[TOC]
1. 分配内存空间
可以使用allocate() 和 allocateDirect()方法为ByteBuffer分配空间，其他buffer类也有该方法
- allocate() : 使用的是java的堆内存，堆内字节缓冲区，读写效率低，会受到GC的影响
- allocateDirect() :使用的是直接内存，直接内存字节缓冲区，读写效率高（零拷贝），不会受GC影响,因为是系统直接内存，所以分配内存要调用操作系统函数，所以分配内存的速度较慢，如果使用不当（资源没得到合理释放），会造成内存泄漏。
import java.nio.ByteBuffer;
/**
* Created by lilinchao
* Date 2022/5/25
* Description 分配空间
*/
public class ByteBufferAllocateDemo {
public static void main(String[] args) {
System.out.println(ByteBuffer.allocate(16).getClass());
System.out.println(ByteBuffer.allocateDirect(16).getClass());
}
}
输出结果
class java.nio.HeapByteBuffer
class java.nio.DirectByteBuffer
2. 向buffer写入数据
有两种办法
- 调用 channel 的 read 方法
int readBytes = channel.read(buf);
- 调用 buffer 自己的 put 方法
buf.put((byte)127);
3. 从buffer读取数据
同样有两种办法
- 调用 channel 的 write 方法
int writeBytes = channel.write(buf);
- 调用 buffer 自己的 get 方法
byte b = buf.get();
get 方法会让 position 读指针向后走，如果想重复读取数据
- 可以调用 rewind 方法将 position 重新置为 0
- 或者调用 get(int i) 方法获取索引 i 的内容，它不会移动读指针
读写示例
import java.nio.ByteBuffer;
import static com.lilinchao.nio.bytebuffer_2.ByteBufferUtil.debugAll;
/**
* Created by lilinchao
* Date 2022/5/25
* Description bytebuffer读写示例
*/
public class TestByteBufferReadWrite {
public static void main(String[] args) {
ByteBuffer buffer = ByteBuffer.allocate(10);
buffer.put((byte) 0x61); //a
debugAll(buffer);
buffer.put(new byte[]{0x62,0x63,0x64}); //b、c、d
debugAll(buffer);
//flip:切换对缓冲区的操作模式 写模式 --> 读模式
buffer.flip();
System.out.println(buffer.get());
//get(i):不会改变索引的位置
System.out.println(buffer.get(2));
debugAll(buffer);
//compact:会把未读完的数据向前压缩，然后切换到写模式
buffer.compact();
debugAll(buffer);
buffer.put(new byte[]{0x65,0x6f});
debugAll(buffer);
// rewind 从头开始读
buffer.flip();
System.out.println((char)buffer.get());
System.out.println((char)buffer.get());
buffer.rewind();
System.out.println((char)buffer.get());
}
}
运行结果
+--------+-------------------- 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...... |
+--------+-------------------------------------------------+----------------+
97
99
+--------+-------------------- 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 6f 00 00 00 00 00 |bcdeo..... |
+--------+-------------------------------------------------+----------------+
b
c
b
4. mark 和 reset
mark 是在读取时，做一个标记，即使 position 改变，只要调用 reset 就能回到 mark 的位置
import java.nio.ByteBuffer;
/**
* Created by lilinchao
* Date 2022/5/25
* Description mark 和 reset方法
*/
public class TestByteBufferMarkAndReset {
public static void main(String[] args) {
ByteBuffer byteBuffer = ByteBuffer.allocate(10);
byteBuffer.put(new byte[]{'a', 'b', 'c', 'd'});
byteBuffer.flip();
System.out.println((char) byteBuffer.get()); // 读取 a
System.out.println((char) byteBuffer.get()); // 读取 b
byteBuffer.mark(); // 加标记 索引为2 的位置
System.out.println((char) byteBuffer.get()); // 读取 c
System.out.println((char) byteBuffer.get()); // 读取 d
byteBuffer.reset(); // 将position 重置到索引为2的位置
System.out.println((char) byteBuffer.get()); // 读取 c
System.out.println((char) byteBuffer.get()); // 读取 d
}
}
运行结果
a
b
c
d
c
d
5. 字符串与 ByteBuffer 互转
ByteBuffer buffer1 = StandardCharsets.UTF_8.encode("你好");
ByteBuffer buffer2 = Charset.forName("utf-8").encode("你好");
debug(buffer1);
debug(buffer2);
CharBuffer buffer3 = StandardCharsets.UTF_8.decode(buffer1);
System.out.println(buffer3.getClass());
System.out.println(buffer3.toString());
输出结果
+-------------------------------------------------+
| 0 1 2 3 4 5 6 7 8 9 a b c d e f |
+--------+-------------------------------------------------+----------------+
|00000000| e4 bd a0 e5 a5 bd |...... |
+--------+-------------------------------------------------+----------------+
+-------------------------------------------------+
| 0 1 2 3 4 5 6 7 8 9 a b c d e f |
+--------+-------------------------------------------------+----------------+
|00000000| e4 bd a0 e5 a5 bd |...... |
+--------+-------------------------------------------------+----------------+
class java.nio.HeapCharBuffer
你好
示例
import java.nio.ByteBuffer;
import java.nio.CharBuffer;
import java.nio.charset.StandardCharsets;
import static com.lilinchao.nio.bytebuffer_2.ByteBufferUtil.debugAll;
/**
* Created by lilinchao
* Date 2022/5/25
* Description 字符串与 ByteBuffer 互转 Demo
*/
public class TestByteBufferString {
public static void main(String[] args) {
//1、字符串转为 ByteBuffer，还是写模式
ByteBuffer buffer = ByteBuffer.allocate(16);
buffer.put("hello".getBytes());
debugAll(buffer);
// 2、Charset 字符集类，自动切换到读模式
ByteBuffer helloBuffer = StandardCharsets.UTF_8.encode("hello");
debugAll(helloBuffer);
// 3、wrap，自动切换到读模式
ByteBuffer buffer3 = ByteBuffer.wrap("hello".getBytes());
debugAll(buffer3);
// 4、转换为字符串
CharBuffer charBuffer = StandardCharsets.UTF_8.decode(helloBuffer);
System.out.println(charBuffer.toString());
// 不起作用
buffer.flip(); // 切换读模式，起作用
CharBuffer charBuffer1 = StandardCharsets.UTF_8.decode(buffer);
System.out.println(charBuffer1.toString());
}
}
运行结果
+--------+-------------------- all ------------------------+----------------+
position: [5], limit: [16]
+-------------------------------------------------+
| 0 1 2 3 4 5 6 7 8 9 a b c d e f |
+--------+-------------------------------------------------+----------------+
|00000000| 68 65 6c 6c 6f 00 00 00 00 00 00 00 00 00 00 00 |hello...........|
+--------+-------------------------------------------------+----------------+
+--------+-------------------- all ------------------------+----------------+
position: [0], limit: [5]
+-------------------------------------------------+
| 0 1 2 3 4 5 6 7 8 9 a b c d e f |
+--------+-------------------------------------------------+----------------+
|00000000| 68 65 6c 6c 6f |hello |
+--------+-------------------------------------------------+----------------+
+--------+-------------------- all ------------------------+----------------+
position: [0], limit: [5]
+-------------------------------------------------+
| 0 1 2 3 4 5 6 7 8 9 a b c d e f |
+--------+-------------------------------------------------+----------------+
|00000000| 68 65 6c 6c 6f |hello |
+--------+-------------------------------------------------+----------------+
hello
hello
6. Scattering Reads（分散读）
分散读取集中写的方法不重要，重要的是思想，可以减少在ByteBuffer之间的拷贝，减少数据的复制次数，提高效率。
示例
- 创建一个文本文件words.txt
onetwothree
- 分散读取示例代码
import java.io.IOException;
import java.io.RandomAccessFile;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
import static com.lilinchao.nio.bytebuffer_2.ByteBufferUtil.debugAll;
/**
* Created by lilinchao
* Date 2022/5/25
* Description 分散读 Demo
*/
public class ScatteringReadsDemo {
public static void main(String[] args) {
try(FileChannel channel = new RandomAccessFile("datas/words.txt", "r").getChannel()) {
ByteBuffer b1 = ByteBuffer.allocate(3); // one
ByteBuffer b2 = ByteBuffer.allocate(3); // two
ByteBuffer b3 = ByteBuffer.allocate(5); // three
channel.read(new ByteBuffer[]{b1, b2, b3});
b1.flip();
b2.flip();
b3.flip();
debugAll(b1);
debugAll(b2);
debugAll(b3);
} catch (IOException e) {
e.printStackTrace();
}
}
}
运行结果
+--------+-------------------- all ------------------------+----------------+
position: [0], limit: [3]
+-------------------------------------------------+
| 0 1 2 3 4 5 6 7 8 9 a b c d e f |
+--------+-------------------------------------------------+----------------+
|00000000| 6f 6e 65 |one |
+--------+-------------------------------------------------+----------------+
+--------+-------------------- all ------------------------+----------------+
position: [0], limit: [3]
+-------------------------------------------------+
| 0 1 2 3 4 5 6 7 8 9 a b c d e f |
+--------+-------------------------------------------------+----------------+
|00000000| 74 77 6f |two |
+--------+-------------------------------------------------+----------------+
+--------+-------------------- all ------------------------+----------------+
position: [0], limit: [5]
+-------------------------------------------------+
| 0 1 2 3 4 5 6 7 8 9 a b c d e f |
+--------+-------------------------------------------------+----------------+
|00000000| 74 68 72 65 65 |three |
+--------+-------------------------------------------------+----------------+
7. Gathering Writes（集中写）
使用如下方式写入，可以将多个 buffer 的数据填充至 channel
import java.io.IOException;
import java.io.RandomAccessFile;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
import static com.lilinchao.nio.bytebuffer_2.ByteBufferUtil.debugAll;
/**
* Created by lilinchao
* Date 2022/5/25
* Description 集中写Demo
*/
public class GatheringWritesDemo {
public static void main(String[] args) {
try (RandomAccessFile file = new RandomAccessFile("datas/words.txt", "rw")) {
FileChannel channel = file.getChannel();
ByteBuffer d = ByteBuffer.allocate(4);
ByteBuffer e = ByteBuffer.allocate(4);
channel.position(11);
d.put(new byte[]{'f', 'o', 'u', 'r'});
e.put(new byte[]{'f', 'i', 'v', 'e'});
d.flip();
e.flip();
debugAll(d);
debugAll(e);
channel.write(new ByteBuffer[]{d, e});
} catch (IOException e) {
e.printStackTrace();
}
}
}
运行结果
+--------+-------------------- all ------------------------+----------------+
position: [0], limit: [4]
+-------------------------------------------------+
| 0 1 2 3 4 5 6 7 8 9 a b c d e f |
+--------+-------------------------------------------------+----------------+
|00000000| 66 6f 75 72 |four |
+--------+-------------------------------------------------+----------------+
+--------+-------------------- all ------------------------+----------------+
position: [0], limit: [4]
+-------------------------------------------------+
| 0 1 2 3 4 5 6 7 8 9 a b c d e f |
+--------+-------------------------------------------------+----------------+
|00000000| 66 69 76 65 |five |
+--------+-------------------------------------------------+----------------+
最后：调试工具类
import io.netty.util.internal.StringUtil;
import java.nio.ByteBuffer;
import static io.netty.util.internal.MathUtil.isOutOfBounds;
import static io.netty.util.internal.StringUtil.NEWLINE;
/**
* @Author: lilinchao
* @Date: 2022/5/25
* @Description: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(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 (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(
" +-------------------------------------------------+" +
NEWLINE + " | 0 1 2 3 4 5 6 7 8 9 a b c d e f |" +
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(NEWLINE +
"+--------+-------------------------------------------------+----------------+");
}
private static void appendHexDumpRowPrefix(StringBuilder dump, int row, int rowStartIndex) {
if (row < HEXDUMP_ROWPREFIXES.length) {
dump.append(HEXDUMP_ROWPREFIXES[row]);
} else {
dump.append(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);
}
}
Java 面试宝典是大明哥全力打造的 Java 精品面试题，它是一份靠谱、强大、详细、经典的 Java 后端面试宝典。它不仅仅只是一道道面试题，而是一套完整的 Java 知识体系，一套你 Java 知识点的扫盲贴。
它的内容包括：
- 大厂真题：Java 面试宝典里面的题目都是最近几年的高频的大厂面试真题。
- 原创内容：Java 面试宝典内容全部都是大明哥原创，内容全面且通俗易懂，回答部分可以直接作为面试回答内容。
- 持续更新：一次购买，永久有效。大明哥会持续更新 3+ 年，累计更新 1000+，宝典会不断迭代更新，保证最新、最全面。
- 覆盖全面：本宝典累计更新 1000+，从 Java 入门到 Java 架构的高频面试题，实现 360° 全覆盖。
- 不止面试：内容包含面试题解析、内容详解、知识扩展，它不仅仅只是一份面试题，更是一套完整的 Java 知识体系。
- 宝典详情：https://www.yuque.com/chenssy/sike-java/xvlo920axlp7sf4k
- 宝典总览：https://www.yuque.com/chenssy/sike-java/yogsehzntzgp4ly1
- 宝典进展：https://www.yuque.com/chenssy/sike-java/en9ned7loo47z5aw
目前 Java 面试宝典累计更新 400+ 道，总字数 42w+。大明哥还在持续更新中，下图是大明哥在 2024-12 月份的更新情况：
想了解详情的小伙伴，扫描下面二维码加大明哥微信【daming091】咨询
同时，大明哥也整理一套目前市面最常见的热点面试题。微信搜[大明哥聊 Java]或扫描下方二维码关注大明哥的原创公众号[大明哥聊 Java] ，回复【面试题】 即可免费领取。
