Netty源码分析-编码器LengthFieldPrepender

package io.netty.handler.codec;

import io.netty.buffer.ByteBuf;
import io.netty.channel.ChannelHandler.Sharable;
import io.netty.channel.ChannelHandlerContext;
import io.netty.util.internal.ObjectUtil;

import java.nio.ByteOrder;
import java.util.List;


/**
 * An encoder that prepends the length of the message.  The length value is
 * prepended as a binary form.
 * <p>
 * For example, <tt>{@link LengthFieldPrepender}(2)</tt> will encode the
 * following 12-bytes string:
 * <pre>
 * +----------------+
 * | "HELLO, WORLD" |    原始消息长度12个字节
 * +----------------+
 * </pre>
 * into the following:
 * <pre>
 * +--------+----------------+
 * + 0x000C | "HELLO, WORLD" |     实际发送消息前面加了长度(0x000C为16进制，转换为10进制为12)
 * +--------+----------------+
 * </pre>
 * If you turned on the {@code lengthIncludesLengthFieldLength} flag in the
 * constructor, the encoded data would look like the following
 * (12 (original data) + 2 (prepended data) = 14 (0xE)):
 * <pre>
 * +--------+----------------+
 * + 0x000E | "HELLO, WORLD" |     如果lengthIncludesLengthFieldLength=true,则长度为14，多加了2个字节
 * +--------+----------------+
 * </pre>
 */
@Sharable
public class LengthFieldPrepender extends MessageToMessageEncoder<ByteBuf> {

    //字节序
    private final ByteOrder byteOrder;
    //标识长度字段的字节数
    private final int lengthFieldLength;
    //内容长度是否包含lengthFieldLength
    private final boolean lengthIncludesLengthFieldLength;
    //偏移量
    private final int lengthAdjustment;

    public LengthFieldPrepender(int lengthFieldLength) {
        this(lengthFieldLength, false);
    }

    
    public LengthFieldPrepender(int lengthFieldLength, boolean lengthIncludesLengthFieldLength) {
        this(lengthFieldLength, 0, lengthIncludesLengthFieldLength);
    }

    public LengthFieldPrepender(int lengthFieldLength, int lengthAdjustment) {
        this(lengthFieldLength, lengthAdjustment, false);
    }

    public LengthFieldPrepender(int lengthFieldLength, int lengthAdjustment, boolean lengthIncludesLengthFieldLength) {
        this(ByteOrder.BIG_ENDIAN, lengthFieldLength, lengthAdjustment, lengthIncludesLengthFieldLength);
    }

    public LengthFieldPrepender(
            ByteOrder byteOrder, int lengthFieldLength,
            int lengthAdjustment, boolean lengthIncludesLengthFieldLength) {
        //表示长度的字节数只能为 1,2,3,4,8
        //lengthFieldLength=1  用1个字节表示长度 最大长度255
        //lengthFieldLength=2  用2个字节表示长度 最大长度65535
        //以此类推.....
        if (lengthFieldLength != 1 && lengthFieldLength != 2 &&
            lengthFieldLength != 3 && lengthFieldLength != 4 &&
            lengthFieldLength != 8) {
            throw new IllegalArgumentException(
                    "lengthFieldLength must be either 1, 2, 3, 4, or 8: " +
                    lengthFieldLength);
        }
        ObjectUtil.checkNotNull(byteOrder, "byteOrder");

        this.byteOrder = byteOrder;
        this.lengthFieldLength = lengthFieldLength;
        this.lengthIncludesLengthFieldLength = lengthIncludesLengthFieldLength;
        this.lengthAdjustment = lengthAdjustment;
    }

    @Override
    protected void encode(ChannelHandlerContext ctx, ByteBuf msg, List<Object> out) throws Exception {
        //消息中的内容长度+偏移量
        int length = msg.readableBytes() + lengthAdjustment;
        if (lengthIncludesLengthFieldLength) {
            //如果lengthIncludesLengthFieldLength=true 则在加上表示长度的字节数
            length += lengthFieldLength;
        }

        //逻辑检查
        if (length < 0) {
            throw new IllegalArgumentException(
                    "Adjusted frame length (" + length + ") is less than zero");
        }

        //根据lengthFieldLength创建ByteBuf
        //然后将计算后的长度大小写入到ByteBuf
        switch (lengthFieldLength) {
        case 1:
            if (length >= 256) {
                throw new IllegalArgumentException(
                        "length does not fit into a byte: " + length);
            }
            out.add(ctx.alloc().buffer(1).order(byteOrder).writeByte((byte) length));
            break;
        case 2:
            if (length >= 65536) {
                throw new IllegalArgumentException(
                        "length does not fit into a short integer: " + length);
            }
            out.add(ctx.alloc().buffer(2).order(byteOrder).writeShort((short) length));
            break;
        case 3:
            if (length >= 16777216) {
                throw new IllegalArgumentException(
                        "length does not fit into a medium integer: " + length);
            }
            out.add(ctx.alloc().buffer(3).order(byteOrder).writeMedium(length));
            break;
        case 4:
            out.add(ctx.alloc().buffer(4).order(byteOrder).writeInt(length));
            break;
        case 8:
            out.add(ctx.alloc().buffer(8).order(byteOrder).writeLong(length));
            break;
        default:
            throw new Error("should not reach here");
        }
        //这里要添加一次引用计数器,因为在父类方法对msg进行了释放，这里要多引用一次
        out.add(msg.retain());
    }
}

客户端发送消息编码器顺序 StringEncoder => LengthFieldPrepender

socketChannel.pipeline().addLast(new LengthFieldPrepender(2));
socketChannel.pipeline().addLast(new StringEncoder());

ctx.channel().writeAndFlush(UUID.randomUUID().toString());

我们发送了一个UUID，长度为36。底层发送的数据包会把36的长度加进去，00 24 一共2个字节，24位16进制等于36

服务器端解码器顺序LengthFieldBasedFrameDecoder=>StringDecoder

LengthFieldBasedFrameDecoder会根据消息中的长度字段解析出后面的字符串，然后交给下一个解码器

StringDecoder 上面已经对数据包进行解码，这里拿到的字节流只包含UUID那部分

ch.pipeline().addLast("lengthDecoder", new LengthFieldBasedFrameDecoder(2000, 0, 2, 0 ,2));
ch.pipeline().addLast("stringDecoder",new StringDecoder(CharsetUtil.UTF_8));

Netty源码分析-编码器LengthFieldPrepender

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