Java插件扩展机制之SPI案例讲解

public interface AdOpFromApolloService {}
public class AdOpFromDbServiceImpl implements AdOpFromDbService {}

-- resources
---- META-INF
-------- services
------------ com.example.spidemocontract.spi.SpiTestDemoService

com.example.spidemocontract.spi.impl.DefaultSpiTestDemoService

public interface SpiTestDemoService {
    void printLog();
    int getOrder();
}


public class DefaultSpiTestDemoService implements SpiTestDemoService {
    @Override
    public int getOrder() {
        return Integer.MAX_VALUE;
    }
    @Override
    public void printLog() {
        System.out.println("输出 DefaultSpiTestDemoService log");
    }
}

<dependency>
    <groupId>com.example</groupId>
    <artifactId>spi-demo-contract</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <scope>compile</scope>
</dependency>

-- resources
---- META-INF
-------- services
------------ com.example.spidemocontract.spi.SpiTestDemoService

com.example.spidemo.spi.OtherSpiTestDemoService

public class OtherSpiTestDemoService implements SpiTestDemoService {
    // 后面我们用SPI类加载器获取时，会根据order排序，越小优先级越高
    @Override
    public int getOrder() {
        return 0;
    }
    @Override
    public void printLog() {
        System.out.println("输出 OtherSpiTestDemoService log");
    }
}

public static void main(String[] args) {
    // 加载SPI
    Iterator<SpiTestDemoService> iterator = ServiceLoader.load(SpiTestDemoService.class).iterator();
    // 实现了ordered,会根据ordered返回值排序，优先级越高，越先取出来
    List<SpiTestDemoService> list = Lists.newArrayList(iterator)
            .stream().sorted(Comparator.comparing(SpiTestDemoService::getOrder))
            .collect(Collectors.toList());
    for (SpiTestDemoService item : list) {
        item.printLog();
    }
}

// 当前接口会在resource/META-INF/services目录下对应文件com.ctrip.framework.apollo.spring.spi.ConfigPropertySourcesProcessorHelper
public interface ConfigPropertySourcesProcessorHelper extends Ordered {
  void postProcessBeanDefinitionReGIStry(BeanDefinitionRegistry registry) throws BeansException;
}

public class DefaultConfigPropertySourcesProcessorHelper implements ConfigPropertySourcesProcessorHelper {
    @Override
    public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) throws BeansException {
        // .....各种注册bean，初始化的流程
    }
    @Override
    public int getOrder() {
        // 优先级排序置为最低。方便其他自定义spi实现类能够覆盖
        return Ordered.LOWEST_PRECEDENCE;
    }
}

public static <S extends Ordered> S loadPrimary(Class<S> clazz) {
    List<S> candidates = loadAllOrdered(clazz);
    return candidates.get(0);
}
public static <S extends Ordered> List<S> loadAllOrdered(Class<S> clazz) {
    Iterator<S> iterator = loadAll(clazz);

    if (!iterator.hasNext()) {
      throw new IllegalStateException(String.fORMat(
          "No implementation defined in /META-INF/services/%s, please check whether the file exists and has the right implementation class!",
          clazz.getName()));
    }
    // 获取迭代中的所有SPI实现实例,然后进行排序，取优先级最高的那个
    List<S> candidates = Lists.newArrayList(iterator);
    Collections.sort(candidates, new Comparator<S>() {
      @Override
      public int compare(S o1, S o2) {
        // the smaller order has higher priority
        return Integer.compare(o1.getOrder(), o2.getOrder());
      }
    });

    return candidates;
}

com.example.app.driver.MyDriver

public class MyDriver extends NonRegisteringDriver implements Driver {
    static {
        try {
            java.sql.DriverManager.registerDriver(new MyDriver());
        } catch (SQLException e) {
            throw new RuntimeException("Can't register driver!", e);
        }
    }
    public MyDriver() throws SQLException {}
    @Override
    public Connection connect(String url, Properties info) throws SQLException {
        System.out.println("MyDriver - 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 connection = super.connect(url, info);
        System.out.println("MyDriver - 数据库连接创建完成!" + connection.toString());
        return connection;
    }
}

String url = "jdbc:Mysql://localhost:3306/test?serverTimezone=UTC";
String user = "root";
String passWord = "root";
Class.forName("com.example.app.driver.MyDriver");
Connection connection = DriverManager.getConnection(url, user, password);

// 获取所有注册的驱动（每个驱动的都遍历一下，其实就是最晚注册那个就用那个了，如果失败就往下一个找）
 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());
            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());
    }
}