package com.yidan.spi;

/**
 * @author wuxuan.chai
 * @date 2021/2/2 6:19 下午
 */
public interface FactoryBeanSpi {
    /**
     * 服务实现类单利
     * @param tClass 实现类
     * @param <T> 类型枚举
     * @return 服务实现类单利
     */
    <T> T getInstance(Class<T> tClass) throws IllegalAccessException, InstantiationException;

    /**
     * 服务
     * @return 服务
     */
    String getService();
}

package com.yidan.spi.services;

import com.yidan.spi.FactoryBeanSpi;

/**
 * @author wuxuan.chai
 * @date 2021/2/2 6:24 下午
 */
public class MySpiFactoryBean implements FactoryBeanSpi {
    @Override
    public <T> T getInstance(Class<T> tClass) throws IllegalAccessException, InstantiationException {
        System.out.println("MySpiFactoryBean在构建：" + tClass.getName());
        return tClass.newInstance();
    }

    @Override
    public String getService() {
        return "MySpiFactoryBean";
    }
}

package com.yidan.spi.services;

import com.yidan.spi.FactoryBeanSpi;

import java.util.Optional;
import java.util.ServiceLoader;
import java.util.stream.StreamSupport;

/**
 * @author wuxuan.chai
 * @date 2021/2/2 6:27 下午
 */
public class MyBeanManager {
    public static FactoryBeanSpi fetchFactoryBean() {
        final ServiceLoader<FactoryBeanSpi> mySpiFactoryBeans = ServiceLoader.load(FactoryBeanSpi.class);
        final Optional<FactoryBeanSpi> factoryBeanOptional = StreamSupport.stream(mySpiFactoryBeans.spliterator(), false).findFirst();
        final FactoryBeanSpi factoryBean = factoryBeanOptional.orElseThrow(() -> new RuntimeException("SPI 加载失败，未找到：" + FactoryBeanSpi.class.getName() + "的实现"));
        return factoryBean;
    }

    public static void main(String[] args) throws InstantiationException, IllegalAccessException {
        final FactoryBeanSpi mySpiFactoryBean = fetchFactoryBean();
        mySpiFactoryBean.getInstance(MyBean.class);
        final String service = mySpiFactoryBean.getService();
        System.out.println(service);
    }
}

package com.yidan.spi.services;

/**
 * @author wuxuan.chai
 * @date 2021/2/2 6:27 下午
 */
public class MyBean {

    public MyBean(){
        System.out.println("MyBean构造方法");
    }
}

//加载驱动
    Class.forName("com.mysql.jdbc.Driver");
    //创建连接
    final Connection connection = DriverManager.getConnection("数据库jdbcURL", "数据库username", "数据库password");
    //预执行
    final PreparedStatement preparedStatement = connection.prepareStatement("select 1");
    //执行
    final ResultSet resultSet = preparedStatement.executeQuery();
    while (resultSet.next()){
        //TODO 获取结果
    }
    
    //释放资源
    preparedStatement.close();
    connection.close();

public class DriverManager {
		/**
     * 通过检查系统属性JDBC来加载初始的JDBC驱动程序，然后使用{@code ServiceLoader}（SPI）机制
     */
    static {
        loadInitialDrivers();
        println("JDBC DriverManager initialized");
    }
  
  	//在加载初始化的驱动中，首先根据环境变量jdbc.drivers去加载驱动，再根据SPI获取驱动实现类，SPI的驱动实现类直接通过AccessController.doPrivileged直接加载到了JVM中，环境变量加载的通过Class.forName加载JVM中，所以我们在调用Driver.getConnection不需要在之前调用Class.forName了
  	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;
        }
        // If the driver is packaged as a Service Provider, load it.
        // Get all the drivers through the classloader
        // exposed as a java.sql.Driver.class service.
        // ServiceLoader.load() replaces the sun.misc.Providers()

        AccessController.doPrivileged(new PrivilegedAction<Void>() {
            public Void run() {

                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.
                 */
                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);
            }
        }
    }
  
  
}