在 Netty 4.x 中服务器没有实现客户端发送的消息

Question

我尝试通过简单地实现以下示例来学习 Netty 4.x。我有一个服务器和一个客户端。在某个时间点，客户端想知道当前的日期时间，他会询问服务器“现在几点？”。当服务器意识到问题时，他会回复当前的日期时间。

我的实现如下

TimeClientInboundHandler.java

public class TimeClientInboundHandler extends ChannelInboundHandlerAdapter {
    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
        ByteBuf byteBuf = (ByteBuf) msg;
        try {
            long currentTimeMillis = (byteBuf.readUnsignedInt() - 2208988800L) * 1000L;
            System.out.println(new Date(currentTimeMillis));
            ctx.close();
        } finally {
            byteBuf.release();
        }
    }

    @Override
    public void channelActive(ChannelHandlerContext ctx) throws Exception {
        ctx.write("What time is it?");
        ctx.flush();
    }

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

TimeServerInboundHandler.java

public class TimeServerInboundHandler extends ChannelInboundHandlerAdapter {

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
        ByteBuf byteBuf = (ByteBuf) msg;
        try {
            byteBuf.readCharSequence(1024, StandardCharsets.UTF_8);
            System.out.println(byteBuf.toString());
        } finally {
            ((ByteBuf) msg).release();
        }
    }

    @Override
    public void channelActive(ChannelHandlerContext ctx) throws Exception {
        final ByteBuf byteBuf = ctx.alloc().buffer(4);
        byteBuf.writeInt((int) (System.currentTimeMillis() / 1000L + 2208988800L));

        final ChannelFuture f = ctx.writeAndFlush(byteBuf);
        f.addListener(new ChannelFutureListener() {
            @Override
            public void operationComplete(ChannelFuture future) throws Exception {
                assert f == future;
                ctx.close();
            }
        });
    }

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

但是，我没有得到预期的结果。具体来说，在服务器端，问题“现在几点？”尚未在控制台上打印出来。

我做错了什么？

Answer 1

原因

这个问题有两个原因。

服务器端无法实现从客户端发送的消息，因为消息（从客户端）以错误的方式写入。而不是ctx.write("What time is it?");，我们应该实现如下ctx.write(Unpooled.coppiedBuffer("What time is it?", StandardCharsets.UTF_8);

此外，在TimeServerInboundHandler.java 中，我们应该删除方法channelActive()。这是因为channelRead() 永远不会被触发，当然channelActive() 在频道处于活动状态时会被触发。

解决方案

TimeClientInboundHandler.java

public class TimeClientInboundHandler extends ChannelInboundHandlerAdapter {
    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) {
        ByteBuf byteBuf = (ByteBuf) msg;
        try {
            System.out.println("Client received: " + byteBuf.toString(StandardCharsets.UTF_8));
            ctx.close();
        } finally {
            byteBuf.release();
        }
    }

    @Override
    public void channelActive(ChannelHandlerContext ctx) {
        ctx.writeAndFlush(Unpooled.copiedBuffer("What time is it?", StandardCharsets.UTF_8));
    }

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

TimeServerInboundHandler.java

public class TimeServerInboundHandler extends ChannelInboundHandlerAdapter {

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) {
        ByteBuf byteBuf = (ByteBuf) msg;
        try {
            System.out.println("Server received: " + byteBuf.toString(StandardCharsets.UTF_8));
            String answer = Utils.getCurrentTime();
            ctx.writeAndFlush(Unpooled.copiedBuffer(answer, StandardCharsets.UTF_8));
        } finally {
            ((ByteBuf) msg).release();
        }
    }

    @Override
    public void channelReadComplete(ChannelHandlerContext ctx) {
        ctx.writeAndFlush(Unpooled.EMPTY_BUFFER)
                .addListener(ChannelFutureListener.CLOSE);
    }

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

Utils.java

public class Utils {
    public static String getCurrentTime() {
        long currentTimeMillis = ((System.currentTimeMillis() / 1000L + 2208988800L) - 2208988800L) * 1000L;
        return new Date(currentTimeMillis).toString();
    }
}