如何使用springboot通过spi机制加载mysql驱动

spring:
  datasource:
    url: jdbc:mysql://localhost:3306/xxxxx?autoReconnect=true&useSSL=false&characterEncoding=utf-8&serverTimezone=Asia/Shanghai
    username: dev
    password: xxxxxx
    platform: mysql

<!--oracle驱动-->
        <dependency>
            <groupId>com.oracle</groupId>
            <artifactId>ojdbc6</artifactId>
            <version>12.1.0.1-atlassian-hosted</version>
        </dependency>
 
        <!--mysql驱动-->
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <scope>runtime</scope>
        </dependency>

 
    /**
    ** 这是一个Configuration类，该类定义了创建HikariDataSource的Bean方法
   ***/
  	@Configuration
	@ConditionalOnClass(HikariDataSource.class)
	@ConditionalOnMissingBean(DataSource.class)
	@ConditionalOnProperty(name = "spring.datasource.type", havingValue = "com.zaxxer.hikari.HikariDataSource",
			matchIfMissing = true)
	static class Hikari {
 
		@Bean
		@ConfigurationProperties(prefix = "spring.datasource.hikari")
		public HikariDataSource dataSource(DataSourceProperties properties) {
            // 使用配置文件中的数据源配置来创建Hikari数据源
			HikariDataSource dataSource = createDataSource(properties, HikariDataSource.class);
			if (StringUtils.hasText(properties.getName())) {
				dataSource.setPoolName(properties.getName());
			}
			return dataSource;
		}
 
	}

abstract class DataSourceConfiguration {
 
	@SuppressWarnings("unchecked")
	protected static <T> T createDataSource(DataSourceProperties properties, Class<? extends DataSource> type) {
        //使用DataSourceProperties数据源配置创建DataSourceBuilder对象（设计模式中的建造者模式）
		return (T) properties.initializeDataSourceBuilder().type(type).build();
	}
 
 
   //下面看下DataSourceBuilder的build方法
    public T build() {
        //在该例子中，type返回的是com.zaxxer.hikari.HikariDataSource类
		Class<? extends DataSource> type = getType();
        //实例化HikariDataSource类
		DataSource result = BeanUtils.instantiateClass(type);
		maybeGetDriverClassName();
        //bind方法中会调用属性的设置，反射机制，在设置driverClassName属性时
		bind(result);
		return (T) result;
	}
 
 
   // HikariConfig的方法，HikariDataSource继承自HikariConfig类
public void setDriverClassName(String driverClassName)
   {
      checkIfSealed();
 
      Class<?> driverClass = null;
      ClassLoader threadContextClassLoader = Thread.currentThread().getContextClassLoader();
      try {
         if (threadContextClassLoader != null) {
            try {
                //加载driverClassName对应的类，即com.mysql.cj.jdbc.Driver类，该类为mysql对应的驱动类
               driverClass = threadContextClassLoader.loadClass(driverClassName);
               LOGGER.debug("Driver class {} found in Thread context class loader {}", driverClassName, threadContextClassLoader);
            }
            catch (ClassNotFoundException e) {
               LOGGER.debug("Driver class {} not found in Thread context class loader {}, trying classloader {}",
                            driverClassName, threadContextClassLoader, this.getClass().getClassLoader());
            }
         }
 
         if (driverClass == null) {
            driverClass = this.getClass().getClassLoader().loadClass(driverClassName);
            LOGGER.debug("Driver class {} found in the HikariConfig class classloader {}", driverClassName, this.getClass().getClassLoader());
         }
      } catch (ClassNotFoundException e) {
         LOGGER.error("Failed to load driver class {} from HikariConfig class classloader {}", driverClassName, this.getClass().getClassLoader());
      }
 
      if (driverClass == null) {
         throw new RuntimeException("Failed to load driver class " + driverClassName + " in either of HikariConfig class loader or Thread context classloader");
      }
 
      try {
         // 创建com.mysql.cj.jdbc.Driver对象，接下来看下com.mysql.cj.jdbc.Driver创建对象过程中发生了什么
         driverClass.newInstance();
         this.driverClassName = driverClassName;
      }
      catch (Exception e) {
         throw new RuntimeException("Failed to instantiate class " + driverClassName, e);
      }
   }
 
 
// com.mysql.cj.jdbc.Driver类
public class Driver extends NonRegisteringDriver implements java.sql.Driver {
    //
    // Register ourselves with the DriverManager
    //
    static {
        try {
            //调用DriverManager注册自身，DriverManager使用CopyOnWriteArrayList来存储已加载的数据库驱动，然后当创建连接时最终会调用DriverManager的getConnection方法，这才是真正面向数据库的，只不过spring的jdbc帮助我们屏蔽了这些细节
            java.sql.DriverManager.registerDriver(new Driver());
        } catch (SQLException E) {
            throw new RuntimeException("Can't register driver!");
        }
    }

static {
        //静态方法，jvm第一次加载该类时会调用该代码块
        loadInitialDrivers();
        println("JDBC DriverManager initialized");
    }
 
    //DriverManager类的loadInitialDrivers方法
 
    private static void loadInitialDrivers() {
        String drivers;
        try {
            drivers = AccessController.doPrivileged(new PrivilegedAction<String>() {
                public String run() {
                    return System.getProperty("jdbc.drivers");
                }
            });
        } catch (Exception ex) {
            drivers = null;
        }
 
        AccessController.doPrivileged(new PrivilegedAction<Void>() {
            public Void run() {
             
                //这就是最终的谜底，最终通过ServiceLoader来加载SPI机制提供的驱动，本文用到了两个，一个是mysql的，一个是oracle的，注意该方法只会在jvm第一次加载DriverManager类时才会调用，所以会一次性加载所有的数据库驱动
                ServiceLoader<Driver> loadedDrivers = ServiceLoader.load(Driver.class);
                Iterator<Driver> driversIterator = loadedDrivers.iterator();
 
                /* Load these drivers, so that they can be instantiated.
                 * It may be the case that the driver class may not be there
                 * i.e. there may be a packaged driver with the service class
                 * as implementation of java.sql.Driver but the actual class
                 * may be missing. In that case a java.util.ServiceConfigurationError
                 * will be thrown at runtime by the VM trying to locate
                 * and load the service.
                 *
                 * Adding a try catch block to catch those runtime errors
                 * if driver not available in classpath but it's
                 * packaged as service and that service is there in classpath.
                 */
                 //下面的代码就是真正完成数据库驱动加载的地方，对应ServiceLoader类的LazyIterator类，所以看下该类的hasNext一级next方法即可，上面已经讲过，这里就不再赘述
                try{
                    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);
            }
        }
    }