Java 讀取檔案顯示進度條：ProgressMonitorInputStream 完整實作

核心類別

• javax.swing.ProgressMonitorInputStream：包裝 InputStream，自動彈出進度對話框。
• javax.swing.ProgressMonitor：可自訂訊息、取消按鈕。

最小可行範例

import javax.swing.*;
import java.io.*;

public class FileProgressDemo {
    public static void main(String[] args) throws IOException {
        File file = new File("large.dat");
        try (ProgressMonitorInputStream pmis = new ProgressMonitorInputStream(
                null, "讀取中: " + file.getName(), new FileInputStream(file))) {

            byte[] buffer = new byte[8192];
            int read;
            while ((read = pmis.read(buffer)) != -1) {
                // 處理資料...
                // pmis.getProgressMonitor().setNote("已讀 " + pmis.getProgressMonitor().getProgress() + " bytes");
            }
        }
    }
}

進階：自訂進度更新邏輯

ProgressMonitorInputStream pmis = new ProgressMonitorInputStream(null, "載入...", new FileInputStream(file));
ProgressMonitor monitor = pmis.getProgressMonitor();
monitor.setMillisToDecideToPopup(0);   // 立即顯示
monitor.setMillisToPopup(0);

long total = file.length();
long readSoFar = 0;
byte[] buf = new byte[4096];
int n;
while ((n = pmis.read(buf)) != -1) {
    readSoFar += n;
    int percent = (int) (readSoFar * 100 / total);
    monitor.setProgress((int) readSoFar);
    monitor.setNote(String.format("已讀 %d%% (%d/%d bytes)", percent, readSoFar, total));
    if (monitor.isCanceled()) throw new IOException("使用者取消");
}

關鍵 API

非 Swing 環境替代方案

• Console：System.out.print("\r進度: " + percent + "%") 配合 \r 回車。
• Log/檔案：定期寫入進度日誌。
• Web：前端輪詢 / WebSocket 推送進度。