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前言
HTTP2终于支持对头部进行压缩传输了，Netty很早就支持HTTP2了，看下Netty对HPACK的实现源码，可以对HPACK理解的更深一下。
HpackDecoder
Netty内置的编解码器Http2FrameCodec专门用来对HTTP2的各种Frame进行编解码，其中就包含将ByteBuf解码为HeadersFrame，解码的工作最终交给了io.netty.handler.codec.http2.HpackDecoder。
状态
HpackDecoder维护了一组状态常量，代表的是当前对Header的读取状态，不同的状态做的事情是不一样的，HpackDecoder通过一个While循环来读取Header，因为一个Frame里面包含若干个Header，根据读取到的数据判断，State会不断变化流转，理解了这些State，再看代码就简单的多了。
| State | 说明 |
|---|---|
| READ_HEADER_REPRESENTATION | 读取header的初试状态 |
| READ_INDEXED_HEADER | 读取被索引的完整header，即name和value均被索引 |
| READ_INDEXED_HEADER_NAME | 读取被索引的header name |
| READ_LITERAL_HEADER_NAME_LENGTH_PREFIX | name未被索引，读取name长度前缀，判断是否使用哈夫曼编码 |
| READ_LITERAL_HEADER_NAME_LENGTH | name未被索引，读取name长度 |
| READ_LITERAL_HEADER_NAME | name未被索引，读取header name |
| READ_LITERAL_HEADER_VALUE_LENGTH_PREFIX | 读取value长度前缀，判断是否使用哈夫曼编码 |
| READ_LITERAL_HEADER_VALUE_LENGTH | 读取value长度 |
| READ_LITERAL_HEADER_VALUE | value未被索引，读取value |
decode()
解码的方法是io.netty.handler.codec.http2.HpackDecoder#decode()，这里直接贴出源码，核心代码已写注释：
private void decode(ByteBuf in, Http2HeadersSink sink) throws Http2Exception {int index = 0;int nameLength = 0;int valueLength = 0;byte state = READ_HEADER_REPRESENTATION;// 初始状态 准备读取Headerboolean huffmanEncoded = false;// 是否使用哈夫曼编码 长度的第1个Bit来标记AsciiString name = null;IndexType indexType = IndexType.NONE;// 索引类型while (in.isReadable()) {// 只要有数据，就循环读switch (state) {case READ_HEADER_REPRESENTATION:byte b = in.readByte();// 读取首字节if (maxDynamicTableSizeChangeRequired && (b & 0xE0) != 0x20) {// HpackEncoder MUST signal maximum dynamic table size changethrow MAX_DYNAMIC_TABLE_SIZE_CHANGE_REQUIRED;}if (b < 0) {// 小于0 即最高位是1，代表name和value均被索引// Indexed Header Fieldindex = b & 0x7F;switch (index) {case 0:throw DECODE_ILLEGAL_INDEX_VALUE;case 0x7F: // 索引号超过了1字节，需要继续读取 才能获取最终索引号state = READ_INDEXED_HEADER;break;default:// 索引号没超 直接从静态/动态表读取即可HpackHeaderField indexedHeader = getIndexedHeader(index);sink.appendToHeaderList((AsciiString) indexedHeader.name,(AsciiString) indexedHeader.value);}} else if ((b & 0x40) == 0x40) {// 01开头 header需要加入到动态表// Literal Header Field with Incremental IndexingindexType = IndexType.INCREMENTAL;index = b & 0x3F;switch (index) {case 0:// name未被索引，读取name长度 再读namestate = READ_LITERAL_HEADER_NAME_LENGTH_PREFIX;break;case 0x3F:// name 索引号超了1字节 需要继续读state = READ_INDEXED_HEADER_NAME;break;default:// name被索引，继续读valuename = readName(index);nameLength = name.length();state = READ_LITERAL_HEADER_VALUE_LENGTH_PREFIX;}} else if ((b & 0x20) == 0x20) {// Dynamic Table Size Update// See https://www.rfc-editor.org/rfc/rfc7541.html#section-4.2throw connectionError(COMPRESSION_ERROR, "Dynamic table size update must happen " +"at the beginning of the header block");} else {// Literal Header Field without Indexing / never IndexedindexType = (b & 0x10) == 0x10 ? IndexType.NEVER : IndexType.NONE;index = b & 0x0F;switch (index) {case 0:state = READ_LITERAL_HEADER_NAME_LENGTH_PREFIX;break;case 0x0F:state = READ_INDEXED_HEADER_NAME;break;default:// Index was stored as the prefixname = readName(index);nameLength = name.length();state = READ_LITERAL_HEADER_VALUE_LENGTH_PREFIX;}}break;case READ_INDEXED_HEADER:// 被索引的header，读取可变长度的索引号，从表中获取headerHpackHeaderField indexedHeader = getIndexedHeader(decodeULE128(in, index));sink.appendToHeaderList((AsciiString) indexedHeader.name,(AsciiString) indexedHeader.value);state = READ_HEADER_REPRESENTATION;break;case READ_INDEXED_HEADER_NAME:// Header Name matches an entry in the Header Tablename = readName(decodeULE128(in, index));nameLength = name.length();state = READ_LITERAL_HEADER_VALUE_LENGTH_PREFIX;break;case READ_LITERAL_HEADER_NAME_LENGTH_PREFIX:// 读取name前缀 判断是否使用哈夫曼编码b = in.readByte();huffmanEncoded = (b & 0x80) == 0x80;index = b & 0x7F;if (index == 0x7f) {state = READ_LITERAL_HEADER_NAME_LENGTH;} else {nameLength = index;state = READ_LITERAL_HEADER_NAME;}break;case READ_LITERAL_HEADER_NAME_LENGTH:// Header Name is a Literal StringnameLength = decodeULE128(in, index);state = READ_LITERAL_HEADER_NAME;break;case READ_LITERAL_HEADER_NAME:// Wait until entire name is readableif (in.readableBytes() < nameLength) {throw notEnoughDataException(in);}name = readStringLiteral(in, nameLength, huffmanEncoded);state = READ_LITERAL_HEADER_VALUE_LENGTH_PREFIX;break;case READ_LITERAL_HEADER_VALUE_LENGTH_PREFIX:b = in.readByte();huffmanEncoded = (b & 0x80) == 0x80;index = b & 0x7F;switch (index) {case 0x7f:state = READ_LITERAL_HEADER_VALUE_LENGTH;break;case 0:insertHeader(sink, name, EMPTY_STRING, indexType);state = READ_HEADER_REPRESENTATION;break;default:valueLength = index;state = READ_LITERAL_HEADER_VALUE;}break;case READ_LITERAL_HEADER_VALUE_LENGTH:// Header Value is a Literal StringvalueLength = decodeULE128(in, index);state = READ_LITERAL_HEADER_VALUE;break;case READ_LITERAL_HEADER_VALUE:// Wait until entire value is readableif (in.readableBytes() < valueLength) {throw notEnoughDataException(in);}AsciiString value = readStringLiteral(in, valueLength, huffmanEncoded);insertHeader(sink, name, value, indexType);state = READ_HEADER_REPRESENTATION;break;default:throw new Error("should not reach here state: " + state);}}if (state != READ_HEADER_REPRESENTATION) {throw connectionError(COMPRESSION_ERROR, "Incomplete header block fragment.");}
}
该方法做的事情：
- 读取首字节，判断最高位是否是1开头。如果是代表name和value都是被索引的，继续读取索引号从静态/动态表获取header即可。
- 判断高位是否是01开头，是的话就需要把读取到的header添加到动态表中，进行维护。
- 如果name被索引，则读取索引号，从静态/动态表获取。如果没有被索引，则判断长度的第1Bit是否是1，如果是代表使用了哈夫曼编码，否则使用常规的ASCII字符编码。
- 读取value的规则类似，也是先判断是否使用哈夫曼编码，再按规则读取。
静态表
Netty通过io.netty.handler.codec.http2.HpackStaticTable类来维护静态表，硬编码写死的，不支持动态修改。
final class HpackStaticTable {static final int NOT_FOUND = -1;// Appendix A: Static Table// https://tools.ietf.org/html/rfc7541#appendix-Aprivate static final List<HpackHeaderField> STATIC_TABLE = Arrays.asList(/* 1 */ newEmptyHeaderField(":authority"),/* 2 */ newHeaderField(":method", "GET"),/* 3 */ newHeaderField(":method", "POST"),/* 4 */ newHeaderField(":path", "/"),/* 5 */ newHeaderField(":path", "/index.html"),/* 6 */ newHeaderField(":scheme", "http"),/* 7 */ newHeaderField(":scheme", "https"),/* 61 */ newEmptyHeaderField("www-authenticate")。。。。。。);
}
动态表
Netty通过io.netty.handler.codec.http2.HpackDynamicTable类来维护动态表，动态表初始是空的，只有读到01开头的Header才会加入到动态表中维护。
final class HpackDynamicTable {// a circular queue of header fieldsHpackHeaderField[] hpackHeaderFields;int head;int tail;private long size;private long capacity = -1;public void add(HpackHeaderField header) {int headerSize = header.size();if (headerSize > capacity) {clear();return;}while (capacity - size < headerSize) {remove();}hpackHeaderFields[head++] = header;size += headerSize;if (head == hpackHeaderFields.length) {head = 0;}}。。。。。。
}