WebSocket08FrameDecoder是Netty协议WS协议的解码器。
public class WebSocket08FrameDecoder extends ByteToMessageDecoder
implements WebSocketFrameDecoder {
//当前解码器状态枚举
enum State {
READING_FIRST,
READING_SECOND,
READING_SIZE,
MASKING_KEY,
PAYLOAD,
CORRUPT
}
//定义opcode
private static final byte OPCODE_CONT = 0x0;
private static final byte OPCODE_TEXT = 0x1;
private static final byte OPCODE_BINARY = 0x2;
private static final byte OPCODE_CLOSE = 0x8;
private static final byte OPCODE_PING = 0x9;
private static final byte OPCODE_PONG = 0xA;
//Websocket最大荷载数据长度，超过该值抛出异常
private final long maxFramePayloadLength;
//是否允许WS扩展
private final boolean allowExtensions;
//是否期望对荷载数据进行掩码-客户端发送的数据必须要掩码
private final boolean expectMaskedFrames;
//是否允许掩码缺失
private final boolean allowMaskMismatch;
//分片发送的数量
private int fragmentedFramesCount;
//当前ws帧是否是完整的
private boolean frameFinalFlag;
//当前ws荷载数据是否已经掩码
private boolean frameMasked;
//RSV1 RSV2 RSV3
private int frameRsv;
//ws帧内 opocde的值
private int frameOpcode;
//荷载数据的长度
private long framePayloadLength;
//掩码
private byte[] maskingKey;
//ws协议PayloadLength表示的长度
private int framePayloadLen1;
//是否收到关闭帧
private boolean receivedClosingHandshake;
//初始状态
private State state = State.READING_FIRST;
@Override
protected void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) throws Exception {
// Discard all data received if closing handshake was received before.
//如果已经收到关闭帧，则丢弃说有字节
if (receivedClosingHandshake) {
in.skipBytes(actualReadableBytes());
return;
}
switch (state) {
case READING_FIRST:
if (!in.isReadable()) {
return;
}
//把荷载数据长度设置为0
framePayloadLength = 0;
// FIN, RSV, OPCODE
//读取ws帧的第一个字节，解析出FIN RSV OPCODE
byte b = in.readByte();
frameFinalFlag = (b & 0x80) != 0; //b & 10000000 得到FIN
frameRsv = (b & 0x70) >> 4; //b & 01110000 完了右移4位 得到RSV
frameOpcode = b & 0x0F; // b & 00001111 得到opcode
//改变状态
state = State.READING_SECOND;
case READING_SECOND:
if (!in.isReadable()) {
return;
}
//读取ws帧的第二个字节
// MASK, PAYLOAD LEN 1
b = in.readByte();
//计算是否需要掩码
frameMasked = (b & 0x80) != 0;
//ws协议PayloadLength表示的长度
framePayloadLen1 = b & 0x7F;
//如果RSV不为0说明使用了WS扩展协议，allowExtensions如果设置为不允许扩展则报错
//目前RSV都为0，还没有扩展协议
if (frameRsv != 0 && !allowExtensions) {
protocolViolation(ctx, "RSV != 0 and no extension negotiated, RSV:" + frameRsv);
return;
}
//如果不允许缺失掩码 并且 客户端又没有掩码 则报错
if (!allowMaskMismatch && expectMaskedFrames != frameMasked) {
protocolViolation(ctx, "received a frame that is not masked as expected");
return;
}
//如果opcpde为一个控制帧 如果 ping pong close
if (frameOpcode > 7) { // control frame (have MSB in opcode set)
// control frames MUST NOT be fragmented
//控制帧必须是一个完整的帧，所有frameFinalFlag必须为true
if (!frameFinalFlag) {
protocolViolation(ctx, "fragmented control frame");
return;
}
//控制帧framePayload必须小于等于125
// control frames MUST have payload 125 octets or less
if (framePayloadLen1 > 125) {
protocolViolation(ctx, "control frame with payload length > 125 octets");
return;
}
//控制帧目前只能是close ping pong，其它目前ws还未定义，出现则报错
// check for reserved control frame opcodes
if (!(frameOpcode == OPCODE_CLOSE || frameOpcode == OPCODE_PING
|| frameOpcode == OPCODE_PONG)) {
protocolViolation(ctx, "control frame using reserved opcode " + frameOpcode);
return;
}
// close frame : if there is a body, the first two bytes of the
// body MUST be a 2-byte unsigned integer (in network byte
// order) representing a getStatus code
//关闭帧framePayloadLen1必为0，不能携带数据
if (frameOpcode == 8 && framePayloadLen1 == 1) {
protocolViolation(ctx, "received close control frame with payload len 1");
return;
}
} else { // data frame
//小于7的都是数据帧
//%x0：表示一个延续帧。当Opcode为0时，表示本次数据传输采用了数据分片，当前收到的数据帧为其中一个数据分片。
//%x1：表示这是一个文本帧（frame）
//%x2：表示这是一个二进制帧（frame）
// check for reserved data frame opcodes
//目前只支持这三种帧，其它抛出异常
if (!(frameOpcode == OPCODE_CONT || frameOpcode == OPCODE_TEXT
|| frameOpcode == OPCODE_BINARY)) {
protocolViolation(ctx, "data frame using reserved opcode " + frameOpcode);
return;
}
//如果是延续帧，那前面必须有一个Text或Binary帧，通过fragmentedFramesCount>0来判断
// check opcode vs message fragmentation state 1/2
if (fragmentedFramesCount == 0 && frameOpcode == OPCODE_CONT) {
protocolViolation(ctx, "received continuation data frame outside fragmented message");
return;
}
//如果fragmentedFramesCount != 0 说明前面出现了text或binary帧，并且fin为false 指示后续还有数据
//但是frameOpcode又不是一个延续帧，说明出现混乱情况报错
//我觉得frameOpcode != OPCODE_PING是一个无效的判断
// check opcode vs message fragmentation state 2/2
if (fragmentedFramesCount != 0 && frameOpcode != OPCODE_CONT && frameOpcode != OPCODE_PING) {
protocolViolation(ctx,
"received non-continuation data frame while inside fragmented message");
return;
}
}
//修改状态
state = State.READING_SIZE;
case READING_SIZE:
// Read frame payload length
//如果payload length=126 后续2个字节是荷载数据的长度
if (framePayloadLen1 == 126) {
if (in.readableBytes() < 2) {
return;
}
//读2个字节，按无符号处理
framePayloadLength = in.readUnsignedShort();
if (framePayloadLength < 126) {
protocolViolation(ctx, "invalid data frame length (not using minimal length encoding)");
return;
}
//127 后续8个字节是何在数据的长度
} else if (framePayloadLen1 == 127) {
if (in.readableBytes() < 8) {
return;
}
//读取8个字节为数据长度
framePayloadLength = in.readLong();
// TODO: check if it's bigger than 0x7FFFFFFFFFFFFFFF, Maybe
// just check if it's negative?
if (framePayloadLength < 65536) {
protocolViolation(ctx, "invalid data frame length (not using minimal length encoding)");
return;
}
} else {
//payload length<125 说明framePayloadLen1本身就表示数据长度
framePayloadLength = framePayloadLen1;
}
//如果荷载数据的长度 大于阈值，抛出异常
if (framePayloadLength > maxFramePayloadLength) {
protocolViolation(ctx, "Max frame length of " + maxFramePayloadLength + " has been exceeded.");
return;
}
if (logger.isDebugEnabled()) {
logger.debug("Decoding WebSocket Frame length={}", framePayloadLength);
}
//转换状态
state = State.MASKING_KEY;
case MASKING_KEY:
//是否有掩码
if (frameMasked) {
if (in.readableBytes() < 4) {
return;
}
//读取4个字节，读取掩码
if (maskingKey == null) {
maskingKey = new byte[4];
}
in.readBytes(maskingKey);
}
//转换状态
state = State.PAYLOAD;
case PAYLOAD:
//可读数据达不到荷载数据长度则等待下一轮事件
if (in.readableBytes() < framePayloadLength) {
return;
}
ByteBuf payloadBuffer = null;
try {
//将荷载数据读到新的缓冲区中
payloadBuffer = readBytes(ctx.alloc(), in, toFrameLength(framePayloadLength));
//切换状态为初始状态，进行下一轮读取。
state = State.READING_FIRST;
//如果有掩码，需要进行XOR二次计算还原出原文
// Unmask data if needed
if (frameMasked) {
unmask(payloadBuffer);
}
// Processing ping/pong/close frames because they cannot be
// fragmented
//根据情况封装不同数据帧
if (frameOpcode == OPCODE_PING) {
out.add(new PingWebSocketFrame(frameFinalFlag, frameRsv, payloadBuffer));
payloadBuffer = null;
return;
}
if (frameOpcode == OPCODE_PONG) {
out.add(new PongWebSocketFrame(frameFinalFlag, frameRsv, payloadBuffer));
payloadBuffer = null;
return;
}
if (frameOpcode == OPCODE_CLOSE) {
//如果是对方发的Close帧则关闭socket
receivedClosingHandshake = true;
checkCloseFrameBody(ctx, payloadBuffer);
out.add(new CloseWebSocketFrame(frameFinalFlag, frameRsv, payloadBuffer));
payloadBuffer = null;
return;
}
// Processing for possible fragmented messages for text and binary
// frames
if (frameFinalFlag) {
//如果是最终的分片则fragmentedFramesCount=0
// Final frame of the sequence. Apparently ping frames are
// allowed in the middle of a fragmented message
if (frameOpcode != OPCODE_PING) {
fragmentedFramesCount = 0;
}
} else {
// Increment counter
//否则fragmentedFramesCount++
fragmentedFramesCount++;
}
// 返回各种帧
if (frameOpcode == OPCODE_TEXT) {
out.add(new TextWebSocketFrame(frameFinalFlag, frameRsv, payloadBuffer));
payloadBuffer = null;
return;
} else if (frameOpcode == OPCODE_BINARY) {
out.add(new BinaryWebSocketFrame(frameFinalFlag, frameRsv, payloadBuffer));
payloadBuffer = null;
return;
} else if (frameOpcode == OPCODE_CONT) {
out.add(new ContinuationWebSocketFrame(frameFinalFlag, frameRsv,
payloadBuffer));
payloadBuffer = null;
return;
} else {
throw new UnsupportedOperationException("Cannot decode web socket frame with opcode: "
+ frameOpcode);
}
} finally {
//释放缓冲区，如果payloadBuffer！=null 说明没有成功返回数据帧
if (payloadBuffer != null) {
payloadBuffer.release();
}
}
case CORRUPT:
if (in.isReadable()) {
// If we don't keep reading Netty will throw an exception saying
// we can't return null if no bytes read and state not changed.
in.readByte();
}
return;
default:
throw new Error("Shouldn't reach here.");
}
}
private void unmask(ByteBuf frame) {
int i = frame.readerIndex();
int end = frame.writerIndex();
ByteOrder order = frame.order();
//把掩码二进制数组转换为int
int intMask = ((maskingKey[0] & 0xFF) << 24)
| ((maskingKey[1] & 0xFF) << 16)
| ((maskingKey[2] & 0xFF) << 8)
| (maskingKey[3] & 0xFF);
//如果是小端序，需要把INT类型的掩码反转
if (order == ByteOrder.LITTLE_ENDIAN) {
intMask = Integer.reverseBytes(intMask);
}
//XOR运算，还原原始值
for (; i + 3 < end; i += 4) {
int unmasked = frame.getInt(i) ^ intMask;
frame.setInt(i, unmasked);
}
for (; i < end; i++) {
frame.setByte(i, frame.getByte(i) ^ maskingKey[i % 4]);
}
}
//抛出异常
private void protocolViolation(ChannelHandlerContext ctx, String reason) {
protocolViolation(ctx, new CorruptedFrameException(reason));
}
//抛出异常，关闭socket
private void protocolViolation(ChannelHandlerContext ctx, CorruptedFrameException ex) {
state = State.CORRUPT;
if (ctx.channel().isActive()) {
Object closeMessage;
if (receivedClosingHandshake) {
closeMessage = Unpooled.EMPTY_BUFFER;
} else {
closeMessage = new CloseWebSocketFrame(1002, null);
}
ctx.writeAndFlush(closeMessage).addListener(ChannelFutureListener.CLOSE);
}
throw ex;
}
}
