前言：开发者用到TCP/IP交互时，偶尔会遇到粘包或者半包的数据，这种情况有时会对我们的程序造成严重的影响，netty框架为解决这种问题提供了若干框架
1. LineBasedFrameDecoder：通过在包尾添加回车换行符 来区分整包消息。
说明：LineBasedFrameDecoder 是从 ByteBuf 的可读字节中找到 或者 ，找到之后就以此为结束，然后将当前读取到的数据组成一行。
使用方法：ch.pipline().addLast(new LineBasedFrameDecoder(1024));//1024是设置每一行的最大长度，如果读到最大长度还没有结束符，会抛出异常，结束当前读取到的数据
2. DelimiterBasedFrameDecoder：通过特殊字符作为分隔符来区分整包消息
说明：客户端和服务端协议中有一致的特殊字符，来代表每一包的结束
如：客户端发送3条消息："你好你好你好$_"
"hellohellohello$_"
"赞赞$_"
服务端为了防止分包粘包，使用方法如下：
ByteBuf delimiter = Unpooled.copiedBuffer("$_".getBytes());
ch.pipline().addLast(new DelimiterBasedFrameDecoder(1024, delimiter));//1024设置每一行的最大长度3.FixedLengthFrameDecoder：用于定常消息，消息固定长度，不够空格补全，发送方和接收方遵循相同规定
使用方法：
ch.pipline().addLast(new FixedLengthFrameDecoder(1024));以上几种方法在实际项目中，存在一定的局限性，协议中可能不会有
这种字符，或者也没有第二种方法提到的特殊字符，第三种方法一般不实用，很少有项目的每条协议字段都是一样的长度。
那就介绍一种科一解决不定长协议的分包粘包方法：LengthFieldBasedFrameDecoder
先看一下LengthFieldBasedFrameDecoder源码中一个构造函数：
public LengthFieldBasedFrameDecoder(
int maxFrameLength,
int lengthFieldOffset, int lengthFieldLength,
int lengthAdjustment, int initialBytesToStrip) {
this(
maxFrameLength,
lengthFieldOffset, lengthFieldLength, lengthAdjustment,
initialBytesToStrip, true);
}
其中有5个参数的定义我用一个协议数据举例
maxFrameLength：单个包的最大长度，根据实际环境定义
lengthFieldOffset：表示数据长度字段开始的偏移量，上图中，表示数据长度的字段前边还有协议头、版本号、日期等字段，一共占了12字节，所以按照这个协议，这个参数填写12
lengthFieldOffset：数据长度字段所占的字节数，上图中协议长度占用2个字节，这个参数写2
lengthAdjustment：lengthAdjustment +数据长度取值 = 数据长度字段之后剩下包的字节数，上图中，除了协议长度和数据内容，后边还有协议尾和CRC认证，占用6个字节，这个参数写6
initialBytesToStrip：从整个包开始，向后忽略的字节数（我写成0）
按照以上协议，我的使用方法就是：ch.pipeline().addLast(new LengthFieldBasedFrameDecoder(1024,12,2,6,0));
这句代码加在自定义Hander的前边，否则无效。这样就解决了粘包和分包的问题
但是我现在遇见的问题是：我的协议数据长度字段和数据内容实际长度可能是不对应的
我的协议是如果长度字段不是8的倍数，要在数据后补0 ，例如：
协议长度字段是0046 46不是8的倍数 要补两个0 所以导致以上所有方法都不适合我，因为我不知道长度究竟会是几，究竟会补几个0，于是我自定义LengthFieldBasedFrameDecoder解决这个问题
源码中LengthFieldBasedFrameDecoder继承的是ByteToMessageDecoder（以上所有方法都是继承这个类）
解决方案：自己创建一个类，继承ByteToMessageDecoder，将LengthFieldBasedFrameDecoder
public class SelfDefineEncodeHandler extends ByteToMessageDecoder {
private final ByteOrder byteOrder;
private final int maxFrameLength;
private final int lengthFieldOffset;
private final int lengthFieldLength;
private final int lengthFieldEndOffset;
private final int lengthAdjustment;
private final int initialBytesToStrip;
private final boolean failFast;
private boolean discardingTooLongFrame;
private long tooLongFrameLength;
private long bytesToDiscard;
/**
* Creates a new instance.
*
* @param maxFrameLength
* the maximum length of the frame. If the length of the frame is
* greater than this value, {@link TooLongFrameException} will be
* thrown.
* @param lengthFieldOffset
* the offset of the length field
* @param lengthFieldLength
* the length of the length field
*/
public SelfDefineEncodeHandler(
int maxFrameLength,
int lengthFieldOffset, int lengthFieldLength) {
this(maxFrameLength, lengthFieldOffset, lengthFieldLength, 0, 0);
}
/**
* Creates a new instance.
*
* @param maxFrameLength
* the maximum length of the frame. If the length of the frame is
* greater than this value, {@link TooLongFrameException} will be
* thrown.
* @param lengthFieldOffset
* the offset of the length field
* @param lengthFieldLength
* the length of the length field
* @param lengthAdjustment
* the compensation value to add to the value of the length field
* @param initialBytesToStrip
* the number of first bytes to strip out from the decoded frame
*/
public SelfDefineEncodeHandler(
int maxFrameLength,
int lengthFieldOffset, int lengthFieldLength,
int lengthAdjustment, int initialBytesToStrip) {
this(
maxFrameLength,
lengthFieldOffset, lengthFieldLength, lengthAdjustment,
initialBytesToStrip, true);
}
/**
* Creates a new instance.
*
* @param maxFrameLength
* the maximum length of the frame. If the length of the frame is
* greater than this value, {@link TooLongFrameException} will be
* thrown.
* @param lengthFieldOffset
* the offset of the length field
* @param lengthFieldLength
* the length of the length field
* @param lengthAdjustment
* the compensation value to add to the value of the length field
* @param initialBytesToStrip
* the number of first bytes to strip out from the decoded frame
* @param failFast
* If <tt>true</tt>, a {@link TooLongFrameException} is thrown as
* soon as the decoder notices the length of the frame will exceed
* <tt>maxFrameLength</tt> regardless of whether the entire frame
* has been read. If <tt>false</tt>, a {@link TooLongFrameException}
* is thrown after the entire frame that exceeds <tt>maxFrameLength</tt>
* has been read.
*/
public SelfDefineEncodeHandler(
int maxFrameLength, int lengthFieldOffset, int lengthFieldLength,
int lengthAdjustment, int initialBytesToStrip, boolean failFast) {
this(
ByteOrder.BIG_ENDIAN, maxFrameLength, lengthFieldOffset, lengthFieldLength,
lengthAdjustment, initialBytesToStrip, failFast);
}
/**
* Creates a new instance.
*
* @param byteOrder
* the {@link ByteOrder} of the length field
* @param maxFrameLength
* the maximum length of the frame. If the length of the frame is
* greater than this value, {@link TooLongFrameException} will be
* thrown.
* @param lengthFieldOffset
* the offset of the length field
* @param lengthFieldLength
* the length of the length field
* @param lengthAdjustment
* the compensation value to add to the value of the length field
* @param initialBytesToStrip
* the number of first bytes to strip out from the decoded frame
* @param failFast
* If <tt>true</tt>, a {@link TooLongFrameException} is thrown as
* soon as the decoder notices the length of the frame will exceed
* <tt>maxFrameLength</tt> regardless of whether the entire frame
* has been read. If <tt>false</tt>, a {@link TooLongFrameException}
* is thrown after the entire frame that exceeds <tt>maxFrameLength</tt>
* has been read.
*/
public SelfDefineEncodeHandler(
ByteOrder byteOrder, int maxFrameLength, int lengthFieldOffset, int lengthFieldLength,
int lengthAdjustment, int initialBytesToStrip, boolean failFast) {
if (byteOrder == null) {
throw new NullPointerException("byteOrder");
}
if (maxFrameLength <= 0) {
throw new IllegalArgumentException(
"maxFrameLength must be a positive integer: " +
maxFrameLength);
}
if (lengthFieldOffset < 0) {
throw new IllegalArgumentException(
"lengthFieldOffset must be a non-negative integer: " +
lengthFieldOffset);
}
if (initialBytesToStrip < 0) {
throw new IllegalArgumentException(
"initialBytesToStrip must be a non-negative integer: " +
initialBytesToStrip);
}
if (lengthFieldOffset > maxFrameLength - lengthFieldLength) {
throw new IllegalArgumentException(
"maxFrameLength (" + maxFrameLength + ") " +
"must be equal to or greater than " +
"lengthFieldOffset (" + lengthFieldOffset + ") + " +
"lengthFieldLength (" + lengthFieldLength + ").");
}
this.byteOrder = byteOrder;
this.maxFrameLength = maxFrameLength;
this.lengthFieldOffset = lengthFieldOffset;
this.lengthFieldLength = lengthFieldLength;
this.lengthAdjustment = lengthAdjustment;
lengthFieldEndOffset = lengthFieldOffset + lengthFieldLength;
this.initialBytesToStrip = initialBytesToStrip;
this.failFast = failFast;
}
@Override
protected final void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) throws Exception {
Object decoded = decode(ctx, in);
if (decoded != null) {
out.add(decoded);
}
}
/**
* Create a frame out of the {@link ByteBuf} and return it.
*
* @param ctx the {@link ChannelHandlerContext} which this {@link ByteToMessageDecoder} belongs to
* @param in the {@link ByteBuf} from which to read data
* @return frame the {@link ByteBuf} which represent the frame or {@code null} if no frame could
* be created.
*/
protected Object decode(ChannelHandlerContext ctx, ByteBuf in) throws Exception {
if (discardingTooLongFrame) {
long bytesToDiscard = this.bytesToDiscard;
int localBytesToDiscard = (int) Math.min(bytesToDiscard, in.readableBytes());
in.skipBytes(localBytesToDiscard);
bytesToDiscard -= localBytesToDiscard;
this.bytesToDiscard = bytesToDiscard;
failIfNecessary(false);
}
if (in.readableBytes() < lengthFieldEndOffset) {
return null;
}
int actualLengthFieldOffset = in.readerIndex() + lengthFieldOffset;
long frameLength = getUnadjustedFrameLength(in, actualLengthFieldOffset, lengthFieldLength, byteOrder);
System.out.println("---------------"+frameLength);
if (frameLength < 0) {
in.skipBytes(lengthFieldEndOffset);
throw new CorruptedFrameException(
"negative pre-adjustment length field: " + frameLength);
}
frameLength += lengthAdjustment + lengthFieldEndOffset;
if (frameLength < lengthFieldEndOffset) {
in.skipBytes(lengthFieldEndOffset);
throw new CorruptedFrameException(
"Adjusted frame length (" + frameLength + ") is less " +
"than lengthFieldEndOffset: " + lengthFieldEndOffset);
}
if (frameLength > maxFrameLength) {
long discard = frameLength - in.readableBytes();
tooLongFrameLength = frameLength;
if (discard < 0) {
// buffer contains more bytes then the frameLength so we can discard all now
in.skipBytes((int) frameLength);
} else {
// Enter the discard mode and discard everything received so far.
discardingTooLongFrame = true;
bytesToDiscard = discard;
in.skipBytes(in.readableBytes());
}
failIfNecessary(true);
return null;
}
// never overflows because it's less than maxFrameLength
int frameLengthInt = (int) frameLength;
if (in.readableBytes() < frameLengthInt) {
return null;
}
if (initialBytesToStrip > frameLengthInt) {
in.skipBytes(frameLengthInt);
throw new CorruptedFrameException(
"Adjusted frame length (" + frameLength + ") is less " +
"than initialBytesToStrip: " + initialBytesToStrip);
}
in.skipBytes(initialBytesToStrip);
// extract frame
int readerIndex = in.readerIndex();
int actualFrameLength = frameLengthInt - initialBytesToStrip;
ByteBuf frame = extractFrame(ctx, in, readerIndex, actualFrameLength);
in.readerIndex(readerIndex + actualFrameLength);
return frame;
}
/**
* Decodes the specified region of the buffer into an unadjusted frame length. The default implementation is
* capable of decoding the specified region into an unsigned 8/16/24/32/64 bit integer. Override this method to
* decode the length field encoded differently. Note that this method must not modify the state of the specified
* buffer (e.g. {@code readerIndex}, {@code writerIndex}, and the content of the buffer.)
*
* @throws DecoderException if failed to decode the specified region
*/
protected long getUnadjustedFrameLength(ByteBuf buf, int offset, int length, ByteOrder order) {
buf = buf.order(order);
long frameLength;
switch (length) {
case 1:
frameLength = buf.getUnsignedByte(offset);
break;
case 2:
frameLength = buf.getUnsignedShort(offset);
break;
case 3:
frameLength = buf.getUnsignedMedium(offset);
break;
case 4:
frameLength = buf.getUnsignedInt(offset);
break;
case 8:
frameLength = buf.getLong(offset);
break;
default:
throw new DecoderException(
"unsupported lengthFieldLength: " + lengthFieldLength + " (expected: 1, 2, 3, 4, or 8)");
}
return frameLength+8-(frameLength%8);
}
private void failIfNecessary(boolean firstDetectionOfTooLongFrame) {
if (bytesToDiscard == 0) {
// Reset to the initial state and tell the handlers that
// the frame was too large.
long tooLongFrameLength = this.tooLongFrameLength;
this.tooLongFrameLength = 0;
discardingTooLongFrame = false;
if (!failFast ||
failFast && firstDetectionOfTooLongFrame) {
fail(tooLongFrameLength);
}
} else {
// Keep discarding and notify handlers if necessary.
if (failFast && firstDetectionOfTooLongFrame) {
fail(tooLongFrameLength);
}
}
}
/**
* Extract the sub-region of the specified buffer.
* <p>
* If you are sure that the frame and its content are not accessed after
* the current {@link #decode(ChannelHandlerContext, ByteBuf)}
* call returns, you can even avoid memory copy by returning the sliced
* sub-region (i.e. <tt>return buffer.slice(index, length)</tt>).
* It's often useful when you convert the extracted frame into an object.
* Refer to the source code of {@link ObjectDecoder} to see how this method
* is overridden to avoid memory copy.
*/
protected ByteBuf extractFrame(ChannelHandlerContext ctx, ByteBuf buffer, int index, int length) {
return buffer.retainedSlice(index, length);
}
private void fail(long frameLength) {
if (frameLength > 0) {
throw new TooLongFrameException(
"Adjusted frame length exceeds " + maxFrameLength +
": " + frameLength + " - discarded");
} else {
throw new TooLongFrameException(
"Adjusted frame length exceeds " + maxFrameLength +
" - discarding");
}
}
}
备忘，参考文章：http://linkedkeeper.com/detail/blog.action?bid=105 里面介绍了更多netty解决粘包分包的方法