Android如何将文件保存到SD卡指定自定义文件夹而非应用专属目录

Got it, let's break down how to save files directly to a custom folder like /storage/9016-4EF8/MyFolder on your SD card—since the default getExternalFilesDir() locks you into the app-specific Android/data directory, we need to work within Android's storage rules based on the OS version.

Android 9 (API 28) and Below #

On older Android versions, you can directly access the SD card root directory with standard storage permissions:

1. Add permissions to your Manifest:

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />

2. Request dynamic permissions (for API 23+):
   You'll need to prompt the user to grant storage access at runtime (since Android 6.0, dangerous permissions require explicit user approval).
3. Create the custom folder and write files:

// First, check if the SD card is mounted and writable
String sdCardPath = Environment.getExternalStorageDirectory().getAbsolutePath();
File myFolder = new File(sdCardPath, "MyFolder");

// Create the folder if it doesn't exist
if (!myFolder.exists()) {
    boolean folderCreated = myFolder.mkdirs();
    if (!folderCreated) {
        // Handle folder creation failure (e.g., log error, show toast)
        return;
    }
}

// Create and write to your target file
File targetFile = new File(myFolder, "my_file.txt");
try (FileOutputStream fos = new FileOutputStream(targetFile)) {
    fos.write("Your content here".getBytes());
} catch (IOException e) {
    e.printStackTrace();
    // Handle write error
}

Android 10 (API 29) #

Android 10 introduced Scoped Storage, which restricts direct access to non-app-specific directories. You have two options here:

Option 1: Temporary Legacy Compatibility (Not Recommended Long-Term) #

If your app targets API 28 or lower, you can add this to your AndroidManifest.xml to bypass Scoped Storage temporarily:

<application
    ...
    android:requestLegacyExternalStorage="true">
</application>

This lets you use the Android 9 code above, but it's not a permanent solution—Google will phase out this flag for newer API levels.

Option 2: Use Storage Access Framework (SAF) (Recommended) #

SAF lets users manually select the target folder, which complies with Scoped Storage rules:

1. Launch the folder picker:

private static final int REQUEST_PICK_FOLDER = 100;

private void openCustomFolderPicker() {
    Intent intent = new Intent(Intent.ACTION_OPEN_DOCUMENT_TREE);
    // Grant persistent read/write permissions to the selected folder
    intent.addFlags(Intent.FLAG_GRANT_READ_URI_PERMISSION);
    intent.addFlags(Intent.FLAG_GRANT_WRITE_URI_PERMISSION);
    intent.addFlags(Intent.FLAG_GRANT_PERSISTABLE_URI_PERMISSION);
    startActivityForResult(intent, REQUEST_PICK_FOLDER);
}

2. Handle the selected folder and write files:

@Override
protected void onActivityResult(int requestCode, int resultCode, @Nullable Intent data) {
    super.onActivityResult(requestCode, resultCode, data);
    if (requestCode == REQUEST_PICK_FOLDER && resultCode == RESULT_OK && data != null) {
        Uri selectedFolderUri = data.getData();
        if (selectedFolderUri != null) {
            // Persist the permission so you don't need to re-select the folder later
            getContentResolver().takePersistableUriPermission(
                selectedFolderUri,
                Intent.FLAG_GRANT_READ_URI_PERMISSION | Intent.FLAG_GRANT_WRITE_URI_PERMISSION
            );

            // Create "MyFolder" inside the selected directory (e.g., SD card root)
            DocumentFile rootFolder = DocumentFile.fromTreeUri(this, selectedFolderUri);
            DocumentFile myCustomFolder = rootFolder.findFile("MyFolder");
            if (myCustomFolder == null) {
                myCustomFolder = rootFolder.createDirectory("MyFolder");
            }

            // Create and write to your file
            DocumentFile targetFile = myCustomFolder.createFile("text/plain", "my_file.txt");
            if (targetFile != null) {
                try (OutputStream os = getContentResolver().openOutputStream(targetFile.getUri())) {
                    os.write("Your content here".getBytes());
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}

Android 11 (API 30) and Above #

Scoped Storage is enforced more strictly here. The only ways to access custom non-app directories are:

1. Storage Access Framework (SAF) (Most Recommended) #

Use the same SAF code as Android 10—this is the most compliant approach and doesn't require special permissions.

2. MANAGE_EXTERNAL_STORAGE Permission (For Specialized Apps Only) #

If your app is a file manager, backup tool, or another type that needs broad storage access, you can request this special permission:

1. Add to Manifest:

<uses-permission android:name="android.permission.MANAGE_EXTERNAL_STORAGE" />

2. Guide users to enable the permission:

Intent intent = new Intent(Settings.ACTION_MANAGE_APP_ALL_FILES_ACCESS_PERMISSION);
Uri appUri = Uri.fromParts("package", getPackageName(), null);
intent.setData(appUri);
startActivity(intent);

Important: Google Play has strict policies for this permission—only apps that meet specific use cases will be approved. Avoid using this unless absolutely necessary.

内容的提问来源于stack exchange，提问作者Young King