看到这里，相信大家已经搞清楚SPI的机制了。这是 jdk 中的 spi机制，springboot 的spi机制稍有不同，区别在于读取录取以及文件内容不同，这部分在后续自动装配原理的时候在说明。

JDBC中的应用

我们开头说，SPI机制为很多框架的拓展提供了可能，其实JDBC就应用到了这一机制。使用JDBC的步骤无非就是加载驱动程序，然后通过DriverManager获取数据库连接。那么它是如何分别是那种数据库的呢？答案就在SPI中。

• 加载

  我们来看看DriverManager类，它在静态代码块立马做了一件比较重要的事情。很明显，它以及通过SPI机制把数据库驱动连接初始化了。

  public class DriverManager {/* Prevent the DriverManager class from being instantiated. */private DriverManager(){}/*** Load the initial JDBC drivers by checking the System property* jdbc.properties and then use the {@code ServiceLoader} mechanism*/static {loadInitialDrivers();println("JDBC DriverManager initialized");}
  }
  

  具体过程还得看loadInitialDrivers方法，它在里面找的是Driver接口的服务类，所以它的文件路径就是：META-INF/services/java.sql.Driver。

  private static void loadInitialDrivers() {String drivers;try {drivers = AccessController.doPrivileged(new PrivilegedAction() {public String run() {return System.getProperty("jdbc.drivers");}});} catch (Exception ex) {drivers = null;}AccessController.doPrivileged(new PrivilegedAction() {public Void run() {// 这里很明显的可以看出它要加载Driver接口的服务，Driver接口的包为：java.sql.Driver// 所以这里要找的就是META-INF/services/java.sql.Driver文件ServiceLoader loadedDrivers = ServiceLoader.load(Driver.class);Iterator driversIterator = loadedDrivers.iterator();try{// 查询之后创建对象while(driversIterator.hasNext()) {driversIterator.next();}} catch(Throwable t) {// Do nothing}return null;}});println("DriverManager.initialize: jdbc.drivers = " + drivers);if (drivers == null || drivers.equals("")) {return;}String[] driversList = drivers.split(":");println("number of Drivers:" + driversList.length);for (String aDriver : driversList) {try {println("DriverManager.Initialize: loading " + aDriver);Class.forName(aDriver, true,ClassLoader.getSystemClassLoader());} catch (Exception ex) {println("DriverManager.Initialize: load failed: " + ex);}}
  }
  

  那么，这个文件在哪呢？我们看看MYSQL的jar包即可，一眼看出就是通过SPI机制进行拓展。

• ·创建实例

  上一步已经找到了MySQL中的com.mysql.cj.jdbc.Driver全限定类名，当调用next方法的时候就会创建实例。mysql的实现类里面就完成了一件事，想DriverManager注册自身的实例。

  public class Driver extends NonRegisteringDriver implements java.sql.Driver {public Driver() throws SQLException {}static {try {// 调用注册方法往registerDrivers集合中加入实例DriverManager.registerDriver(new Driver());} catch (SQLException var1) {throw new RuntimeException("Can't register driver!");}}
  }
  

• 创建 Connection

  在DriverManager.getConnection方法就是创建连接的地方，它通过循环已注册的数据库驱动程序，调用其connect方法，获取连接并返回。

  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;// 循环已经注册的Driver实例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());// 创建连接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 we got here nobody could connect.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");
  }
  

手写自己的数据库连接驱动

既然我们已经知道JDBC是怎么创建数据库连接的，那我们是不是可以创建一个我们自己的 Driver 文件。

我们首先创建实现类，继承自MySQL中的 NonRegisteringDriver，还要实现java.sql.Driver接口。这样在调用connect的时候就会调用到此类，但实际的创建过程还是靠MYSQL完成。

public class MyDriver extends NonRegisteringDriver implements Driver {// 仿照 mysql 的Driver , 先进行注册static {try {DriverManager.registerDriver(new MyDriver());} catch (SQLException var1) {throw new RuntimeException("Can't register driver!");}}public MyDriver() throws SQLException {}// 重写连接方法@Overridepublic Connection connect(String url, Properties info) throws SQLException {System.out.println("准备创建数据库连接 url:" + url);System.out.println("JDBC 配置信息：" + info);info.setProperty("user","root");Connection connect = super.connect(url, info);System.out.println("数据库连接创建完成：" + connect.toString());return connect;}
}

然后按照 SPI 的配置，我们去classpath下创建相关文件 java.sql.Driver （接口的全限定类名）：

内容为实现类的全限定类名。