Java检测文件是否UTF8编码

介绍UTF-8编码规则

1. UTF-8 编码字符理论上可以最多到 6 个字节长, 然而 16 位 BMP 字符最多只用到 3 字节长. Bigendian UCS-4 字节串的排列顺序是预定的.
2. 字节 0xFE 和 0xFF 在 UTF-8 编码中从未用到.
3. 下列字节串用来表示一个字符. 用到哪个串取决于该字符在 Unicode 中的序号.
4. U-00000000 - U-0000007F: 0xxxxxxx
5. U-00000080 - U-000007FF: 110xxxxx 10xxxxxx
6. U-00000800 - U-0000FFFF: 1110xxxx 10xxxxxx 10xxxxxx
7. U-00010000 - U-001FFFFF: 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx
8. U-00200000 - U-03FFFFFF: 111110xx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx
9. U-04000000 - U-7FFFFFFF: 1111110x 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx
10. xxx 的位置由字符编码数的二进制表示的位填入. 越靠右的 x 具有越少的特殊意义. 只用最短的那个足够表达一个字符编码数的多字节串. 注意在多字节串中, 第一个字节的开头"1"的数目就是整个串中字节的数目.
11. 例如: Unicode 字符 U+00A9 = 1010 1001 (版权符号) 在 UTF-8 里的编码为:
12. 11000010 10101001 = 0xC2 0xA9
13. 而字符 U+2260 = 0010 0010 0110 0000 (不等于) 编码为:
14. 11100010 10001001 10100000 = 0xE2 0x89 0xA0
15. 特殊规则: 文件头三个字节用16进制表示是EFBBBF, 此规则不通用, 由编辑工具定义.
16. 这种编码的官方名字拼写为 UTF-8, 其中 UTF 代表 UCS Transformation Format. 请勿在任何文档中用其他名字 (比如 utf8 或 UTF_8) 来表示 UTF-8, 当然除非你指的是一个变量名而不是这种编码本身.

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源码实现:

1. package com.yy.game.test;
2. import java.io.BufferedInputStream;
3. import java.io.File;
4. import java.io.FileInputStream;
5. import java.io.IOException;
6. import java.io.InputStream;
7. import java.nio.CharBuffer;
8. import java.nio.MappedByteBuffer;
9. import java.nio.channels.FileChannel;
10. import java.nio.channels.FileChannel.MapMode;
11. import java.nio.charset.Charset;
12. import java.nio.charset.CharsetDecoder;
13. import java.nio.charset.CoderResult;
14. public class UTF8Checker {
15.         public static void main(String[] args) throws IOException {
16.                 File dir = new File("F:\test");
17.                 for (File file : dir.listFiles()) {
18.                         System.out.format("%s: %s, %s%n", file, check(file), check2(file));
19.                 }
20.         }
21.         /**
22.          * JDK自带API实现
23.          */
24.         @SuppressWarnings("resource")
25.         public static boolean check2(File file) throws IOException {
26.                 long start = System.nanoTime();
27.                 FileChannel fc = null;
28.                 try {
29.                         fc = new FileInputStream(file).getChannel();
30.                         MappedByteBuffer buf = fc.map(MapMode.READ_ONLY, 0, fc.size());
31.                         Charset utf8 = Charset.forName("UTF-8");
32.                         CharsetDecoder decoder = utf8.newDecoder();
33.                         CharBuffer cbuf = CharBuffer.allocate((int) (buf.limit() * decoder.averageCharsPerByte()));
34.                         CoderResult result = decoder.decode(buf, cbuf, true);
35.                         return !result.isError();
36.                 } finally {
37.                         if (fc != null) {
38.                                 fc.close();
39.                         }
40.                         long end = System.nanoTime();
41.                         System.out.println("used(ns):" + (end - start));
42.                 }
43.         }
44.         /**
45.          * 自定义实现
46.          */
47.         public static boolean check(File file) throws IOException {
48.                 long start = System.nanoTime();
49.                 InputStream in = null;
50.                 try {
51.                         in = new BufferedInputStream(new FileInputStream(file));
52.                         StreamBuffer sbuf = new StreamBuffer(in, 1024);
53.                         if (sbuf.next() == 0xEF && sbuf.next() == 0xBB && sbuf.next() == 0xBF) {
54.                                 return true;
55.                         }
56.                         sbuf.redo();
57.                         // 1. U-00000000 - U-0000007F: 0xxxxxxx
58.                         // 2. U-00000080 - U-000007FF: 110xxxxx 10xxxxxx
59.                         // 3. U-00000800 - U-0000FFFF: 1110xxxx 10xxxxxx 10xxxxxx
60.                         // 4. U-00010000 - U-001FFFFF: 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx
61.                         // 5. U-00200000 - U-03FFFFFF: 111110xx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx
62.                         // 6. U-04000000 - U-7FFFFFFF: 1111110x 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx
63.                         for (int ch = 0; (ch = sbuf.next()) != -1;) {
64.                                 int n = 0;
65.                                 if (ch <= 0x7F) {
66.                                         n = 1;
67.                                 } else if (ch <= 0xBF) {
68.                                         return false;
69.                                 } else if (ch <= 0xDF) {
70.                                         n = 2;
71.                                 } else if (ch <= 0xEF) {
72.                                         n = 3;
73.                                 } else if (ch <= 0xF7) {
74.                                         n = 4;
75.                                 } else if (ch <= 0xFB) {
76.                                         n = 5;
77.                                 } else if (ch <= 0xFD) {
78.                                         n = 6;
79.                                 } else {
80.                                         return false;
81.                                 }
82.                                 while (--n > 0) {
83.                                         if ((sbuf.next() & 0x80) != 0x80) {
84.                                                 return false;
85.                                         }
86.                                 }
87.                         }
88.                         return true;
89.                 } finally {
90.                         if (in != null) {
91.                                 in.close();
92.                         }
93.                         long end = System.nanoTime();
94.                         System.out.println("used(ns):" + (end - start));
95.                 }
96.         }
97.         static class StreamBuffer {
98.                 final InputStream in;
99.                 final byte[] buf;
100.                 int pos = -1;// 初始值为-1,表示指针尚未移动.
101.                 int len;
102.                 public StreamBuffer(InputStream in, int size) {
103.                         this.in = in;
104.                         if (size < 3) {
105.                                 size = 3;
106.                         }
107.                         this.buf = new byte[size];
108.                 }
109.                 public void redo() {
110.                         this.pos = 0;
111.                 }
112.                 public int next() throws IOException {
113.                         if (len > 0 || pos < 0) {
114.                                 if (++pos == len) {
115.                                         if ((len = in.read(buf)) == 0) {
116.                                                 return -1;
117.                                         }
118.                                         pos = 0;
119.                                 }
120.                                 return this.buf[this.pos] & 0xFF;
121.                         } else {
122.                                 return -1;
123.                         }
124.                 }
125.         }
126. }

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在本机测试, JDK原生API需要创建CharBuffer,性能明显慢了25%以上.

1. used(ns):472420
2. used(ns):4490075
3. F: est334d5fd-b8a7-48f4-9099-f6011c7e5a48.sql: true, true
4. used(ns):122515
5. used(ns):343490
6. F: est334d5fd-b8a7-48f4-9099-f6011c7e5a482.sql: false, false
7. used(ns):55164
8. used(ns):82425
9. F: est est.sql: false, false

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