一文带你了解Java中的SPI机制

    private static final String PREFIX = "META-INF/services/";


  private ServiceLoader(Class<S> svc, ClassLoader cl) {
        service = Objects.requireNonNull(svc, "Service interface cannot be null");
        loader = (cl == null) ? ClassLoader.getSystemClassLoader() : cl;
        acc = (System.getSecurityManager() != null) ? AccessController.getContext() : null;
        reload();
    }

	/** 
     * 
     * 在调用该方法之后，迭代器方法的后续调用将延迟地从头开始查找和实例化提供程序，就像新创建的加载程序所做的		  那样
     */
   public void reload() {
        providers.clear(); //清除此加载程序的提供程序缓存，以便重新加载所有提供程序。
        lookupIterator = new LazyIterator(service, loader);
    }

	private class LazyIterator implements Iterator<S>{

        Class<S> service;
        ClassLoader loader;
        Enumeration<URL> configs = null;
        Iterator<String> pending = null;
        String nextName = null;


        private boolean hasNextService() {
            if (nextName != null) {
                return true;
            }
            if (configs == null) {
                try {
                    //找到配置文件
                    String fullName = PREFIX + service.getName();
                    //加载配置文件中的内容
                    if (loader == null)
                        configs = ClassLoader.getSystemResources(fullName);
                    else
                        configs = loader.getResources(fullName);
                } catch (IOException x) {
                    fail(service, "Error locating configuration files", x);
                }
            }
            while ((pending == null) || !pending.hasNext()) {
                if (!configs.hasMoreElements()) {
                    return false;
                }
                //解析配置文件
                pending = parse(service, configs.nextElement());
            }
            //获取配置文件中内容
            nextName = pending.next();
            return true;
        }
    }

		/** 
     	* 
     	*  通过反射 实例化配置文件中的具体实现类
    	 */
		private S nextService() {
            if (!hasNextService())
                throw new NoSuchElementException();
            String cn = nextName;
            nextName = null;
            Class<?> c = null;
            try {
                c = Class.forName(cn, false, loader);
            } catch (ClassNotFoundException x) {
                fail(service,
                     "Provider " + cn + " not found");
            }
            if (!service.isAssignableFrom(c)) {
                fail(service,
                     "Provider " + cn  + " not a subtype");
            }
            try {
                S p = service.cast(c.newInstance());
                providers.put(cn, p);
                return p;
            } catch (Throwable x) {
                fail(service,
                     "Provider " + cn + " could not be instantiated",
                     x);
            }
            throw new Error();          // This cannot happen
        }

public interface IService {

    /**
     * 获取价格
     * @return
     */
    String getPrice();

    /**
     * 获取规格信息
     * @return
     */
    String getSpecifications();
}

public class GoodServiceImpl implements IService {

    @Override
    public String getPrice() {
        return "2000.00元";
    }

    @Override
    public String getSpecifications() {
        return "200g/件";
    }
}

public class MedicalServiceImpl implements IService {

    @Override
    public String getPrice() {
        return "3022.12元";
    }

    @Override
    public String getSpecifications() {
        return "30粒/盒";
    }
}

public class Main {
    public static void main(String[] args) {
        final ServiceLoader<IService> serviceLoader = ServiceLoader.load(IService.class);
        serviceLoader.forEach(service -> {
            System.out.println(service.getPrice() + "=" + service.getSpecifications());
        });
    }
}