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Android 中，格式化文件大小（Formatter.formatFileSize），Android8 之前用 1024 为单位，之后采用 1000 为单位。

Formatter 的源码解析

查看 Android 28（Android P）源码

public static BytesResult formatBytes(Resources res, long sizeBytes, int flags) {
        final int unit = ((flags & FLAG_IEC_UNITS) != 0) ? 1024 : 1000;
        final boolean isNegative = (sizeBytes < 0);
        float result = isNegative ? -sizeBytes : sizeBytes;
        int suffix = com.android.internal.R.string.byteShort;
        long mult = 1;
        if (result > 900) {
            suffix = com.android.internal.R.string.kilobyteShort;
            mult = unit;
            result = result / unit;
        }
        ......
     }

formatFileSize 方法源码如下：

 /* <p>As of O, the prefixes are used in their standard meanings in the SI system, so kB = 1000
     * bytes, MB = 1,000,000 bytes, etc.</p>
     *
     * <p class="note">In {@link android.os.Build.VERSION_CODES#N} and earlier, powers of 1024 are
     * used instead, with KB = 1024 bytes, MB = 1,048,576 bytes, etc.</p>
*/
public static String formatFileSize(@Nullable Context context, long sizeBytes) {
        if (context == null) {
            return "";
        }
        final BytesResult res = formatBytes(context.getResources(), sizeBytes, FLAG_SI_UNITS);
        return bidiWrap(context, context.getString(com.android.internal.R.string.fileSizeSuffix,
                res.value, res.units));
    }

意思就是在 Android 7 之后单位就变了,使用标准的单位制含义，即国际单位制，就像 1km = 1000 byte 一样;
在 Android 7 及更早的版本是 1024，即 1k = 1024B，这里就不贴代码了，感兴趣的同学可以去看源码。

解决方案

有两个方案，一个是反射设置 FLAG_SI_UNITS 的值，使得与 FLAG_IEC_UNITS 相与不为 0 ，
另外一个方案重新封装一个类，如下,用法与 Android 提供的一样

public class Formatter {
    /** * get file format size * * @param context context * @param roundedBytes file size * @return file format size (like 2.12k) */
    public static String formatFileSize(Context context, long roundedBytes) {
        return formatFileSize(context, roundedBytes, false, Locale.US);
    }

    public static String formatFileSize(Context context, long roundedBytes, Locale locale) {
        return formatFileSize(context, roundedBytes, false, locale);
    }


    private static String formatFileSize(Context context, long roundedBytes, boolean shorter, Locale locale) {
        if (context == null) {
            return "";
        }
        float result = roundedBytes;
        String suffix = "B";
        if (result > 900) {
            suffix = "KB";
            result = result / 1024;
        }
        if (result > 900) {
            suffix = "MB";
            result = result / 1024;
        }
        if (result > 900) {
            suffix = "GB";
            result = result / 1024;
        }
        if (result > 900) {
            suffix = "TB";
            result = result / 1024;
        }
        if (result > 900) {
            suffix = "PB";
            result = result / 1024;
        }
        String value;
        if (result < 1) {
            value = String.format(locale, "%.2f", result);
        } else if (result < 10) {
            if (shorter) {
                value = String.format(locale, "%.1f", result);
            } else {
                value = String.format(locale, "%.2f", result);
            }
        } else if (result < 100) {
            if (shorter) {
                value = String.format(locale, "%.0f", result);
            } else {
                value = String.format(locale, "%.2f", result);
            }
        } else {
            value = String.format(locale, "%.0f", result);
        }
        return String.format("%s%s", value, suffix);
    }

}