JAVA使用ServiceLoader加载服务

ServiceLoader是SPI的是一种实现，所谓SPI，即Service Provider Interface，用于一些服务提供给第三方实现或者扩展，可以增强框架的扩展或者替换一些组件。
其实关于ServiceLoader，我们平时虽然很少用到，但是却在背后为我们做了很多事情，最常见的就是JDBC的操作了，相信大家对DriverManager类并不陌生，我们可以通过该类加载驱动并获得数据库连接，在这个类中其实就用到了ServiceLoader。

private static void loadInitialDrivers() {
    String drivers;
    try {
        drivers = AccessController.doPrivileged(new PrivilegedAction() {
            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() {
        public Void run() {

            ServiceLoader loadedDrivers = ServiceLoader.load(Driver.class);
            Iterator 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);
        }
    }
}

接下来我们就还看看如何使用ServiceLoader，假设我们需要一个日志的服务，那么首先我们需要定义一个服务的接口

package com.jinggujin.service;

public interface LogService {
   void log(String msg);
}

针对这个服务我们再来提供一个实现

package com.jinggujin.service;

public class DefaultLogService implements LogService {

   @Override
   public void log(String msg) {
      System.out.println(msg);
   }
}

好了下面我们需要提供我们的服务，我们需要在META-INF中添加services文件夹，然后在该文件夹中创建一个文件，文件名为服务接口的全限定类名，那么这里完整的文件路径应为：META-INF/services/com.jinggujin.service.LogService，最后我们需要在该文件中添加服务实现的全限定类名，一行数据一个实现全限定类名，文件内容如下：

com.jinggujin.service.DefaultLogService

准备工作已经完成，我们就来使用ServiceLoader加载我们提供的服务。

package test;

import java.util.ServiceLoader;

import org.junit.Test;

import com.jinggujin.service.LogService;

public class ServiceLoaderTest {
   @Test
   public void test() {
      ServiceLoader loader = ServiceLoader.load(LogService.class);

      for (LogService service : loader) {
         System.out.println(service.getClass());
      }
   }
}

运行结果如下：

class com.jinggujin.service.DefaultLogService