用java编写modbustcp主站

下面是一个简单的Java程序，用于实现ModbusTCP主站的读取功能：

import java.net.InetAddress;
import java.net.Socket;
import java.nio.ByteBuffer;
import java.nio.ByteOrder;
import java.util.Arrays;

public class ModbusTCPMaster {
    private Socket socket;
    private ByteBuffer buffer;
    private int transactionId = 0;

    public ModbusTCPMaster(String ipAddress, int port) throws Exception {
        socket = new Socket(InetAddress.getByName(ipAddress), port);
        buffer = ByteBuffer.allocate(1024);
        buffer.order(ByteOrder.BIG_ENDIAN);
    }

    private int getNextTransactionId() {
        return transactionId++;
    }

    public int[] readHoldingRegisters(int slaveId, int startAddress, int numRegisters) throws Exception {
        // Modbus TCP Header
        buffer.clear();
        buffer.putInt(getNextTransactionId()); // Transaction ID
        buffer.putShort((short) 0); // Protocol ID
        buffer.putShort((short) 6); // Length
        buffer.put((byte) slaveId); // Slave ID
        buffer.put((byte) 3); // Function Code
        buffer.putShort((short) startAddress); // Start Address
        buffer.putShort((short) numRegisters); // Number of Registers

        // Send request
        socket.getOutputStream().write(buffer.array(), 0, 12);

        // Wait for response
        buffer.clear();
        int bytesRead = socket.getInputStream().read(buffer.array(), 0, 1024);
        if (bytesRead < 9) {
            throw new Exception("Invalid response");
        }

        // Parse response
        int responseTransactionId = buffer.getInt();
        int responseProtocolId = buffer.getShort();
        int responseLength = buffer.getShort() & 0xFFFF;
        int responseSlaveId = buffer.get() & 0xFF;
        int responseFunctionCode = buffer.get() & 0xFF;
        int responseNumBytes = buffer.get() & 0xFF;

        if (responseFunctionCode == 0x83) {
            int exceptionCode = buffer.get() & 0xFF;
            throw new Exception("Exception code: " + exceptionCode);
        }

        if (responseFunctionCode != 0x03 || responseSlaveId != slaveId) {
            throw new Exception("Invalid response");
        }

        if (responseNumBytes != numRegisters * 2) {
            throw new Exception("Invalid response");
        }

        int[] values = new int[numRegisters];
        for (int i = 0; i < numRegisters; i++) {
            values[i] = buffer.getShort() & 0xFFFF;
        }

        return values;
    }

    public void close() throws Exception {
        socket.close();
    }

    public static void main(String[] args) {
        try {
            ModbusTCPMaster master = new ModbusTCPMaster("192.168.1.100", 502);
            int[] values = master.readHoldingRegisters(1, 0, 10);
            System.out.println(Arrays.toString(values));
            master.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

这个程序实现了读取从站的保持寄存器（Holding Registers）的功能。主要步骤如下：

1. 创建Socket连接到从站的IP地址和端口号。
2. 构造Modbus TCP请求消息，并发送到从站。
3. 等待从站的响应消息。
4. 解析从站的响应消息，提取保持寄存器的值。
5. 关闭Socket连接。

在实现实际应用中，需要根据具体的Modbus通讯协议和应用场景进行修改和优化。