1.
服务接口和服务扩展之间是一对多关系，当没有明确指明默认实现时并不是根据接口类型随机指定，而是根据默认扩展来查找，Protocol$Adaptive代码如下所示
```
String extName = (url.getProtocol() == null ? "dubbo" : url.getProtocol());
if (extName == null)
throw new IllegalStateException("Fail to get
extension(org.apache.dubbo.rpc.Protocol) name from url(" + url.toString() + ")
use keys([protocol])");
org.apache.dubbo.rpc.Protocol extension = (org.apache.dubbo.rpc.Protocol)
ExtensionLoader
.getExtensionLoader(org.apache.dubbo.rpc.Protocol.class).getExtension(extName);
```
2. @SPI注解在dubbo框架中是必要参数，ExtensionLoader.java 代码如下所示：
```
public static <T> ExtensionLoader<T> getExtensionLoader(Class<T> type) {
if (!withExtensionAnnotation(type)) {
throw new IllegalArgumentException("Extension type(" + type +
") is not extension, because WITHOUT @" +
SPI.class.getSimpleName() + " Annotation!");
}
}
```
3. @SPI 中value 值是非必填参数
```
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE})
public @interface SPI {
/**
* default extension name
*/
String value() default "";
}
```
结论：是否应修改@SPI的value值为必须参数，避免在代码中重复判断是否采用默认值。
@diecui1202
[ Full content available at:
https://github.com/apache/incubator-dubbo/issues/2398 ]
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