Java高级文件操作之随机访问文件完整教学

import java.nio.file.*;
import java.nio.channels.FileChannel;
import java.nio.ByteBuffer;
import java.io.IOException;

import static java.nio.file.StandardOpenOption.*;

public class RandomAccessDemo {
    public static void main(String[] args) {
        Path file = Paths.get("file.txt");

        String s = "I was here!\n";
        byte[] data = s.getBytes();
        ByteBuffer out = ByteBuffer.wrap(data);       // 包装要写入的数据
        ByteBuffer copy = ByteBuffer.allocate(12);    // 用于存储文件开头的 12 字节

        try (FileChannel fc = FileChannel.open(file, READ, WRITE)) {

            // 1️⃣ 读取文件前 12 个字节
            int nread;
            do {
                nread = fc.read(copy);
            } while (nread != -1 && copy.hasRemaining());

            // 2️⃣ 写入 "I was here!" 到文件开头
            fc.position(0);  // 跳到文件开头
            while (out.hasRemaining()) {
                fc.write(out);
            }
            out.rewind();  // 复位缓冲区，用于后续写入

            // 3️⃣ 移动到文件末尾，并复制前 12 个字节到末尾
            long length = fc.size();
            fc.position(length); // 文件末尾
            copy.flip();         // 切换为读模式
            while (copy.hasRemaining()) {
                fc.write(copy);
            }

            // 4️⃣ 如果需要再次写入 "I was here!" 到文件末尾，需要指定out.rewind();
            while (out.hasRemaining()) {
                fc.write(out);
            }

            System.out.println("文件操作完成！");
        } catch (IOException e) {
            System.err.println("I/O 异常: " + e.getMessage());
        }
    }
}

import java.io.RandomAccessFile;
import java.io.IOException;
public class RandomAccessFileDemo {
    public static void main(String[] args) {
        String filePath = "employee.dat";
        try (RandomAccessFile raf = new RandomAccessFile(filePath, "rw")) {
            // 定义记录结构：ID（4字节）+ 姓名（UTF-8，最大20字节）+ 年龄（4字节）+ 工资（8字节）
            // 每条记录固定长度 = 4 + 20 + 4 + 8 = 36 字节（简化，实际UTF-8长度可变）
            // 为简化，这里用固定长度的字符串（20个字符）
            // 写入记录1
            raf.seek(0);
            raf.writeInt(1001);
            raf.writeUTF("张三");    // 实际长度不定，此处简化
            // 我们使用固定长度写入
            raf.writeInt(28);
            raf.writeDouble(5000.0);
            // 写入记录2
            raf.seek(36); // 跳到第二条记录
            raf.writeInt(1002);
            raf.writeUTF("李四");
            raf.writeInt(35);
            raf.writeDouble(6200.0);
            // 读取记录2（从开头偏移36字节）
            raf.seek(36);
            int id = raf.readInt();
            String name = raf.readUTF();
            int age = raf.readInt();
            double salary = raf.readDouble();
            System.out.printf("ID: %d, 姓名: %s, 年龄: %d, 工资: %.2f%n", id, name, age, salary);
            // 更新记录1的工资
            raf.seek(0 + 4 + 20 + 4); // 跳过ID(4) + 姓名(20) + 年龄(4)
            raf.writeDouble(5500.0);
            // 再次读取记录1验证
            raf.seek(0);
            id = raf.readInt();
            name = raf.readUTF();
            age = raf.readInt();
            salary = raf.readDouble();
            System.out.printf("更新后: ID: %d, 姓名: %s, 年龄: %d, 工资: %.2f%n", id, name, age, salary);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

import java.io.RandomAccessFile;
import java.io.IOException;
import java.nio.charset.StandardCharsets;
public class FixedLengthRecord {
    private static final int ID_SIZE = 4;
    private static final int NAME_SIZE = 20; // 字节数
    private static final int AGE_SIZE = 4;
    private static final int SALARY_SIZE = 8;
    private static final int RECORD_SIZE = ID_SIZE + NAME_SIZE + AGE_SIZE + SALARY_SIZE;
    // 写入记录
    public static void writeRecord(RandomAccessFile raf, int id, String name, int age, double salary) throws IOException {
        raf.writeInt(id);
        byte[] nameBytes = name.getBytes(StandardCharsets.UTF_8);
        byte[] nameBuf = new byte[NAME_SIZE];
        System.arraycopy(nameBytes, 0, nameBuf, 0, Math.min(nameBytes.length, NAME_SIZE));
        raf.write(nameBuf);
        raf.writeInt(age);
        raf.writeDouble(salary);
    }
    // 读取记录
    public static void readRecord(RandomAccessFile raf, int recordNum) throws IOException {
        raf.seek((long) recordNum * RECORD_SIZE);
        int id = raf.readInt();
        byte[] nameBuf = new byte[NAME_SIZE];
        raf.readFully(nameBuf);
        String name = new String(nameBuf, StandardCharsets.UTF_8).trim();
        int age = raf.readInt();
        double salary = raf.readDouble();
        System.out.printf("ID: %d, 姓名: %s, 年龄: %d, 工资: %.2f%n", id, name, age, salary);
    }
    public static void main(String[] args) throws IOException {
        try (RandomAccessFile raf = new RandomAccessFile("fixed.dat", "rw")) {
            // 写入三条记录
            writeRecord(raf, 1, "张三", 25, 5000);
            writeRecord(raf, 2, "李四", 30, 6000);
            writeRecord(raf, 3, "王五", 28, 5500);
            // 读取第二条记录
            readRecord(raf, 1); // 索引从0开始
            // 更新第二条记录的工资
            raf.seek(1 * RECORD_SIZE + ID_SIZE + NAME_SIZE + AGE_SIZE);
            raf.writeDouble(6800.0);
            System.out.println("更新后：");
            readRecord(raf, 1);
        }
    }
}

import java.io.RandomAccessFile;
import java.io.IOException;
public class DownloadStatus {
    private static final String STATUS_FILE = "download.status";
    // 保存已下载的字节数
    public static void saveProgress(long bytes) throws IOException {
        try (RandomAccessFile raf = new RandomAccessFile(STATUS_FILE, "rw")) {
            raf.setLength(0); // 清空
            raf.writeLong(bytes);
        }
    }
    // 读取已下载的字节数
    public static long loadProgress() throws IOException {
        try (RandomAccessFile raf = new RandomAccessFile(STATUS_FILE, "r")) {
            if (raf.length() < 8) {
                return 0;
            }
            return raf.readLong();
        }
    }
    public static void main(String[] args) throws IOException {
        long progress = loadProgress();
        System.out.println("上次下载进度: " + progress + " 字节");
        // 模拟下载，每10秒更新一次进度
        for (long i = progress; i < 1024 * 1024 * 10; i += 1024 * 10) {
            // 模拟下载
            saveProgress(i);
            System.out.println("当前进度: " + i);
            try { Thread.sleep(100); } catch (InterruptedException e) {}
        }
        System.out.println("下载完成！");
    }
}