Protocol Extension

public interface Protocol {
    /**
     * Export remote service: <br>
     * 1. Should save address info for the request when the protocol receives it: RpcContext.getContext().setRemoteAddress();<br>
     * 2. export() must be implemented as idempotent, that is, it should not introduce side effect when the implementation gets called with the same Invoker for more than once.
     * 3. Invoker is passed by the framework, and the protocol should not care about it. <br>
     * 
     * @param <T> Service type
     * @param invoker Service invoker
     * @return exporter The reference of service exporter, used for cancelling service export.
     * @throws RpcException throw when there's any error during service export, e.g. the port is occupied
     */
    <T> Exporter<T> export(Invoker<T> invoker) throws RpcException;
 
    /**
     * Reference remote service: <br>
     * 1. When user calls `invoke()` method of `Invoker` object which's returned from `refer()` call, the protocol needs to correspondingly execute `invoke()` method of `Invoker` object passed from remote `export()` method associated with the same URL. <br>
     * 2. It's protocol's responsibility to implement `Invoker` which's returned from `refer()`. Generally speaking, protocol sends remote request in the `Invoker` implementation. <br>
     * 3. When there's check=false set in URL, the implementation must not throw exception but try to recover when connection fails.
     * 
     * @param <T> Service type
     * @param type Service type
     * @param url URL address for the remote service
     * @return invoker service's local proxy
     * @throws RpcException throw when there's any error while connecting to the service provider
     */
    <T> Invoker<T> refer(Class<T> type, URL url) throws RpcException;
 
}

<!-- declare protocol, if id is not set, then use the value of name for id -->
<dubbo:protocol id="xxx1" name="xxx" />
<!-- reference protocol, if protocol's attribute is not set, then protocol configuration will be scanned automatically from ApplicationContext -->
<dubbo:service protocol="xxx1" />
<!-- default value for referenced protocol, it will be used if protocol attribute is not configured in <dubbo:service> --> 
<dubbo:provider protocol="xxx1" />

src
 |-main
    |-java
        |-com
            |-xxx
                |-XxxProtocol.java (Protocol implementation)
                |-XxxExporter.java (Exporter implementation)
                |-XxxInvoker.java (Invoker implementation)
    |-resources
        |-META-INF
            |-dubbo
                |-org.apache.dubbo.rpc.Protocol (plain text file with the content: xxx=com.xxx.XxxProtocol)

package com.xxx;
 
import org.apache.dubbo.rpc.Protocol;
 
public class XxxProtocol implements Protocol {
    public <T> Exporter<T> export(Invoker<T> invoker) throws RpcException {
        return new XxxExporter(invoker);
    }
    public <T> Invoker<T> refer(Class<T> type, URL url) throws RpcException {
        return new XxxInvoker(type, url);
    }
}

package com.xxx;
 
import org.apache.dubbo.rpc.support.AbstractExporter;
 
public class XxxExporter<T> extends AbstractExporter<T> {
    public XxxExporter(Invoker<T> invoker) throws RemotingException{
        super(invoker);
        // ...
    }
    public void unexport() {
        super.unexport();
        // ...
    }
}

package com.xxx;
 
import org.apache.dubbo.rpc.support.AbstractInvoker;
 
public class XxxInvoker<T> extends AbstractInvoker<T> {
    public XxxInvoker(Class<T> type, URL url) throws RemotingException{
        super(type, url);
    }
    
    @Override
    protected Result doInvoke(Invocation invocation) throws Throwable {
        // ...
    }
}

xxx=com.xxx.XxxProtocol