Java 文件讀寫全攻略：流分類、核心 API、高性能模式與避坑指南

Java IO 流體系總覽

                    Object
                     ↑
            ┌────────┴────────┐
            │                 │
        InputStream       OutputStream      (字節流，處理二進制：圖片、視頻、音頻)
            │                 │
    ┌───────┴───────┐ ┌─────┴─────┐
    │               │ │           │
FileInputStream  ...  FileOutputStream ...

            │                 │
            ▼                 ▼
        Reader            Writer          (字符流，處理文本：TXT、JSON、CSV、XML)
            │                 │
    ┌───────┴───────┐ ┌─────┴─────┐
    │               │ │           │
FileReader       ...  FileWriter ...

            │                 │
            ▼                 ▼
    BufferedReader    BufferedWriter  (緩衝裝飾器，提升性能，按行讀寫)
            │                 │
            ▼                 ▼
    InputStreamReader OutputStreamWriter (字節↔字符橋梁，指定編碼)

讀文件四大核心模式

1. 字節流讀取 (二進制文件)

public static byte[] readBytes(String path) throws IOException {
    try (FileInputStream fis = new FileInputStream(path);
         ByteArrayOutputStream baos = new ByteArrayOutputStream()) {
        byte[] buffer = new byte[8192];
        int len;
        while ((len = fis.read(buffer)) != -1) {
            baos.write(buffer, 0, len);
        }
        return baos.toByteArray();
    }
}

2. 字符流讀取 (文本文件，指定編碼)

public static String readText(String path, Charset charset) throws IOException {
    try (InputStreamReader isr = new InputStreamReader(
            new FileInputStream(path), charset);
         BufferedReader br = new BufferedReader(isr)) {
        
        StringBuilder sb = new StringBuilder();
        String line;
        while ((line = br.readLine()) != null) {
            sb.append(line).append(System.lineSeparator());
        }
        return sb.toString();
    }
}

3. 按行讀取 (最常用，內存友好)

// Java 7+ try-with-resources
public static void processLines(String path, Charset charset, Consumer processor) 
        throws IOException {
    try (BufferedReader br = new BufferedReader(
            new InputStreamReader(new FileInputStream(path), charset))) {
        String line;
        while ((line = br.readLine()) != null) {
            processor.accept(line);
        }
    }
}

// Java 8+ Stream API (自動關閉)
public static List readAllLines(String path, Charset charset) throws IOException {
    try (Stream lines = Files.lines(Paths.get(path), charset)) {
        return lines.collect(Collectors.toList());
    }
}

4. 隨機訪問 (大文件斷點續傳、分片讀取)

public static byte[] readRandom(String path, long offset, int length) throws IOException {
    try (RandomAccessFile raf = new RandomAccessFile(path, "r")) {
        raf.seek(offset);
        byte[] buf = new byte[length];
        int read = raf.read(buf);
        return Arrays.copyOf(buf, read);
    }
}

寫文件核心模式

1. 字節流寫入

public static void writeBytes(String path, byte[] data, boolean append) throws IOException {
    try (FileOutputStream fos = new FileOutputStream(path, append);
         BufferedOutputStream bos = new BufferedOutputStream(fos)) {
        bos.write(data);
    }
}

2. 字符流寫入 (指定編碼)

public static void writeText(String path, String content, Charset charset, boolean append) 
        throws IOException {
    try (OutputStreamWriter osw = new OutputStreamWriter(
            new FileOutputStream(path, append), charset);
         BufferedWriter bw = new BufferedWriter(osw)) {
        bw.write(content);
    }
}

3. 便捷工具類 (Java 7+ NIO.2 Files)

// 一行代碼讀寫 (小文件)
byte[] data = Files.readAllBytes(Paths.get(path));
String text = Files.readString(Paths.get(path), StandardCharsets.UTF_8);
Files.writeString(Paths.get(path), text, StandardCharsets.UTF_8, 
    StandardOpenOption.CREATE, StandardOpenOption.APPEND);
Files.write(Paths.get(path), data, StandardOpenOption.CREATE, StandardOpenOption.APPEND);

// 大文件流式
try (InputStream in = Files.newInputStream(Paths.get(src));
     OutputStream out = Files.newOutputStream(Paths.get(dest), 
         StandardOpenOption.CREATE, StandardOpenOption.APPEND)) {
    in.transferTo(out); // Java 9+ 零拷貝
}

性能對比與選型建議

編碼處理鐵律

1. 顯式指定 Charset：StandardCharsets.UTF_8，永不依賴平台預設編碼
2. InputStreamReader/OutputStreamWriter 是字節↔字符唯一橋梁
3. 屬性文件 Properties.load JDK 11+ 支持 UTF-8，舊版需自行轉碼
4. BOM 頭處理：BOMInputStream (Apache Commons IO) 自動檢測並剝離

資源管理鐵律

// ✅ Java 7+ try-with-resources (自動關閉，異常安全)
try (Resource r = acquire()) { use(r); }

// ✅ Java 8+ Stream 結束自動關閉
try (Stream s = Files.lines(path)) { ... }

// ❌ 手動 close 易漏、異常路徑不關閉
Resource r = acquire();
try { use(r); } finally { if (r != null) r.close(); }

常見坑點避雷

現代化替代方案

• Apache Commons IO：FileUtils.readFileToString、IOUtils.copy、Tailer 尾部監聽
• Google Guava：Files.asCharSource(path, UTF_8).readLines()、ByteStreams
• Okio：Okio.buffer(Okio.source(file)).readUtf8()、現代 Kotlin/Java 首選
• Netty ByteBuf：高性能網絡編程場景

總結

Java 文件讀寫核心在於「字節流處理二進制，字符流處理文本，緩衝流提升性能，編碼顯式指定，資源自動管理」。掌握 Files (NIO.2)、BufferedReader/Writer、InputStreamReader/OutputStreamWriter 三大組合，覆蓋 95% 業務場景。大文件必用流式處理，小文件享受 Files.readString 便利。遵循「零信任編碼、try-with-resources 管理資源、零拷貝傳輸大文件」三大鐵律，構建健壯、高性能、跨平台的文件處理代碼。