Java检测文件是否UTF8编码

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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
11. 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.
12. 例如: Unicode 字符 U+00A9 = 1010 1001 (版权符号) 在 UTF-8 里的编码为:
13. 11000010 10101001 = 0xC2 0xA9
14. 而字符 U+2260 = 0010 0010 0110 0000 (不等于) 编码为:
15. 11100010 10001001 10100000 = 0xE2 0x89 0xA0
17. 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.
19. 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;
3. import java.io.BufferedInputStream;
4. import java.io.File;
5. import java.io.FileInputStream;
6. import java.io.IOException;
7. import java.io.InputStream;
8. import java.nio.CharBuffer;
9. import java.nio.MappedByteBuffer;
10. import java.nio.channels.FileChannel;
11. import java.nio.channels.FileChannel.MapMode;
12. import java.nio.charset.Charset;
13. import java.nio.charset.CharsetDecoder;
14. import java.nio.charset.CoderResult;
16. public class UTF8Checker {
18.         public static void main(String[] args) throws IOException {
19.                 File dir = new File("F:\\test");
20.                 for (File file : dir.listFiles()) {
21.                         System.out.format("%s: %s, %s%n", file, check(file), check2(file));
22.                 }
23.         }
25.         /**
26.          * JDK自带API实现
27.          */
28.         @SuppressWarnings("resource")
29.         public static boolean check2(File file) throws IOException {
30.                 long start = System.nanoTime();
31.                 FileChannel fc = null;
32.                 try {
34.                         fc = new FileInputStream(file).getChannel();
36.                         MappedByteBuffer buf = fc.map(MapMode.READ_ONLY, 0, fc.size());
38.                         Charset utf8 = Charset.forName("UTF-8");
39.                         CharsetDecoder decoder = utf8.newDecoder();
40.                         CharBuffer cbuf = CharBuffer.allocate((int) (buf.limit() * decoder.averageCharsPerByte()));
41.                         CoderResult result = decoder.decode(buf, cbuf, true);
42.                         return !result.isError();
43.                 } finally {
44.                         if (fc != null) {
45.                                 fc.close();
46.                         }
47.                         long end = System.nanoTime();
48.                         System.out.println("used(ns):" + (end - start));
49.                 }
51.         }
53.         /**
54.          * 自定义实现
55.          */
56.         public static boolean check(File file) throws IOException {
57.                 long start = System.nanoTime();
58.                 InputStream in = null;
59.                 try {
60.                         in = new BufferedInputStream(new FileInputStream(file));
61.                         StreamBuffer sbuf = new StreamBuffer(in, 1024);
62.                         if (sbuf.next() == 0xEF && sbuf.next() == 0xBB && sbuf.next() == 0xBF) {
63.                                 return true;
64.                         }
65.                         sbuf.redo();
66.                         // 1. U-00000000 - U-0000007F: 0xxxxxxx
67.                         // 2. U-00000080 - U-000007FF: 110xxxxx 10xxxxxx
68.                         // 3. U-00000800 - U-0000FFFF: 1110xxxx 10xxxxxx 10xxxxxx
69.                         // 4. U-00010000 - U-001FFFFF: 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx
70.                         // 5. U-00200000 - U-03FFFFFF: 111110xx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx
71.                         // 6. U-04000000 - U-7FFFFFFF: 1111110x 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx
72.                         for (int ch = 0; (ch = sbuf.next()) != -1;) {
73.                                 int n = 0;
74.                                 if (ch <= 0x7F) {
75.                                         n = 1;
76.                                 } else if (ch <= 0xBF) {
77.                                         return false;
78.                                 } else if (ch <= 0xDF) {
79.                                         n = 2;
80.                                 } else if (ch <= 0xEF) {
81.                                         n = 3;
82.                                 } else if (ch <= 0xF7) {
83.                                         n = 4;
84.                                 } else if (ch <= 0xFB) {
85.                                         n = 5;
86.                                 } else if (ch <= 0xFD) {
87.                                         n = 6;
88.                                 } else {
89.                                         return false;
90.                                 }
91.                                 while (--n > 0) {
92.                                         if ((sbuf.next() & 0x80) != 0x80) {
93.                                                 return false;
94.                                         }
95.                                 }
96.                         }
97.                         return true;
98.                 } finally {
99.                         if (in != null) {
100.                                 in.close();
101.                         }
102.                         long end = System.nanoTime();
103.                         System.out.println("used(ns):" + (end - start));
104.                 }
106.         }
108.         static class StreamBuffer {
110.                 final InputStream in;
111.                 final byte[] buf;
112.                 int pos = -1;// 初始值为-1,表示指针尚未移动.
113.                 int len;
115.                 public StreamBuffer(InputStream in, int size) {
116.                         this.in = in;
117.                         if (size < 3) {
118.                                 size = 3;
119.                         }
120.                         this.buf = new byte[size];
121.                 }
123.                 public void redo() {
124.                         this.pos = 0;
125.                 }
127.                 public int next() throws IOException {
128.                         if (len > 0 || pos < 0) {
129.                                 if (++pos == len) {
130.                                         if ((len = in.read(buf)) == 0) {
131.                                                 return -1;
132.                                         }
133.                                         pos = 0;
134.                                 }
135.                                 return this.buf[this.pos] & 0xFF;
136.                         } else {
137.                                 return -1;
138.                         }
139.                 }
141.         }
143. }

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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:\test\b334d5fd-b8a7-48f4-9099-f6011c7e5a48.sql: true, true
4. used(ns):122515
5. used(ns):343490
6. F:\test\b334d5fd-b8a7-48f4-9099-f6011c7e5a482.sql: false, false
7. used(ns):55164
8. used(ns):82425
9. F:\test\test.sql: false, false

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Java检测文件是否UTF8编码

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原文地址：http://www.cnblogs.com/zolo/p/5849180.html