Druid之driver怎么注册的,sql执行怎么对应数据库厂商实现的方法

1.Sun公司在数据库的链接方式中只定义了接口java.sql.Driver，并未提供对各个数据库产品的链接实现方式，并且要求各数据库厂商在实现java.sql.Driver接口的同时，要将驱动类注册到java.sql.DriverManager中。

1)譬如mysql怎么把驱动类注册到DriverManager，通过静态代码块调用DriverManager注册方法进行注册

com.mysql.cj.jdbc.Driver，最重要的方法也就是建立连接Connection connect(String url, java.util.Properties info),所以连接池的建立连接都要调用这个方法

    static {
        try {
            java.sql.DriverManager.registerDriver(new Driver());
        } catch (SQLException E) {
            throw new RuntimeException("Can't register driver!");
        }
    }

registerDriver其实是把驱动放进CopyOnWriteArrayList里

    private final static CopyOnWriteArrayList<DriverInfo> registeredDrivers = new CopyOnWriteArrayList<>();
    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) {
            registeredDrivers.addIfAbsent(new DriverInfo(driver, da));
        } else {
            // This is for compatibility with the original DriverManager
            throw new NullPointerException();
        }

        println("registerDriver: " + driver);

    }

2.Druid或者说其他连接池怎么加载Driver，或者我们自己用jdbc怎么加载Drvier，用类加载器把对应数据库的Driver加载进来

Druid的实现，其他也类似

public static Driver createDriver(ClassLoader classLoader, String driverClassName) throws SQLException {
        Class<?> clazz = null;
        if (classLoader != null) {
            try {
                clazz = classLoader.loadClass(driverClassName);
            } catch (ClassNotFoundException e) {
                // skip
            }
        }

        if (clazz == null) {
            try {
                ClassLoader contextLoader = Thread.currentThread().getContextClassLoader();
                if (contextLoader != null) {
                    clazz = contextLoader.loadClass(driverClassName);
                }
            } catch (ClassNotFoundException e) {
                // skip
            }
        }

        if (clazz == null) {
            try {
                clazz = Class.forName(driverClassName);
            } catch (ClassNotFoundException e) {
                throw new SQLException(e.getMessage(), e);
            }
        }

        try {
            return (Driver) clazz.newInstance();
        } catch (IllegalAccessException e) {
            throw new SQLException(e.getMessage(), e);
        } catch (InstantiationException e) {
            throw new SQLException(e.getMessage(), e);
        }
    }

3.我们拿到连接，怎么执行sql，怎么设置事务，跟Driver一样，sun提供了一个接口java.sql.Connection,各大厂商实现

例如:mysql的实现类是这个com.mysql.cj.jdbc.ConnectionImpl，怎么执行sql，加载mysql的Drvier，调用connect拿到的连接就是ConnectionImpl的实例,剩下的sql执行，不用说都是用mysql的实现

public java.sql.Connection connect(String url, Properties info) throws SQLException {

        try {
            if (!ConnectionUrl.acceptsUrl(url)) {
                /*
                 * According to JDBC spec:
                 * The driver should return "null" if it realizes it is the wrong kind of driver to connect to the given URL. This will be common, as when the
                 * JDBC driver manager is asked to connect to a given URL it passes the URL to each loaded driver in turn.
                 */
                return null;
            }

            ConnectionUrl conStr = ConnectionUrl.getConnectionUrlInstance(url, info);
            switch (conStr.getType()) {
                case SINGLE_CONNECTION:
                    return com.mysql.cj.jdbc.ConnectionImpl.getInstance(conStr.getMainHost());

                case FAILOVER_CONNECTION:
                case FAILOVER_DNS_SRV_CONNECTION:
                    return FailoverConnectionProxy.createProxyInstance(conStr);

                case LOADBALANCE_CONNECTION:
                case LOADBALANCE_DNS_SRV_CONNECTION:
                    return LoadBalancedConnectionProxy.createProxyInstance(conStr);

                case REPLICATION_CONNECTION:
                case REPLICATION_DNS_SRV_CONNECTION:
                    return ReplicationConnectionProxy.createProxyInstance(conStr);

                default:
                    return null;
            }

        } catch (UnsupportedConnectionStringException e) {
            // when Connector/J can't handle this connection string the Driver must return null
            return null;

        } catch (CJException ex) {
            throw ExceptionFactory.createException(UnableToConnectException.class,
                    Messages.getString("NonRegisteringDriver.17", new Object[] { ex.toString() }), ex);
        }
    }