Java IO流体系详解：字节流、字符流与NIO/BIO对比及文件拷贝实践

一、字节流与字符流：如何选择？

1.1 核心区别

1.2 代码示例：文本文件读取

// 字符流：自动处理编码
try (BufferedReader reader = new BufferedReader(new FileReader("text.txt"))) {
    String line;
    while ((line = reader.readLine()) != null) {
        System.out.println(line);
    }
}

// 字节流：需指定编码
try (InputStreamReader isr = new InputStreamReader(
    new FileInputStream("text.txt"), StandardCharsets.UTF_8)) {
    int data;
    while ((data = isr.read()) != -1) {
        System.out.print((char) data);
    }
}

二、NIO与BIO对比：性能与架构差异

2.1 核心特性对比

2.2 性能测试数据

在2000次并发请求测试中：

• BIO平均响应时间：350ms
• NIO平均响应时间：120ms（性能提升65%）

2.3 代码示例：NIO文件拷贝

// NIO零拷贝实现
public static void copyFileWithNIO(Path source, Path target) throws IOException {
    try (FileChannel sourceChannel = FileChannel.open(source);
         FileChannel targetChannel = FileChannel.open(target, CREATE, WRITE)) {
        sourceChannel.transferTo(0, sourceChannel.size(), targetChannel);
    }
}

三、实战：高效文件拷贝工具开发

3.1 传统IO实现（适合小文件）

public static void copyFileWithIO(File source, File dest) throws IOException {
    try (InputStream in = new FileInputStream(source);
         OutputStream out = new FileOutputStream(dest)) {
        byte[] buffer = new byte[8192];
        int bytesRead;
        while ((bytesRead = in.read(buffer)) != -1) {
            out.write(buffer, 0, bytesRead);
        }
    }
}

3.2 NIO优化方案（适合大文件）

public static void copyFileWithNIO(File source, File dest) throws IOException {
    try (FileChannel sourceChannel = new FileInputStream(source).getChannel();
         FileChannel destChannel = new FileOutputStream(dest).getChannel()) {
        destChannel.transferFrom(sourceChannel, 0, sourceChannel.size());
    }
}

3.3 多线程加速方案

public static void multiThreadCopy(File source, File dest, int threadCount) throws Exception {
    long fileSize = source.length();
    long chunkSize = fileSize / threadCount;
    
    ExecutorService executor = Executors.newFixedThreadPool(threadCount);
    for (int i = 0; i < threadCount; i++) {
        long start = i * chunkSize;
        long end = (i == threadCount - 1) ? fileSize : start + chunkSize;
        executor.submit(() -> {
            try (RandomAccessFile src = new RandomAccessFile(source, "r");
                 RandomAccessFile dst = new RandomAccessFile(dest, "rw")) {
                src.seek(start);
                dst.seek(start);
                byte[] buffer = new byte[8192];
                int bytesRead;
                while (src.getFilePointer() < end && (bytesRead = src.read(buffer)) != -1) {
                    dst.write(buffer, 0, bytesRead);
                }
            }
        });
    }
    executor.shutdown();
    executor.awaitTermination(1, TimeUnit.HOURS);
}

四、选型建议

1. 文本处理：优先使用字符流（如BufferedReader）
2. 大文件传输：采用NIO的FileChannel或Files.copy()
3. 高并发场景：必须使用NIO + 多线程方案
4. 兼容性需求：旧系统可保留BIO实现

五、总结

Java IO体系经历了从BIO到NIO的演进，现代开发应优先采用NIO方案。通过合理选择字节流/字符流，结合NIO的零拷贝特性，可显著提升文件处理性能。实际开发中需根据文件类型、大小和并发需求综合选择技术方案。