Android 4.4.4在Samsung S4上保存到SD

Question

I'm attempting to save files to my SD but i cannot get it to, I even tried moving the app to the SD to see if I can. I don't really care where it ends up on there but this isn;t working:

    String state = Environment.getExternalStorageState();
    File filesDir;

    if (Environment.MEDIA_MOUNTED.equals(state)) {
        // We can read and write the media
        filesDir = getExternalFilesDir(null);
        Log.i(Utils.TAG, "We can read and write the media: " + filesDir.getAbsolutePath()); // This is the local on the phone
    } else {
        // Load another directory, probably local memory
        filesDir = getFilesDir();
        Log.i(Utils.TAG, "Load another directory, probably local memory: " + filesDir.getAbsolutePath());
    }

    try {
       // Creates a trace file in the primary external storage space of the 
       // current application.
       // If the file does not exists, it is created.
       //File traceFile = new File(((Context)this).getExternalFilesDir(null), "TraceFile.txt"); //This one saves to the internal file folder
        File traceFile = new File(filesDir, "TraceFile.txt");

        Log.i(Utils.TAG, traceFile.getAbsolutePath());

       if (!traceFile.exists())
          traceFile.createNewFile();
                                // Adds a line to the trace file
       BufferedWriter writer = new BufferedWriter(new FileWriter(traceFile, true /*append*/));
       writer.write("This is a test trace file.");
       writer.close();
                               // Refresh the data so it can seen when the device is plugged in a
                               // computer. You may have to unplug and replug the device to see the 
                               // latest changes. This is not necessary if the user should not modify
                               // the files.
        MediaScannerConnection.scanFile((Context)(this),
                                         new String[] { traceFile.toString() },
                                         null,
                                         null);

    } catch (IOException e) {
        Log.i(Utils.TAG, "Unable to write to the TraceFile.txt file.");
    }

However, this gave me the SD file but I couldn't write to it:

public HashSet<String> getExternalMounts() {
    final HashSet<String> out = new HashSet<String>();
    String reg = "(?i).*vold.*(vfat|ntfs|exfat|fat32|ext3|ext4).*rw.*";
    String s = "";
    try {
        final Process process = new ProcessBuilder().command("mount")
                .redirectErrorStream(true).start();
        process.waitFor();
        final InputStream is = process.getInputStream();
        final byte[] buffer = new byte[1024];
        while (is.read(buffer) != -1) {
            s = s + new String(buffer);
        }
        is.close();
    } catch (final Exception e) {
        e.printStackTrace();
    }

    // parse output
    final String[] lines = s.split("\n");
    for (String line : lines) {
        if (!line.toLowerCase(Locale.US).contains("asec")) {
            if (line.matches(reg)) {
                String[] parts = line.split(" ");
                for (String part : parts) {
                    if (part.startsWith("/"))
                        if (!part.toLowerCase(Locale.US).contains("vold"))
                            out.add(part);
                }
            }
        }
    }
    return out;
}

Answer 1

Android 4.4 implemented numerous SD card limits, and there were (and still are) a lot of apps which broke during this period, because of issues with write limitations to the SD card. Your code itself seems fine to me, but my believe is that it's Android 4.4's SD Card limits which ensure that it doesn't work. How to fix that (without root access) is beyond me.

Answer 2

Android 4.4+ allows you to write to removable media, but only in select spots obtained by methods like:

• getExternalFilesDirs()
• getExternalCacheDirs()
• getExternalMediaDirs() (this one was added in Android 5.0 IIRC)

Note the plural on those method names. If they return 2+ entries, the second and subsequent ones should be on removable media, in a directory that is unique for your app. You can read and write to those directories without any <uses-permission> element.

You might also consider the Storage Access Framework ( ACTION_OPEN_DOCUMENT and kin), to allow the user to choose where to place the file, whether that be on external storage or removable storage or Google Drive or whatever.