前幾天在專案中需要讀取使用者上傳過來的txt檔案，但不確定txt檔案的字符集

UTF-16、UTF-8(帶BOM)、Unicode可以根據前三個位元組區別 

public String getTxtEncode(FileInputStream in) throws IOException{
        byte[] head = new byte[3];  
        in.read(head);    
        String code = "GBK";  
        if (head[0] == -1 && head[1] == -2 )  
            code = "UTF-16";  
        if (head[0] == -2 && head[1] == -1 )  
            code = "Unicode";
        //帶BOM
        if(head[0]==-17 && head[1]==-69 && head[2] ==-65)  
            code = "UTF-8";  
        if("Unicode".equals(code)){
         code = "UTF-16";
        }
        return code;
 }
 

通過在google上搜索發現不帶BOM的識別是Java遺留的一個bug，呵呵，終於找到根源了，Java提供了此bug的解決方案

package com.justsy.sts.utf8;

import java.io.*;  

/** 
 * This inputstream will recognize unicode BOM marks and will skip bytes if 
 * getEncoding() method is called before any of the read(...) methods. 
 *  
 * Usage pattern: String enc = "ISO-8859-1"; // or NULL to use systemdefault 
 * FileInputStream fis = new FileInputStream(file); UnicodeInputStream uin = new 
 * UnicodeInputStream(fis, enc); enc = uin.getEncoding(); // check and skip 
 * possible BOM bytes InputStreamReader in; if (enc == null) in = new 
 * InputStreamReader(uin); else in = new InputStreamReader(uin, enc); 
 */  
public class UnicodeInputStream extends InputStream {  
    PushbackInputStream internalIn;  
    boolean isInited = false;  
    String defaultEnc;  
    String encoding;  
  
    private static final int BOM_SIZE = 4;  
  
    public UnicodeInputStream(InputStream in, String defaultEnc) {  
        internalIn = new PushbackInputStream(in, BOM_SIZE);  
        this.defaultEnc = defaultEnc;  
    }  
  
    public String getDefaultEncoding() {  
        return defaultEnc;  
    }  
  
    public String getEncoding() {  
        if (!isInited) {  
            try {  
                init();  
            } catch (IOException ex) {  
                IllegalStateException ise = new IllegalStateException(  
                        "Init method failed.");  
                ise.initCause(ise);  
                throw ise;  
            }  
        }  
        return encoding;  
    }  
  
    /** 
     * 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 (isInited)  
            return;  
  
        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);  
  
        isInited = true;  
    }  
  
    public void close() throws IOException {  
        // init();  
        isInited = true;  
        internalIn.close();  
    }  
  
    public int read() throws IOException {  
        // init();  
        isInited = true;  
        return internalIn.read();  
    }  
}
 

通過使用上述InputStream類的實現可以正確的讀取出不帶BOM和帶BOM的字符集

package com.justsy.sts.utf8;

import java.io.BufferedReader;  
import java.io.File;  
import java.io.FileInputStream;  
import java.io.IOException;  
import java.io.InputStreamReader;
import java.nio.charset.Charset;
  
public class UTF8Test {  
    public static void main(String[] args) throws IOException {  
        File f  = new File("D:"+File.separator+"Order.txt");  
        FileInputStream in = new FileInputStream(f);  
        String dc  = Charset.defaultCharset().name();
        UnicodeInputStream uin = new UnicodeInputStream(in,dc);
        BufferedReader br = new BufferedReader(new InputStreamReader(uin));  
        String line = br.readLine();  
        while(line != null)  
        {  
            System.out.println(line);  
            line = br.readLine();  
        }  
    }  
}
 

結合Java提供的方案，我們就可以比較完整的判別出各種字元集了

 public String getTxtEncode(FileInputStream in) throws IOException{
  
  String dc  = Charset.defaultCharset().name();
        UnicodeInputStream uin = new UnicodeInputStream(in,dc);
        
        if("UTF-8".equals(uin.getEncoding())){
         uin.close();
         return "UTF-8";
        }
        uin.close();
        
        byte[] head = new byte[3];  
        in.read(head);    
        String code = "GBK";  
        if (head[0] == -1 && head[1] == -2 )  
            code = "UTF-16";  
        if (head[0] == -2 && head[1] == -1 )  
            code = "Unicode";
        //帶BOM
        if(head[0]==-17 && head[1]==-69 && head[2] ==-65)  
            code = "UTF-8";  
        if("Unicode".equals(code)){
         code = "UTF-16";
        }
        return code;
 }