MINA消息过长

MINA在处理网络编程问题的时候不失为一个好助手，但使用过程中总会碰到大大小小的问题，最近在使用时便碰到了如此的一个小问题，出现问题的时候查阅源码是个好习惯。

问题

用MINA发送字符串消息的时候，出现了消息过长的问题。

1	org.apache.mina.common.BufferDataException: Line is too long

消息编码和解码的时候用到了TextLineCodecFactory，其实消息过长的问题出现在了解码的时候。

/**
 * Decode a line using the default delimiter on the current system
 */
private void decodeAuto(Context ctx, IoSession session, IoBuffer in, ProtocolDecoderOutput out)
        throws CharacterCodingException, ProtocolDecoderException {
    int matchCount = ctx.getMatchCount();
    // Try to find a match
    int oldPos = in.position();
    int oldLimit = in.limit();
    while (in.hasRemaining()) {
        byte b = in.get();
        boolean matched = false;
        switch (b) {
        case '\r':
            // Might be Mac, but we don't auto-detect Mac EOL
            // to avoid confusion.
            matchCount++;
            break;
        case '\n':
            // UNIX
            matchCount++;
            matched = true;
            break;
        default:
            matchCount = 0;
        }
        if (matched) {
            // Found a match.
            int pos = in.position();
            in.limit(pos);
            in.position(oldPos);
            ctx.append(in);
            in.limit(oldLimit);
            in.position(pos);
            if (ctx.getOverflowPosition() == 0) {
                IoBuffer buf = ctx.getBuffer();
                buf.flip();
                buf.limit(buf.limit() - matchCount);
                try {
                    byte[] data = new byte[buf.limit()];
                    buf.get(data);
                    CharsetDecoder decoder = ctx.getDecoder();
                    CharBuffer buffer = decoder.decode(ByteBuffer.wrap(data));
                    String str = buffer.toString();
                    writeText(session, str, out);
                } finally {
                    buf.clear();
                }
            } else {
                int overflowPosition = ctx.getOverflowPosition();
                ctx.reset();
                throw new RecoverableProtocolDecoderException("Line is too long: " + overflowPosition);
            }
            oldPos = pos;
            matchCount = 0;
        }
    }
    // Put remainder to buf.
    in.position(oldPos);
    ctx.append(in);
    ctx.setMatchCount(matchCount);
}

当读取到消息结束标识后，剩余的空间不够用，所以抛出异常。

办法

TextLineCodecFactory lineCodec=new TextLineCodecFactory();
lineCodec.setDecoderMaxLineLength(1024*1024); //1M
filterChain.addLast("codec", new ProtocolCodecFilter(lineCodec));

给decode中每一行的读取长度做了定义，默认是1K，可以按照自己的需要来改动。

多看看

TextLineCodecFactory中设置decoder的长度

/**
 * Sets the allowed maximum size of the line to be decoded.
 * If the size of the line to be decoded exceeds this value, the
 * decoder will throw a {@link BufferDataException}.  The default
 * value is <tt>1024</tt> (1KB).
 * <p>
 * This method does the same job with {@link TextLineDecoder#setMaxLineLength(int)}.
 * 
 * @param maxLineLength the maximum decoded line length
 */
public void setDecoderMaxLineLength(int maxLineLength) {
    decoder.setMaxLineLength(maxLineLength);
}

做了这样的设置之后，每次再读取一行数据的时候就会按照1M的数据量去读。

MINA中的ProtocolCodecFactory自己有4个实现类，来进行编码处理。

/**
 * Provides {@link ProtocolEncoder} and {@link ProtocolDecoder} which translates
 * binary or protocol specific data into message object and vice versa.
 * <p>
 * Please refer to
 * <a href="../../../../../xref-examples/org/apache/mina/examples/reverser/ReverseProtocolProvider.html"><code>ReverserProtocolProvider</code></a>
 * example.
 *
 * @author <a href="http://mina.apache.org">Apache MINA Project</a>
 */
public interface ProtocolCodecFactory {
    /**
     * Returns a new (or reusable) instance of {@link ProtocolEncoder} which
     * encodes message objects into binary or protocol-specific data.
     * 
     * @param session The current session
     * @return The encoder instance
     * @throws Exception If an error occurred while retrieving the encoder
     */
    ProtocolEncoder getEncoder(IoSession session) throws Exception;
    /**
     * Returns a new (or reusable) instance of {@link ProtocolDecoder} which
     * decodes binary or protocol-specific data into message objects.
     * 
     * @param session The current session
     * @return The decoder instance
     * @throws Exception If an error occurred while retrieving the decoder
     */
    ProtocolDecoder getDecoder(IoSession session) throws Exception;
}

类名	作用	备注
TextLineCodecFactory	文本消息，按行处理，识别回车换行符
DemuxingProtocolCodecFactory	转送消息到对应CodeFactory
PrefixedStringCodecFactory	处理有固定前缀长度的字符串
ObjectSerializationCodecFactory	Java对象的序列化	传输对象必须实现序列化接口，且服务端和客户端包路径必须一致