Java 文本文件處理精要：Reader/Writer 體系深度解析

設計動機：彌補 byte 流的文本短板

早期 Java 僅提供 InputStream/OutputStream 字節流，處理文本需手動處理編碼轉換，極其繁瑣。引入 Reader/Writer 抽象類及其子類，確立「字節流 ↔ 字符流」橋梁模式，徹底簡化文本 I/O。

核心類層次與職責

讀取文本的三種範式

1. 基礎 FileReader（單字符，教學用）

FileReader fr = new FileReader("ming.txt");
int ch;
while ((ch = fr.read()) != -1) {
    System.out.print((char) ch);
}
fr.close();

2. 顯式編碼 InputStreamReader（生產必備）

InputStreamReader isr = new InputStreamReader(
    new FileInputStream("ming.txt"), "UTF-8");
int ch;
while ((ch = isr.read()) != -1) {
    System.out.print((char) ch);
}
isr.close();

3. 緩衝逐行 BufferedReader（高性能標準）

BufferedReader br = new BufferedReader(
    new InputStreamReader(new FileInputStream("ming.txt"), "UTF-8"));
String line;
while ((line = br.readLine()) != null) {
    System.out.println(line);
}
br.close();

寫入文本的三種範式

1. FileWriter 直接寫入

FileWriter fw = new FileWriter("hello.txt");
String s = "hello world";
fw.write(s, 0, s.length());
fw.flush(); // 強制刷新緩衝區
fw.close();

2. OutputStreamWriter 指定編碼

OutputStreamWriter osw = new OutputStreamWriter(
    new FileOutputStream("hello2.txt"), "UTF-8");
osw.write(s, 0, s.length());
osw.flush();
osw.close();

3. PrintWriter 格式化輸出（推薦）

// true = autoFlush，遇 println/printf/format 自動刷新
PrintWriter pw = new PrintWriter(
    new OutputStreamWriter(new FileOutputStream("hello3.txt"), "UTF-8"), true);
pw.println(s);
pw.close();

完整整合示例

測試文件 ming.txt 內容：

hello world i like java language

import java.io.*;

public class TestFile2 {
    public static void main(String[] args) throws IOException {
        // 1. FileReader 逐字符
        FileReader fr = new FileReader("ming.txt");
        int ch;
        while ((ch = fr.read()) != -1) System.out.print((char) ch);

        // 2. InputStreamReader 逐字符
        InputStreamReader isr = new InputStreamReader(new FileInputStream("ming.txt"));
        while ((ch = isr.read()) != -1) System.out.print((char) ch);

        // 3. BufferedReader 逐行
        BufferedReader br = new BufferedReader(new InputStreamReader(new FileInputStream("ming.txt")));
        String line;
        while ((line = br.readLine()) != null) System.out.println(line);

        // 寫入演示
        FileWriter fw = new FileWriter("hello.txt");
        String s = "hello world";
        fw.write(s, 0, s.length()); fw.flush();

        OutputStreamWriter osw = new OutputStreamWriter(new FileOutputStream("hello2.txt"));
        osw.write(s, 0, s.length()); osw.flush();

        PrintWriter pw = new PrintWriter(new OutputStreamWriter(new FileOutputStream("hello3.txt")), true);
        pw.println(s);

        // 統一關閉（實際建議 try-with-resources）
        fr.close(); isr.close(); br.close();
        fw.close(); osw.close(); pw.close();
    }
}

避坑指南

• 編碼顯式化：永遠不要依賴平台默認編碼，構造橋梁類時必須指定 StandardCharsets.UTF_8。
• 緩衝必加：直接用 FileReader/Writer 處理大文件性能極差，務必包裝 BufferedReader/Writer。
• flush 與 close：flush() 僅刷緩衝區，close() 會先 flush 再釋放文件句柄，二者不可互相替代。
• 資源管理：JDK 7+ 必須使用 try-with-resources，避免異常路徑導致洩漏。