浅析Java SPI 与 dubbo SPI

public interface Robot {
    
    void sayHello();
}

public class RobotA implements Robot {
    public RobotA() {
        System.out.println("Happy RobotA is loaded");
    }
    @Override
    public void sayHello() {
        System.out.println("i am a very very happy Robot ");
    }
    public void sayBye(){}
}

public class RobotB implements Robot {
    public RobotB() {
        System.out.println("SB RobotB is loaded");
    }
    @Override
    public void sayHello() {
        System.out.println("i am a da sha bi ");
    }
    public void sayBye(){}
}

ServiceLoader<Robot> serviceLoader=ServiceLoader.load(Robot.class);
serviceLoader.forEach(Robot::sayHello);

public static <S> ServiceLoader<S> load(Class<S> service) {
    ClassLoader cl = Thread.currentThread().getContextClassLoader();
    return ServiceLoader.load(service, cl);
}

public static <S> ServiceLoader<S> load(Class<S> service,
                                        ClassLoader loader)
{
    return new ServiceLoader<>(service, loader);
}

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();
}
public void reload() {
    //这里的provider是一个对于已实例化对象的缓存,为Map类型
        providers.clear();
        lookupIterator = new LazyIterator(service, loader);
    }

private LazyIterator(Class<S> service, ClassLoader loader) {
    this.service = service;
    this.loader = loader;
}

public Iterator<S> iterator() {
    return new Iterator<S>() {

        Iterator<Map.Entry<String,S>> knownProviders
            = providers.entrySet().iterator();

        public boolean hasNext() {
            if (knownProviders.hasNext())
                return true;
            return lookupIterator.hasNext();
        }

        public S next() {
            if (knownProviders.hasNext())
                return knownProviders.next().getValue();
            return lookupIterator.next();
        }

        public void remove() {
            throw new UnsupportedOperationException();
        }

    };

 public S next() {
            if (knownProviders.hasNext())
                return knownProviders.next().getValue();
            return lookupIterator.next();
        }

public S next() {
    if (acc == null) {
        return nextService();
    } else {
        PrivilegedAction<S> action = new PrivilegedAction<S>() {
            public S run() { return nextService(); }
        };
        return AccessController.doPrivileged(action, acc);
    }
}

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");
}

//获取文件
String fullName = PREFIX + service.getName();
if (loader == null)
    configs = ClassLoader.getSystemResources(fullName);
else
    configs = loader.getResources(fullName);
//解析文件配置
while ((pending == null) || !pending.hasNext()) {
                if (!configs.hasMoreElements()) {
                    return false;
                }
                pending = parse(service, configs.nextElement());
            }
            nextName = pending.next();

@SPI
public interface Robot {
    
    void sayHello();
}

public class RobotA implements Robot {
    public RobotA() {
        System.out.println("Happy RobotA is loaded");
    }
    @Override
    public void sayHello() {
        System.out.println("i am a very very happy Robot ");
    }
    public void sayBye(){}
}

public class RobotB implements Robot {
    public RobotB() {
        System.out.println("SB RobotB is loaded");
    }
    @Override
    public void sayHello() {
        System.out.println("i am a da sha bi ");
    }
    public void sayBye(){}
}

robotA = cn.testlove.double_dubbo.inter.impl.RobotA
robotB=cn.testlove.double_dubbo.inter.impl.RobotB

ExtensionLoader<Robot> extensionLoader= ExtensionLoader.getExtensionLoader(Robot.class);
Robot robotB = extensionLoader.getExtension("robotB");

ExtensionLoader<T> loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);
if (loader == null) {
    EXTENSION_LOADERS.putIfAbsent(type, new ExtensionLoader<T>(type));
    loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);
}

//从缓存中找
final Holder<Object> holder = getOrCreateHolder(name);
Object instance = holder.get();
//双重检查
if (instance == null) {
    synchronized (holder) {
        instance = holder.get();
        if (instance == null) {
            instance = createExtension(name);
            holder.set(instance);
        }
    }
}

Class<?> clazz = getExtensionClasses().get(name);

T instance = (T) EXTENSION_INSTANCES.get(clazz);
if (instance == null) {
    EXTENSION_INSTANCES.putIfAbsent(clazz, clazz.newInstance());
    instance = (T) EXTENSION_INSTANCES.get(clazz);
}
injectExtension(instance);
Set<Class<?>> wrapperClasses = cachedWrapperClasses;
if (CollectionUtils.isNotEmpty(wrapperClasses)) {
    for (Class<?> wrapperClass : wrapperClasses) {
        instance = injectExtension((T) wrapperClass.getConstructor(type).newInstance(instance));
    }
}
initExtension(instance);
return instance;

Map<String, Class<?>> classes = cachedClasses.get();
if (classes == null) {
    synchronized (cachedClasses) {
        classes = cachedClasses.get();
        if (classes == null) {
            classes = loadExtensionClasses();
            cachedClasses.set(classes);
        }
    }
}
return classes;

private final ConcurrentMap<Class<?>, String> cachedNames = new ConcurrentHashMap<>();
private final Holder<Map<String, Class<?>>> cachedClasses = new Holder<>()    
private final Map<String, Object> cachedActivates = new ConcurrentHashMap<>();
private final ConcurrentMap<String, Holder<Object>> cachedInstances = new ConcurrentHashMap<>();
private final Holder<Object> cachedAdaptiveInstance = new Holder<>();
private volatile Class<?> cachedAdaptiveClass = null;
private String cachedDefaultName;