grpc拦截器

在grpc的报文中可以增加报文头，用于标注消息的元数据。

服务端拦截器

在服务端可以继承ServerInterceptor来实现服务端的拦截器，用于操作报文头：

public class MyServerInterceptor implements ServerInterceptor {
    //服务端header的key
    static final Metadata.Key<String> CUSTOM_HEADER_KEY =
            Metadata.Key.of("serverHeader", Metadata.ASCII_STRING_MARSHALLER);

    @Override
    public <ReqT, RespT> ServerCall.Listener<ReqT> interceptCall(ServerCall<ReqT, RespT> call, Metadata headers, ServerCallHandler<ReqT, RespT> next) {
        //输出客户端传递过来的header
		System.out.println("header received from client:" + headers);
        
		return next.startCall(new ForwardingServerCall.SimpleForwardingServerCall<ReqT, RespT>(call) {
            @Override
            public void sendHeaders(Metadata responseHeaders) {
				//在返回中增加header
                responseHeaders.put(CUSTOM_HEADER_KEY, "response");
                super.sendHeaders(responseHeaders);
            }
        }, headers);
    }
}

客户端拦截器

类似的，需要继承ClientInterceptor实现客户端的拦截器

public class MyClientInterceptor implements ClientInterceptor {
    //客户端header的key
    static final Metadata.Key<String> CUSTOM_HEADER_KEY =
            Metadata.Key.of("clientHeader", Metadata.ASCII_STRING_MARSHALLER);

    @Override
    public <ReqT, RespT> ClientCall<ReqT, RespT> interceptCall(MethodDescriptor<ReqT, RespT> method, CallOptions callOptions, Channel next) {
        return new ForwardingClientCall.SimpleForwardingClientCall<ReqT, RespT>(next.newCall(method, callOptions)) {
            @Override
            public void start(Listener<RespT> responseListener, Metadata headers) {
				//放入客户端的header
                headers.put(CUSTOM_HEADER_KEY, "request");
                super.start(new ForwardingClientCallListener.SimpleForwardingClientCallListener<RespT>(responseListener) {
                    @Override
                    public void onHeaders(Metadata headers) {
                        //输出服务端传递回来的header
                        System.out.println("header received from server:" + headers);
                        super.onHeaders(headers);
                    }
                }, headers);
            }
        };
    }
}

调用

在完成两端的拦截器的代码就需要使用它们了，它们的用法是在客户端channel或服务端的构建过程中将它们注入到对应的客户端/服务端header的key，下面我们在上次代码的基础上进行修改

//服务端
public class IntceptorServer {

    private int port;
    private Server server;
    public IntceptorServer(int port) throws IOException {
        this.port=port;
        server= ServerBuilder.forPort(port)
				//将MyServerInterceptor注册到服务端
                .addService(ServerInterceptors.intercept(new HelloServiceImpl(),new MyServerInterceptor()))
                .build();
        server.start();
        System.out.println("Server started, listening on " + port );
    }

    private void blockUntilShutdown() throws InterruptedException {
        while(true){
            server.awaitTermination();
        }
    }
    public static void main(String[] args) throws Exception {
        (new IntceptorServer(8080)).blockUntilShutdown();
    }
}

//客户端
public class InterceporClient {
    static final Metadata.Key<String> ATTCHED_HEADER =
            Metadata.Key.of("attached_header", Metadata.ASCII_STRING_MARSHALLER);

    public static void  main(String[] args) throws InterruptedException {
        final ManagedChannel originalChannel = ManagedChannelBuilder.forAddress("127.0.0.1", 8080).usePlaintext(true).build();
		//将MyClientInterceptor和原有的channel结合，生成包含拦截器的channel
        Channel channel = ClientInterceptors.intercept(originalChannel, new MyClientInterceptor());
        HelloServiceGrpc.HelloServiceBlockingStub blockingStub = HelloServiceGrpc.newBlockingStub(channel);

        ProtoObj.Person person = ProtoObj.Person.newBuilder().setMyName("World").build();
        System.out.println(blockingStub.simpleHello(person).getString());
		
		//如果只需要在客户端传送header，而不需要接受服务端的header可以简单调用MetadataUtils.attachHeaders注册meta数据而不用定义Interceptor
		blockingStub = HelloServiceGrpc.newBlockingStub(originalChannel);
        Metadata meta=new Metadata();
        meta.put(ATTCHED_HEADER, "attched");
        HelloServiceGrpc.HelloServiceBlockingStub s=MetadataUtils.attachHeaders(blockingStub,meta);
        System.out.println(s.simpleHello(person).getString());
		
		//关闭originalChannel
        originalChannel.shutdown().awaitTermination(5, TimeUnit.SECONDS);

    }
}

----服务端输出----
header received from client:Metadata(content-type=application/grpc,user-agent=grpc-java-netty/1.2.0,clientheader=request,grpc-accept-encoding=gzip,grpc-census-bin=)
World calling
header received from client:Metadata(content-type=application/grpc,user-agent=grpc-java-netty/1.2.0,attached_header=attched,grpc-accept-encoding=gzip,grpc-census-bin=)
World calling

----客户端输出----
header received from server:Metadata(content-type=application/grpc,serverheader=response,grpc-encoding=identity,grpc-accept-encoding=gzip)
hello, World
hello, World

这样在两端就可以看到对应的header

总结

通过header的内容可以对调用的流程进行一定的控制，以达到认证等功能。