【オススメあり】Javaでファイル操作する方法の徹底比較 (JDK標準 / Apache Commons / Google Guava)

public static void jdk1_0Style(String srcPath, String destPath) throws IOException {
    File srcFile = new File(srcPath);
    File destFile = new File(destPath);

    BufferedReader reader = null;
    OutputStream writer = null;
    try {
        reader = new BufferedReader(new InputStreamReader(new FileInputStream(srcFile), StandardCharsets.UTF_8));
        writer = new BufferedOutputStream(new FileOutputStream(destFile));

        String line;
        while ((line = reader.readLine()) != null) { // 1行読み出し
            // プラットフォームに応じた改行文字を付加
            line += System.getProperty("line.separator");
            // Shift-JIS binary 取得
            byte[] binary = line.getBytes(Charset.forName("SJIS"));
            // ファイルに書き込み
            writer.write(binary, 0, binary.length);
        }
    } finally {
        if (writer != null) {
            writer.close();
        }
        if (reader != null) {
            reader.close();
        }
    }
}

public static void jdk1_7Style(String srcPath, String destPath) throws IOException {
    Path srcFile = Paths.get(srcPath);
    Path destFile = Paths.get(destPath);

    try (BufferedReader br = Files.newBufferedReader(srcFile, StandardCharsets.UTF_8)) {
        try (BufferedWriter bw = Files.newBufferedWriter(destFile, Charset.forName("SJIS"))) {
            String line;
            while ((line = br.readLine()) != null) {
                // ファイルに書き込み
                bw.write(line + System.lineSeparator());
            }
        }
    }
}

public static void jdk1_8Style(String srcPath, String destPath) throws IOException {
    Path srcFile = Paths.get(srcPath);
    Path destFile = Paths.get(destPath);

    try (BufferedReader br = Files.newBufferedReader(srcFile, StandardCharsets.UTF_8)) {
        try (BufferedWriter bw = Files.newBufferedWriter(destFile, Charset.forName("SJIS"))) {
            br.lines()
                    .map(s -> s + System.lineSeparator())
//                  .forEach(bw::write) // 例外が出なければこれで良いのだが
                    .forEach(s -> {
                        try {
                            // ファイルに書き込み
                            bw.write(s);
                        } catch (IOException e) {
                            // 例外処理をしないといけないのがイマイチ
                            e.printStackTrace();
                        }
                    });
        }
    }
}

public static void apacheCommonsStyle(String srcPath, String destPath) throws IOException {
    File srcFile = new File(srcPath);
    File destFile = new File(destPath);

    // LineIterator を使う場合
    try (LineIterator it = FileUtils.lineIterator(srcFile, "UTF-8")) {
        try (FileOutputStream out = FileUtils.openOutputStream(destFile)) {
            while (it.hasNext()) {
                String line = it.nextLine() + System.lineSeparator();
                // ファイルに書き込み
                out.write(line.getBytes(Charset.forName("SJIS")));

                // FileUtils.write(destFile, line, "SJIS", true); // 毎回ファイルオープンするので非常に低速
            }
        }
    }

    // BufferedReader / BufferedOutputStream を使う場合 (apache commons)
    try (BufferedReader br = new BufferedReader(new InputStreamReader(FileUtils.openInputStream(srcFile), StandardCharsets.UTF_8))) {
        try (BufferedOutputStream out = new BufferedOutputStream(FileUtils.openOutputStream(destFile))) {
            String line;
            while ((line = br.readLine()) != null) {
                line += System.lineSeparator();
                // ファイルに書き込み
                out.write(line.getBytes(Charset.forName("SJIS")));
            }
        }
    }
}

public static void googleGuavaStyle(String srcPath, String destPath) throws IOException {
    File srcFile = new File(srcPath);
    File destFile = new File(destPath);

    // Source file
    CharSource charSource = Files.asCharSource(srcFile, StandardCharsets.UTF_8);
    // Dest file
    CharSink charSink = Files.asCharSink(destFile, Charset.forName("SJIS"));
    // ファイルの中身をコピーする
    charSource.copyTo(charSink);
}