我在使用DriverManager时发现的问题

小白今天在连接数据库的时候，心里面突然想起之前有大佬说没有必要添加一条"com.mysql.jdbc.Driver",当时也实验了确实可行，但是这个可是驱动地址，难道是不用添加是根据驱动jar包自动识别出来吗？

开始了磕磕绊绊的探索，

 public void connection4() throws Exception {

        //1.驱动地址，url，user，passwrod
        String driverClassName  = "com.mysql.jdbc.Driver";
        String url = "jdbc:mysql://localhost:3307/mysql_test";
        String user = "root";
        String password = "root";

        //2.建立驱动
        Class<?> aClass = Class.forName(driverClassName);
        Driver driver = (Driver)aClass.newInstance();
        //3.注册驱动
        DriverManager.registerDriver(driver);

        //4.创建连接
        Connection connection = DriverManager.getConnection(url,user,password);

        //在这里，我有个疑问，我想试着看一下通过DriverManager.registerDriver注册的驱动到底有几个？

        //01.先得到驱动
        Enumeration<Driver> drivers = DriverManager.getDrivers();

        int nums = 0;
        //使用while循环来遍历驱动的个数
        while(drivers.hasMoreElements()) {
            nums ++;
            //打印出驱动
            System.out.println(drivers.nextElement());
        }
        //打印出驱动个数
        System.out.println("驱动个数:" + nums);
    }

打印输出，发现居然有3个驱动，我自己只注册了1个。

带着个疑问，我试着注释了建立注册驱动。

public void connection4() throws Exception {

        //1.驱动地址，url，user，passwrod
//      String driverClassName  = "com.mysql.jdbc.Driver";
        String url = "jdbc:mysql://localhost:3307/mysql_test";
        String user = "root";
        String password = "root";

        //2.建立驱动
//        Class<?> aClass = Class.forName(driverClassName);
//        Driver driver = (Driver)aClass.newInstance();
//        //3.注册驱动 DriverManger.registerDriver
//        DriverManager.registerDriver(driver);

        //4.创建连接
        Connection connection = DriverManager.getConnection(url,user,password);

        //在这里，我有个疑问，我想试着看一下通过DriverManager.registerDriver注册的驱动到底有几个？

        //01.先得到驱动
        Enumeration<Driver> drivers = DriverManager.getDrivers();

        int nums = 0;
        //使用while循环来遍历驱动的个数
        while(drivers.hasMoreElements()) {
            nums ++;
            //打印出驱动
            System.out.println(drivers.nextElement());
        }

        //打印出驱动个数
        System.out.println("驱动个数:" + nums);

    }

问题来了，看来我只是注册了1个驱动，多余的两个是从哪里来的呢？

为此我去翻看了一下api文档里面关于DriverManager有这样一段话，

The DriverManager methods getConnection and getDrivers have been enhanced to support the Java Standard Edition Service Provider mechanism. JDBC 4.0 Drivers must include the file META-INF/services/java.sql.Driver. This file contains the name of the JDBC drivers implementation of java.sql.Driver. For example, to load the my.sql.Driver class, the META-INF/services/java.sql.Driver file would contain the entry:（大概是在META-INF包里面包含了my.sql.Driver的类）

 my.sql.Driver

Applications no longer need to explictly load JDBC drivers using Class.forName(). Existing programs which currently load JDBC drivers using Class.forName() will continue to work without modification.

When the method getConnection is called, the DriverManager will attempt to locate a suitable driver from amongst those loaded at initialization and those loaded explicitly using the same classloader as the current applet or application.

确实有两个驱动类，所以自动生成的两个驱动是从这里来的。。

后面大概是在讲，在JDBC 4.0以后 可以不用再很明确的去建立驱动，这样可以简化一些代码。

DriverManager使用过程中发现的问题得到解决。