RabbitMQ-java-client版本

1. com.rabbitmq:amqp-client:4.3.0
2. RabbitMQ版本宣告: 3.6.15

Command

1. Command介面是AMQP方法-引數的容器介面，帶有可選的內容頭(content header)和內容體(content body)

2. 原始碼

   /**
    * Interface to a container for an AMQP method-and-arguments, with optional content header and body.
    * AMQP方法和引數的容器介面，帶有可選的content header and body
    */
    
   
   public interface Command {
       /**
        * Retrieves the {@link Method} held within this Command. Downcast to
        * concrete (implementation-specific!) subclasses as necessary.
        *
        * @return the command's method.
        */
       Method getMethod();
   
       /**
        * Retrieves the ContentHeader subclass instance held as part of this Command, if any.
        *
        * Downcast to one of the inner classes of AMQP,
        * for instance {@link AMQP.BasicProperties}, as appropriate.
        *
        * @return the Command's {@link ContentHeader}, or null if none
        */
    
   
       ContentHeader getContentHeader();
   
       /**
        * Retrieves the body byte array that travelled as part of this
        * Command, if any.
        *
        * @return the Command's content body, or null if none
        */
       byte[] getContentBody();
   }
   
   

AMQCommand

1. MQCommand委託多個CommandAssembler類進行位元組流與協議的轉換，CommandAssembler
   是執行緒安全的
2. 核心方法原始碼,將Command轉換為多個Frame並且傳送

 public void transmit(AMQChannel channel) throws IOException {
        int channelNumber = channel.getChannelNumber();
        AMQConnection connection = channel.getConnection();

        synchronized (assembler) {
            Method m = this.assembler.getMethod();
            connection.writeFrame(m.toFrame(channelNumber));
            if (m.hasContent()) {
                byte[] body = this.assembler.getContentBody();

                connection.writeFrame(this.assembler.getContentHeader()
                        .toFrame(channelNumber, body.length));
                //如果body長度超過client與server協商出的最大幀長度，將分多個Frame傳送
                int frameMax = connection.getFrameMax();
                int bodyPayloadMax = (frameMax == 0) ? body.length : frameMax
                        - EMPTY_FRAME_SIZE;

                for (int offset = 0; offset < body.length; offset += bodyPayloadMax) {
                    int remaining = body.length - offset;

                    int fragmentLength = (remaining < bodyPayloadMax) ? remaining
                            : bodyPayloadMax;
                    Frame frame = Frame.fromBodyFragment(channelNumber, body,
                            offset, fragmentLength);
                    connection.writeFrame(frame);
                }
            }
        }

        connection.flush();
    }

CommandAssembler

1. 原始碼分析，CommandAssembler是執行緒安全的

       final class CommandAssembler {
           private static final byte[] EMPTY_BYTE_ARRAY = new byte[0];
       
           /** Current state, used to decide how to handle each incoming frame.
            * 當前狀態，用於決定如何處理每個傳入的幀，不過這個名字取名CAState真的好嗎????(哈哈)
            * */
           private enum CAState {
               EXPECTING_METHOD, EXPECTING_CONTENT_HEADER, EXPECTING_CONTENT_BODY, COMPLETE
           }
           //
           private CAState state;
       
           /** The method for this command */
           private Method method;
           
           /** The content header for this command */
           private AMQContentHeader contentHeader;
       
           /** The fragments of this command's content body - a list of byte[] */
           private final List<byte[]> bodyN;
           /** sum of the lengths of all fragments 所有內容幀body陣列加起來的長度*/
           private int bodyLength;
       
           /** No bytes of content body not yet accumulated */
           private long remainingBodyBytes;
       
           public CommandAssembler(Method method, AMQContentHeader contentHeader, byte[] body) {
               this.method = method;
               this.contentHeader = contentHeader;
               this.bodyN = new ArrayList<byte[]>(2);
               this.bodyLength = 0;
               this.remainingBodyBytes = 0;
               appendBodyFragment(body);
               if (method == null) {
                   this.state = CAState.EXPECTING_METHOD;
               } else if (contentHeader == null) {
                   this.state = method.hasContent() ? CAState.EXPECTING_CONTENT_HEADER : CAState.COMPLETE;
               } else {
                   this.remainingBodyBytes = contentHeader.getBodySize() - this.bodyLength;
                   updateContentBodyState();
               }
           }
       
           public synchronized Method getMethod() {
               return this.method;
           }
       
           public synchronized AMQContentHeader getContentHeader() {
               return this.contentHeader;
           }
       
           /** @return true if the command is complete */
           public synchronized boolean isComplete() {
               return (this.state == CAState.COMPLETE);
           }
       
           /** Decides whether more body frames are expected */
           private void updateContentBodyState() {
               this.state = (this.remainingBodyBytes > 0) ? CAState.EXPECTING_CONTENT_BODY : CAState.COMPLETE;
           }
       
           private void consumeMethodFrame(Frame f) throws IOException {
               if (f.type == AMQP.FRAME_METHOD) {
                   //如果是方法幀就讀取方法幀payload中的流，這裡的f.getInputStream()僅僅是payload，不包含幀頭資料
                   this.method = AMQImpl.readMethodFrom(f.getInputStream());
                   this.state = this.method.hasContent() ? CAState.EXPECTING_CONTENT_HEADER : CAState.COMPLETE;
               } else {
                   throw new UnexpectedFrameError(f, AMQP.FRAME_METHOD);
               }
           }
       
           private void consumeHeaderFrame(Frame f) throws IOException {
               if (f.type == AMQP.FRAME_HEADER) {
                   //內容頭幀
                   this.contentHeader = AMQImpl.readContentHeaderFrom(f.getInputStream());
                   this.remainingBodyBytes = this.contentHeader.getBodySize();
                   updateContentBodyState();
               } else {
                   throw new UnexpectedFrameError(f, AMQP.FRAME_HEADER);
               }
           }
       
           private void consumeBodyFrame(Frame f) {
               if (f.type == AMQP.FRAME_BODY) {
                   byte[] fragment = f.getPayload();
                   this.remainingBodyBytes -= fragment.length;
                   updateContentBodyState();
                   if (this.remainingBodyBytes < 0) {
                       throw new UnsupportedOperationException("%%%%%% FIXME unimplemented");
                   }
                   appendBodyFragment(fragment);
               } else {
                   throw new UnexpectedFrameError(f, AMQP.FRAME_BODY);
               }
           }
       
           /** Stitches together a fragmented content body into a single byte array
            * 合併內容幀body陣列
            * */
           private byte[] coalesceContentBody() {
               if (this.bodyLength == 0) return EMPTY_BYTE_ARRAY;
               if (this.bodyN.size() == 1) return this.bodyN.get(0);
       
               byte[] body = new byte[bodyLength];
               int offset = 0;
               for (byte[] fragment : this.bodyN) {
                   System.arraycopy(fragment, 0, body, offset, fragment.length);
                   offset += fragment.length;
               }
               this.bodyN.clear();
               this.bodyN.add(body);
               return body;
           }
       
           public synchronized byte[] getContentBody() {
               return coalesceContentBody();
           }
       
           private void appendBodyFragment(byte[] fragment) {
               if (fragment == null || fragment.length == 0) return;
               bodyN.add(fragment);
               bodyLength += fragment.length;
           }
       
           /**
            * 處理Frame,根據當前的狀態獲取相關的Frame
            * @param f frame to be incorporated
            * @return true if command becomes complete
            * @throws IOException if error reading frame
            */
           public synchronized boolean handleFrame(Frame f) throws IOException
           {
               switch (this.state) {
                 case EXPECTING_METHOD:          consumeMethodFrame(f); break;
                 case EXPECTING_CONTENT_HEADER:  consumeHeaderFrame(f); break;
                 case EXPECTING_CONTENT_BODY:    consumeBodyFrame(f);   break;
       
                 default:
                     throw new AssertionError("Bad Command State " + this.state);
               }
               return isComplete();
           }
       }