java SPI机制的使用及原理

/*** * 抽象支付 */
public interface Pay {
   

    void pay();
}

/** * @Auther: * @Date: 2020/5/2 14:28 * @Description: 支付宝付款 */
public class AliPay implements Pay {
   
    @Override
    public void pay() {
   
        System.out.println("使用支付宝pay....");
    }
}

/** * @Auther: * @Date: 2020/5/2 14:28 * @Description: 微信pay */
public class WechatPay implements Pay {
   
    @Override
    public void pay() {
   
        System.out.println("使用微信支付....");
    }
}

/** * @Auther: * @Date: 2020/5/2 14:29 * @Description:银行卡pay */
public class BankCardPay implements Pay {
   
    @Override
    public void pay() {
   
        System.out.println("使用银行卡支付....");
    }
}

/**
 * @Auther:
 * @Date: 2020/5/2 14:36
 * @Description:
 */
public class Context {


    private final Pay pay;

    public Context(Pay pay) {
        this.pay = pay;
    }

    public void invokeStrategy(){
        pay.pay();
    }
}

/** * @Auther: * @Date: 2020/5/2 14:35 * @Description: */
public class PayTest {
   

    public static void main(String[] args) {
   
        //ali
        Context aliContext = new Context(new AliPay());
        aliContext.invokeStrategy();
        //wechat
        Context wechatContext = new Context(new WechatPay());
        wechatContext.invokeStrategy();
    }
}

com.taolong.dubbo.spi.strategy.AliPay

public void invokeStrategy(){
   
    ServiceLoader<Pay> payServiceLoader = ServiceLoader.load(Pay.class);
    Iterator<Pay> iterator = payServiceLoader.iterator();
    if (iterator.hasNext()){
   
        iterator.next().pay();
    }
}

 public static void main(String[] args) {
   
// //ali
// Context aliContext = new Context(new AliPay());
// aliContext.invokeStrategy();
// //wechat
// Context wechatContext = new Context(new WechatPay());
// wechatContext.invokeStrategy();
        Context context = new Context();
        context.invokeStrategy();
    }

/** * 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");
}

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() {
   

            //重点看这里
            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);
        }
    }
}