Java讀帶有BOM的UTF-8文件亂碼

通過java寫的UTF-8文件，使用Java可以正確的讀，但是如果用記事本將相同的內容使用UTF-8格式保存，則在使用程序讀取是會從文件中多讀出一個不可見字符，多一個?

實例：

新建一個文本：內容爲“測試BOM”的txt文本，另存爲UTF-8.

處理帶BOM的類UnicodeReader

package com.java.io;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.PushbackInputStream;
import java.io.Reader;
/**
version: 1.1 / 2007-01-25
- changed BOM recognition ordering (longer boms first)
網絡地址：http://koti.mbnet.fi/akini/java/unicodereader/UnicodeReader.java.txt
Original pseudocode   : Thomas Weidenfeller
Implementation tweaked: Aki Nieminen
http://www.unicode.org/unicode/faq/utf_bom.html
BOMs:
  00 00 FE FF    = UTF-32, big-endian
  FF FE 00 00    = UTF-32, little-endian
  EF BB BF       = UTF-8,
  FE FF          = UTF-16, big-endian
  FF FE          = UTF-16, little-endian
Win2k Notepad:
  Unicode format = UTF-16LE
***/
/**
* Generic unicode textreader, which will use BOM mark
* to identify the encoding to be used. If BOM is not found
* then use a given default or system encoding.
*/
public class UnicodeReader extends Reader {
  PushbackInputStream internalIn;
  InputStreamReader   internalIn2 = null;
  String              defaultEnc;
  private static final int BOM_SIZE = 4;
  /**
   *
   * @param in  inputstream to be read
   * @param defaultEnc default encoding if stream does not have
   *                   BOM marker. Give NULL to use system-level default.
   */
  UnicodeReader(InputStream in, String defaultEnc) {
     internalIn = new PushbackInputStream(in, BOM_SIZE);
     this.defaultEnc = defaultEnc;
  }
  public String getDefaultEncoding() {
     return defaultEnc;
  }
  /**
   * Get stream encoding or NULL if stream is uninitialized.
   * Call init() or read() method to initialize it.
   */
  public String getEncoding() {
     if (internalIn2 == null) return null;
     return internalIn2.getEncoding();
  }
  /**
   * Read-ahead four bytes and check for BOM marks. Extra bytes are
   * unread back to the stream, only BOM bytes are skipped.
   */
  protected void init() throws IOException {
     if (internalIn2 != null) return;
     String encoding;
     byte bom[] = new byte[BOM_SIZE];
     int n, unread;
     n = internalIn.read(bom, 0, bom.length);
     if ( (bom[0] == (byte)0x00) && (bom[1] == (byte)0x00) &&
                 (bom[2] == (byte)0xFE) && (bom[3] == (byte)0xFF) ) {
        encoding = "UTF-32BE";
        unread = n - 4;
     } else if ( (bom[0] == (byte)0xFF) && (bom[1] == (byte)0xFE) &&
                 (bom[2] == (byte)0x00) && (bom[3] == (byte)0x00) ) {
        encoding = "UTF-32LE";
        unread = n - 4;
     } else if (  (bom[0] == (byte)0xEF) && (bom[1] == (byte)0xBB) &&
           (bom[2] == (byte)0xBF) ) {
        encoding = "UTF-8";
        unread = n - 3;
     } else if ( (bom[0] == (byte)0xFE) && (bom[1] == (byte)0xFF) ) {
        encoding = "UTF-16BE";
        unread = n - 2;
     } else if ( (bom[0] == (byte)0xFF) && (bom[1] == (byte)0xFE) ) {
        encoding = "UTF-16LE";
        unread = n - 2;
     } else {
        // Unicode BOM mark not found, unread all bytes
        encoding = defaultEnc;
        unread = n;
     }  
     //System.out.println("read=" + n + ", unread=" + unread);
     if (unread > 0) internalIn.unread(bom, (n - unread), unread);
     // Use given encoding
     if (encoding == null) {
        internalIn2 = new InputStreamReader(internalIn);
     } else {
        internalIn2 = new InputStreamReader(internalIn, encoding);
     }
  }
  public void close() throws IOException {
     init();
     internalIn2.close();
  }
  public int read(char[] cbuf, int off, int len) throws IOException {
     init();
     return internalIn2.read(cbuf, off, len);
  }
}

測試類

package com.java.io;
import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.InputStreamReader;
import java.nio.charset.Charset;
public class BomRead {
    /**
     * 讀帶有BOM的UTF-8文件亂碼
     * @param args
     */
    public static void main(String[] args)throws Exception {
         File file  = new File("E:\\JS_Exercise\\JavaExercise\\BOM.txt");
         FileInputStream in = new FileInputStream(file);
         BufferedReader br = new BufferedReader(new InputStreamReader(in, "UTF-8"));
         String line = null;
         System.out.println("處理前：");
         while((line = br.readLine()) != null){
             System.out.println(line);
         }
                                                                      
          File file2  = new File("E:\\JS_Exercise\\JavaExercise\\BOM.txt");
          FileInputStream in2 = new FileInputStream(file2);
          BufferedReader br2 = new BufferedReader(new UnicodeReader(in2, "UTF-8"));
          String line2 = null;
          System.out.println("處理後：");
          while((line2 = br2.readLine()) != null){
              System.out.println(line2);
          }
    }
}

輸出結果

處理前：

?測試BOM

處理後：

測試BOM

另一種解決方式

從目前來看1.6只是解決了讀取帶有BOM文件失敗的問題，還是不能區別處理有BOM和無BOM的UTF-8編碼的文件，從Bug ID:4508058裏的描述可以看出，這個問題將作爲一個不會修改的問題關閉，對於BOM編碼的識別將由應用程序自己來處理，原因可從另處一個bug處查看到，因爲Unicode對於BOM的編碼的規定可能發生變化。也就是說對於一個UTF-8的文件，應用程序需要知道這個文件有沒有寫BOM，然後自己決定處理BOM的方式。

因此在遇到此問題的時候可以特殊問題特殊處理。