code: @Test public void testRpcFilter() throws Exception {
    DemoService service=new DemoServiceImpl();
    URL url=URL.valueOf("dubbo://127.0.0.1:9010/org.apache.dubbo.rpc.DemoService?service.filter=echo");
    protocol.export(proxy.getInvoker(service,DemoService.class,url));
    service=proxy.getProxy(protocol.refer(DemoService.class,url));
    Assert.assertEquals("123",service.echo("123"));
    EchoService echo=proxy.getProxy(protocol.refer(EchoService.class,url));
    Assert.assertEquals(echo.$echo("test"),"test");
    Assert.assertEquals(echo.$echo("abcdefg"),"abcdefg");
    Assert.assertEquals(echo.$echo(1234),1234);
  }
  