去除UTF-8 BOM表頭的InputStreamReader

如果你曾經使用Java遇過讀一些文字檔前面出現??等亂碼的話, 表示該份文件有加入bom(byte order mark), 基本上bom的用途原本是用來讓程式辨別該份文檔的編碼格式, Microsoft在Windows 2000以後的Notepad存UTF-8的檔案會加上 BOM(Byte Order Mark, U+FEFF), 主要是因為UTF-8和ASCII是相容的, 為了避免使用者自己忘記用甚麼存, 造成UTF-8檔案用 ASCII 模式開看到是亂碼, 所以在檔案最前面加上BOM.

看到這裡可能有很多人會開始%#~!微軟...

但是碰到問題總是得解決的, 下面這個改寫InputStreamReader的UnicodeInputStreamReader就能夠讓你免去上述煩惱, 所有功能仍然和InputStreamReader相同, 只是在constructor中加入判斷bom表頭的機制, 使用PushbackInputStream當讀取到相關格式時能夠移動檔案開始位置.

使用方式: BufferedReader in = new BufferedReader(new UnicodeInputStreamReader(new FileInputStream(file), encode));

UnicodeInputStreamReader:

package com.progking.io;

import java.io.IOException;
import java.io.PushbackInputStream;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.UnsupportedEncodingException;

import sun.nio.cs.StreamDecoder;

public class UnicodeInputStreamReader extends InputStreamReader
{
   private static final int BOM_SIZE = 4;
   private final StreamDecoder decoder;
   private PushbackInputStream pushBack;
   private String encode;
   private String defaultEnc;

   public UnicodeInputStreamReader(InputStream input, String defaultEnc) throws UnsupportedEncodingException
   {
       super(input);
       try {
           this.defaultEnc = defaultEnc;
           pushBack = new PushbackInputStream(input, BOM_SIZE);
           init();
       } catch (Exception e) {
           e.printStackTrace();
       }
       decoder = StreamDecoder.forInputStreamReader(pushBack, this, encode);
   }

   private void init() throws IOException
   {
       byte[] bom = new byte[BOM_SIZE];
       int n, unread;

       // 初始讀取一次
       n = pushBack.read(bom, 0, bom.length);

       // 比對表頭
       if ((bom[0] == (byte) 0x00) && (bom[1] == (byte) 0x00) && (bom[2] == (byte) 0xFE) && (bom[3] == (byte) 0xFF)) {
           encode = "UTF-32BE";
           unread = n - 4;
       } else if ((bom[0] == (byte) 0xFF) && (bom[1] == (byte) 0xFE) && (bom[2] == (byte) 0x00) && (bom[3] == (byte) 0x00)) {
           encode = "UTF-32LE";
           unread = n - 4;
       } else if ((bom[0] == (byte) 0xEF) && (bom[1] == (byte) 0xBB) && (bom[2] == (byte) 0xBF)) {
           encode = "UTF-8";
           unread = n - 3;
       } else if ((bom[0] == (byte) 0xFE) && (bom[1] == (byte) 0xFF)) {
           encode = "UTF-16BE";
           unread = n - 2;
       } else if ((bom[0] == (byte) 0xFF) && (bom[1] == (byte) 0xFE)) {
           encode = "UTF-16LE";
           unread = n - 2;
       } else {
           // 如果沒有找到任何表頭, 則退回長度等於原先總長
           encode = defaultEnc;
           unread = n;
       }
       System.out.println("has BOM=" + ((unread == n) ? false : true) + ", encode=" + encode + ", read=" + n + ", unread=" + unread);
       // 計算應該退回多少byte
       if (unread > 0)
           pushBack.unread(bom, (n - unread), unread);
   }

   public String getEncoding()
   {
       return encode;
   }

   public int read() throws IOException
   {
       return decoder.read();
   }

   public int read(char cbuf[], int offset, int length) throws IOException
   {
       return decoder.read(cbuf, offset, length);
   }

   public boolean ready() throws IOException
   {
       return decoder.ready();
   }

   public void close() throws IOException
   {
       decoder.close();
   }
}