[Java] 深入理解Java中的SPI机制

• 数据库驱动加载接口实现类的加载
  

• 日志门面接口实现类加载
  

• Spring
  

• Dubbo
  

public interface IShout {
  void shout();
}
public class Cat implements IShout {
  @Override
  public void shout() {
  System.out.println("miao miao");
  }
}
public class Dog implements IShout {
  @Override
  public void shout() {
  System.out.println("wang wang");
  }
}

- src
  -main
  -resources
    - META-INF
    - services
      - org.foo.demo.IShout

org.foo.demo.animal.Dog
org.foo.demo.animal.Cat

public class SPIMain {
  public static void main(String[] args) {
  ServiceLoader<IShout> shouts = ServiceLoader.load(IShout.class);
  for (IShout s : shouts) {
    s.shout();
  }
  }
}

wang wang
miao miao

public final class ServiceLoader<S> implements Iterable<S>{
private static final String PREFIX = "META-INF/services/";
 
  // 代表被加载的类或者接口
  private final Class<S> service;
 
  // 用于定位，加载和实例化providers的类加载器
  private final ClassLoader loader;
 
  // 创建ServiceLoader时采用的访问控制上下文
  private final AccessControlContext acc;
 
  // 缓存providers，按实例化的顺序排列
  private LinkedHashMap<String,S> providers = new LinkedHashMap<>();
 
  // 懒查找迭代器
  private LazyIterator lookupIterator;
 
  ......
}

• loader(ClassLoader类型，类加载器)
  
• acc(AccessControlContext类型，访问控制器)
  
• providers(LinkedHashMap<String,S>类型，用于缓存加载成功的类)
  
• lookupIterator(实现迭代器功能)
  

try {
  String fullName = PREFIX + service.getName();
  if (loader == null)
  configs = ClassLoader.getSystemResources(fullName);
  else
  configs = loader.getResources(fullName);
} catch (IOException x) {
  fail(service, "Error locating configuration files", x);
}