偶然在看網友們在Google Play Store 裡的留言，發現有人說會crash, 他用的 Android 是 6.0 版，窮到爆的我，怎麼可能有錢去買裝置來測，還好前陣子下載了 Genymotion，正好解決 Android 6.0 遇到的問題。

實際 debug 後看到的錯誤訊息是：

java.io.IOException: open failed: ENOENT (No such file or directory)

在 AndroidManifest.xml 也有宣告許可權：

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

寫入檔案前，也有先create dirs：

final File dir = new File(root_path);
dir.mkdirs(); //create folders where write files

試到這裡，這樣還是不行。

建立檔案先，取出許可權看看：

int permissionCheck = ContextCompat.checkSelfPermission(getActivity(),
Manifest.permission.WRITE_EXTERNAL_STORAGE);

permissionCheck 結果是 true, 有許可權，為什麼還是不能寫入 @[email protected]

最後解法是用這一段 code，新的 Android 存取外部共用空間時，不能偷偷摸摸存取，要裝置主人的同意。

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        /* Request user permissions in runtime */
 

        ActivityCompat.requestPermissions(MainActivity.this,
            new String[] {
                    Manifest.permission.READ_EXTERNAL_STORAGE,
                    Manifest.permission.WRITE_EXTERNAL_STORAGE
            },
            100);
        /* Request user permissions in runtime */

        String storagePath = Environment.getExternalStorageDirectory().getAbsolutePath();
        String rootPath = storagePath + "/test";
        String fileName = "/test.zip";

        File root = new File(rootPath);
        if(!root.mkdirs()) {
            Log.i("Test", "This path is already exist: " + root.getAbsolutePath());
        }

        File file = new File(rootPath + fileName);
        try {
            int permissionCheck = ContextCompat.checkSelfPermission(MainActivity.this, android.Manifest.permission.WRITE_EXTERNAL_STORAGE);
            if (permissionCheck == PackageManager.PERMISSION_GRANTED) {
                if (!file.createNewFile()) {
                    Log.i("Test", "This file is already exist: " + file.getAbsolutePath());
                }
            }

        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    @TargetApi(23)
    @Override
    public void onRequestPermissionsResult(int requestCode, String[] permissions, int[] grantResults) {
        switch (requestCode) {
            case 100:
                if (grantResults.length > 0
                        || grantResults[0] == PackageManager.PERMISSION_GRANTED
                        || grantResults[1] == PackageManager.PERMISSION_GRANTED) {
                    /* User checks permission. */

                } else {
                    Toast.makeText(MainActivity.this, "Permission is denied.", Toast.LENGTH_SHORT).show();
                    finish();
                }
                return;
        }
    }
}

另一個解法是盡量不要去存取共用空間，改用 getExternalCacheDir()。

public static void storeSnapshot(List<WifiReading> readings, String fileName, Context c) {

		String state = Environment.getExternalStorageState();

		if (Environment.MEDIA_MOUNTED.equals(state)) {
			// We can read and write the external storage
			try {
				JSONArray json = readingsToJson(readings);
				File root = c.getExternalCacheDir();
				File jsonFile = new File(root, constructFileName(fileName));
				FileOutputStream out;
				out = new FileOutputStream(jsonFile);
				out.write(json.toString().getBytes());
				out.close();
				U.showToast(constructFileName(fileName));
				Log.d(TAG, "Successfully stored snapshot to SDCard");
			} catch (Exception e) {
				U.showToast("Could not save the snapshot on the SDCard.");
				Log.e(TAG, "Could not save the snapshot on the SDCard.", e);
			}
		} else {
			U.showToast("Cannot write to external storage.");
		}
	}

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Requesting Permissions at Run Time

Beginning in Android 6.0 (API level 23), users grant permissions to apps while the app is running, not when they install the app.