javaIO之随机读写

javaIO包提供了很多可以读写文件的类，但是如果想在文件的指定位置读写，就需要使用RandomAccessFile

public class App {
    public static void main(String[] args) throws IOException {
        {
            String s1 = "ggg\n";
            String s2 = "ggg,hhh\n";
            String fileName = "C:\\Users\\G007112\\Desktop\\test123.txt";
//            update(fileName, 6, s1, s2);
            append(fileName, "zzzzz");

        }
    }

    /**
     * 在文件指定位置插入内容
     *
     * @param fileName
     * @param pos
     * @param s
     * @throws IOException
     */
    public static void insert(String fileName, int pos, String s) throws IOException {
        File tmp = File.createTempFile("tmp", null);
        try {
            File file = new File(fileName);
            tmp.deleteOnExit();
            try (RandomAccessFile rw = new RandomAccessFile(file, "rw")) {
                try (FileOutputStream tmpOut = new FileOutputStream(tmp);
                     FileInputStream tmpIn = new FileInputStream(tmp)) {
                    rw.seek(pos);
                    byte[] buffer = new byte[64];
                    //读取指针之后的所有内容
                    int hasRead = 0;
                    while ((hasRead = rw.read(buffer)) > 0) {
                        tmpOut.write(buffer, 0, hasRead);
                    }
                    //重新定位指针
                    rw.seek(pos);
                    rw.write(s.getBytes(StandardCharsets.UTF_8));
                    while ((hasRead = tmpIn.read(buffer)) > 0) {
                        rw.write(buffer, 0, hasRead);
                    }
                }
            }

        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }

    /**
     * 在文件指定位置替换内容
     *
     * @param fileName
     * @param pos
     * @param s0       原字符串
     * @param s1       目标字符串
     * @throws IOException
     */
    public static void update(String fileName, int pos, String s0, String s1) throws IOException {
        File tmp = File.createTempFile("tmp", null);
        try {
            File file = new File(fileName);
            tmp.deleteOnExit();
            try (RandomAccessFile rw = new RandomAccessFile(file, "rw")) {
                try (FileOutputStream tmpOut = new FileOutputStream(tmp);
                     FileInputStream tmpIn = new FileInputStream(tmp)) {
                    rw.seek(pos + s0.length());
                    byte[] buffer = new byte[64];
                    //读取指针之后的所有内容
                    int hasRead = 0;
                    //先把s1和s0之后的内容缓存
                    tmpOut.write(s1.getBytes(StandardCharsets.UTF_8), 0, s1.length());
                    while ((hasRead = rw.read(buffer)) > 0) {
                        tmpOut.write(buffer, 0, hasRead);
                    }
                    //重新定位指针
                    rw.seek(pos);
                    while ((hasRead = tmpIn.read(buffer)) > 0) {
                        rw.write(buffer, 0, hasRead);
                    }
                }
            }

        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }

    /**
     * 在文件末尾追加数据
     *
     * @param fileName
     * @param s
     */
    public static void append(String fileName, String s) {
        File file = new File(fileName);
        try (RandomAccessFile rf = new RandomAccessFile(file, "rw")) {
            long length = file.length();
            rf.seek(length);
            rf.write(s.getBytes(StandardCharsets.UTF_8));
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }
}