byte[] bs = str.getBytes();
newstr = new String(bs, "UTF-8");

InputStreamReader read = new InputStreamReader(new FileInputStream(A.txt),"UTF-8"); 
        BufferedReader br = new BufferedReader(read);
        BufferedWriter bw = new BufferedWriter(new FileWriter(B.txt));

import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;

public class FileProcess {

    public static void main(String[] args) {
        FileInputStream fis = null;
        FileOutputStream fos = null;
        try{
            fis = new FileInputStream(new File("d:\\a.txt"));
            fos = new FileOutputStream(new File("d:\\b.txt"));
            byte[] buffer = new byte[fis.available()];
            fis.read(buffer);
            String str = new String(buffer, "GBK");
            fos.write(str.getBytes("utf-8"));
        }catch(Exception e){
            e.printStackTrace();
        }finally{
            if(fis != null){
                try{
                    fis.close();
                }catch(Exception e){
                    
                }
            }
            if(fos != null){
                try{
                    fos.close();
                }catch(Exception e){
                    
                }
            }
        }
    }
}

------解决方案--------------------
我试了一下，好像没有问题，你看看

Java code
import java.io.BufferedReader;
import java.io.Closeable;
import java.io.File;
import java.io.FileOutputStream;
import java.io.FileReader;
import java.io.IOException;
import java.io.OutputStream;
import java.io.OutputStreamWriter;

public class Test01 {
    
    public static void main(String[] args) {
        File in = new File("test.txt");
        File out = new File("test2.txt");
        BufferedReader br = null;
        OutputStreamWriter ow = null;        
        try {
            br = new BufferedReader(new FileReader(in));
            ow = new OutputStreamWriter(new FileOutputStream(out), "utf-8");
            for(String str = null, separator = System.getProperty("line.separator"); (str = br.readLine()) != null;) {
                ow.write(str);
                ow.write(separator);
            }
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            close(br);
            close(ow);
        }
        System.out.println("ok");
    }

    private static void close(Closeable stream) {
        try {
            stream.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}