Netty in Java 2

如何處理 Stream-based Transport

在 TCP/IP 的 stream-based transport 中，接收資料後，會存放在 socket receive buffer，但資料是一連串的 bytes，這代表說，即使用兩個訊息，以獨立的資料封包傳送，在 OS 也只會認為是一連串的 bytes，不代表解讀時也是這樣。

假設 OS TCP/IP 收到這三個資料包要傳送

ABC   DEF   GHI

在接收端可能會讀取到

AB    CDEFG    H   I

因此接收端必須自己整理 bytes 資料，並恢復為原本的狀態，才能正確解讀資料

ABC   DEF   GHI

方案1

在 Time client 的例子中，雖然 32 bits 資料很少，不太可能分片，但也有可能因為 traffic 增加，而出現這種狀況。

最簡單的方式，就是建立一個內部 buffer，累積到 4 bytes，就能繼續處理。

TimeClientHandler1.java

package netty.time;

import io.netty.buffer.ByteBuf;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelInboundHandlerAdapter;

import java.util.Date;

public class TimeClientHandler1 extends ChannelInboundHandlerAdapter {
    private ByteBuf buf;

    @Override
    public void handlerAdded(ChannelHandlerContext ctx) {
        // 在 channel 產生時，建立一個 4 bytes buffer
        buf = ctx.alloc().buffer(4);
    }

    @Override
    public void handlerRemoved(ChannelHandlerContext ctx) {
        // 當 channel 被移除時，就 release buffer
        buf.release();
        buf = null;
    }

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) {
        ByteBuf m = (ByteBuf) msg;
        // 一次讀取一個 byte，寫入 ByteBuf
        buf.writeBytes(m);
        m.release();

        // 檢查是否已經累積到 4 bytes
        if (buf.readableBytes() >= 4) {
            long currentTimeMillis = (buf.readUnsignedInt() - 2208988800L) * 1000L;
            System.out.println(new Date(currentTimeMillis));
            ctx.close();
        }
    }

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

方案2

將訊息組合的部份拆開，移到 Decoder

TimeDecoder.java

package netty.time.stream;

import io.netty.buffer.ByteBuf;
import io.netty.channel.ChannelHandlerContext;
import io.netty.handler.codec.ByteToMessageDecoder;

import java.util.List;

// 處理碎片問題
public class TimeDecoder extends ByteToMessageDecoder {

    // 每次收到資料時，都會用內部的 buffer，呼叫這個 method
    @Override
    protected void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) {
        // 當 in 不足 4 bytes 時，就不寫入到 out
        if (in.readableBytes() < 4) {
            return;
        }
        // 當 in 有 4 bytes
        out.add(in.readBytes(4));
    }
}

TimeDecoder1.java

TimeDecoder 可用另一種簡化的方式實作

package netty.time.stream;

import io.netty.buffer.ByteBuf;
import io.netty.channel.ChannelHandlerContext;
import io.netty.handler.codec.ReplayingDecoder;

import java.util.List;

// 改用 ReplayingDecoder，可簡化 TimeDecoder 的寫法
public class TimeDecoder1 extends ReplayingDecoder<Void> {
    @Override
    protected void decode(
            ChannelHandlerContext ctx, ByteBuf in, List<Object> out) {
        out.add(in.readBytes(4));
    }
}

TimeClient.java

修改 ChannelInitializer 的部分，加上 Decoder

package netty.time.stream;

import io.netty.bootstrap.Bootstrap;
import io.netty.channel.ChannelFuture;
import io.netty.channel.ChannelInitializer;
import io.netty.channel.ChannelOption;
import io.netty.channel.EventLoopGroup;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.SocketChannel;
import io.netty.channel.socket.nio.NioSocketChannel;

public class TimeClient {
    public static void main(String[] args) throws Exception {

        String host = "localhost";
        int port = 8080;
        if (args.length > 0) {
            host = args[0];
            port = Integer.parseInt(args[1]);
        }
        EventLoopGroup workerGroup = new NioEventLoopGroup();

        try {
            // client 要使用 Bootstrap，跟 ServerBootstrap 不同
            Bootstrap b = new Bootstrap();
            // client 只需要一個 worker EventLoopGroup
            b.group(workerGroup);
            // client 要改用 NioSocketChannel，跟 NioServerSocketChannel 不同
            b.channel(NioSocketChannel.class);
            // 不要使用 childOption
            b.option(ChannelOption.SO_KEEPALIVE, true);
//            b.handler(new ChannelInitializer<SocketChannel>() {
//                @Override
//                public void initChannel(SocketChannel ch) throws Exception {
//                    ch.pipeline().addLast(new TimeClientHandler());
//                }
//            });
            // 要在 TimeClientHandler 之前，使用 TimeDecoder
            b.handler(new ChannelInitializer<SocketChannel>() {
                @Override
                public void initChannel(SocketChannel ch) throws Exception {
                    ch.pipeline().addLast(new TimeDecoder(), new TimeClientHandler());
                }
            });

            // Start the client.
            // 呼叫 connect 而不是 bind
            ChannelFuture f = b.connect(host, port).sync();

            // Wait until the connection is closed.
            f.channel().closeFuture().sync();
        } finally {
            workerGroup.shutdownGracefully();
        }
    }
}

TimeClientHandler.java

程式不變

package netty.time.stream;

import io.netty.buffer.ByteBuf;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelInboundHandlerAdapter;

import java.util.Date;

public class TimeClientHandler extends ChannelInboundHandlerAdapter {
    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) {
        // 收到的資料是 ByteBuf
        ByteBuf m = (ByteBuf) msg;
        try {
            long currentTimeMillis = (m.readUnsignedInt() - 2208988800L) * 1000L;
            System.out.println(new Date(currentTimeMillis));
            ctx.close();
        } finally {
            m.release();
        }
    }

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

POJO

改用 POJO java class，讓 protocol 解讀更清楚

UnixTime.java

package netty.time.pojo;

import java.util.Date;

public class UnixTime {

    private final long value;

    public UnixTime() {
        this(System.currentTimeMillis() / 1000L + 2208988800L);
    }

    public UnixTime(long value) {
        this.value = value;
    }

    public long value() {
        return value;
    }

    @Override
    public String toString() {
        return new Date((value() - 2208988800L) * 1000L).toString();
    }
}

修改 TimeDecoder

TimeDecoder.java

package netty.time.pojo;

import io.netty.buffer.ByteBuf;
import io.netty.channel.ChannelHandlerContext;
import io.netty.handler.codec.ByteToMessageDecoder;

import java.util.List;

// 處理碎片問題
public class TimeDecoder extends ByteToMessageDecoder {

    @Override
    protected void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) {
        if (in.readableBytes() < 4) {
            return;
        }

        out.add(new UnixTime(in.readUnsignedInt()));
    }
}

TimeClientHandler.java

package netty.time.pojo;

import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelInboundHandlerAdapter;

public class TimeClientHandler extends ChannelInboundHandlerAdapter {
    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) {
        UnixTime m = (UnixTime) msg;
        System.out.println(m);
        ctx.close();
    }

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

TimeClient.java

package netty.time.pojo;

import io.netty.bootstrap.Bootstrap;
import io.netty.channel.ChannelFuture;
import io.netty.channel.ChannelInitializer;
import io.netty.channel.ChannelOption;
import io.netty.channel.EventLoopGroup;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.SocketChannel;
import io.netty.channel.socket.nio.NioSocketChannel;

public class TimeClient {
    public static void main(String[] args) throws Exception {

        String host = "localhost";
        int port = 8080;
        if (args.length > 0) {
            host = args[0];
            port = Integer.parseInt(args[1]);
        }
        EventLoopGroup workerGroup = new NioEventLoopGroup();

        try {
            // client 要使用 Bootstrap，跟 ServerBootstrap 不同
            Bootstrap b = new Bootstrap();
            // client 只需要一個 worker EventLoopGroup
            b.group(workerGroup);
            // client 要改用 NioSocketChannel，跟 NioServerSocketChannel 不同
            b.channel(NioSocketChannel.class);
            // 不要使用 childOption
            b.option(ChannelOption.SO_KEEPALIVE, true);
//            b.handler(new ChannelInitializer<SocketChannel>() {
//                @Override
//                public void initChannel(SocketChannel ch) throws Exception {
//                    ch.pipeline().addLast(new TimeClientHandler());
//                }
//            });
            // 要在 TimeClientHandler 之前，使用 TimeDecoder
            b.handler(new ChannelInitializer<SocketChannel>() {
                @Override
                public void initChannel(SocketChannel ch) throws Exception {
                    ch.pipeline().addLast(new TimeDecoder(), new TimeClientHandler());
                }
            });

            // Start the client.
            // 呼叫 connect 而不是 bind
            ChannelFuture f = b.connect(host, port).sync();

            // Wait until the connection is closed.
            f.channel().closeFuture().sync();
        } finally {
            workerGroup.shutdownGracefully();
        }
    }
}

POJO Server

UnixTime.java

package netty.time.pojoserver;

import java.util.Date;

public class UnixTime {

    private final long value;

    public UnixTime() {
        this(System.currentTimeMillis() / 1000L + 2208988800L);
    }

    public UnixTime(long value) {
        this.value = value;
    }

    public long value() {
        return value;
    }

    @Override
    public String toString() {
        return new Date((value() - 2208988800L) * 1000L).toString();
    }
}

TimeServerHandler.java

package netty.time.pojoserver;

import io.netty.channel.ChannelFuture;
import io.netty.channel.ChannelFutureListener;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelInboundHandlerAdapter;

public class TimeServerHandler extends ChannelInboundHandlerAdapter {

    @Override
    public void channelActive(ChannelHandlerContext ctx) {
        ChannelFuture f = ctx.writeAndFlush(new UnixTime());
        f.addListener(ChannelFutureListener.CLOSE);
    }

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

TimeEncoder.java

package netty.time.pojoserver;

import io.netty.buffer.ByteBuf;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelOutboundHandlerAdapter;
import io.netty.channel.ChannelPromise;

public class TimeEncoder extends ChannelOutboundHandlerAdapter {
    @Override
    public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) {
        UnixTime m = (UnixTime) msg;
        ByteBuf encoded = ctx.alloc().buffer(4);
        encoded.writeInt((int)m.value());
        // 將 ChannelPromise 傳給 Netty，讓 netty 可決定 success or failure
        ctx.write(encoded, promise);
    }
}

TimeEncoder1.java

簡化寫法

package netty.time.pojoserver;

import io.netty.buffer.ByteBuf;
import io.netty.channel.ChannelHandlerContext;
import io.netty.handler.codec.MessageToByteEncoder;

public class TimeEncoder1 extends MessageToByteEncoder<UnixTime> {
    @Override
    protected void encode(ChannelHandlerContext ctx, UnixTime msg, ByteBuf out) {
        out.writeInt((int)msg.value());
    }
}

TimeServer.java

package netty.time.pojoserver;

import io.netty.bootstrap.ServerBootstrap;
import io.netty.channel.ChannelFuture;
import io.netty.channel.ChannelInitializer;
import io.netty.channel.ChannelOption;
import io.netty.channel.EventLoopGroup;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.SocketChannel;
import io.netty.channel.socket.nio.NioServerSocketChannel;

public class TimeServer {

    private int port;

    public TimeServer(int port) {
        this.port = port;
    }

    public void run() throws Exception {
        // NioEventLoopGroup 是 multithread event loop，處理 IO operation
        // 第一個 NioEventLoopGroup 處理 incoming connection，稱為 boss
        // 第二個 NioEventLoopGroup 處理已接收連線的 traffic，稱為 worker
        EventLoopGroup bossGroup = new NioEventLoopGroup();
        EventLoopGroup workerGroup = new NioEventLoopGroup();
        try {
            // ServerBootstrap 是 helper class，可設定 server
            ServerBootstrap b = new ServerBootstrap();
            b.group(bossGroup, workerGroup)
                    .channel(NioServerSocketChannel.class)  // 使用 NioServerSocketChannel 接受 incoming connection
                    .childHandler(new ChannelInitializer<SocketChannel>() {
                        @Override
                        public void initChannel(SocketChannel ch) throws Exception {
                            ch.pipeline().addLast(new TimeEncoder(), new TimeServerHandler());
                        }
                    })
                    // 設定 ChannelOption, ChannelConfig 可取得所有可設定的參數
                    // option 是設定 NioServerSocketChannel 的參數
                    .option(ChannelOption.SO_BACKLOG, 128)
                    // childOption 是設定 NioSocketChannel 的參數
                    .childOption(ChannelOption.SO_KEEPALIVE, true);

            // Bind and start to accept incoming connections.
            // 綁定 TCP Port
            ChannelFuture f = b.bind(port).sync(); // (7)

            // 這邊開始會等待所有 server socket 關閉
            // 但這個例子不會發生這種狀況
            // 如果要以正常方式關閉 server，可呼叫以下 method
            f.channel().closeFuture().sync();
        } finally {
            workerGroup.shutdownGracefully();
            bossGroup.shutdownGracefully();
        }
    }

    public static void main(String[] args) throws Exception {
        int port = 8080;
        if (args.length > 0) {
            port = Integer.parseInt(args[0]);
        }

        new TimeServer(port).run();
    }
}

References

Netty.docs: User guide for 4.x