graph TD
A[注册一个Driver] -->B(建立数据库连接)
B --> C(创建一个Statement)
C-->D(执行SQL语句,获取结果)
D-->F(关闭JDBC对象)

Class.forName("com.mysql.jdbc.Driver")

public class Driver extends com.mysql.cj.jdbc.Driver {
    public Driver() throws SQLException {
        super();
    }
}

public class Driver extends NonRegisteringDriver implements java.sql.Driver {
    static {
        try {
            // 调用DriverManager 注册自己（Driver）
            java.sql.DriverManager.registerDriver(new Driver());
        } catch (SQLException E) {
            throw new RuntimeException("Can't register driver!");
        }
    }
    public Driver() throws SQLException {
        // Class.forName().newInstance() 所需要的
    }
}

    public static synchronized void registerDriver(java.sql.Driver driver)
        throws SQLException {
        registerDriver(driver, null);
    }
    public static synchronized void registerDriver(java.sql.Driver driver,
            DriverAction da)
        throws SQLException {
        /* Register the driver if it has not already been added to our list */
        if(driver != null) {
            // 将驱动相关信息加到registeredDrivers
            registeredDrivers.addIfAbsent(new DriverInfo(driver, da));
        } else {
            // This is for compatibility with the original DriverManager
            throw new NullPointerException();
        }
        println("registerDriver: " + driver);
    }

    @CallerSensitive
    public static Class<?> forName(String className)
                throws ClassNotFoundException {
        Class<?> caller = Reflection.getCallerClass();
        return forName0(className, true, ClassLoader.getClassLoader(caller), caller);
    }

    public static <S> ServiceLoader<S> load(Class<S> service) {
        ClassLoader cl = Thread.currentThread().getContextClassLoader();
        return ServiceLoader.load(service, cl);
    }

connection=DriverManager.getConnection(URL,USER,PASSWROD);

    public static Connection getConnection(String url,
        String user, String password) throws SQLException {
        java.util.Properties info = new java.util.Properties();
        if (user != null) {
            info.put("user", user);
        }
        if (password != null) {
            info.put("password", password);
        }
        return (getConnection(url, info, Reflection.getCallerClass()));
    }

    private static Connection getConnection(
        String url, java.util.Properties info, Class<?> caller) throws SQLException {
        /*
         * When callerCl is null, we should check the application's
         * (which is invoking this class indirectly)
         * classloader, so that the JDBC driver class outside rt.jar
         * can be loaded from here.
         */
        ClassLoader callerCL = caller != null ? caller.getClassLoader() : null;
        synchronized(DriverManager.class) {
            // synchronize loading of the correct classloader.
            if (callerCL == null) {
                callerCL = Thread.currentThread().getContextClassLoader();
            }
        }
        if(url == null) {
            throw new SQLException("The url cannot be null", "08001");
        }
        println("DriverManager.getConnection(\"" + url + "\")");
        // Walk through the loaded registeredDrivers attempting to make a connection.
        // Remember the first exception that gets raised so we can reraise it.
        SQLException reason = null;
        for(DriverInfo aDriver : registeredDrivers) {
            // If the caller does not have permission to load the driver then
            // skip it.
            if(isDriverAllowed(aDriver.driver, callerCL)) {
                try {
                    println("    trying " + aDriver.driver.getClass().getName());
                    Connection con = aDriver.driver.connect(url, info);
                    if (con != null) {
                        // Success!
                        println("getConnection returning " + aDriver.driver.getClass().getName());
                        return (con);
                    }
                } catch (SQLException ex) {
                    if (reason == null) {
                        reason = ex;
                    }
                }
            } else {
                println("    skipping: " + aDriver.getClass().getName());
            }
        }
        if (reason != null)    {
            println("getConnection failed: " + reason);
            throw reason;
        }
        println("getConnection: no suitable driver found for "+ url);
        throw new SQLException("No suitable driver found for "+ url, "08001");
    }

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
public class DBUtil {
    private static String URL="jdbc:mysql://127.0.0.1:3306/test";
    private static String USER="root";
    private static String PASSWROD ="123456";
    private static Connection connection=null;
    static{
        try {
            Class.forName("com.mysql.jdbc.Driver");
            // 获取数据库连接
            connection=DriverManager.getConnection(URL,USER,PASSWROD);
            System.out.println("连接成功");
        } catch (ClassNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (SQLException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
    // 返回数据库连接
    public static Connection getConnection(){
        return connection;
    }
}