MainActivity.java

package com.twitter.externalstorage;

import android.Manifest;
import android.app.Activity;
import android.content.pm.PackageManager;
import android.os.AsyncTask;
import android.os.Bundle;
import android.os.Environment;
import android.util.Log;
import android.view.View;
import android.widget.Toast;

import java.io.Closeable;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;

public class MainActivity extends Activity {

    private static final int PERMISSION_REQUEST_WRITE_EXTERNAL_STORAGE = 1;

    private static final String TAG = "WES";

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

        findViewById(R.id.button).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                if (checkSelfPermission(Manifest.permission.WRITE_EXTERNAL_STORAGE) == PackageManager.PERMISSION_GRANTED) {
                    copyToStorage();
                } else {
                    requestPermissions(new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE}, PERMISSION_REQUEST_WRITE_EXTERNAL_STORAGE);
                }
            }
        });
    }

    @Override
    public void onRequestPermissionsResult(int requestCode, String[] permissions, int[] grantResults) {
        if (requestCode == PERMISSION_REQUEST_WRITE_EXTERNAL_STORAGE) {
            if (grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                copyToStorage();

            } else {
                Toast.makeText(this, "permission denied", Toast.LENGTH_SHORT).show();
            }
        }
    }

    private void copyToStorage() {
        final File sourceDir = getExternalFilesDir(null);
        final String identifier = "hello";
        final File sourceFile = new File(sourceDir, identifier);
        try {
            FileOutputStream fileOutputStream = new FileOutputStream(sourceFile);
            fileOutputStream.write("hello contents".getBytes());
        } catch (IOException e) {
            Log.d(TAG, "failed to create src file");
        }
        if (sourceFile.exists()) {
            Log.d(TAG, "src: " + sourceFile.toString());
        } else {
            throw new IllegalStateException("src file doesn't exist");
        }

        new AsyncTask<Void, Void, Boolean>() {
            @Override
            protected Boolean doInBackground(Void... params) {
                final String destFile = getDestFile();
                Log.d(TAG, "dest: " + destFile);
                return copyFile(sourceFile, new File(destFile));
            }

            @Override
            protected void onPostExecute(Boolean success) {
                final String result = success ? "success" : "failure";
                Toast.makeText(MainActivity.this, result, Toast.LENGTH_SHORT).show();
            }
        }.execute();
    }

    private String getDestFile() {
        final File baseDir = Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_PICTURES);
        baseDir.mkdirs();
        return (new StringBuffer())
                .append(baseDir.toString())
                .append(File.separator)
                .append("hellocopy")
                .toString();
    }

    private static boolean copyFile(File srcFile, File destFile) {
        InputStream inStream = null;
        try {
            inStream = new FileInputStream(srcFile);
            return copyToFile(inStream, destFile);
        } catch (FileNotFoundException e) {
            Log.d(TAG, e.toString());
            return false;
        } finally {
            closeSilently(inStream);
        }
    }

    private static boolean copyToFile(InputStream is, File file) {
        boolean success = false;
        if (createPathForFile(file)) {
            FileOutputStream out = null;
            try {
                // throws FileNotFoundException: open failed: EACCES (Permission denied)
                out = new FileOutputStream(file);
                final int bytesLen = readFullyWriteTo(is, out, 4 * 1024);
                success = bytesLen != 0;
                out.flush();
            } catch (IOException e) {
                Log.d(TAG, e.toString());
            } finally {
                closeSilently(out);
                if (!success) {
                    // noinspection ResultOfMethodCallIgnored
                    file.delete();
                }
            }
        }
        return success;
    }

    private static boolean createPathForFile(File file) {
        final File dir = file.getParentFile();
        return dir == null || dir.exists() || dir.mkdirs();
    }

    private static int readFullyWriteTo(InputStream is, OutputStream os,
                                       int bufferSize) throws IOException {
        final byte[] buf = new byte[bufferSize];
        int bytesRead;
        int totalBytes = 0;
        while ((bytesRead = is.read(buf)) != -1) {
            if (os != null) {
                os.write(buf, 0, bytesRead);
            }
            totalBytes += bytesRead;
        }
        return totalBytes;
    }

    private static void closeSilently(Closeable closeable) {
        if (closeable != null) {
            try {
                closeable.close();
            } catch (IOException ignore) {
            }
        }
    }
}