如何使用netty客戶端獲取服務器響應

Question

我想寫一個基於 netty 的客戶端。 它應該有方法public String send(String msg); 哪個應該從服務器或將來返回響應 - 無關緊要。 它也應該是多線程的。 像這樣：

public class Client {
public static void main(String[] args) throws InterruptedException {
    Client client = new Client();

}

private Channel channel;

public Client() throws InterruptedException {
    EventLoopGroup loopGroup = new NioEventLoopGroup();

    Bootstrap b = new Bootstrap();
    b.group(loopGroup).channel(NioSocketChannel.class).handler(new ChannelInitializer<SocketChannel>() {
        @Override
        protected void initChannel(SocketChannel ch) throws Exception {
            ch.pipeline().addLast(new StringDecoder()).
                    addLast(new StringEncoder()).
                    addLast(new ClientHandler());
        }
    });
    channel = b.connect("localhost", 9091).sync().channel();
}

public String sendMessage(String msg) {
    channel.writeAndFlush(msg);
    return ??????????;
}

}

在調用 writeAndFlush(); 后，我不知道如何從服務器檢索響應； 我該怎么辦？

我也使用 Netty 4.0.18.Final

Answer 1

為該方法返回Future<String>很簡單，我們將實現以下方法簽名：

public Futute<String> sendMessage(String msg) {

當您熟悉異步編程結構時，這相對容易做到。 為了解決設計問題，我們將執行以下步驟：

1. 寫入消息時，將Promise<String>添加到ArrayBlockingQueue<Promise>

   這將用作最近發送的消息列表，並允許我們更改Future<String>對象的返回結果。

2. 當消息返回到處理程序時，根據Queue的頭部解析它

   這使我們能夠得到正確的未來改變。

3. 更新Promise<String>

   我們調用promise.setSuccess()來最終設置對象的狀態，這將傳播回未來的對象。

示例代碼

public class ClientHandler extends SimpleChannelInboundHandler<String> {
    private ChannelHandlerContext ctx;
    private BlockingQueue<Promise<String>> messageList = new ArrayBlockingQueue<>(16);

    @Override
    public void channelActive(ChannelHandlerContext ctx) {
        super.channelActive(ctx);
        this.ctx = ctx;
    }

    @Override
    public void channelInactive(ChannelHandlerContext ctx) {
        super.channelInactive(ctx);
        synchronized(this){
            Promise<String> prom;
            while((prom = messageList.poll()) != null) 
                prom.setFailure(new IOException("Connection lost"));
            messageList = null;
        }
    }

    public Future<String> sendMessage(String message) {
        if(ctx == null) 
            throw new IllegalStateException();
        return sendMessage(message, ctx.executor().newPromise());
    }

    public Future<String> sendMessage(String message, Promise<String> prom) {
        synchronized(this){
            if(messageList == null) {
                // Connection closed
                prom.setFailure(new IllegalStateException());
            } else if(messageList.offer(prom)) { 
                // Connection open and message accepted
                ctx.writeAndFlush(message).addListener();
            } else { 
                // Connection open and message rejected
                prom.setFailure(new BufferOverflowException());
            }
            return prom;
        }
    }
    @Override
    protected void messageReceived(ChannelHandlerContext ctx, String msg) {
        synchronized(this){
            if(messageList != null) {
                 messageList.poll().setSuccess(msg);
            }
        }
    }
}

文件分解

• private ChannelHandlerContext ctx;

  用於存儲我們對 ChannelHandlerContext 的引用，我們使用它來創建承諾

• private BlockingQueue<Promise<String>> messageList = new ArrayBlockingQueue<>();

  我們將過去的消息保留在此列表中，以便我們可以更改未來的結果

• public void channelActive(ChannelHandlerContext ctx)

  當連接變為活動時由 netty 調用。 在這里初始化我們的變量。

• public void channelInactive(ChannelHandlerContext ctx)

  當連接因錯誤或正常連接關閉而變為非活動狀態時由 netty 調用。

• protected void messageReceived(ChannelHandlerContext ctx, String msg)

  當有新消息到達時由netty調用，這里挑出隊列的頭部，然后我們對其調用setsuccess。

警告提示

使用期貨時，您需要注意一件事，如果未來尚未完成，請不要從 1 個 netty 線程調用 get()，不遵循此簡單規則將導致死鎖或BlockingOperationException .

Answer 2

您可以在 netty 項目中找到示例。 我們可以將結果保存到最后一個處理程序的自定義字段中。 在下面的代碼中，我們想要的是 handler.getFactorial() 。

參考http://www.lookatsrc.com/source/io/netty/example/factorial/FactorialClient.java?a=io.netty:netty-all

因子客戶端.java

public final class FactorialClient {

    static final boolean SSL = System.getProperty("ssl") != null;
    static final String HOST = System.getProperty("host", "127.0.0.1");
    static final int PORT = Integer.parseInt(System.getProperty("port", "8322"));
    static final int COUNT = Integer.parseInt(System.getProperty("count", "1000"));

    public static void main(String[] args) throws Exception {
        // Configure SSL.
        final SslContext sslCtx;
        if (SSL) {
            sslCtx = SslContextBuilder.forClient()
                .trustManager(InsecureTrustManagerFactory.INSTANCE).build();
        } else {
            sslCtx = null;
        }

        EventLoopGroup group = new NioEventLoopGroup();
        try {
            Bootstrap b = new Bootstrap();
            b.group(group)
             .channel(NioSocketChannel.class)
             .handler(new FactorialClientInitializer(sslCtx));

            // Make a new connection.
            ChannelFuture f = b.connect(HOST, PORT).sync();

            // Get the handler instance to retrieve the answer.
            FactorialClientHandler handler =
                (FactorialClientHandler) f.channel().pipeline().last();

            // Print out the answer.
            System.err.format("Factorial of %,d is: %,d", COUNT, handler.getFactorial());
        } finally {
            group.shutdownGracefully();
        }
    }
}

public class FactorialClientHandler extends SimpleChannelInboundHandler<BigInteger> {

    private ChannelHandlerContext ctx;
    private int receivedMessages;
    private int next = 1;
    final BlockingQueue<BigInteger> answer = new LinkedBlockingQueue<BigInteger>();

    public BigInteger getFactorial() {
        boolean interrupted = false;
        try {
            for (;;) {
                try {
                    return answer.take();
                } catch (InterruptedException ignore) {
                    interrupted = true;
                }
            }
        } finally {
            if (interrupted) {
                Thread.currentThread().interrupt();
            }
        }
    }

    @Override
    public void channelActive(ChannelHandlerContext ctx) {
        this.ctx = ctx;
        sendNumbers();
    }

    @Override
    public void channelRead0(ChannelHandlerContext ctx, final BigInteger msg) {
        receivedMessages ++;
        if (receivedMessages == FactorialClient.COUNT) {
            // Offer the answer after closing the connection.
            ctx.channel().close().addListener(new ChannelFutureListener() {
                @Override
                public void operationComplete(ChannelFuture future) {
                    boolean offered = answer.offer(msg);
                    assert offered;
                }
            });
        }
    }

    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) {
        cause.printStackTrace();
        ctx.close();
    }

    private void sendNumbers() {
        // Do not send more than 4096 numbers.
        ChannelFuture future = null;
        for (int i = 0; i < 4096 && next <= FactorialClient.COUNT; i++) {
            future = ctx.write(Integer.valueOf(next));
            next++;
        }
        if (next <= FactorialClient.COUNT) {
            assert future != null;
            future.addListener(numberSender);
        }
        ctx.flush();
    }

    private final ChannelFutureListener numberSender = new ChannelFutureListener() {
        @Override
        public void operationComplete(ChannelFuture future) throws Exception {
            if (future.isSuccess()) {
                sendNumbers();
            } else {
                future.cause().printStackTrace();
                future.channel().close();
            }
        }
    };
}

Answer 3

調用channel.writeAndFlush(msg); 已經返回 ChannelFuture。 要處理此方法調用的結果，您可以像這樣向 future 添加一個偵聽器：

future.addListener(new ChannelFutureListener() {
    public void operationComplete(ChannelFuture future) {
        // Perform post-closure operation
        // ...
    }
}); 

（這是從 Netty 文檔中獲取的，請參閱： Netty doc ）