方法一：使用Apache的common-io

1.maven專案pom.xml
    <dependency>
        <groupId>commons-io</groupId>
        <artifactId>commons-io</artifactId>
        <version>2.5</version>
    </dependency>

2.程式碼

import java.io.File;
import java.io.IOException;
import java.util.Collection;
 

import org.apache.commons.io.FileUtils;

public class Main {
public static void main(String[] args) {
    //GBK編碼格式原始碼路徑 
    String srcDirPath = "D:\\ENews\\src"; 
    //轉為UTF-8編碼格式原始碼路徑 
    String utf8DirPath = "D:\\ENews\\UTF8\\src"; 
    //獲取所有java檔案 
    Collection<File> javaGbkFileCol =  FileUtils.listFiles(new
 
 File(srcDirPath), new String[]{"java"}, true); 
        for (File javaGbkFile : javaGbkFileCol) { 
              //UTF8格式檔案路徑 
              String utf8FilePath = utf8DirPath+javaGbkFile.getAbsolutePath().substring(srcDirPath.length()); 
               //使用GBK讀取資料，然後用UTF-8寫入資料 
              try {
                FileUtils.writeLines(new
 
 File(utf8FilePath), "UTF-8", FileUtils.readLines(javaGbkFile, "GBK"));
            } catch (IOException e) {
                e.printStackTrace();
            }        
        }
    }
}

方法二：

import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.io.UnsupportedEncodingException;
import java.io.Writer;
import java.util.ArrayList;
import java.util.List;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class ConverEncoding {

   public static void main(String[] args) throws Exception {
      String srcDir = "E:\\project\\ReadFile\\src";
      List<String> files = new ArrayList<String>();
      fetchFileList(srcDir, files, ".java");
      for(String fileName : files){
         convert(fileName, "GBK", fileName, "UTF-8");
      }
   }
   public static void convert(String oldFile, String oldCharset, String newFlie, String newCharset){
      BufferedReader bin;
      FileOutputStream fos;
      StringBuffer content = new StringBuffer();
      try {
         System.out.println(oldFile);
         bin = new BufferedReader(new InputStreamReader(new FileInputStream(oldFile), "gbk"));
         String line = null;
         while((line=bin.readLine())!=null){
            content.append(line);
            content.append(System.getProperty("line.separator"));
         }
         bin.close();
         File dir = new File(newFlie.substring(0, newFlie.lastIndexOf("\\")));
         if(!dir.exists()){
            dir.mkdirs();
         }
         fos = new FileOutputStream(newFlie);
         Writer out = new OutputStreamWriter(fos, newCharset);
         out.write(content.toString());
         out.close();
         fos.close();
      } catch (UnsupportedEncodingException e) {
         e.printStackTrace();
      } catch (FileNotFoundException e) {
         e.printStackTrace();
      } catch (IOException e) {
         e.printStackTrace();
      }
   }
   public static void fetchFileList(String strPath, List<String> filelist, final String regex) {
      File dir = new File(strPath);
        File[] files = dir.listFiles();
        Pattern p = Pattern.compile(regex);
        if (files == null)
            return;
        for (int i = 0; i < files.length; i++) {
            if (files[i].isDirectory()) {
               fetchFileList(files[i].getAbsolutePath(), filelist, regex);
            } else {
                String strFileName = files[i].getAbsolutePath().toLowerCase();
                Matcher m = p.matcher(strFileName);
                if(m.find()){
                  filelist.add(strFileName);
                }
            }
        }
    }
}