2.NIO-ByteBuffer结构和入门使用

1.2、ByteBuffer

1.2.1、使用

代码示例

准备一个文本，内容：1234567890abcd

channel操作文本

@Slf4j
public class TestByteBuffer {
    public static void main(String[] args) {
        try {
            //FileChannel
            //1.输入或输出流获得FileChannel,2.RandomAccessFile
            FileChannel channel = new FileInputStream("data.txt").getChannel();
            //准备缓冲区
            ByteBuffer buffer = ByteBuffer.allocate(10);
            while (true) {
                //从channel中读取数据，写入buffer
                int len = channel.read(buffer);
                log.debug("读取到的字节数：{}", len);
                if (len == -1) {
                    break;
                }

                //切换为读模式
                buffer. flip();
                while (buffer.hasRemaining()) {
                    byte b = buffer.get();
                    log.debug("读取到的数据：{}",(char)b);
                }

                //切换为写模式
                buffer.clear();
            }

        } catch (Exception e) {
            throw new RuntimeException(e);
        }
       
    }
}

结果：

08:30:55.937 [main] DEBUG com.jpy.TestByteBuffer - 读取到的字节数：10
08:30:55.940 [main] DEBUG com.jpy.TestByteBuffer - 读取到的数据：1
08:30:55.940 [main] DEBUG com.jpy.TestByteBuffer - 读取到的数据：2
08:30:55.940 [main] DEBUG com.jpy.TestByteBuffer - 读取到的数据：3
08:30:55.940 [main] DEBUG com.jpy.TestByteBuffer - 读取到的数据：4
08:30:55.940 [main] DEBUG com.jpy.TestByteBuffer - 读取到的数据：5
08:30:55.940 [main] DEBUG com.jpy.TestByteBuffer - 读取到的数据：6
08:30:55.941 [main] DEBUG com.jpy.TestByteBuffer - 读取到的数据：7
08:30:55.941 [main] DEBUG com.jpy.TestByteBuffer - 读取到的数据：8
08:30:55.941 [main] DEBUG com.jpy.TestByteBuffer - 读取到的数据：9
08:30:55.941 [main] DEBUG com.jpy.TestByteBuffer - 读取到的数据：0
08:30:55.941 [main] DEBUG com.jpy.TestByteBuffer - 读取到的字节数：4
08:30:55.941 [main] DEBUG com.jpy.TestByteBuffer - 读取到的数据：a
08:30:55.941 [main] DEBUG com.jpy.TestByteBuffer - 读取到的数据：b
08:30:55.941 [main] DEBUG com.jpy.TestByteBuffer - 读取到的数据：c
08:30:55.941 [main] DEBUG com.jpy.TestByteBuffer - 读取到的数据：d
08:30:55.941 [main] DEBUG com.jpy.TestByteBuffer - 读取到的字节数：-1

ByteBuffer使用说明

1. 向 buffer 写入数据，例如调用 channel.read(buffer)
2. 调用 flip() 切换至读模式
3. 从 buffer 读取数据，例如调用 buffer.get()
4. 调用 clear() 或 compact() 切换至写模式
5. 重复 1~4 步骤

1.2.2、ByteBuffer结构

ByteBuffer属性：

• capactity 容量
• postion 指针/小标
• limit

一开始：

写模式：postion是写入位置，limit=capacity

flip后，读模式，limit=读取限制

读取abcd后，

clear后，

compact方法，是把未读完部分向前压缩，然后切换至写模式

compact比clear更耗性能，compact会保存未读取数据，将新数据追加到未读取之后。

clear会清楚未读取的数据

调试工具类

public class ByteBufferUtil {
    private static final char[] BYTE2CHAR = new char[256];
    private static final char[] HEXDUMP_TABLE = new char[256 * 4];
    private static final String[] HEXPADDING = new String[16];
    private static final String[] HEXDUMP_ROWPREFIXES = new String[65536 >>> 4];
    private static final String[] BYTE2HEX = new String[256];
    private static final String[] BYTEPADDING = new String[16];

    static {
        final char[] DIGITS = "0123456789abcdef".toCharArray();
        for (int i = 0; i < 256; i++) {
            HEXDUMP_TABLE[i << 1] = DIGITS[i >>> 4 & 0x0F];
            HEXDUMP_TABLE[(i << 1) + 1] = DIGITS[i & 0x0F];
        }

        int i;

        // Generate the lookup table for hex dump paddings
        for (i = 0; i < HEXPADDING.length; i++) {
            int padding = HEXPADDING.length - i;
            StringBuilder buf = new StringBuilder(padding * 3);
            for (int j = 0; j < padding; j++) {
                buf.append("   ");
            }
            HEXPADDING[i] = buf.toString();
        }

        // Generate the lookup table for the start-offset header in each row (up to 64KiB).
        for (i = 0; i < HEXDUMP_ROWPREFIXES.length; i++) {
            StringBuilder buf = new StringBuilder(12);
            buf.append(StringUtil.NEWLINE);
            buf.append(Long.toHexString(i << 4 & 0xFFFFFFFFL | 0x100000000L));
            buf.setCharAt(buf.length() - 9, '|');
            buf.append('|');
            HEXDUMP_ROWPREFIXES[i] = buf.toString();
        }

        // Generate the lookup table for byte-to-hex-dump conversion
        for (i = 0; i < BYTE2HEX.length; i++) {
            BYTE2HEX[i] = ' ' + StringUtil.byteToHexStringPadded(i);
        }

        // Generate the lookup table for byte dump paddings
        for (i = 0; i < BYTEPADDING.length; i++) {
            int padding = BYTEPADDING.length - i;
            StringBuilder buf = new StringBuilder(padding);
            for (int j = 0; j < padding; j++) {
                buf.append(' ');
            }
            BYTEPADDING[i] = buf.toString();
        }

        // Generate the lookup table for byte-to-char conversion
        for (i = 0; i < BYTE2CHAR.length; i++) {
            if (i <= 0x1f || i >= 0x7f) {
                BYTE2CHAR[i] = '.';
            } else {
                BYTE2CHAR[i] = (char) i;
            }
        }
    }

    /**
     * 打印所有内容
     * @param buffer
     */
    public static void debugAll(ByteBuffer buffer) {
        int oldlimit = buffer.limit();
        buffer.limit(buffer.capacity());
        StringBuilder origin = new StringBuilder(256);
        appendPrettyHexDump(origin, buffer, 0, buffer.capacity());
        System.out.println("+--------+-------------------- all ------------------------+----------------+");
        System.out.printf("position: [%d], limit: [%d]\n", buffer.position(), oldlimit);
        System.out.println(origin);
        buffer.limit(oldlimit);
    }

    /**
     * 打印可读取内容
     * @param buffer
     */
    public static void debugRead(ByteBuffer buffer) {
        StringBuilder builder = new StringBuilder(256);
        appendPrettyHexDump(builder, buffer, buffer.position(), buffer.limit() - buffer.position());
        System.out.println("+--------+-------------------- read -----------------------+----------------+");
        System.out.printf("position: [%d], limit: [%d]\n", buffer.position(), buffer.limit());
        System.out.println(builder);
    }

    private static void appendPrettyHexDump(StringBuilder dump, ByteBuffer buf, int offset, int length) {
        if (MathUtil.isOutOfBounds(offset, length, buf.capacity())) {
            throw new IndexOutOfBoundsException(
                    "expected: " + "0 <= offset(" + offset + ") <= offset + length(" + length
                            + ") <= " + "buf.capacity(" + buf.capacity() + ')');
        }
        if (length == 0) {
            return;
        }
        dump.append(
                "         +-------------------------------------------------+" +
                        StringUtil.NEWLINE + "         |  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |" +
                        StringUtil.NEWLINE + "+--------+-------------------------------------------------+----------------+");

        final int startIndex = offset;
        final int fullRows = length >>> 4;
        final int remainder = length & 0xF;

        // Dump the rows which have 16 bytes.
        for (int row = 0; row < fullRows; row++) {
            int rowStartIndex = (row << 4) + startIndex;

            // Per-row prefix.
            appendHexDumpRowPrefix(dump, row, rowStartIndex);

            // Hex dump
            int rowEndIndex = rowStartIndex + 16;
            for (int j = rowStartIndex; j < rowEndIndex; j++) {
                dump.append(BYTE2HEX[getUnsignedByte(buf, j)]);
            }
            dump.append(" |");

            // ASCII dump
            for (int j = rowStartIndex; j < rowEndIndex; j++) {
                dump.append(BYTE2CHAR[getUnsignedByte(buf, j)]);
            }
            dump.append('|');
        }

        // Dump the last row which has less than 16 bytes.
        if (remainder != 0) {
            int rowStartIndex = (fullRows << 4) + startIndex;
            appendHexDumpRowPrefix(dump, fullRows, rowStartIndex);

            // Hex dump
            int rowEndIndex = rowStartIndex + remainder;
            for (int j = rowStartIndex; j < rowEndIndex; j++) {
                dump.append(BYTE2HEX[getUnsignedByte(buf, j)]);
            }
            dump.append(HEXPADDING[remainder]);
            dump.append(" |");

            // Ascii dump
            for (int j = rowStartIndex; j < rowEndIndex; j++) {
                dump.append(BYTE2CHAR[getUnsignedByte(buf, j)]);
            }
            dump.append(BYTEPADDING[remainder]);
            dump.append('|');
        }

        dump.append(StringUtil.NEWLINE +
                "+--------+-------------------------------------------------+----------------+");
    }

    private static void appendHexDumpRowPrefix(StringBuilder dump, int row, int rowStartIndex) {
        if (row < HEXDUMP_ROWPREFIXES.length) {
            dump.append(HEXDUMP_ROWPREFIXES[row]);
        } else {
            dump.append(StringUtil.NEWLINE);
            dump.append(Long.toHexString(rowStartIndex & 0xFFFFFFFFL | 0x100000000L));
            dump.setCharAt(dump.length() - 9, '|');
            dump.append('|');
        }
    }

    public static short getUnsignedByte(ByteBuffer buffer, int index) {
        return (short) (buffer.get(index) & 0xFF);
    }
}

工具类测试

public class TestByteBufferReadWriter {
    public static void main(String[] args) {
        ByteBuffer buffer = ByteBuffer.allocate(10);
        buffer.put((byte) 0x61);
        ByteBufferUtil.debugAll(buffer);
        buffer.put(new byte[]{0x62, 0x63, 0x64});
        ByteBufferUtil.debugAll(buffer);

        System.out.println("===========读取===========");
        //读
        //直接读是读不到的，因为position=4,
        buffer.flip();//会把position=0
        System.out.println(buffer.get());
        ByteBufferUtil.debugAll(buffer);

        //compact,压缩
        buffer.compact();
        ByteBufferUtil.debugAll(buffer);
        buffer.put(new byte[]{0x65, 0x66});
        ByteBufferUtil.debugAll(buffer);

        buffer.flip();
        ByteBufferUtil.debugAll(buffer);
    }
}

+--------+-------------------- all ------------------------+----------------+
position: [1], limit: [10]
         +-------------------------------------------------+
         |  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |
+--------+-------------------------------------------------+----------------+
|00000000| 61 00 00 00 00 00 00 00 00 00                   |a.........      |
+--------+-------------------------------------------------+----------------+
+--------+-------------------- all ------------------------+----------------+
position: [4], limit: [10]
         +-------------------------------------------------+
         |  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |
+--------+-------------------------------------------------+----------------+
|00000000| 61 62 63 64 00 00 00 00 00 00                   |abcd......      |
+--------+-------------------------------------------------+----------------+
===========读取===========
97
+--------+-------------------- all ------------------------+----------------+
position: [1], limit: [4]
         +-------------------------------------------------+
         |  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |
+--------+-------------------------------------------------+----------------+
|00000000| 61 62 63 64 00 00 00 00 00 00                   |abcd......      |
+--------+-------------------------------------------------+----------------+
+--------+-------------------- all ------------------------+----------------+
position: [3], limit: [10]
         +-------------------------------------------------+
         |  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |
+--------+-------------------------------------------------+----------------+
|00000000| 62 63 64 64 00 00 00 00 00 00                   |bcdd......      |
+--------+-------------------------------------------------+----------------+
+--------+-------------------- all ------------------------+----------------+
position: [5], limit: [10]
         +-------------------------------------------------+
         |  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |
+--------+-------------------------------------------------+----------------+
|00000000| 62 63 64 65 66 00 00 00 00 00                   |bcdef.....      |
+--------+-------------------------------------------------+----------------+
+--------+-------------------- all ------------------------+----------------+
position: [0], limit: [5]
         +-------------------------------------------------+
         |  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |
+--------+-------------------------------------------------+----------------+
|00000000| 62 63 64 65 66 00 00 00 00 00                   |bcdef.....      |
+--------+-------------------------------------------------+----------------+