我不明白为什么 mkdir() 返回 false？

Question

I am trying to write my bitmap files to a filepath in a directory I create in androids external storage but when I run my code I recieve a null pointer exception from bitmap.compress (bitmap.CompressFormat.JPEG, 0, outputStream) . From debugging I have found that my bitmap object is not null & that the outputStream object is null so the null pointer exception is occuring when I create the outputStream object & I recieve a java.io.FileNotFoundException . From debugging I know that the java.io.FileNotFoundException is occuring because dir.mkdir () returns false so no directory is created for my bitmap files.

I don't understand why dir.mkdir () returns false because I instantiate dir & check if the external storage is writable in isExternalStorageWritable() and add <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" /> in my manifest file and have created askPermission() , onRequestPermissionsResult() & isStoragePermissionGranted () that ask the user for permission for the external storage. So I don't understand why dir.mkdir () returns false and the directory is not created.

This is the class where the exception occurs:

public class BitmapFiles {

public String imagePath;
public String name;
private Bitmap bitmap;
private int resourceId;
private Context context;
private OutputStream outputStream;
private File file;

public BitmapFiles (Context context, int resourceId, String name) {
    this.resourceId = resourceId;
    this.name = name;
    this.context = context;

    convertToBitmap();
    saveBitmap();
    setImagePath();
}

public void convertToBitmap () {
    bitmap = BitmapFactory.decodeResource(context.getResources(), resourceId);

}

public void saveBitmap () {
    File filePath = Environment.getExternalStorageDirectory ();
    File dir = new File (filePath.getAbsolutePath() + "/Items");

    if (isExternalStorageWritable()) {
        Log.d ("red", "external storage is writable");//I recieve this in the log cat
    }

    if (!dir.exists ()) {
        try {
            boolean direct = dir.mkdir();//Returns false

            if (dir.exists()) {
                Log.d ("blue", "directory exists now");//I don't recieve this in the log cat
            }

            if (!direct) {
                Log.d ("green", "the directory was not created");//I recieve this in the log cat
            }

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

    } else {
        Log.d ("yellow", "the directory already exists");
    }

    file = new File (dir, name + ".jpg");

    try {
        outputStream = new FileOutputStream(file);
    } catch (FileNotFoundException e) {
        e.printStackTrace ();
    }

    if (outputStream == null) {
        Log.d ("grey", "outputstream is null");//I recieve this in log cat
    }

    bitmap.compress (Bitmap.CompressFormat.JPEG, 0, outputStream);

}

public boolean isExternalStorageWritable () {
    String state = Environment.getExternalStorageState();

    if (Environment.MEDIA_MOUNTED.equals (state)) {
        return true;
    }

    return false;
}

}

This is the class where I ask the user for permission:

public class GroceryItem extends AppCompatActivity {

private static final int STORAGE_PERMISSION_CODE = 101;

protected void onCreate (Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_grocery_item);

    askForPermission(Manifest.permission.WRITE_EXTERNAL_STORAGE, STORAGE_PERMISSION_CODE);

}

public void askForPermission (String permission, int requestCode) {
    if (isStoragePermissionGranted()) {
        ActivityCompat.requestPermissions(this, new String [] {permission}, requestCode);
    } else {
        Toast.makeText (this, "Permission already granted", Toast.LENGTH_SHORT)
                .show();

    }
}

public void onRequestPermissionsResult (int requestCode, String [] permissions, int [] grantResults) {
    super.onRequestPermissionsResult (requestCode, permissions, grantResults);

    if (requestCode == STORAGE_PERMISSION_CODE) {
        if (grantResults.length > 0 && grantResults [0] == PackageManager.PERMISSION_GRANTED) {
            Toast.makeText(this, "Storage Permission Granted", Toast.LENGTH_SHORT)
                    .show();
        } else {
            Toast.makeText(this, "Storage Permission Denied", Toast.LENGTH_SHORT)
                    .show();
        }
    }


}

public boolean isStoragePermissionGranted () {

    if (ContextCompat.checkSelfPermission(this, Manifest.permission.WRITE_EXTERNAL_STORAGE) != PackageManager.PERMISSION_GRANTED) {
        Toast.makeText(this, "permission granted", Toast.LENGTH_SHORT)
                .show();
        return true;
    }

    return false;
}

}

This is the exception I recieve:

2020-09-23 10:18:20.829 32212-32263/com.myapp.groceryapp W/System.err: java.io.FileNotFoundException: /storage/emulated/0/Items/faan.jpg: open failed: ENOENT (No such file or directory)
2020-09-23 10:18:20.829 32212-32263/com.myapp.groceryapp W/System.err:     at libcore.io.IoBridge.open(IoBridge.java:496)
2020-09-23 10:18:20.829 32212-32263/com.myapp.groceryapp W/System.err:     at java.io.FileOutputStream.<init>(FileOutputStream.java:235)
2020-09-23 10:18:20.830 32212-32263/com.myapp.groceryapp W/System.err:     at java.io.FileOutputStream.<init>(FileOutputStream.java:186)
2020-09-23 10:18:20.830 32212-32263/com.myapp.groceryapp W/System.err:     at com.myapp.groceryapp.BitmapFiles.saveBitmap(BitmapFiles.java:82)
2020-09-23 10:18:20.830 32212-32263/com.myapp.groceryapp W/System.err:     at com.myapp.groceryapp.BitmapFiles.<init>(BitmapFiles.java:30)
2020-09-23 10:18:20.830 32212-32263/com.myapp.groceryapp W/System.err:     at com.myapp.groceryapp.MyDatabaseHelper.createBitmapFiles(MyDatabaseHelper.java:42)
2020-09-23 10:18:20.830 32212-32263/com.myapp.groceryapp W/System.err:     at com.myapp.groceryapp.MyDatabaseHelper.upgradeDatabase(MyDatabaseHelper.java:37)
2020-09-23 10:18:20.830 32212-32263/com.myapp.groceryapp W/System.err:     at com.myapp.groceryapp.MyDatabaseHelper.onCreate(MyDatabaseHelper.java:33)
2020-09-23 10:18:20.830 32212-32263/com.myapp.groceryapp W/System.err:     at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:412)
2020-09-23 10:18:20.830 32212-32263/com.myapp.groceryapp W/System.err:     at android.database.sqlite.SQLiteOpenHelper.getReadableDatabase(SQLiteOpenHelper.java:341)
2020-09-23 10:18:20.830 32212-32263/com.myapp.groceryapp W/System.err:     at com.myapp.groceryapp.GroceryItem.accessDataBase(GroceryItem.java:44)
2020-09-23 10:18:20.830 32212-32263/com.myapp.groceryapp W/System.err:     at com.myapp.groceryapp.GroceryItem$StartDatabase.doInBackground(GroceryItem.java:101)
2020-09-23 10:18:20.830 32212-32263/com.myapp.groceryapp W/System.err:     at com.myapp.groceryapp.GroceryItem$StartDatabase.doInBackground(GroceryItem.java:93)
2020-09-23 10:18:20.830 32212-32263/com.myapp.groceryapp W/System.err:     at android.os.AsyncTask$3.call(AsyncTask.java:378)
2020-09-23 10:18:20.830 32212-32263/com.myapp.groceryapp W/System.err:     at java.util.concurrent.FutureTask.run(FutureTask.java:266)
2020-09-23 10:18:20.830 32212-32263/com.myapp.groceryapp W/System.err:     at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:289)
2020-09-23 10:18:20.830 32212-32263/com.myapp.groceryapp W/System.err:     at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1167)
2020-09-23 10:18:20.830 32212-32263/com.myapp.groceryapp W/System.err:     at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:641)
2020-09-23 10:18:20.830 32212-32263/com.myapp.groceryapp W/System.err:     at java.lang.Thread.run(Thread.java:919)
2020-09-23 10:18:20.830 32212-32263/com.myapp.groceryapp W/System.err: Caused by: android.system.ErrnoException: open failed: ENOENT (No such file or directory)
2020-09-23 10:18:20.830 32212-32263/com.myapp.groceryapp W/System.err:     at libcore.io.Linux.open(Native Method)
2020-09-23 10:18:20.830 32212-32263/com.myapp.groceryapp W/System.err:     at libcore.io.ForwardingOs.open(ForwardingOs.java:167)
2020-09-23 10:18:20.830 32212-32263/com.myapp.groceryapp W/System.err:     at libcore.io.BlockGuardOs.open(BlockGuardOs.java:252)
2020-09-23 10:18:20.830 32212-32263/com.myapp.groceryapp W/System.err:     at libcore.io.ForwardingOs.open(ForwardingOs.java:167)
2020-09-23 10:18:20.830 32212-32263/com.myapp.groceryapp W/System.err:     at android.app.ActivityThread$AndroidOs.open(ActivityThread.java:7581)
2020-09-23 10:18:20.830 32212-32263/com.myapp.groceryapp W/System.err:     at libcore.io.IoBridge.open(IoBridge.java:482)
2020-09-23 10:18:20.830 32212-32263/com.myapp.groceryapp W/System.err:  ... 18 more
2020-09-23 10:18:20.830 32212-32263/com.myapp.groceryapp D/outty: outputstream is null
2020-09-23 10:18:20.836 32212-32263/com.myapp.groceryapp E/AndroidRuntime: FATAL EXCEPTION: AsyncTask #1
Process: com.myapp.groceryapp, PID: 32212
java.lang.RuntimeException: An error occurred while executing doInBackground()
    at android.os.AsyncTask$4.done(AsyncTask.java:399)
    at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:383)
    at java.util.concurrent.FutureTask.setException(FutureTask.java:252)
    at java.util.concurrent.FutureTask.run(FutureTask.java:271)
    at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:289)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1167)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:641)
    at java.lang.Thread.run(Thread.java:919)
 Caused by: java.lang.NullPointerException
    at android.graphics.Bitmap.compress(Bitmap.java:1407)
    at com.myapp.groceryapp.BitmapFiles.saveBitmap(BitmapFiles.java:95)
    at com.myapp.groceryapp.BitmapFiles.<init>(BitmapFiles.java:30)
    at com.myapp.groceryapp.MyDatabaseHelper.createBitmapFiles(MyDatabaseHelper.java:42)
    at com.myapp.groceryapp.MyDatabaseHelper.upgradeDatabase(MyDatabaseHelper.java:37)
    at com.myapp.groceryapp.MyDatabaseHelper.onCreate(MyDatabaseHelper.java:33)
    at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:412)
    at android.database.sqlite.SQLiteOpenHelper.getReadableDatabase(SQLiteOpenHelper.java:341)
    at com.myapp.groceryapp.GroceryItem.accessDataBase(GroceryItem.java:44)
    at com.myapp.groceryapp.GroceryItem$StartDatabase.doInBackground(GroceryItem.java:101)
    at com.myapp.groceryapp.GroceryItem$StartDatabase.doInBackground(GroceryItem.java:93)
    at android.os.AsyncTask$3.call(AsyncTask.java:378)
    at java.util.concurrent.FutureTask.run(FutureTask.java:266)
    at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:289) 
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1167) 
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:641) 
    at java.lang.Thread.run(Thread.java:919) 

Answer 1

The NullPointerException and FileNotFoundException are caused by incorrect error recover code. Put simply, your code tries too hard to write that bitmap, in situations where it is simply not possible to do so. (If the directory doesn't exist after your attempt to create it, there is no way you could possibly save the bitmap. Persistence is futile!)

The root cause of the cascade of problems is that mkdir() is (in some circumstances) by creating the the required directory.

I don't understand why mkdir() returns false.

This could be happening for a variety of reasons, including some of the following:

• The parent directory for the directory may not exist
• The parent directory's ownership and permissions may be wrong
• The app may not be permitted to write in this file system.
• The device could be read only
• The device could be corrupted or experiencing hardware errors
• The file system could be so full that a directory cannot be created.

The problem is that it is not possible to find out which of those possible causes is the actual cause. File.mkdir() doesn't tell you, and doesn't provide a way to find out. java.io.File is a clunky old interface ... and you shouldn't be using it anymore. (It was superseded in Java 7!)

The first step in solving this is to change your application to using the NIO2 Files and Path APIs. In particular, you need to use one of:

• Files.createDirectory(Path dir, FileAttribute<?>... attrs) ( javadoc )
• Files.createDirectories(Path dir, FileAttribute<?>... attrs) ( javadoc )

These both will throw an exception if the directory could not be created. In addition, createDirectory will throw an exception if the directory already exists.

In either case, the exception and its message should tell you why the operation failed. This will help you to figure out what is really going on ... and hence work out how to fix it.