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Nifty Modbus TCP implementation.
There is a newer version: 0.17.0
/* ==================================================================
 * TcpModbusMessageEncoder.java - 29/11/2022 7:26:32 am
 *
 * Copyright 2022 SolarNetwork.net Dev Team
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License as
 * published by the Free Software Foundation; either version 2 of
 * the License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
 * General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
 * 02111-1307 USA
 * ==================================================================
 */

package net.solarnetwork.io.modbus.tcp.netty;

import java.util.List;
import java.util.concurrent.ConcurrentMap;
import java.util.function.IntSupplier;
import io.netty.buffer.ByteBuf;
import io.netty.channel.ChannelHandlerContext;
import io.netty.handler.codec.MessageToMessageEncoder;
import net.solarnetwork.io.modbus.ModbusMessage;
import net.solarnetwork.io.modbus.ModbusMessageReply;
import net.solarnetwork.io.modbus.tcp.SimpleTransactionIdSupplier;

/**
 * Encoder of {@link ModbusMessage} to TCP encapsulated {@link ByteBuf} Modbus
 * frame.
 *
 * @author matt
 * @version 1.0
 */
public class TcpModbusMessageEncoder extends MessageToMessageEncoder {

	/** A mapping of transaction pendingMessages to pair requests/responses. */
	private final ConcurrentMap pendingMessages;

	/** A provider of transaction IDs. */
	private final IntSupplier transactionIdSupplier;

	/**
	 * Constructor.
	 * 
	 * @param pendingMessages
	 *        a mapping of transaction IDs to associated pendingMessages, to
	 *        handle request and response pairing
	 * @throws IllegalArgumentException
	 *         if any argument is {@literal null}
	 */
	public TcpModbusMessageEncoder(ConcurrentMap pendingMessages) {
		this(pendingMessages, SimpleTransactionIdSupplier.INSTANCE);
	}

	/**
	 * Constructor.
	 * 
	 * @param pendingMessages
	 *        a mapping of transaction IDs to associated pendingMessages, to
	 *        handle request and response pairing
	 * @param transactionIdSupplier
	 *        a TCP Modbus transaction ID supplier; only values from 1-65535
	 *        should be supplied
	 * @throws IllegalArgumentException
	 *         if any argument is {@literal null}
	 */
	public TcpModbusMessageEncoder(ConcurrentMap pendingMessages,
			IntSupplier transactionIdSupplier) {
		super();
		if ( pendingMessages == null ) {
			throw new IllegalArgumentException("The pendingMessages argument must not be null.");
		}
		this.pendingMessages = pendingMessages;
		if ( transactionIdSupplier == null ) {
			throw new IllegalArgumentException("The transactionIdSupplier argument must not be null.");
		}
		this.transactionIdSupplier = transactionIdSupplier;
	}

	@Override
	protected void encode(ChannelHandlerContext ctx, ModbusMessage msg, List