Druid 数据库连接池如何根据url加载Driver

获取数据库连接首先从getConnection开始：

DruidDataSource.class

 @Override
    public DruidPooledConnection getConnection() throws SQLException {
        return getConnection(maxWait);
    }

第二步进入getConnection(maxWait)方法：

DruidDataSource.class

public DruidPooledConnection getConnection(long maxWaitMillis) throws SQLException {
        init();

        if (filters.size() > 0) {
            FilterChainImpl filterChain = new FilterChainImpl(this);
            return filterChain.dataSource_connect(this, maxWaitMillis);
        } else {
            return getConnectionDirect(maxWaitMillis);
        }
    }

第三步进入init()方法：

public void init() throws SQLException {
    
           ......
            if (this.driver == null) {
                if (this.driverClass == null || this.driverClass.isEmpty()) {
                    this.driverClass = JdbcUtils.getDriverClassName(this.jdbcUrl);
                }
           ......
               
    }

第四步进入关键的JdbcUtils.getDriverClassName(this.jdbcUrl)方法：

public static String getDriverClassName(String rawUrl) throws SQLException {
        if (rawUrl == null) {
            return null;
        }
        
        if (rawUrl.startsWith("jdbc:derby:")) {
            return "org.apache.derby.jdbc.EmbeddedDriver";
        } else if (rawUrl.startsWith("jdbc:mysql:")) {
            if (mysql_driver_version_6 == null) {
                mysql_driver_version_6 = Utils.loadClass("com.mysql.cj.jdbc.Driver") != null;
            }

            if (mysql_driver_version_6) {
                return MYSQL_DRIVER_6;
            } else {
                return MYSQL_DRIVER;
            }
        } else if (rawUrl.startsWith("jdbc:log4jdbc:")) {
            return LOG4JDBC_DRIVER;
        } else if (rawUrl.startsWith("jdbc:mariadb:")) {
            return MARIADB_DRIVER;
        } else if (rawUrl.startsWith("jdbc:oracle:") //
                   || rawUrl.startsWith("JDBC:oracle:")) {
            return ORACLE_DRIVER;
        } else if (rawUrl.startsWith("jdbc:alibaba:oracle:")) {
            return ALI_ORACLE_DRIVER;
        } else if (rawUrl.startsWith("jdbc:microsoft:")) {
            return "com.microsoft.jdbc.sqlserver.SQLServerDriver";
        } else if (rawUrl.startsWith("jdbc:sqlserver:")) {
            return "com.microsoft.sqlserver.jdbc.SQLServerDriver";
        } else if (rawUrl.startsWith("jdbc:sybase:Tds:")) {
            return "com.sybase.jdbc2.jdbc.SybDriver";
        } else if (rawUrl.startsWith("jdbc:jtds:")) {
            return "net.sourceforge.jtds.jdbc.Driver";
        } else if (rawUrl.startsWith("jdbc:fake:") || rawUrl.startsWith("jdbc:mock:")) {
            return "com.alibaba.druid.mock.MockDriver";
        } else if (rawUrl.startsWith("jdbc:postgresql:")) {
            return POSTGRESQL_DRIVER;
        } else if (rawUrl.startsWith("jdbc:edb:")) {
            return ENTERPRISEDB_DRIVER;
        } else if (rawUrl.startsWith("jdbc:odps:")) {
            return ODPS_DRIVER;
        } else if (rawUrl.startsWith("jdbc:hsqldb:")) {
            return "org.hsqldb.jdbcDriver";
        } else if (rawUrl.startsWith("jdbc:db2:")) {
            return DB2_DRIVER;
        } else if (rawUrl.startsWith("jdbc:sqlite:")) {
            return SQLITE_DRIVER;
        } else if (rawUrl.startsWith("jdbc:ingres:")) {
            return "com.ingres.jdbc.IngresDriver";
        } else if (rawUrl.startsWith("jdbc:h2:")) {
            return H2_DRIVER;
        } else if (rawUrl.startsWith("jdbc:mckoi:")) {
            return "com.mckoi.JDBCDriver";
        } else if (rawUrl.startsWith("jdbc:cloudscape:")) {
            return "COM.cloudscape.core.JDBCDriver";
        } else if (rawUrl.startsWith("jdbc:informix-sqli:")) {
            return "com.informix.jdbc.IfxDriver";
        } else if (rawUrl.startsWith("jdbc:timesten:")) {
            return "com.timesten.jdbc.TimesTenDriver";
        } else if (rawUrl.startsWith("jdbc:as400:")) {
            return "com.ibm.as400.access.AS400JDBCDriver";
        } else if (rawUrl.startsWith("jdbc:sapdb:")) {
            return "com.sap.dbtech.jdbc.DriverSapDB";
        } else if (rawUrl.startsWith("jdbc:JSQLConnect:")) {
            return "com.jnetdirect.jsql.JSQLDriver";
        } else if (rawUrl.startsWith("jdbc:JTurbo:")) {
            return "com.newatlanta.jturbo.driver.Driver";
        } else if (rawUrl.startsWith("jdbc:firebirdsql:")) {
            return "org.firebirdsql.jdbc.FBDriver";
        } else if (rawUrl.startsWith("jdbc:interbase:")) {
            return "interbase.interclient.Driver";
        } else if (rawUrl.startsWith("jdbc:pointbase:")) {
            return "com.pointbase.jdbc.jdbcUniversalDriver";
        } else if (rawUrl.startsWith("jdbc:edbc:")) {
            return "ca.edbc.jdbc.EdbcDriver";
        } else if (rawUrl.startsWith("jdbc:mimer:multi1:")) {
            return "com.mimer.jdbc.Driver";
        } else if (rawUrl.startsWith("jdbc:dm:")) {
            return JdbcConstants.DM_DRIVER;
        } else if (rawUrl.startsWith("jdbc:kingbase:")) {
            return JdbcConstants.KINGBASE_DRIVER;
        } else if (rawUrl.startsWith("jdbc:gbase:")) {
            return JdbcConstants.GBASE_DRIVER;
        } else if (rawUrl.startsWith("jdbc:xugu:")) {
            return JdbcConstants.XUGU_DRIVER;
        } else if (rawUrl.startsWith("jdbc:hive:")) {
            return JdbcConstants.HIVE_DRIVER;
        } else if (rawUrl.startsWith("jdbc:hive2:")) {
            return JdbcConstants.HIVE_DRIVER;
        } else if (rawUrl.startsWith("jdbc:phoenix:thin:")) {
            return "org.apache.phoenix.queryserver.client.Driver";
        } else if (rawUrl.startsWith("jdbc:phoenix://")) {
            return JdbcConstants.PHOENIX_DRIVER;
        } else if (rawUrl.startsWith("jdbc:kylin:")) {
            return JdbcConstants.KYLIN_DRIVER;
        } else if (rawUrl.startsWith("jdbc:elastic:")) {
            return JdbcConstants.ELASTIC_SEARCH_DRIVER;
        } else if (rawUrl.startsWith("jdbc:clickhouse:")) {
            return JdbcConstants.CLICKHOUSE_DRIVER;
        } else if(rawUrl.startsWith("jdbc:presto:")) {
            return JdbcConstants.PRESTO_DRIVER;
        }else {
            throw new SQLException("unkow jdbc driver : " + rawUrl);
        }
    }

可以看出是根据配置的url进行匹配获取Driver驱动的全类名，再通过反射的方式实例化Driver驱动。

比如url: jdbc:mysql://localhost:3306就对应于下述代码：

if (rawUrl.startsWith("jdbc:mysql:")) {
            if (mysql_driver_version_6 == null) {
                mysql_driver_version_6 = Utils.loadClass("com.mysql.cj.jdbc.Driver") != null;
            }

            if (mysql_driver_version_6) {
                return MYSQL_DRIVER_6;
            } else {
                return MYSQL_DRIVER;
            }
        }

最终对应于com.mysql.jdbc.Driver的全类名。

另外：虽然代码中匹配了绝大多数jdbc连接的Driver驱动，但如果恰好用到一个未覆盖到的驱动，那就需要显式的配置driverClassName属性为驱动的全类名。