SPI service discovery mechanism

author : TaoYuanming of the Western Wei Dynasty
Blog : https://blog.springlearn.cn/

TaoYuanming of the Western Wei Dynasty
Don't laugh at the young Jianghu dream , Who doesn't dream of Jianghu

One 、 What is? SPI

SPI , Its full name is Service Provider Interface, Is a service discovery mechanism .JDK Medium SPI It is through ClassPath The next path META-INF/services Folder lookup extension file , Automatically load the classes defined in the file .

In Xiaobian's understanding , I think it is a kind of thought . That is, find the interface of the service , Famous for its name : Service discovery mechanism thought . Many open source frameworks have borrowed this idea , such as dubbo、jdbc.

Two 、SPI stay JDK How to use

SPI stay JDK in , We can use ServiceLoader Class to use .

1. The premise to prepare

public interface SpiService {
    String say();
}

Two implementation classes

public class ASpiServiceImpl implements SpiService {
    static {
        System.out.println("static init a");
    }

    {
        System.out.println("init a");
    }

    @Override
    public String say() {
        return "A";
    }
}

public class BSpiServiceImpl implements SpiService {
    static {
        System.out.println("static init b");
    }

    {
        System.out.println("init b");
    }
    @Override
    public String say() {
        return "B";
    }
}

2. To configure

stay resources Created in META-INF/services Catalog

│  └── resources
│      └── META-INF
│          └── services
│              └── com.github.easylog.spi.SpiService

com.github.easylog.spi.SpiService The contents of the document

com.github.easylog.spi.impl.ASpiServiceImpl
com.github.easylog.spi.impl.BSpiServiceImpl

3. Use

adopt ServiceLoader Class we can load all configured implementation classes , And deal with the implementation class . It should be noted that , see 4 Use caution .

public class SpiTester {
    public static void main(String[] args) {
        ServiceLoader<SpiService> spiServices = ServiceLoader.load(SpiService.class);
        Iterator<SpiService> iterator = spiServices.iterator();
        while (iterator.hasNext()) {
            SpiService next = iterator.next();
            System.out.println(next.say());
        }
    }
}

4. Use caution

You can take a look at the two implementation classes previously declared in the Xiaobian , Both static and non static code blocks are defined . Normally, when this bytecode is loaded , Will execute the content in the static code block , But it is not executed in actual operation , There's a reason .

You can see that the second parameter is false. That is, no initialization is performed during loading .

3、 ... and 、Dubbo The idea of service discovery

The characteristic of service discovery is : Code is not hard coded , But configurable . Just put the implementation classes to be supported under the specified configuration file , It will be loaded automatically . Then the code only cares about use . We can use this idea to realize , Extensions to the framework , For example, as mentioned above .Dubbo Make use of SPI The mind of , Load user-defined filters .

This idea is especially suitable for service expansion . This idea is now used in most open source frameworks .

1. Define filters

@Activate(group = { Constants.PROVIDER })
public class ProviderHelloFilter implements Filter {
  
    @Override
    public Result invoke(Invoker<?> invoker, Invocation invocation) throws RpcException {
        System.out.pringln("hello ok!");
        return invoker.invoke(invocation);
    }

}

2. Add configuration file

META-INF/dubbo/Interal/com.alibaba.dubbo.rpc.Filter

Default supported filters

utilize SPI principle , We customize a filter

3. Use

Actually API Follow JDK Use in ServiceLoader The way , Very similar . The only difference is Dubbo Is used in ExtensionLoader. because dubbo Some special enhancements have been made in . For example, you can customize an alias in the configuration file key. Pictured above hello Namely key. adopt getExtension(“hello”) You can get the specified implementation class .

public class SpiTester {
    public static void main(String[] args) throws Exception{
        ExtensionLoader<Filter> filterExtensionLoader = ExtensionLoader.getExtensionLoader(Filter.class);
        Set<String> supportedExtensions = filterExtensionLoader.getSupportedExtensions();
        System.out.println(supportedExtensions);
        //[accesslog, activelimit, cache...]
        Filter hello = filterExtensionLoader.getExtension("hello");
        //[email protected]
        System.out.println(hello);
    }
    
}

** So have you learned this thought ? **

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