    public static void main(String [] args) {
        try {
            String url = "jdbc:mysql://localhost:3306/projects";
            Connection conn = DriverManager.getConnection(url, "user1", "123");
            System.out.println("Connected successfully!");
            conn.close();
        } catch (SQLException e) {
            System.out.println("Connection failed:");
            e.printStackTrace();
        }
    }
}

Causes

• The MySQL server is not running.
• The specified database URL is incorrect.
• Firewall is blocking access to the MySQL server.
• The MySQL server is not listening on the specified port (default is 3306).
• Invalid user credentials.

Solutions

• Ensure that the MySQL server is running (check with `mysql.server status`).
• Double-check the JDBC URL for accuracy, including the host, port, and database name.
• Verify that your network allows connections to the MySQL server (check firewall settings).
• Inspect MySQL server configuration to confirm that it is listening on the expected port. Use `SHOW VARIABLES LIKE 'port';` in MySQL.
• Confirm that the use of the correct credentials to access the database.

Mistake: Not starting the MySQL server before connection attempts.

Solution: Ensure that the MySQL server is up and running before executing your Java program.

Mistake: Using incorrect JDBC URL syntax.

Solution: Verify the format: jdbc:mysql://[host]:[port]/[database]?user=[username]&password=[password].