Dubbo中的SPI机制实现，自定义对接SPI

SPI在java中称为Service Provider Interface，Dubbo中对java的spi机制自己实现相对应的逻辑。
我们以Dubbo中的Protociol为例，一般我们去获取对应的Protocol的时候都是通过如下方式：

ExtensionLoader.getExtensionLoader(Protocol.class).getAdaptiveExtension();
// 或者
ExtensionLoader.getExtensionLoader(Protocol.class).getExtension(extName);

我们看下Dubbo中具体怎么实现，

public static  ExtensionLoader getExtensionLoader(Class type) {
        if (type == null) {
            throw new IllegalArgumentException("Extension type == null");
        }
        if (!type.isInterface()) {
            throw new IllegalArgumentException("Extension type (" + type + ") is not an interface!");
        }
        if (!withExtensionAnnotation(type)) {
            throw new IllegalArgumentException("Extension type (" + type +
                    ") is not an extension, because it is NOT annotated with @" + SPI.class.getSimpleName() + "!");
        }

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

可以看到，Dubbo中是每个通过SPI加载类型都有一个对应的ExtensionLoader，我们看看具体怎么加载类的：

 public T getExtension(String name) {
        T extension = getExtension(name, true);
        if (extension == null) {
            throw new IllegalArgumentException("Not find extension: " + name);
        }
        return extension;
    }
public T getExtension(String name, boolean wrap) {
        if (StringUtils.isEmpty(name)) {
            throw new IllegalArgumentException("Extension name == null");
        }
        if ("true".equals(name)) {
            return getDefaultExtension();
        }
        final Holder