文件读取和写入：打开本地管理器选择txt

拓展：文件管理工具类—FileUtil 实现的功能：获得外部存储绝对路径、创建文件夹、创建文件、遍历文件夹下的所有文件、获得指定文件、删除文件夹里的所有文件、删除文件、向文件中的指定位置添加String内容、修改文件内容（覆盖或者添加String）、读取文件内容String、重命名文件、复制文件夹、复制文件、拷贝图片文件、修改文件内容String（相对路径）。。。

一、功能描述

打开文件管理器，选择本地txt，按行读取内容，按一定格式整理每行内容，显示在界面上，并保存副本txt在同一个文件夹里。
注意：
1.请把txt保存格式修改为utf-8，否则文本会乱码（打开文件，另存为，修改保存格式）
2.动态申请写入权限！！！

二、运行效果

三、技术点讲解

1、声明内存读取权限

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

2、动态请求内存读取权限

 private String[] permissions = {Manifest.permission.READ_EXTERNAL_STORAGE, Manifest.permission.WRITE_EXTERNAL_STORAGE};

  /**
     * 开始提交请求权限
     */
    private void startRequestPermission() {
        ActivityCompat.requestPermissions(MainActivity.this, permissions, RESULT_SUCCESS);
    }

    /**
     * 用户权限 申请 的回调方法
     *
     * @param requestCode
     * @param permissions
     * @param grantResults
     */
    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
        super.onRequestPermissionsResult(requestCode, permissions, grantResults);
        if (requestCode == RESULT_SUCCESS) {
            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
                if (grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                    isGetPermission = true;
                }
            } else {
                isGetPermission = true;
            }
        }
        if (!isGetPermission) {
            llMain.setVisibility(View.GONE);
        } else {
            llMain.setVisibility(View.VISIBLE);
        }
    }

3、读取txt文本

  try {
  //1.从assets文件夹里，读取txt
//            AssetManager am = getAssets();
//            InputStream is = am.open("bbb.txt");
//            InputStreamReader reader = new InputStreamReader(is, "UTF-8");
//            BufferedReader bufferedReader = new BufferedReader(reader);
//2.打开文件管理区，读取选择的txt文件
            BufferedReader bufferedReader = new BufferedReader(new FileReader(readFilePath));
            contentStrBuf = new StringBuffer("");
            String str;
            //按行读取txt
            while ((str = bufferedReader.readLine()) != null) {
                    lineCount++;
                    contentStrBuf.append(lineCount+"、");
                    contentStrBuf.append(str);
                    contentStrBuf.append("\r\n");//追加换行符
            }
            tvSaveInfo.setText("行数：" + lineCount + "；\n保存路径：" + saveHolderPath + "/" + saveFileName);
            tvContent.setText(contentStrBuf.toString());
            bufferedReader.close();
        } catch (IOException e) {
            e.printStackTrace();
        } catch (Exception e) {
            e.printStackTrace();
        }

4、写入文本并保存新txt文件

 try {
            //创建一个带缓冲区的输出流
            String state = Environment.getExternalStorageState();
            if (state.equals(Environment.MEDIA_MOUNTED)) {
//1.指定存储地址与文件名
//                File sdDir = Environment.getExternalStorageDirectory();//外部存储绝对路径
//                String fileHoldName = "文件夹";
//                File fileDir = new File(savePath + "/" + fileHoldName);
//                if (!fileDir.exists()) {
//                    fileDir.mkdir();
//                }
//                File file = new File(fileDir.getAbsolutePath(), "data.txt");
//2.自动保存在读取的txt文件夹里
                File file = new File(saveHolderPath + "/" + saveFileName);
                FileOutputStream fos = new FileOutputStream(file);
                OutputStreamWriter writer = new OutputStreamWriter(fos, "utf-8");
                writer.write(content);
                writer.close();
                fos.close();
                Toast.makeText(this, "保存成功", Toast.LENGTH_SHORT).show();
            } else {
                Toast.makeText(this, "保存失败,SD卡不可用！", Toast.LENGTH_SHORT).show();
            }
        } catch (IOException e) {
            e.printStackTrace();
        } catch (Exception e) {
            e.printStackTrace();
        }

5、打开文件管理器

Intent intent = new Intent(Intent.ACTION_GET_CONTENT);
                //intent.setType(“image/*”);//选择图片
                //intent.setType(“audio/*”); //选择音频
                //intent.setType(“video/*”); //选择视频 （mp4 3gp 是android支持的视频格式）
                //intent.setType(“video/*;image/*”);//同时选择视频和图片
                intent.setType("*/*");//无类型限制
                intent.addCategory(Intent.CATEGORY_OPENABLE);
                startActivityForResult(intent, 1);

url解析

    public String getRealPathFromURI(Uri contentUri) {
        String res = null;
        String[] proj = {MediaStore.Images.Media.DATA};
        Cursor cursor = getContentResolver().query(contentUri, proj, null, null, null);
        if (null != cursor && cursor.moveToFirst()) {
            ;
            int column_index = cursor.getColumnIndexOrThrow(MediaStore.Images.Media.DATA);
            res = cursor.getString(column_index);
            cursor.close();
        }
        return res;
    }

    /**
     * 专为Android4.4设计的从Uri获取文件绝对路径，以前的方法已不好使
     */
    @SuppressLint("NewApi")
    public String getPath(final Context context, final Uri uri) {

        final boolean isKitKat = Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT;

        // DocumentProvider
        if (isKitKat && DocumentsContract.isDocumentUri(context, uri)) {
            // ExternalStorageProvider
            if (isExternalStorageDocument(uri)) {
                final String docId = DocumentsContract.getDocumentId(uri);
                final String[] split = docId.split(":");
                final String type = split[0];

                if ("primary".equalsIgnoreCase(type)) {
                    return Environment.getExternalStorageDirectory() + "/" + split[1];
                }
            }
            // DownloadsProvider
            else if (isDownloadsDocument(uri)) {

                final String id = DocumentsContract.getDocumentId(uri);
                final Uri contentUri = ContentUris.withAppendedId(
                        Uri.parse("content://downloads/public_downloads"), Long.valueOf(id));

                return getDataColumn(context, contentUri, null, null);
            }
            // MediaProvider
            else if (isMediaDocument(uri)) {
                final String docId = DocumentsContract.getDocumentId(uri);
                final String[] split = docId.split(":");
                final String type = split[0];

                Uri contentUri = null;
                if ("image".equals(type)) {
                    contentUri = MediaStore.Images.Media.EXTERNAL_CONTENT_URI;
                } else if ("video".equals(type)) {
                    contentUri = MediaStore.Video.Media.EXTERNAL_CONTENT_URI;
                } else if ("audio".equals(type)) {
                    contentUri = MediaStore.Audio.Media.EXTERNAL_CONTENT_URI;
                }

                final String selection = "_id=?";
                final String[] selectionArgs = new String[]{split[1]};

                return getDataColumn(context, contentUri, selection, selectionArgs);
            }
        }
        // MediaStore (and general)
        else if ("content".equalsIgnoreCase(uri.getScheme())) {
            return getDataColumn(context, uri, null, null);
        }
        // File
        else if ("file".equalsIgnoreCase(uri.getScheme())) {
            return uri.getPath();
        }
        return null;
    }

    /**
     * Get the value of the data column for this Uri. This is useful for
     * MediaStore Uris, and other file-based ContentProviders.
     *
     * @param context       The context.
     * @param uri           The Uri to query.
     * @param selection     (Optional) Filter used in the query.
     * @param selectionArgs (Optional) Selection arguments used in the query.
     * @return The value of the _data column, which is typically a file path.
     */
    public String getDataColumn(Context context, Uri uri, String selection,
                                String[] selectionArgs) {

        Cursor cursor = null;
        final String column = "_data";
        final String[] projection = {column};

        try {
            cursor = context.getContentResolver().query(uri, projection, selection, selectionArgs,
                    null);
            if (cursor != null && cursor.moveToFirst()) {
                final int column_index = cursor.getColumnIndexOrThrow(column);
                return cursor.getString(column_index);
            }
        } finally {
            if (cursor != null)
                cursor.close();
        }
        return null;
    }

    /**
     * @param uri The Uri to check.
     * @return Whether the Uri authority is ExternalStorageProvider.
     */
    public boolean isExternalStorageDocument(Uri uri) {
        return "com.android.externalstorage.documents".equals(uri.getAuthority());
    }

    /**
     * @param uri The Uri to check.
     * @return Whether the Uri authority is DownloadsProvider.
     */
    public boolean isDownloadsDocument(Uri uri) {
        return "com.android.providers.downloads.documents".equals(uri.getAuthority());
    }

    /**
     * @param uri The Uri to check.
     * @return Whether the Uri authority is MediaProvider.
     */
    public boolean isMediaDocument(Uri uri) {
        return "com.android.providers.media.documents".equals(uri.getAuthority());
    }

6、获取到选择的txt

 @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        if (resultCode == Activity.RESULT_OK) {
            Uri uri = data.getData();
            if ("file".equalsIgnoreCase(uri.getScheme())) {//使用第三方应用打开
                readFilePath = uri.getPath();
                setData();
                return;
            }
            if (Build.VERSION.SDK_INT > Build.VERSION_CODES.KITKAT) {//4.4以后
                readFilePath = getPath(this, uri);
            } else {//4.4以下下系统调用方法
                readFilePath = getRealPathFromURI(uri);
            }
            setData();
        }
    }