你在用什么 Files.readString() ? 我建议使用扫描仪或类似设备逐行扫描：

Scanner in = new Scanner(new File(filepath)); // be sure to catch or throw the exception
for (int lineNum = 1; in.hasNextLine(); lineNum++) {
    String line = in.nextLine();
    int index = line.indexOf(s2);
    if (index >= 0) {
        System.out.println("Match found on line " + lineNum + ", column " + (index + 1));
    }
}

但是，如果您需要将其存储为一大块文本，也许您可以利用这个版本的indexof，它为搜索提供一个起始索引。

for (int startIndex = 0, endIndex = 0, lineNum = 1; true; lineNum++) {

    endIndex = s1.indexOf("\n", startIndex);
    String line;
    if (endIndex < 0) {
        // No newline found. Continue to the end of the string.
        line = s1.substring(startIndex);
    } else {
        line = s1.substring(startIndex, endIndex);
    }

    int matchIndex = line.indexOf(s2);
    if (matchIndex >= 0) {
        System.out.println("Match found on line " + lineNum + ", column " + (matchIndex + 1));
    }

    if (endIndex < 0) {
        break;
    }

    startIndex = endIndex + 1;
}