Android – cannot use libgdx to read and write to external storage

My creation method is as follows:

@Override
public void create () {
    batch = new SpriteBatch();

    FileHandle file = Gdx.files.external("file.txt");
    file.writeString("My god, it's full of stars", false);
}

I also include:

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

The exception I get is this:

02-13 14:45:51.858 12439-12466/com.sNowdevs.tweetiebirds E/AndroidRuntime: FATAL EXCEPTION: GLThread 1120
Process: com.sNowdevs.tweetiebirds, PID: 12439
com.badlogic.gdx.utils.GdxRuntimeException: Error writing file: file.txt (External)
    at com.badlogic.gdx.files.FileHandle.writeString(FileHandle.java:353)
    at com.badlogic.gdx.files.FileHandle.writeString(FileHandle.java:339)
    at com.sNowdevs.tweetiebirds.TweetieBirdsGame.create(TweetieBirdsGame.java:22)
    at com.badlogic.gdx.backends.android.AndroidGraphics.onSurfaceChanged(AndroidGraphics.java:254)
    at android.opengl.GLSurfaceView$GLThread.guardedRun(GLSurfaceView.java:1519)
    at android.opengl.GLSurfaceView$GLThread.run(GLSurfaceView.java:1240)
 Caused by: com.badlogic.gdx.utils.GdxRuntimeException: Error writing file: file.txt (External)
    at com.badlogic.gdx.files.FileHandle.writer(FileHandle.java:330)
    at com.badlogic.gdx.files.FileHandle.writeString(FileHandle.java:350)
    at com.badlogic.gdx.files.FileHandle.writeString(FileHandle.java:339) 
    at com.sNowdevs.tweetiebirds.TweetieBirdsGame.create(TweetieBirdsGame.java:22) 
    at com.badlogic.gdx.backends.android.AndroidGraphics.onSurfaceChanged(AndroidGraphics.java:254) 
    at android.opengl.GLSurfaceView$GLThread.guardedRun(GLSurfaceView.java:1519) 
    at android.opengl.GLSurfaceView$GLThread.run(GLSurfaceView.java:1240) 
 Caused by: java.io.FileNotFoundException: /storage/emulated/0/file.txt: open Failed: EACCES (Permission denied)
    at libcore.io.IoBridge.open(IoBridge.java:452)
    at java.io.FileOutputStream.<init>(FileOutputStream.java:87)
    at com.badlogic.gdx.files.FileHandle.writer(FileHandle.java:322)
    at com.badlogic.gdx.files.FileHandle.writeString(FileHandle.java:350) 
    at com.badlogic.gdx.files.FileHandle.writeString(FileHandle.java:339) 
    at com.sNowdevs.tweetiebirds.TweetieBirdsGame.create(TweetieBirdsGame.java:22) 
    at com.badlogic.gdx.backends.android.AndroidGraphics.onSurfaceChanged(AndroidGraphics.java:254) 
    at android.opengl.GLSurfaceView$GLThread.guardedRun(GLSurfaceView.java:1519) 
    at android.opengl.GLSurfaceView$GLThread.run(GLSurfaceView.java:1240) 
 Caused by: android.system.ErrnoException: open Failed: EACCES (Permission denied)
    at libcore.io.Posix.open(Native Method)
    at libcore.io.BlockGuardOs.open(BlockGuardOs.java:186)
    at libcore.io.IoBridge.open(IoBridge.java:438)
    at java.io.FileOutputStream.<init>(FileOutputStream.java:87) 
    at com.badlogic.gdx.files.FileHandle.writer(FileHandle.java:322) 
    at com.badlogic.gdx.files.FileHandle.writeString(FileHandle.java:350) 
    at com.badlogic.gdx.files.FileHandle.writeString(FileHandle.java:339) 
    at com.sNowdevs.tweetiebirds.TweetieBirdsGame.create(TweetieBirdsGame.java:22) 
    at com.badlogic.gdx.backends.android.AndroidGraphics.onSurfaceChanged(AndroidGraphics.java:254) 
    at android.opengl.GLSurfaceView$GLThread.guardedRun(GLSurfaceView.java:1519) 
    at android.opengl.GLSurfaceView$GLThread.run(GLSurfaceView.java:1240) 

I used nexus 5 with Android 6 marshmallow for testing. After searching, I found that Android 6 uses runtime permissions, but libgdx developers say it can even run on Android 6. Is there a fix with or without runtime permissions?

resolvent:

I solve the problem by checking whether the user has granted permission to the external storage. If the user is not granted the request permission in the following ways, I can solve the problem:

            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
                if (this.checkSelfPermission(Manifest.permission.WRITE_EXTERNAL_STORAGE) != PackageManager.PERMISSION_GRANTED) {
                     this.requestPermissions(new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE}, WRITE_REQUEST_CODE);
            }

All of these are Android specific code, so they are in the androidlauncher class