摘要

在詳解http報文一文中，詳細介紹了http報文的文字結構。那麼作為服務端，web容器是如何解析http報文的呢？本文以jetty和undertow容器為例，來解析web容器是如何處理http報文的。

在前文中我們從概覽中可以瞭解到，http報文其實就是一定規則的字串，那麼解析它們，就是解析字串，看看是否滿足http協議約定的規則。

start-line: 起始行,描述請求或響應的基本資訊

*( header-field CRLF ): 頭

CRLF

[message-body]: 訊息body，實際傳輸的資料

jetty

以下程式碼都是jetty9.4.12版本

如何解析這麼長的字串呢，jetty是通過狀態機來實現的。具體可以看下org.eclipse.jetty.http.HttpParse

 public enum State
    {
        START,
        METHOD,
        
![](https://img2018.cnblogs.com/blog/1147363/201910/1147363-20191009220439773-204646534.png),
        SPACE1,
        STATUS,
        URI,
        SPACE2,
        REQUEST_VERSION,
        REASON,
        PROXY,
        HEADER,
        CONTENT,
        EOF_CONTENT,
        CHUNKED_CONTENT,
        CHUNK_SIZE,
        CHUNK_PARAMS,
        CHUNK,
        TRAILER,
        END,
        CLOSE,  // The associated stream/endpoint should be closed
        CLOSED  // The associated stream/endpoint is at EOF
    }
 

總共分成了21種狀態，然後進行狀態間的流轉。在parseNext方法中分別對起始行 -> header -> body content分別解析

public boolean parseNext(ByteBuffer buffer)
    {
        try
        {
            // Start a request/response
            if (_state==State.START)
            {
                // 快速判斷
                if (quickStart(buffer))
                    return true;
            }

            // Request/response line 轉換
            if (_state.ordinal()>= State.START.ordinal() && _state.ordinal()<State.HEADER.ordinal())
            {
                if (parseLine(buffer))
                    return true;
            }

            // headers轉換
            if (_state== State.HEADER)
            {
                if (parseFields(buffer))
                    return true;
            }

            // content轉換
            if (_state.ordinal()>= State.CONTENT.ordinal() && _state.ordinal()<State.TRAILER.ordinal())
            {
                // Handle HEAD response
                if (_responseStatus>0 && _headResponse)
                {
                    setState(State.END);
                    return handleContentMessage();
                }
                else
                {
                    if (parseContent(buffer))
                        return true;
                }
            }
         
        return false;
    }
 

整體流程

整體有三條路徑

1. 開始 -> start-line -> header -> 結束
2. 開始 -> start-line -> header -> content -> 結束
3. 開始 -> start-line -> header -> chunk-content -> 結束
   

起始行

start-line = request-line(請求起始行）/（響應起始行）status-line

1. 請求報文解析狀態遷移
   請求行：START -> METHOD -> SPACE1 -> URI -> SPACE2 -> REQUEST_VERSION

2. 響應報文解析狀態遷移
   響應行：START -> RESPONSE_VERSION -> SPACE1 -> STATUS -> SPACE2 -> REASON

header 頭

HEADER 的狀態只有一種了，在jetty的老版本中還區分了HEADER_IN_NAM, HEADER_VALUE, HEADER_IN_VALUE等，9.4中都去除了。為了提高匹配效率，jetty使用了Trie樹快速匹配header頭。

static
    {
        CACHE.put(new HttpField(HttpHeader.CONNECTION,HttpHeaderValue.CLOSE));
        CACHE.put(new HttpField(HttpHeader.CONNECTION,HttpHeaderValue.KEEP_ALIVE));
      // 以下省略了很多了通用header頭

content

請求體：

1. CONTENT -> END，這種是普通的帶Content-Length頭的報文，HttpParser一直執行CONTENT狀態，直到最後ContentLength達到了指定的數量，則進入END狀態
2. chunked分塊傳輸的資料
   CHUNKED_CONTENT -> CHUNK_SIZE -> CHUNK -> CHUNK_END -> END

undertow

undertow是另一種web容器，它的處理方式與jetty有什麼不同呢
狀態機種類不一樣了，io.undertow.util.HttpString.ParseState

    public static final int VERB = 0;
    public static final int PATH = 1;
    public static final int PATH_PARAMETERS = 2;
    public static final int QUERY_PARAMETERS = 3;
    public static final int VERSION = 4;
    public static final int AFTER_VERSION = 5;
    public static final int HEADER = 6;
    public static final int HEADER_VALUE = 7;
    public static final int PARSE_COMPLETE = 8;

具體處理流程在HttpRequestParser抽象類中

public void handle(ByteBuffer buffer, final ParseState currentState, final HttpServerExchange builder) throws BadRequestException {
        if (currentState.state == ParseState.VERB) {
            //fast path, we assume that it will parse fully so we avoid all the if statements

            // 快速處理GET
            final int position = buffer.position();
            if (buffer.remaining() > 3
                    && buffer.get(position) == 'G'
                    && buffer.get(position + 1) == 'E'
                    && buffer.get(position + 2) == 'T'
                    && buffer.get(position + 3) == ' ') {
                buffer.position(position + 4);
                builder.setRequestMethod(Methods.GET);
                currentState.state = ParseState.PATH;
            } else {
                try {
                    handleHttpVerb(buffer, currentState, builder);
                } catch (IllegalArgumentException e) {
                    throw new BadRequestException(e);
                }
            }
            // 處理path
            handlePath(buffer, currentState, builder);
           // 處理版本
            if (failed) {
                handleHttpVersion(buffer, currentState, builder);
                handleAfterVersion(buffer, currentState);
            }
            // 處理header
            while (currentState.state != ParseState.PARSE_COMPLETE && buffer.hasRemaining()) {
                handleHeader(buffer, currentState, builder);
                if (currentState.state == ParseState.HEADER_VALUE) {
                    handleHeaderValue(buffer, currentState, builder);
                }
            }
            return;
        }
        handleStateful(buffer, currentState, builder);
    }

與jetty不同的是對content的處理，在header處理完以後，將資料放到io.undertow.server.HttpServerExchange,然後根據型別，有不同的content讀取方式，比如處理固定長度的，FixedLengthStreamSourceConduit。

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參考

http://www.blogjava.net/DLevin/archive/2014/04/19/411673.html

https://www.ph0ly.com/2018/10/06/jetty/connection/http-parser/

https://webtide.com/http-trailers-in-jetty/

http://undertow.io/undertow-docs/undertow-docs-2.0