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  6. Custom TCP Protocol

Custom TCP Protocol

When using TCP to communicate with an external program, it may be necessary to write a custom Mule protocol. The first step is to get a complete description of how the external program delimits messages within the TCP stream. The next is to implement the protocol as a Java class.

  • All protocols must implement the interface org.mule.transport.tcp.TcpProtocol, which contains three methods:

    • Object read(InputStream is) reads a message from the TCP socket

    • write(OutputStream os, Object data) writes a message to the TCP socket

    • ResponseOutputStream createResponse(Socket socket) creates a stream to which a response can be written.

  • Protocols which process byte-streams rather than serialized Mule messages can inherit much useful infrastructure by subclassing org.mule.transport.tcp.protocols.AbstractByteProtocol This class

    • implements createResponse

    • handles converting messages to byte arrays, allowing subclasses to implement only the simpler method writeByteArray(OutputStream os, byte[] data)

    • provides methods safeRead(InputStream is, byte[] buffer) and safeRead(InputStream is, byte[] buffer, int size) that handle the situation where data is not currently available when doing non-blocking reads from the TCP socket

Suppose we want to communicate with a server that has a simple protocol: all messages are terminated by >>>. The protocol class would look like this:

package org.mule.transport.tcp.integration;

import org.mule.transport.tcp.protocols.AbstractByteProtocol;

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;

public class CustomByteProtocol extends AbstractByteProtocol
{

    /**
     * Create a CustomByteProtocol object.
     */
    public CustomByteProtocol()
    {
        super(false); // This protocol does not support streaming.
    }

    /**
     * Write the message's bytes to the socket,
     * then terminate each message with '>>>'.
     */
    @Override
    protected void writeByteArray(OutputStream os, byte[] data) throws IOException
    {
        super.writeByteArray(os, data);
        os.write('>');
        os.write('>');
        os.write('>');
    }

    /**
     * Read bytes until we see '>>>', which ends the message
     */
    public Object read(InputStream is) throws IOException
    {
        ByteArrayOutputStream baos = new ByteArrayOutputStream();
        int count = 0;
        byte read[] = new byte[1];

        while (true)
        {
            // if no bytes are currently avalable, safeRead()
            // will wait until some arrive
            if (safeRead(is, read) < 0)
            {
                // We've reached EOF.  Return null, so that our
                // caller will know there are no
                // remaining messages
                return null;
            }
            byte b = read[0];
            if (b == '>')
            {
                count++;
                if (count == 3)
                {
                    return baos.toByteArray();
                }
            }
            else
            {
                for (int i = 0; i < count; i++)
                {
                    baos.write('>');
                }
                count = 0;
                baos.write(b);
            }
        }
    }
}