druid数据源根据url自动加载相应驱动的原理

# 数据源配置
spring:
    datasource:
        type: com.alibaba.druid.pool.DruidDataSource
        #driverClassName: com.mysql.cj.jdbc.Driver
        druid:
            # 主库数据源
            master:
                url: jdbc:mysql://localhost:3306/ry-vue?useUnicode=true&characterEncoding=utf8
                username: root
                password: root

上述为一个常见的springboot使用druid作为数据源的配置，可以看到driverClassName的配置是可以注释掉的，那druid是如何知道该加载哪一种数据库驱动呢，下面一起分析一下。在DataSource的getConnection调用时必须知道加载的驱动是什么，才能拿到相关的Connection，于是从这一步开始分析：

1配置的数据源DruidDataSource：

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

2下一步：

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);
        }
    }

3下一步进入inint方法：

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

4下一步进入JdbcUtils.getDriverClassName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);
        }
    }

从这个方法可以看到druid是根据我们配置的url信息进行查找返回类名字符串，比如我们配置的为：

url: jdbc:mysql://localhost:3306/ry-vue?useUnicode=true&characterEncoding=utf8

那根据jdbc:mysql即返回mysql的驱动类名，后续再通过反射进行加载使用。

---

druid官网说明：Druid是根据url前缀来识别DriverClass的，这样使得配置更方便简洁。

前缀	DriverCLass	描述信息
jdbc:odps	com.aliyun.odps.jdbc.OdpsDriver
jdbc:derby	org.apache.derby.jdbc.EmbeddedDriver
jdbc:mysql	com.mysql.jdbc.Driver
jdbc:oracle	oracle.jdbc.driver.OracleDriver
jdbc:microsoft	com.microsoft.jdbc.sqlserver.SQLServerDriver
jdbc:sybase:Tds	com.sybase.jdbc2.jdbc.SybDriver
jdbc:jtds	net.sourceforge.jtds.jdbc.Driver
jdbc:postgresql	org.postgresql.Driver
jdbc:fake	com.alibaba.druid.mock.MockDriver
jdbc:mock	com.alibaba.druid.mock.MockDriver
jdbc:hsqldb	org.hsqldb.jdbcDriver
jdbc:db2	COM.ibm.db2.jdbc.app.DB2Driver	DB2的JDBC Driver十分混乱，这个匹配不一定对
jdbc:sqlite	org.sqlite.JDBC
jdbc:ingres	com.ingres.jdbc.IngresDriver
jdbc:h2	org.h2.Driver
jdbc:mckoi	com.mckoi.JDBCDriver
jdbc:cloudscape	COM.cloudscape.core.JDBCDriver
jdbc:informix-sqli	com.informix.jdbc.IfxDriver
jdbc:timesten	com.timesten.jdbc.TimesTenDriver
jdbc:as400	com.ibm.as400.access.AS400JDBCDriver
jdbc:sapdb	com.sap.dbtech.jdbc.DriverSapDB
jdbc:JSQLConnect	com.jnetdirect.jsql.JSQLDriver
jdbc:JTurbo	com.newatlanta.jturbo.driver.Driver
jdbc:firebirdsql	org.firebirdsql.jdbc.FBDriver
jdbc:interbase	interbase.interclient.Driver
jdbc:pointbase	com.pointbase.jdbc.jdbcUniversalDriver
jdbc:edbc	ca.edbc.jdbc.EdbcDriver
jdbc:mimer:multi1	com.mimer.jdbc.Driver