tomcat 源码研究之http报文解析

一，我们知道tomcat作为web服务器 已经在行业中应用多年了，深入各方面。但是作为一个java方向的web开发者，我们是否对其源码有过深入的研究呢？当然了tomcat本身涉及的知识点很多，我在这里只是讲他的一方面应用。

众所周知，客户端发送了http请求以后，tomcat服务器会接受到http请求封装成request，但是tomcat是怎么进行解析的呢?下面就是我要讲述的内容。

二，再说解析http请求报文之前我们先说说http请求在tomcat中所走的流程
当tomcat启动以后，Connector的组件会找到相应protocolhandler（这个在我们server.xml文件中配置的http1.1的protocol和ajp的protocol，tomcat默认是这样的配置）去处理监听对应的请求，而http的protocolhandler即Http11Protocol这个类会用JIoEndpoint这类去负责socket监听
代码如下

protected class Acceptor extends AbstractEndpoint.Acceptor {

        @Override
        public void run() {

            int errorDelay = 0;

            // Loop until we receive a shutdown command
            while (running) {
               //代码省略...
                try {
                    //if we have reached max connections, wait
                	//超过最大连接数将经行等待
                    countUpOrAwaitConnection();

                    Socket socket = null;
                    try {
                        // Accept the next incoming connection from the server
                        // socket
                    	//获得socket 在这儿监听停顿
                        socket = serverSocketFactory.acceptSocket(serverSocket);
                    } catch (IOException ioe) {
                        countDownConnection();
                        // Introduce delay if necessary
                        errorDelay = handleExceptionWithDelay(errorDelay);
                        // re-throw
                        throw ioe;
                    }
                    // Successful accept, reset the error delay
                    errorDelay = 0;

                    // Configure the socket
                    if (running && !paused && setSocketOptions(socket)) {
                        // Hand this socket off to an appropriate processor
                    	//当有 请求到达时 ,这儿负责socket请求的处理
                    	if (!processSocket(socket)) {
                            countDownConnection();
                            // Close socket right away
                            closeSocket(socket);
                        }
                  //代码省略...
       
    }

接下来是processSocket(Socket socket) 作用是封装socket请求，进入连接池 等待解析处理代码如下

protected boolean processSocket(Socket socket) {
        // Process the request from this socket
        try {
            SocketWrapper wrapper = new SocketWrapper(socket);
            wrapper.setKeepAliveLeft(getMaxKeepAliveRequests());
            wrapper.setSecure(isSSLEnabled());
            // During shutdown, execu4tor may be null - avoid NPE
            if (!running) {
                return false;
            }
            //把socket请求封装成SocketProcessor 然后放入线程池进行处理
            getExecutor().execute(new SocketProcessor(wrapper));
         //代码省略......
    }

//接着到了SocketProcessor类的run方法中

   public void run() {
            boolean launch = false;
            synchronized (socket) {
                try {
                    SocketState state = SocketState.OPEN;

                    try {
                        // SSL handshake
                        serverSocketFactory.handshake(socket.getSocket());
                    } catch (Throwable t) {
                        ExceptionUtils.handleThrowable(t);
                        if (log.isDebugEnabled()) {
                            log.debug(sm.getString("endpoint.err.handshake"), t);
                        }
                        // Tell to close the socket
                        state = SocketState.CLOSED;
                    }
                     //这里socket非close状态下进入process处理函数
                     //以下的if else语句中还有其他条件进入的process函数 我在这里就只讲这处的
                    if ((state != SocketState.CLOSED)) {
                        if (status == null) {
                            state = handler.process(socket, SocketStatus.OPEN_READ);
                        } else {
                            state = handler.process(socket,status);
                        }
                     }
               //代码省略....

上述代码中handler恰恰是类Http11Processor 它process函数在父类中如下

   public SocketState process(SocketWrapper socketWrapper)
        throws IOException {
        RequestInfo rp = request.getRequestProcessor();
        rp.setStage(org.apache.coyote.Constants.STAGE_PARSE);

        // Setting up the I/O
        setSocketWrapper(socketWrapper);
        getInputBuffer().init(socketWrapper, endpoint);
        getOutputBuffer().init(socketWrapper, endpoint);

        // Flags
        keepAlive = true;
        comet = false;
        openSocket = false;
        sendfileInProgress = false;
        readComplete = true;
        if (endpoint.getUsePolling()) {
            keptAlive = false;
        } else {
            keptAlive = socketWrapper.isKeptAlive();
        }

        if (disableKeepAlive()) {
            socketWrapper.setKeepAliveLeft(0);
        }

        while (!getErrorState().isError() && keepAlive && !comet && !isAsync() &&
                upgradeInbound == null &&
                httpUpgradeHandler == null && !endpoint.isPaused()) {
            // Parsing the request header
            try {
                setRequestLineReadTimeout();
                //取得http报头的第一行 格式如： GET /cuidiwhere/article/details/12361425 HTTP/1.1
                if (!getInputBuffer().parseRequestLine(keptAlive)) {
                    if (handleIncompleteRequestLineRead()) {
                        break;
                    }
                }
     //http头
 //=======================================================================
                if (endpoint.isPaused()) {
                    response.setStatus(503);
                    setErrorState(ErrorState.CLOSE_CLEAN, null);
                } else {
                    keptAlive = true;
                    // Set this every time in case limit has been changed via JMX
                    request.getMimeHeaders().setLimit(endpoint.getMaxHeaderCount());
                    // Currently only NIO will ever return false here
                    //解析http header头的内容
                    /**
                     * 格式如下：
                     * 	Host: blog.csdn.net
						Connection: keep-alive
						Pragma: no-cache
						Cache-Control: no-cache
						Upgrade-Insecure-Requests: 1
						User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/57.0.2987.133 Safari/537.36
						Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp;
					*/
                    if (!getInputBuffer().parseHeaders()) {
                        // We've read part of the request, don't recycle it
                        // instead associate it with the socket
                        openSocket = true;
                        readComplete = false;
                        break;
                    }
                    if (!disableUploadTimeout) {
                        setSocketTimeout(connectionUploadTimeout);
                    }
                }
            } catch (IOException e) {
                if (getLog().isDebugEnabled()) {
                    getLog().debug(
                            sm.getString("http11processor.header.parse"), e);
                }
                setErrorState(ErrorState.CLOSE_NOW, e);
                break;
            } catch (Throwable t) {
                ExceptionUtils.handleThrowable(t);
                UserDataHelper.Mode logMode = userDataHelper.getNextMode();
                if (logMode != null) {
                    String message = sm.getString(
                            "http11processor.header.parse");
                    switch (logMode) {
                        case INFO_THEN_DEBUG:
                            message += sm.getString(
                                    "http11processor.fallToDebug");
                            //$FALL-THROUGH$
                        case INFO:
                            getLog().info(message);
                            break;
                        case DEBUG:
                            getLog().debug(message);
                    }
                }
                // 400 - Bad Request
                response.setStatus(400);
                setErrorState(ErrorState.CLOSE_CLEAN, t);
                getAdapter().log(request, response, 0);
            }

            if (!getErrorState().isError()) {
                // Setting up filters, and parse some request headers
                rp.setStage(org.apache.coyote.Constants.STAGE_PREPARE);
                try {
                	//准备request的信息
                    prepareRequest();
                } catch (Throwable t) {
                    ExceptionUtils.handleThrowable(t);
                    if (getLog().isDebugEnabled()) {
                        getLog().debug(sm.getString(
                                "http11processor.request.prepare"), t);
                    }
                    // 500 - Internal Server Error
                    response.setStatus(500);
                    setErrorState(ErrorState.CLOSE_CLEAN, t);
                    getAdapter().log(request, response, 0);
                }
            }
          
             if (maxKeepAliveRequests == 1) {
                keepAlive = false;
            } else if (maxKeepAliveRequests > 0 &&
                    socketWrapper.decrementKeepAlive() <= 0) {
                keepAlive = false;
            }
            

            // Process the request in the adapter
            if (!getErrorState().isError()) {
                try {
                    rp.setStage(org.apache.coyote.Constants.STAGE_SERVICE);
                    //处理解析好的请求
                    adapter.service(request, response);
                    // Handle when the response was committed before a serious
                    // error occurred.  Throwing a ServletException should both
                    // set the status to 500 and set the errorException.
                    // If we fail here, then the response is likely already
                    // committed, so we can't try and set headers.
                    if(keepAlive && !getErrorState().isError() && (
                            response.getErrorException() != null ||
                                    (!isAsync() &&
                                    statusDropsConnection(response.getStatus())))) {
                        setErrorState(ErrorState.CLOSE_CLEAN, null);
                    }
                    setCometTimeouts(socketWrapper);
//省略代码

上述代码两个重要的函数 getInputBuffer().parseRequestLine(keptAlive) 和 getInputBuffer().parseHeaders() 来解析http请求头的
二，我们知道http报文格式如下图



（1），getInputBuffer().parseRequestLine（boolean）这个方法是用来解析“请求行”
代码如下

 public boolean parseRequestLine(boolean useAvailableDataOnly)
    
        throws IOException {

        int start = 0;

        //
        // Skipping blank lines
        //
      //首先读取一段字节buf  然后通过循环判断buf中的每个字节是否是\r\n 然后跳过
        /**
         * 用do while 循环是来去除空格的
         */
        byte chr = 0;
        do {

            // Read new bytes if needed
            if (pos >= lastValid) {
                if (!fill())
                    throw new EOFException(sm.getString("iib.eof.error"));
            }
            // Set the start time once we start reading data (even if it is
            // just skipping blank lines)
            if (request.getStartTime() < 0) {
                request.setStartTime(System.currentTimeMillis());
            }
            chr = buf[pos++];
        } while ((chr == Constants.CR) || (chr == Constants.LF));
//当循环跳出时说明 此时pos已经不是\r\n需要回退一步
        pos--;

        // Mark the current buffer position
        start = pos;

        //
        // Reading the method name
        // Method name is always US-ASCII
        //

        boolean space = false;
        /**
         * 当请求头没有 空格换行符时开始读取请求行
         * 以下是读取请求行中method方法的
         */
        while (!space) {

            // Read new bytes if needed
            if (pos >= lastValid) {
                if (!fill())
                    throw new EOFException(sm.getString("iib.eof.error"));
            }

            // Spec says no CR or LF in method name
            if (buf[pos] == Constants.CR || buf[pos] == Constants.LF) {
                throw new IllegalArgumentException(
                        sm.getString("iib.invalidmethod"));
            }
            // Spec says single SP but it also says be tolerant of HT
            //当遇到空格和制表符时停止读取 表示method已经读取完 存入request中
            if (buf[pos] == Constants.SP || buf[pos] == Constants.HT) {
                space = true;
                request.method().setBytes(buf, start, pos - start);
            }

            pos++;

        }

        /** 去除空格
         * while 循环是来去除空白字符时
         */
        // Spec says single SP but also says be tolerant of multiple and/or HT
        while (space) {
            // Read new bytes if needed
            if (pos >= lastValid) {
                if (!fill())
                    throw new EOFException(sm.getString("iib.eof.error"));
            }
            if (buf[pos] == Constants.SP || buf[pos] == Constants.HT) {
                pos++;
            } else {
                space = false;
            }
        }

        // Mark the current buffer position
        start = pos;
        int end = 0;
        int questionPos = -1;

        //
        // Reading the URI
        //

        boolean eol = false;
       /**
        * 当没有空白字符时，读取uri
        */
        while (!space) {

            // Read new bytes if needed
            if (pos >= lastValid) {
                if (!fill())
                    throw new EOFException(sm.getString("iib.eof.error"));
            }

            // Spec says single SP but it also says be tolerant of HT
            if (buf[pos] == Constants.SP || buf[pos] == Constants.HT) {
                space = true;
                end = pos;
            } else if ((buf[pos] == Constants.CR) 
                       || (buf[pos] == Constants.LF)) {
                // HTTP/0.9 style request
                eol = true;
                space = true;
                end = pos;
            } else if ((buf[pos] == Constants.QUESTION) 
                       && (questionPos == -1)) {
                questionPos = pos;
            }

            pos++;

        }
       //读取请求参数以后的参数 ，即？后面的请求参数
        request.unparsedURI().setBytes(buf, start, end - start);
        if (questionPos >= 0) {
            request.queryString().setBytes(buf, questionPos + 1, 
                                           end - questionPos - 1);
            request.requestURI().setBytes(buf, start, questionPos - start);
        } else {
            request.requestURI().setBytes(buf, start, end - start);
        }

        // Spec says single SP but also says be tolerant of multiple and/or HT
        /** 去除空格
         * while 循环是来去除空格的
         */
        while (space) {
            // Read new bytes if needed
            if (pos >= lastValid) {
                if (!fill())
                    throw new EOFException(sm.getString("iib.eof.error"));
            }
            if (buf[pos] == Constants.SP || buf[pos] == Constants.HT) {
                pos++;
            } else {
                space = false;
            }
        }

        // Mark the current buffer position
        start = pos;
        end = 0;

        //
        // Reading the protocol
        // Protocol is always US-ASCII
        //
        /**
         * 查看是否读取到末尾 如果末尾 
         * 那么就暂停
         */
        while (!eol) {

            // Read new bytes if needed
            if (pos >= lastValid) {
                if (!fill())
                    throw new EOFException(sm.getString("iib.eof.error"));
            }

            if (buf[pos] == Constants.CR) {
                end = pos;
            } else if (buf[pos] == Constants.LF) {
                if (end == 0)
                    end = pos;
                eol = true;
            }

            pos++;

        }
        /**
         * 根据end和start计算出协议头的字节数  格式如HTTP/1.1
         */
        if ((end - start) > 0) {
            request.protocol().setBytes(buf, start, end - start);
        } else {
            request.protocol().setString("");
        }
        
        return true;

    }

上述代码就是一个解析请求行的过程
大致流程如下 
跳过空白字符->读取Method方法根据空白字判断结束->跳过空白字符->读取uri如果有请求参数则通过？判断->跳过空白字符->读取最后一部分，如果遇到空白字符则请求头完->根据start和end计算出协议所占字节数

（2）getInputBuffer().parseHeaders() 这个方法用来解析的请求头的
方法如下

   @Override
    public boolean parseHeaders()
        throws IOException {
        if (!parsingHeader) {
            throw new IllegalStateException(
                    sm.getString("iib.parseheaders.ise.error"));
        }
        /**
         * 每循环一次表示读取一次key:value
         * parseHeader()表示读取一行的key：value
         * http报头可能有多行的key:value组成
         */
        while (parseHeader()) {
            // Loop until we run out of headers
        }

        parsingHeader = false;
        end = pos;
        return true;
    }


    /**
     * Parse an HTTP header.
     * 
     * @return false after reading a blank line (which indicates that the
     * HTTP header parsing is done
     */
    @SuppressWarnings("null") // headerValue cannot be null
    private boolean parseHeader()
        throws IOException {

        //
        // Check for blank line
        //

        byte chr = 0;
        /**
         *跳过空白字符
         *如果遇到特殊不符合的字符则返回不进行解析 
         */
        while (true) {

            // Read new bytes if needed
            if (pos >= lastValid) {
                if (!fill())
                    throw new EOFException(sm.getString("iib.eof.error"));
            }

            chr = buf[pos];

            if (chr == Constants.CR) {
                // Skip
            	/**
            	 * 在清楚空白字符时 如果再次遇到换行符，那么就结束所有的http头解析
            	 * 因为在上一次key：value中已经解析了换行符了，如果再次开头遇到
            	 * 那么就表示解析完了
            	 * 因为两次同时遇到换行符只能是消息体了
            	 */
            } else if (chr == Constants.LF) {
                pos++;
                return false;
            } else {
                break;
            }

            pos++;

        }

        // Mark the current buffer position
        int start = pos;

        //
        // Reading the header name
        // Header name is always US-ASCII
        //

        boolean colon = false;
        MessageBytes headerValue = null;

        while (!colon) {

            // Read new bytes if needed
            if (pos >= lastValid) {
                if (!fill())
                    throw new EOFException(sm.getString("iib.eof.error"));
            }
            //遇到冒号：表示前面读取的字符就是key值 然后设置colon为空true 跳出此次循环 
            //并将key存入headerValue中了
            if (buf[pos] == Constants.COLON) {
                colon = true;
                headerValue = headers.addValue(buf, start, pos - start);
            } else if (!HTTP_TOKEN_CHAR[buf[pos]]) {
                // If a non-token header is detected, skip the line and
                // ignore the header
                skipLine(start);
                return true;
            }

            chr = buf[pos];
            //大写转化成小写 将key的大写形式转成成小写
            if ((chr >= Constants.A) && (chr <= Constants.Z)) {
                buf[pos] = (byte) (chr - Constants.LC_OFFSET);
            }

            pos++;

        }

        // Mark the current buffer position
        start = pos;
        int realPos = pos;

        //
        // Reading the header value (which can be spanned over multiple lines)
        //

        /**
         * pos读到冒号：点上  key:（此处可能有空格换行等待空白字符）value 
         */
        boolean eol = false;
        boolean validLine = true;

        while (validLine) {

            boolean space = true;

            // Skipping spaces
            /**
             * 去除value之前的各种空白字符
             */
            while (space) {

                // Read new bytes if needed
                if (pos >= lastValid) {
                    if (!fill())
                        throw new EOFException(sm.getString("iib.eof.error"));
                }

                if ((buf[pos] == Constants.SP) || (buf[pos] == Constants.HT)) {
                    pos++;
                } else {
                    space = false;
                }

            }

            int lastSignificantChar = realPos;

            // Reading bytes until the end of the line
            while (!eol) {

                // Read new bytes if needed
                if (pos >= lastValid) {
                    if (!fill())
                        throw new EOFException(sm.getString("iib.eof.error"));
                }

                if (buf[pos] == Constants.CR) {
                    // Skip
               //当遇到\n时表示表已经读取完此行的key:value了
                } else if (buf[pos] == Constants.LF) {
                    eol = true;
                } else if (buf[pos] == Constants.SP) {
                    buf[realPos] = buf[pos];
                    realPos++;
                } else {
                    buf[realPos] = buf[pos];
                    realPos++;
                    lastSignificantChar = realPos;
                }

                pos++;

            }

            realPos = lastSignificantChar;

            // Checking the first character of the new line. If the character
            // is a LWS, then it's a multiline header

            // Read new bytes if needed
            if (pos >= lastValid) {
                if (!fill())
                    throw new EOFException(sm.getString("iib.eof.error"));
            }

            chr = buf[pos];
            if ((chr != Constants.SP) && (chr != Constants.HT)) {
                validLine = false;
            } else {
                eol = false;
                // Copying one extra space in the buffer (since there must
                // be at least one space inserted between the lines)
                buf[realPos] = chr;
                realPos++;
            }

        }
        /**
         * 跳出循环后计算出本次读取的最后字节即value的值
         * 至此一行key:value读取完了
         * 
         * 
         */
        // Set the header value
        headerValue.setBytes(buf, start, realPos - start);

        return true;

    }

以上是解析 请求头中的key:value 方法
通过冒号:定位key的字节 通过换行符定位value的值通过多次循环解析多个key-value
如果两次同时遇到回车换行符表示解析完成

（3）整个消息头就是这样解析的 但是我们的消息体是怎么读取的
要知道 http的get方法直接是读取的请求参数 uri问号后面的部分。
但是post却是怎么读取的呢？ tomcat采取了延迟解析 只有用到的时候才解析
我们都知道我们获取参数是通过 request的getParameterxxx（)一堆方法的和getInputStream()这类方法才用到来获取消息体的
但是这些方法最终都指向了
方法 Request的 parseParameters
代码如下

  protected void parseParameters() {

        parametersParsed = true;

        Parameters parameters = coyoteRequest.getParameters();
        boolean success = false;
        try {
            // Set this every time in case limit has been changed via JMX
            parameters.setLimit(getConnector().getMaxParameterCount());

            // getCharacterEncoding() may have been overridden to search for
            // hidden form field containing request encoding
            String enc = getCharacterEncoding();

            boolean useBodyEncodingForURI = connector.getUseBodyEncodingForURI();
            if (enc != null) {
                parameters.setEncoding(enc);
                if (useBodyEncodingForURI) {
                    parameters.setQueryStringEncoding(enc);
                }
            } else {
                parameters.setEncoding
                    (org.apache.coyote.Constants.DEFAULT_CHARACTER_ENCODING);
                if (useBodyEncodingForURI) {
                    parameters.setQueryStringEncoding
                        (org.apache.coyote.Constants.DEFAULT_CHARACTER_ENCODING);
                }
            }

            parameters.handleQueryParameters();

            if (usingInputStream || usingReader) {
                success = true;
                return;
            }

            if( !getConnector().isParseBodyMethod(getMethod()) ) {
                success = true;
                return;
            }

            String contentType = getContentType();
            if (contentType == null) {
                contentType = "";
            }
            int semicolon = contentType.indexOf(';');
            if (semicolon >= 0) {
                contentType = contentType.substring(0, semicolon).trim();
            } else {
                contentType = contentType.trim();
            }

            //如果消息体是文件流 则用parseParts()解析
            if ("multipart/form-data".equals(contentType)) {
                parseParts();
                success = true;
                return;
            }

            if (!("application/x-www-form-urlencoded".equals(contentType))) {
                success = true;
                return;
            }

            int len = getContentLength();

            if (len > 0) {
                int maxPostSize = connector.getMaxPostSize();
                if ((maxPostSize > 0) && (len > maxPostSize)) {
                    if (context.getLogger().isDebugEnabled()) {
                        context.getLogger().debug(
                                sm.getString("coyoteRequest.postTooLarge"));
                    }
                    checkSwallowInput();
                    return;
                }
                byte[] formData = null;
                if (len < CACHED_POST_LEN) {
                    if (postData == null) {
                        postData = new byte[CACHED_POST_LEN];
                    }
                    formData = postData;
                } else {
                    formData = new byte[len];
                }
                try {
                	//如果消息体是form参数 则用ReadPostBody解析
                    if (readPostBody(formData, len) != len) {
                        return;
                    }
                } catch (IOException e) {
                    // Client disconnect
                    if (context.getLogger().isDebugEnabled()) {
                        context.getLogger().debug(
                                sm.getString("coyoteRequest.parseParameters"), e);
                    }
                    return;
                }
                //存放到Parametes对象中
                parameters.processParameters(formData, 0, len);
            } else if ("chunked".equalsIgnoreCase(
                    coyoteRequest.getHeader("transfer-encoding"))) {
                byte[] formData = null;
                try {
                    formData = readChunkedPostBody();
                } catch (IOException e) {
                    // Client disconnect or chunkedPostTooLarge error
                    if (context.getLogger().isDebugEnabled()) {
                        context.getLogger().debug(
                                sm.getString("coyoteRequest.parseParameters"), e);
                    }
                    return;
                }
                if (formData != null) {
                    parameters.processParameters(formData, 0, formData.length);
                }
            }
            success = true;
        } finally {
            if (!success) {
                parameters.setParseFailed(true);
            }
        }

    }

这里我们只分析 form表单消息体
readPostBody这个方法

protected int readPostBody(byte body[], int len)
        throws IOException {

        int offset = 0;
        do {
            int inputLen = getStream().read(body, offset, len - offset);
            if (inputLen <= 0) {
                return offset;
            }
            offset += inputLen;
        } while ((len - offset) > 0);
        return len;

    }

getStream 是获得就是 inputBuffer对象的封装
而inputBuffer 里面却是coyoteRequest 这个封装

    public void setCoyoteRequest(org.apache.coyote.Request coyoteRequest) {
        this.coyoteRequest = coyoteRequest;
        inputBuffer.setRequest(coyoteRequest);
    }

当inputBuffer读取字节时

     @Override
    public int realReadBytes(byte cbuf[], int off, int len)
            throws IOException {

        if (closed) {
            return -1;
        }
        if (coyoteRequest == null) {
            return -1;
        }

        if(state == INITIAL_STATE) {
            state = BYTE_STATE;
        }
       //coyoteRequest 恰恰是doRead方法
        int result = coyoteRequest.doRead(bb);

        return result;

    }

doReader恰恰又回到了 parseHeaders()和 parseRequestLine（）这两个方法所在类InternalInputBuffer
代码如下

protected class InputStreamInputBuffer 
        implements InputBuffer {


        /**
         * Read bytes into the specified chunk.
         */
        @Override
        public int doRead(ByteChunk chunk, Request req ) 
            throws IOException {

            if (pos >= lastValid) {
            	//字节不够则填充字节到buf中
                if (!fill())
                    return -1;
            }

            int length = lastValid - pos;
            chunk.setBytes(buf, pos, length);
            pos = lastValid;

            return (length);
        }
  }

上述类恰好是InternalInputBuffer这类的嵌套类
就这样完成了inputStream中的消息体字节的读取