Java SPI机制使用

1. Service Provider Interface SPI描述

接口提供者优先规定接口要求，然后交由具体的实现者对接口进行内容的实现；

2. 实现

//1. 定义接口
public Interface Demo_interface{
    //具体实现者必须要实现的内容
    void doexcute();
}
//2.实现接口
public class Demo_Impl implements Demo_interface{
  void doexcute(){
      //pass  
  }
}
//3. 使用ServiceLoader读取文件加载对象（ServiceLoader是主进程模块调用的）
ServiceLoader<Demo_interface> demoServiceLoader = ServiceLoader.load(Demo_interface.class);

3 举例说明

以 JDBC 的 PostgreSQL 实现为例(Mysql同理)

• 一 ： 实现类模块文件结构需要保持下图所示，其中文件名称java.sql.Drive就是JDBC定义需要被实现的接口，内容org.postgresql.Driver表明的是实现类的路径
  
• 二 ：JDBC步骤说明；注册驱动 ==> 创建连接 ==> 构建执行器 ==> 获取结果集 ==> 关闭连接

//注册驱动，这里的DriverManager将会调用ServiceLoader将实现类注入
public static void register() throws SQLException {
        if (isRegistered()) {
            throw new IllegalStateException("Driver is already registered. It can only be registered once.");
        } else {
            Driver registeredDriver = new Driver();
            DriverManager.registerDriver(registeredDriver);
            Driver.registeredDriver = registeredDriver;
        }
    }
// 驱动初始化，DriverManager调用ServiceLoader（DriverManager属于java.sql包下）
//调取时机，此方法是写在static块中，所以和class一起，故class.forName("")即可注册驱动
 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);
            }
        }
    }