Java處理Modbus-RTU協議（另闢蹊徑改源碼）

開始

看到這篇文章 https://www.cnblogs.com/yangming1996/p/6549800.html
，發現可以用以下類來處理

        ByteArrayInputStream bInputStream = new ByteArrayInputStream(bytesData);
        DataInputStream dataInputStream = new DataInputStream(bInputStream);

問題

但是當我用

byte[] bytesData = new byte[] { 0x01, 0x03, 0x04,  0x02, (byte)0x92,  (byte) 0xFF, (byte)0x9B, 0x5A, 0x3D };
ByteArrayInputStream bInputStream = new ByteArrayInputStream(bytesData);
DataInputStream dataInputStream = new DataInputStream(bInputStream);



        // 地址 功能 返回有效字節數   溼度數據 溫度數據     校驗低位 校驗高位
        //  01   03     04             02 92    FF 9B          5A       3D

        // 計算： 0292 ->  658   溼度  65.8 %RH
        //        FF9B -> -101   溫度 -10.1 ℃

dataInputStream.skipBytes(3);

int value1 = dataInputStream.readShort();//讀 0x0292
int value2 = dataInputStream.readShort();//讀 0xFF9B
double valuef1 = value1/10.0;
double valuef2 = value2/10.0;

JVM裏是這樣的

這是正常的，因爲Java把0x92認爲是有符號的int，超出了byte的範圍[-128~127]，詳見我另一篇文章 https://www.cnblogs.com/yucloud/p/Java_byte.html

發現value1和value2的數值都不對，理論上應該是 658和-101，猜測可能是read()沒有取位，果然看了源碼發現： DataInputStream的readShort()使用了read()，

但這裏read()只是繼承了其父類 FilterInputStream 的 in.read()，

並沒有改動，並沒有取低位byte&0xFF，因此不適合用於讀取 byte 數組。

小解決

因此這裏我改爲

import java.io.DataInputStream;
import java.io.IOException;
import java.io.InputStream;

public class testModBusRtuDataInputStream extends DataInputStream{

    public testModBusRtuDataInputStream(InputStream in) {
        super(in);
    }

    @Override
    public int read() throws IOException {
        return super.read()&0xFF;
    }

}

然後用 testModBusRtuDataInputStream 代替 DataInputStream 使用即可。

更進一步的改造

當然，我們的需求是把 0x0292 讀取爲 65.8
0292 -> 658 溼度 65.8 %RH
然後如果想實現這個，甚至更多功能，就得改造源碼了

DataInputStream 繼承了 FilterInputStream 且實現了 DataInput接口

public class DataInputStream extends FilterInputStream implements DataInput {
	...代碼略...
}

其方法如下

想繼承DataInputStream，但發現不能Override DataInputStream裏使用到的DataInput的接口

其中的 DataInput 接口，它有如下接口方法

方案：
複製源碼並把類改成自定義類，如 RtuDataInputStream.java
這樣就可以根據需求來修改 DataInput 的方法，如修改 readFloat()
DataInputStream.java 裏是這樣的

修改後的 RtuDataInputStream.java

        ...其他代碼略...
    @Override
    public int read() throws IOException {
        // TODO Auto-generated method stub
        return super.read()&0xFF;
    }

        ...其他代碼略...
    public final float readFloat() throws IOException {
        return (float) (readShort()/10.0);
    }
        ...其他代碼略...

然後把main()裏用到 DataInputStream 都替換成 RtuDataInputStream 即可，因爲這兩個類現在有相同的父類和接口，相當於是兄弟類了

這樣就可以讀取 RTU 的溫溼度值了

byte[] bytesData = new byte[] { 0x01, 0x03, 0x04,  0x02, (byte)0x92,  (byte) 0xFF, (byte)0x9B, 0x5A, 0x3D };
ByteArrayInputStream bInputStream = new ByteArrayInputStream(bytesData);
RtuDataInputStream dataInputStream = new RtuDataInputStream(bInputStream);
//注意這裏都換成了 RtuDataInputStream 自定義類


        // 地址 功能 返回有效字節數   溼度數據 溫度數據     校驗低位 校驗高位
        //  01   03     04             02 92    FF 9B          5A       3D

        // 計算： 0292 ->  658   溼度  65.8 %RH
        //        FF9B -> -101   溫度 -10.1 ℃

dataInputStream.skipBytes(3);

float value1 = dataInputStream.readFloat();//讀 0x0292 爲 65.8
float value2 = dataInputStream.readFloat();//讀 0xFF9B 爲 -10.1

測試代碼和效果

import java.io.ByteArrayInputStream;
import java.io.IOException;


public class ModbusRtuParser {


    public static void main(String[] args) throws IOException { 

        byte[] hexData = new byte[] { 0x01, 0x03, 0x04, 0x02, 0x1F, 0x01, 0x4E, 0x4B, (byte) 0xE9 };
        byte[] hexTestData = new byte[] { 0x01, 0x03, 0x04, 0x03, (byte)0xc9, 0x01, 0x26, (byte)0xaa, 0x03};
        byte[] hexNagData = new byte[] { 0x01, 0x03, 0x04,  0x02, (byte)0x92,  (byte) 0xFF, (byte)0x9B, 0x5A, 0x3D };
        
        byte[] bytesData = new byte[hexNagData.length];
        for (int i=0; i<bytesData.length; i++)
            bytesData[i] = (byte) (0xFF&hexNagData[i]);

                /******************************************** 例子 *******************************************/

        // 地址 功能   始地址  數據長度 校驗低位 校驗高位
        //  01   03    00 00    00 02      C4      0B

        // 地址 功能 返回有效字節數   溼度數據 溫度數據     校驗低位 校驗高位
        //  01   03     04             02 92    FF 9B          5A       3D

        // 計算： 0292 ->  658   溼度  65.8 %RH
        //        FF9B -> -101   溫度 -10.1 ℃

        // TX: 01 03 00 00 00 02 C4 0B
        // RX: 01 03 04 02 1F 01 4E 4B E9



        
        ByteArrayInputStream bInputStream = new ByteArrayInputStream(bytesData);
        //DataInputStream dataInputStream = new DataInputStream(bInputStream);
        ModBusRtuDataInputStream dataInputStream = new ModBusRtuDataInputStream(bInputStream);
        boolean isMarkSupported = dataInputStream.markSupported();

        dataInputStream.skipBytes(2);
        int validDataCnt = dataInputStream.readByte();
        dataInputStream.reset();


        if (dataInputStream.available() != validDataCnt+5)
            throw new IOException();

        
        int id = dataInputStream.readByte();
        int funCode = dataInputStream.readByte();

        int valDataCnt = dataInputStream.readByte();
        int value1 = dataInputStream.readShort();
        int value2 = dataInputStream.readShort();

        dataInputStream.reset();
        dataInputStream.skipBytes(3);
        double valuef1 = dataInputStream.readFloat();
        double valuef2 = value2/10.0;
        
        //ByteArrayOutputStream bOutputStream = new ByteArrayOutputStream();//32
        //DataOutputStream  dataOutputStream = new DataOutputStream(bOutputStream);

    }
    
    
}

不要問爲什麼沒有打印，因爲個人習慣用調試器看結果

改造後的自定義類源碼

改動的例子源碼太長了，摺疊起來：
自定義類 ModBusRtuDataInputStream.java 源碼在此
@hasEdited 代表該方法被修改了，沒有這個符號就代表和原 DataInputStream 基本一致

**展開查看 ModBusRtuDataInputStream.java 源碼**

/*
 * Copyright (c) 1994, 2016, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 * 
 *  @hasEdited
 *  @NOTE: 照着需求修改 java.io.DataInputStream 的....
 *  微軟 JDK11.0.8
 */
import java.io.*;



/**
 * A data input stream lets an application read primitive Java data
 * types from an underlying input stream in a machine-independent
 * way. An application uses a data output stream to write data that
 * can later be read by a data input stream.
 * <p>
 * DataInputStream is not necessarily safe for multithreaded access.
 * Thread safety is optional and is the responsibility of users of
 * methods in this class.
 *
 * @author  Arthur van Hoff
 * @see     java.io.DataOutputStream
 * @since   1.0
 */
public
class ModBusRtuDataInputStream extends FilterInputStream implements DataInput {

    /**
     * Creates a DataInputStream that uses the specified
     * underlying InputStream.
     *
     * @param  in   the specified input stream
     */
    public ModBusRtuDataInputStream(InputStream in) {
        super(in);
    }

    
    /**
     * @author: 
     * @hasEdited
     * 這裏只需要讀取byte，因此需要取 &0xFF
     */
    @Override
    public int read() throws IOException {
        return super.read()&0xFF;
    }



    /**
     * working arrays initialized on demand by readUTF
     */
    private byte bytearr[] = new byte[80];
    private char chararr[] = new char[80];

    /**
     * Reads some number of bytes from the contained input stream and
     * stores them into the buffer array <code>b</code>. The number of
     * bytes actually read is returned as an integer. This method blocks
     * until input data is available, end of file is detected, or an
     * exception is thrown.
     *
     * <p>If <code>b</code> is null, a <code>NullPointerException</code> is
     * thrown. If the length of <code>b</code> is zero, then no bytes are
     * read and <code>0</code> is returned; otherwise, there is an attempt
     * to read at least one byte. If no byte is available because the
     * stream is at end of file, the value <code>-1</code> is returned;
     * otherwise, at least one byte is read and stored into <code>b</code>.
     *
     * <p>The first byte read is stored into element <code>b[0]</code>, the
     * next one into <code>b[1]</code>, and so on. The number of bytes read
     * is, at most, equal to the length of <code>b</code>. Let <code>k</code>
     * be the number of bytes actually read; these bytes will be stored in
     * elements <code>b[0]</code> through <code>b[k-1]</code>, leaving
     * elements <code>b[k]</code> through <code>b[b.length-1]</code>
     * unaffected.
     *
     * <p>The <code>read(b)</code> method has the same effect as:
     * <blockquote><pre>
     * read(b, 0, b.length)
     * </pre></blockquote>
     *
     * @param      b   the buffer into which the data is read.
     * @return     the total number of bytes read into the buffer, or
     *             <code>-1</code> if there is no more data because the end
     *             of the stream has been reached.
     * @exception  IOException if the first byte cannot be read for any reason
     * other than end of file, the stream has been closed and the underlying
     * input stream does not support reading after close, or another I/O
     * error occurs.
     * @see        java.io.FilterInputStream#in
     * @see        java.io.InputStream#read(byte[], int, int)
     */
    public final int read(byte b[]) throws IOException {
        return in.read(b, 0, b.length);
    }

    /**
     * Reads up to <code>len</code> bytes of data from the contained
     * input stream into an array of bytes.  An attempt is made to read
     * as many as <code>len</code> bytes, but a smaller number may be read,
     * possibly zero. The number of bytes actually read is returned as an
     * integer.
     *
     * <p> This method blocks until input data is available, end of file is
     * detected, or an exception is thrown.
     *
     * <p> If <code>len</code> is zero, then no bytes are read and
     * <code>0</code> is returned; otherwise, there is an attempt to read at
     * least one byte. If no byte is available because the stream is at end of
     * file, the value <code>-1</code> is returned; otherwise, at least one
     * byte is read and stored into <code>b</code>.
     *
     * <p> The first byte read is stored into element <code>b[off]</code>, the
     * next one into <code>b[off+1]</code>, and so on. The number of bytes read
     * is, at most, equal to <code>len</code>. Let <i>k</i> be the number of
     * bytes actually read; these bytes will be stored in elements
     * <code>b[off]</code> through <code>b[off+</code><i>k</i><code>-1]</code>,
     * leaving elements <code>b[off+</code><i>k</i><code>]</code> through
     * <code>b[off+len-1]</code> unaffected.
     *
     * <p> In every case, elements <code>b[0]</code> through
     * <code>b[off]</code> and elements <code>b[off+len]</code> through
     * <code>b[b.length-1]</code> are unaffected.
     *
     * @param      b     the buffer into which the data is read.
     * @param off the start offset in the destination array <code>b</code>
     * @param      len   the maximum number of bytes read.
     * @return     the total number of bytes read into the buffer, or
     *             <code>-1</code> if there is no more data because the end
     *             of the stream has been reached.
     * @exception  NullPointerException If <code>b</code> is <code>null</code>.
     * @exception  IndexOutOfBoundsException If <code>off</code> is negative,
     * <code>len</code> is negative, or <code>len</code> is greater than
     * <code>b.length - off</code>
     * @exception  IOException if the first byte cannot be read for any reason
     * other than end of file, the stream has been closed and the underlying
     * input stream does not support reading after close, or another I/O
     * error occurs.
     * @see        java.io.FilterInputStream#in
     * @see        java.io.InputStream#read(byte[], int, int)
     */
    public final int read(byte b[], int off, int len) throws IOException {
        return in.read(b, off, len);
    }

    /**
     * See the general contract of the {@code readFully}
     * method of {@code DataInput}.
     * <p>
     * Bytes
     * for this operation are read from the contained
     * input stream.
     *
     * @param   b   the buffer into which the data is read.
     * @throws  NullPointerException if {@code b} is {@code null}.
     * @throws  EOFException  if this input stream reaches the end before
     *          reading all the bytes.
     * @throws  IOException   the stream has been closed and the contained
     *          input stream does not support reading after close, or
     *          another I/O error occurs.
     * @see     java.io.FilterInputStream#in
     */
    public final void readFully(byte b[]) throws IOException {
        readFully(b, 0, b.length);
    }

    /**
     * See the general contract of the {@code readFully}
     * method of {@code DataInput}.
     * <p>
     * Bytes
     * for this operation are read from the contained
     * input stream.
     *
     * @param      b     the buffer into which the data is read.
     * @param      off   the start offset in the data array {@code b}.
     * @param      len   the number of bytes to read.
     * @exception  NullPointerException if {@code b} is {@code null}.
     * @exception  IndexOutOfBoundsException if {@code off} is negative,
     *             {@code len} is negative, or {@code len} is greater than
     *             {@code b.length - off}.
     * @exception  EOFException  if this input stream reaches the end before
     *             reading all the bytes.
     * @exception  IOException   the stream has been closed and the contained
     *             input stream does not support reading after close, or
     *             another I/O error occurs.
     * @see        java.io.FilterInputStream#in
     */
    public final void readFully(byte b[], int off, int len) throws IOException {
        if (len < 0)
            throw new IndexOutOfBoundsException();
        int n = 0;
        while (n < len) {
            int count = in.read(b, off + n, len - n);
            if (count < 0)
                throw new EOFException();
            n += count;
        }
    }

    /**
     * See the general contract of the <code>skipBytes</code>
     * method of <code>DataInput</code>.
     * <p>
     * Bytes for this operation are read from the contained
     * input stream.
     *
     * @param      n   the number of bytes to be skipped.
     * @return     the actual number of bytes skipped.
     * @exception  IOException  if the contained input stream does not support
     *             seek, or the stream has been closed and
     *             the contained input stream does not support
     *             reading after close, or another I/O error occurs.
     */
    public final int skipBytes(int n) throws IOException {
        int total = 0;
        int cur = 0;

        while ((total<n) && ((cur = (int) in.skip(n-total)) > 0)) {
            total += cur;
        }

        return total;
    }

    /**
     * See the general contract of the <code>readBoolean</code>
     * method of <code>DataInput</code>.
     * <p>
     * Bytes for this operation are read from the contained
     * input stream.
     *
     * @return     the <code>boolean</code> value read.
     * @exception  EOFException  if this input stream has reached the end.
     * @exception  IOException   the stream has been closed and the contained
     *             input stream does not support reading after close, or
     *             another I/O error occurs.
     * @see        java.io.FilterInputStream#in
     */
    public final boolean readBoolean() throws IOException {
        int ch = in.read();
        if (ch < 0)
            throw new EOFException();
        return (ch != 0);
    }

    /**
     * See the general contract of the <code>readByte</code>
     * method of <code>DataInput</code>.
     * <p>
     * Bytes
     * for this operation are read from the contained
     * input stream.
     *
     * @return     the next byte of this input stream as a signed 8-bit
     *             <code>byte</code>.
     * @exception  EOFException  if this input stream has reached the end.
     * @exception  IOException   the stream has been closed and the contained
     *             input stream does not support reading after close, or
     *             another I/O error occurs.
     * @see        java.io.FilterInputStream#in
     */
    public final byte readByte() throws IOException {
        int ch = in.read();
        if (ch < 0)
            throw new EOFException();
        return (byte)(ch);
    }

    /**
     * See the general contract of the <code>readUnsignedByte</code>
     * method of <code>DataInput</code>.
     * <p>
     * Bytes
     * for this operation are read from the contained
     * input stream.
     *
     * @return     the next byte of this input stream, interpreted as an
     *             unsigned 8-bit number.
     * @exception  EOFException  if this input stream has reached the end.
     * @exception  IOException   the stream has been closed and the contained
     *             input stream does not support reading after close, or
     *             another I/O error occurs.
     * @see         java.io.FilterInputStream#in
     */
    public final int readUnsignedByte() throws IOException {
        int ch = in.read();
        if (ch < 0)
            throw new EOFException();
        return ch;
    }

    /**
     * See the general contract of the <code>readShort</code>
     * method of <code>DataInput</code>.
     * <p>
     * Bytes
     * for this operation are read from the contained
     * input stream.
     *
     * @return     the next two bytes of this input stream, interpreted as a
     *             signed 16-bit number.
     * @exception  EOFException  if this input stream reaches the end before
     *               reading two bytes.
     * @exception  IOException   the stream has been closed and the contained
     *             input stream does not support reading after close, or
     *             another I/O error occurs.
     * @see        java.io.FilterInputStream#in
     */
    public final short readShort() throws IOException {
        int ch1 = in.read();
        int ch2 = in.read();
        if ((ch1 | ch2) < 0)
            throw new EOFException();
        return (short)((ch1 << 8) + (ch2 << 0));
    }

    /**
     * See the general contract of the <code>readUnsignedShort</code>
     * method of <code>DataInput</code>.
     * <p>
     * Bytes
     * for this operation are read from the contained
     * input stream.
     *
     * @return     the next two bytes of this input stream, interpreted as an
     *             unsigned 16-bit integer.
     * @exception  EOFException  if this input stream reaches the end before
     *             reading two bytes.
     * @exception  IOException   the stream has been closed and the contained
     *             input stream does not support reading after close, or
     *             another I/O error occurs.
     * @see        java.io.FilterInputStream#in
     */
    public final int readUnsignedShort() throws IOException {
        int ch1 = in.read();
        int ch2 = in.read();
        if ((ch1 | ch2) < 0)
            throw new EOFException();
        return (ch1 << 8) + (ch2 << 0);
    }

    /**
     * See the general contract of the <code>readChar</code>
     * method of <code>DataInput</code>.
     * <p>
     * Bytes
     * for this operation are read from the contained
     * input stream.
     *
     * @return     the next two bytes of this input stream, interpreted as a
     *             <code>char</code>.
     * @exception  EOFException  if this input stream reaches the end before
     *               reading two bytes.
     * @exception  IOException   the stream has been closed and the contained
     *             input stream does not support reading after close, or
     *             another I/O error occurs.
     * @see        java.io.FilterInputStream#in
     */
    public final char readChar() throws IOException {
        int ch1 = in.read();
        int ch2 = in.read();
        if ((ch1 | ch2) < 0)
            throw new EOFException();
        return (char)((ch1 << 8) + (ch2 << 0));
    }

    /**
     * See the general contract of the <code>readInt</code>
     * method of <code>DataInput</code>.
     * <p>
     * Bytes
     * for this operation are read from the contained
     * input stream.
     *
     * @return     the next four bytes of this input stream, interpreted as an
     *             <code>int</code>.
     * @exception  EOFException  if this input stream reaches the end before
     *               reading four bytes.
     * @exception  IOException   the stream has been closed and the contained
     *             input stream does not support reading after close, or
     *             another I/O error occurs.
     * @see        java.io.FilterInputStream#in
     */
    public final int readInt() throws IOException {
        int ch1 = in.read();
        int ch2 = in.read();
        int ch3 = in.read();
        int ch4 = in.read();
        if ((ch1 | ch2 | ch3 | ch4) < 0)
            throw new EOFException();
        return ((ch1 << 24) + (ch2 << 16) + (ch3 << 8) + (ch4 << 0));
    }

    private byte readBuffer[] = new byte[8];

    /**
     * See the general contract of the <code>readLong</code>
     * method of <code>DataInput</code>.
     * <p>
     * Bytes
     * for this operation are read from the contained
     * input stream.
     *
     * @return     the next eight bytes of this input stream, interpreted as a
     *             <code>long</code>.
     * @exception  EOFException  if this input stream reaches the end before
     *               reading eight bytes.
     * @exception  IOException   the stream has been closed and the contained
     *             input stream does not support reading after close, or
     *             another I/O error occurs.
     * @see        java.io.FilterInputStream#in
     */
    public final long readLong() throws IOException {
        readFully(readBuffer, 0, 8);
        return (((long)readBuffer[0] << 56) +
                ((long)(readBuffer[1] & 255) << 48) +
                ((long)(readBuffer[2] & 255) << 40) +
                ((long)(readBuffer[3] & 255) << 32) +
                ((long)(readBuffer[4] & 255) << 24) +
                ((readBuffer[5] & 255) << 16) +
                ((readBuffer[6] & 255) <<  8) +
                ((readBuffer[7] & 255) <<  0));
    }

    /**
     * See the general contract of the <code>readFloat</code>
     * method of <code>DataInput</code>.
     * <p>
     * Bytes
     * for this operation are read from the contained
     * input stream.
     * 
     * @hasEdited
     * 
     * @return     the next four bytes of this input stream, interpreted as a
     *             <code>float</code>.
     * @exception  EOFException  if this input stream reaches the end before
     *               reading four bytes.
     * @exception  IOException   the stream has been closed and the contained
     *             input stream does not support reading after close, or
     *             another I/O error occurs.
     * @see        java.io.DataInputStream#readInt()
     * @see        java.lang.Float#intBitsToFloat(int)
     */
    public final float readFloat() throws IOException {
        return (float) (readShort()/10.0);
    }

    /**
     * See the general contract of the <code>readDouble</code>
     * method of <code>DataInput</code>.
     * <p>
     * Bytes
     * for this operation are read from the contained
     * input stream.
     *
     * @return     the next eight bytes of this input stream, interpreted as a
     *             <code>double</code>.
     * @exception  EOFException  if this input stream reaches the end before
     *               reading eight bytes.
     * @exception  IOException   the stream has been closed and the contained
     *             input stream does not support reading after close, or
     *             another I/O error occurs.
     * @see        java.io.DataInputStream#readLong()
     * @see        java.lang.Double#longBitsToDouble(long)
     */
    public final double readDouble() throws IOException {
        return Double.longBitsToDouble(readLong());
    }

    private char lineBuffer[];

    /**
     * See the general contract of the <code>readLine</code>
     * method of <code>DataInput</code>.
     * <p>
     * Bytes
     * for this operation are read from the contained
     * input stream.
     *
     * @deprecated This method does not properly convert bytes to characters.
     * As of JDK&nbsp;1.1, the preferred way to read lines of text is via the
     * <code>BufferedReader.readLine()</code> method.  Programs that use the
     * <code>DataInputStream</code> class to read lines can be converted to use
     * the <code>BufferedReader</code> class by replacing code of the form:
     * <blockquote><pre>
     *     DataInputStream d =&nbsp;new&nbsp;DataInputStream(in);
     * </pre></blockquote>
     * with:
     * <blockquote><pre>
     *     BufferedReader d
     *          =&nbsp;new&nbsp;BufferedReader(new&nbsp;InputStreamReader(in));
     * </pre></blockquote>
     *
     * @return     the next line of text from this input stream.
     * @exception  IOException  if an I/O error occurs.
     * @see        java.io.BufferedReader#readLine()
     * @see        java.io.FilterInputStream#in
     */
    @Deprecated
    public final String readLine() throws IOException {
        char buf[] = lineBuffer;

        if (buf == null) {
            buf = lineBuffer = new char[128];
        }

        int room = buf.length;
        int offset = 0;
        int c;

loop:   while (true) {
            switch (c = in.read()) {
              case -1:
              case '\n':
                break loop;

              case '\r':
                int c2 = in.read();
                if ((c2 != '\n') && (c2 != -1)) {
                    if (!(in instanceof PushbackInputStream)) {
                        this.in = new PushbackInputStream(in);
                    }
                    ((PushbackInputStream)in).unread(c2);
                }
                break loop;

              default:
                if (--room < 0) {
                    buf = new char[offset + 128];
                    room = buf.length - offset - 1;
                    System.arraycopy(lineBuffer, 0, buf, 0, offset);
                    lineBuffer = buf;
                }
                buf[offset++] = (char) c;
                break;
            }
        }
        if ((c == -1) && (offset == 0)) {
            return null;
        }
        return String.copyValueOf(buf, 0, offset);
    }

    /**
     * See the general contract of the <code>readUTF</code>
     * method of <code>DataInput</code>.
     * <p>
     * Bytes
     * for this operation are read from the contained
     * input stream.
     *
     * @return     a Unicode string.
     * @exception  EOFException  if this input stream reaches the end before
     *               reading all the bytes.
     * @exception  IOException   the stream has been closed and the contained
     *             input stream does not support reading after close, or
     *             another I/O error occurs.
     * @exception  UTFDataFormatException if the bytes do not represent a valid
     *             modified UTF-8 encoding of a string.
     * @see        java.io.DataInputStream#readUTF(java.io.DataInput)
     */
    public final String readUTF() throws IOException {
        return readUTF(this);
    }

    /**
     * Reads from the
     * stream <code>in</code> a representation
     * of a Unicode  character string encoded in
     * <a href="DataInput.html#modified-utf-8">modified UTF-8</a> format;
     * this string of characters is then returned as a <code>String</code>.
     * The details of the modified UTF-8 representation
     * are  exactly the same as for the <code>readUTF</code>
     * method of <code>DataInput</code>.
     *
     * @param      in   a data input stream.
     * @return     a Unicode string.
     * @exception  EOFException            if the input stream reaches the end
     *               before all the bytes.
     * @exception  IOException   the stream has been closed and the contained
     *             input stream does not support reading after close, or
     *             another I/O error occurs.
     * @exception  UTFDataFormatException  if the bytes do not represent a
     *               valid modified UTF-8 encoding of a Unicode string.
     * @see        java.io.DataInputStream#readUnsignedShort()
     */
    public static final String readUTF(DataInput in) throws IOException {
        int utflen = in.readUnsignedShort();
        byte[] bytearr = null;
        char[] chararr = null;
        if (in instanceof ModBusRtuDataInputStream) {
            ModBusRtuDataInputStream dis = (ModBusRtuDataInputStream)in;
            if (dis.bytearr.length < utflen){
                dis.bytearr = new byte[utflen*2];
                dis.chararr = new char[utflen*2];
            }
            chararr = dis.chararr;
            bytearr = dis.bytearr;
        } else {
            bytearr = new byte[utflen];
            chararr = new char[utflen];
        }

        int c, char2, char3;
        int count = 0;
        int chararr_count=0;

        in.readFully(bytearr, 0, utflen);

        while (count < utflen) {
            c = (int) bytearr[count] & 0xff;
            if (c > 127) break;
            count++;
            chararr[chararr_count++]=(char)c;
        }

        while (count < utflen) {
            c = (int) bytearr[count] & 0xff;
            switch (c >> 4) {
                case 0: case 1: case 2: case 3: case 4: case 5: case 6: case 7:
                    /* 0xxxxxxx*/
                    count++;
                    chararr[chararr_count++]=(char)c;
                    break;
                case 12: case 13:
                    /* 110x xxxx   10xx xxxx*/
                    count += 2;
                    if (count > utflen)
                        throw new UTFDataFormatException(
                            "malformed input: partial character at end");
                    char2 = (int) bytearr[count-1];
                    if ((char2 & 0xC0) != 0x80)
                        throw new UTFDataFormatException(
                            "malformed input around byte " + count);
                    chararr[chararr_count++]=(char)(((c & 0x1F) << 6) |
                                                    (char2 & 0x3F));
                    break;
                case 14:
                    /* 1110 xxxx  10xx xxxx  10xx xxxx */
                    count += 3;
                    if (count > utflen)
                        throw new UTFDataFormatException(
                            "malformed input: partial character at end");
                    char2 = (int) bytearr[count-2];
                    char3 = (int) bytearr[count-1];
                    if (((char2 & 0xC0) != 0x80) || ((char3 & 0xC0) != 0x80))
                        throw new UTFDataFormatException(
                            "malformed input around byte " + (count-1));
                    chararr[chararr_count++]=(char)(((c     & 0x0F) << 12) |
                                                    ((char2 & 0x3F) << 6)  |
                                                    ((char3 & 0x3F) << 0));
                    break;
                default:
                    /* 10xx xxxx,  1111 xxxx */
                    throw new UTFDataFormatException(
                        "malformed input around byte " + count);
            }
        }
        // The number of chars produced may be less than utflen
        return new String(chararr, 0, chararr_count);
    }
}