@Extension, @spi annotation principle

stay dubbo Source code , It involves some dubbo Custom comments , This note mainly records the principle and use of these annotations , Now that you know that , It's convenient for us to treat dubbo Second expansion

@SPI

dubbo Of SPI Mechanism , and Java Native SPI Mechanism compared to , Added name, You can specify the corresponding name, When we use it , According to name, Get the extension class you want to use , If you get the implementation class , Appoint name by true, The default return is @spi Specified in the annotation name The corresponding implementation class
Simply speaking ：@SPI Specified in the annotation value value , Is the default implementation class of the current interface , such as , The following screenshot , It means Protocol The default implementation class is DubboProtocol

Where can this be reflected ？

com.alibaba.dubbo.common.extension.ExtensionLoader#loadExtensionClasses
In this method , Is the file added to the specified file directory , First of all, it will be based on type, Get the corresponding @SPI annotation , there type, Is the corresponding interface ; therefore , Here you can see , Will be able to @SPI In the annotations value Corresponding key, Set to CacheDefaultName In this variable

com.alibaba.dubbo.common.extension.ExtensionLoader#getExtension
In this method , It is based on the name, Find the corresponding implementation class , such as ： At present type yes protocol, If you participate dubbo, That is to find key yes dubbo Of protocol Implementation class , Usually dubboProtocol

Here we will see , If so, then name yes true, Indicates that the default implementation class is to be obtained
Here we will see , It's from CacheDefaultName From

public T getDefaultExtension() {
    
   getExtensionClasses();
   if (null == cachedDefaultName || cachedDefaultName.length() == 0
           || "true".equals(cachedDefaultName)) {
    
       return null;
   }
   return getExtension(cachedDefaultName);
}

therefore ：@SPI annotation , Is added to the interface , Represents the default implementation class of the interface , At the time of acquisition , If you enter name yes true, Will take the default implementation class

@Extension

Usually in use dubbo Of SPI Mechanism , We need to use it in the document by ourselves key = Full class name ; In this format name; But if we don't specify in the file , That can be used @Extension annotation , Add to the implementation class , And designate key（ there key It is equivalent to key）, Because in dubbo Source code , When parsing files , If not specified name, Will resolve whether the implementation class has been added @Extension
What needs to be noted here is ： This note and @SPI Note between the two , I don't think it matters much , because @SPI Annotated value, We are designating name by true When , Will be with @SPI Notes have something to do with
and @Extension The note is key A setting method of
com.alibaba.dubbo.common.extension.ExtensionLoader#loadClass
In this method , There's a piece of code

The context of this method , A brief introduction , stay dubbo Source code , Will be added to type All corresponding implementation classes , According to the implementation class , To distinguish between , The current implementation class is a wrapper class ？ Or adaptive extension class ？ Or ordinary implementation classes , If it is an ordinary implementation , Will execute the method in the screenshot , When executed , If we do not specify key = Implementation class , At this time, we will judge the implementation class ( there clazz It is already an implementation class , because dubbo According to the full class name in the file , Generate a clazz) Is there any addition @Extension annotation

private String findAnnotationName(Class<?> clazz) {
    
    com.alibaba.dubbo.common.Extension extension = clazz.getAnnotation(com.alibaba.dubbo.common.Extension.class);
    if (extension == null) {
    
        String name = clazz.getSimpleName();
        if (name.endsWith(type.getSimpleName())) {
    
            name = name.substring(0, name.length() - type.getSimpleName().length());
        }
        return name.toLowerCase();
    }
    return extension.value();
}

therefore ,@Extension The meaning of annotation is : If we are in the document , The implementation class is not specified key, Then we will judge whether the implementation class is used @Extension annotation , Yes , Will use the corresponding annotation value As a key