Chapter 5 data-centric-Data Access (1), Chapter 5 Access

Chapter 5 data-centric-Data Access (1), Chapter 5 Access
Chapter 5 data-centric-Data Access

A typical desktop operating system provides a public file system that any application can use to store and read files, this file can also be read by other application software (with some permission control settings ). Android uses a different system. In Android, all application software data (including files) is private to the application software. However, Android also provides a standard way to use software to open private data to other applications. This section describes how an application stores and obtains data, opens data to other applications, requests data from other applications, and opens them up.

Available storage methods include file storage, SQLite database, SharedPreferences, and content provider ). We will introduce it in detail in this chapter.

5.1 File Operations 5.1.1 read and write general text files

Let's take a look at the common Java APIs for file or folder operations:

String path = File. getPath (); // relative path String path = File. getAbsoultePath (); // absolute path String parentPath = File. getParent (); // obtain the parent directory of a File or folder String Name = File. getName (); File. mkDir (); // create a folder File. createNewFile (); // create a File File. isDirectory (); // identifies a File or folder File [] files = File. listFiles (); // list all files and folder names in the folder File. renameTo (dest); // modify the folder and File name File. delete (); // delete a folder or File

 

In Android, files under the SD card are often operated:

Environment. getExternalStorageState (). equals (android. OS. Environment. MEDIA_MOUNTED ); // Determine whether the SD card is inserted File skRoot = Environment. getExternalStorageDirectory (); // get the root directory of the SD card File fileRoot = Context. getFilesDir () + "//"; // obtain the private root directory

 

Similar to the traditional Java program that implements I/O, in Android, it provides the openFileInput and openFileOuput methods to read files on the device.

The example code is as follows:

String FILE_NAME = "tempfile. tmp"; // determine the file name to operate on FileOutputStream fos = openFileOutput (FILE_NAME, Context. MODE_PRIVATE); // Initialization FileInputStream FCM = openFileInput (FILE_NAME); // create a write stream

 

Experience Sharing: By default, files created using the openFileOutput () method can only be used by the called applications. Other applications cannot read the files. If you need to share data in different applications, you can use Content Provider. We will introduce Content Provider later. For Android files, you must have the following permissions: <Uses-permission android: name = "android. permission. WRITE_EXTERNAL_STORAGE"/>

 

In Android development, where are resource files generally stored?

If your application requires some additional resource files, such as MP3 files used to test whether the music player you write works properly, you can put these files under/res/raw/of the application, for example, mydatafileworkflow. In this case, you can use getResources in your application to obtain resources and open the file using the openRawResource method (resource file name without a suffix). The implementation code is as follows:

Resources myResources = getResources (); InputStream myFile = myResources. openRawResource (R. raw. myfilename );

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Next let's take a look at the next example to read the resource file test.txt under the/res/raw/directory.

// Import omitted Public class ReadTextTest extends Activity {   Public static final int REFRESH = 0x000001; Private TextView text = null;   @ Override Public void onCreate (Bundle savedInstanceState ){ Super. onCreate (savedInstanceState ); Text = new TextView (this ); Text. setBackgroundColor (0xff000000 ); Text. setTextColor (0 xffffffff ); Text. setGravity (Gravity. CENTER ); InputStreamReader inputStreamReader = null; InputStream inputStream = getResources (). openRawResource (R. raw. test ); Try { InputStreamReader = new InputStreamReader (inputStream, "UTF-8 "); } Catch (UnsupportedEncodingException e1 ){ E1.printStackTrace (); } BufferedReader reader = new BufferedReader (inputStreamReader ); StringBuffer sb = new StringBuffer (""); String line; Try { While (line = reader. readLine ())! = Null ){ Sb. append (line ); Sb. append ("\ n "); } } Catch (IOException e ){ E. printStackTrace (); } Text. setText (sb. toString ()); SetContentView (text ); } }

 

Create a new test.txt file under/res/raw/directory, add the "test read txt file" text, save as UTF-8 format.

Figure 5-1 shows the running result:

Figure 5-1 result of reading txt document