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Android的数据持久化存储  

2012-09-16 10:39:32|  分类: 数据存储 |  标签: |举报 |字号 订阅

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原文: http://developer.android.com/guide/topics/data/data-storage.html#filesInternal
一、简介
Android系统为我们提供了五种数据持久化存储的方式,以满足不同的需求。
他们分别是:
Shared Preferences
Store private primitive data in key-value pairs.
Internal Storage
Store private data on the device memory.把数据持久化存储到手机内部存储空间。他主要用于私有数据存储。
External Storage
Store public data on the shared external storage.把数据持久化存储到手机外部SD卡中。他主要用于非隐秘数据存储。
SQLite Databases
Store structured data in a private database.
Network Connection
Store data on the web with your own network server.
Android provides a way for you to expose even your private data to other applications — with a content provider. A content provider is an optional component that exposes read/write access to your application data, subject to whatever restrictions you want to impose. For more information about using content providers, see the Content Providers documentation.
Android系统中专门提供了Content Providers 这么一个组件来实现把你的应用程序的数据暴露给别的应用程序,这样别的应用程序也可以读写你的应用程序的数据。
二、Shared Preferences

The SharedPreferences class provides a general framework that allows you to save and retrieve persistent key-value pairs of primitive data types. You can use SharedPreferences to save any primitive data: booleans, floats, ints, longs, and strings. This data will persist across user sessions (even if your application is killed).

User Preferences

Shared preferences are not strictly for saving "user preferences," such as what ringtone a user has chosen. If you're interested in creating user preferences for your application, seePreferenceActivity, which provides an Activity framework for you to create user preferences, which will be automatically persisted (using shared preferences).

To get a SharedPreferences object for your application, use one of two methods:

  • getSharedPreferences() - Use this if you need multiple preferences files identified by name, which you specify with the first parameter.
  • getPreferences() - Use this if you need only one preferences file for your Activity. Because this will be the only preferences file for your Activity, you don't supply a name.

To write values:

  1. Call edit() to get a SharedPreferences.Editor.
  2. Add values with methods such as putBoolean() and putString().
  3. Commit the new values with commit()

To read values, use SharedPreferences methods such as getBoolean() and getString().

Here is an example that saves a preference for silent keypress mode in a calculator:

public class Calc extends Activity {
    public static final String PREFS_NAME = "MyPrefsFile";
    @Override
    protected void onCreate(Bundle state){
       super.onCreate(state);
       . . .
       /* Restore preferences*/
       SharedPreferences settings = getSharedPreferences(PREFS_NAME, 0);
       boolean silent = settings.getBoolean("silentMode", false);
       setSilent(silent);
    }
    @Override
    protected void onStop(){
       super.onStop();
      /* We need an Editor object to make preference changes.*/
      /* All objects are from android.context.Context*/
      SharedPreferences settings = getSharedPreferences(PREFS_NAME, 0);
      SharedPreferences.Editor editor = settings.edit();
      editor.putBoolean("silentMode", mSilentMode);
      /* Commit the edits!*/
      editor.commit();
    }
}

关于SharedPreferences的更多内容请阅读《SharedPreferences简介

三、Internal Storage

3.1、基本知识

You can save files directly on the device's internal storage. By default, files saved to the internal storage are private to your application and other applications cannot access them (nor can the user). When the user uninstalls your application, these files are removed.

To create and write a private file to the internal storage:

  1. Call openFileOutput() with the name of the file and the operating mode. This returns aFileOutputStream.
  2. Write to the file with write().
  3. Close the stream with close().
对于使用Context对象的openFileOutput()openFileInput()来进行数据持久化存储的这种方式,你的数据文件将存储在内部存储空间的/data/data/你的应用程序的包名/files/目录下,无法指定更深一级的目录,而且默认是Context.MODE_PRIVATE模式,即别的应用程序不能访问它。
示例1

String FILENAME = "hello_file";
String string = "hello world!";
FileOutputStream fos = openFileOutput(FILENAME, Context.MODE_PRIVATE);
fos.write(string.getBytes());
fos.close();

MODE_PRIVATE will create the file (or replace a file of the same name) and make it private to your application. Other modes available are: MODE_APPENDMODE_WORLD_READABLE, and MODE_WORLD_WRITEABLE.
示例2

String FILENAME = "hello_file";
String string = "hello world!";
FileOutputStream fos = openFileOutput(FILENAME, Context.MODE_WORLD_READABLE);
fos.write(string.getBytes());
fos.close();

示例3

String FILENAME = "hello_file";
String string = "hello world!";
FileOutputStream fos = openFileOutput(FILENAME, Context.MODE_APPEND|Context.MODE_WORLD_READABLE);
fos.write(string.getBytes());
fos.close();

注意:在很多手机上,虽然我们使用openFileOutput(FILENAME, Context.MODE_WORLD_READABLE)的方式来创建文件,而且使用ls -l看到该文件对别的应用程序来说其实已经有读的权限,但是别的应用程序实际上还是无法读取这些。这时我们需要在创建该文件的应用程序中对getFilesDir()目录执行"chmod 705"的操作来解决该问题。具体可以参照《Android中一些数据存储函数的封装》中的getInternalStorageDirectory()方法。
3.2、Saving cache files

If you'd like to cache some data, rather than store it persistently, you should use getCacheDir() to open a Filethat represents the internal directory where your application should save temporary cache files.

When the device is low on internal storage space, Android may delete these cache files to recover space. However, you should not rely on the system to clean up these files for you. You should always maintain the cache files yourself and stay within a reasonable limit of space consumed, such as 1MB. When the user uninstalls your application, these files are removed.

3.3、Other useful methods
getFilesDir()
Gets the absolute path to the filesystem directory where your internal files are saved.
getDir()
Creates (or opens an existing) directory within your internal storage space.
deleteFile()
Deletes a file saved on the internal storage.
fileList()
Returns an array of files currently saved by your application.
3.4、通过JAVA传统方式访问
你也可以通过绝对路径以JAVA传统方式访问内部存储空间。但是以这种方式创建的文件是对私有,创建它的应用程序对该文件是可读可写,但是别的应用程序并不能直接访问它。不是所有的内部存储空间应用程序都可以访问,默认情况下只能访问“/data/data/你的应用程序的包名”这个路径下的文件。
实际1

File file = new File("/data/data/"+getPackageName()+"/1.txt");
Log.i(TAG, "path:" + file.getAbsolutePath());
OutputStream out = null;
try {
out = new BufferedOutputStream(new FileOutputStream(file));
out.write("Hello World".getBytes());
} catch (FileNotFoundException e1) {
e1.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
} finally {
if (out != null) {
try {
out.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}

运行结果
09-24 22:42:51.468: I/robin(24089): path:/data/data/com.lenovo.robin.test/1.txt
四、External Storage
4.1、简介

Every Android-compatible device supports a shared "external storage" that you can use to save files. This can be a removable storage media (such as an SD card) or an internal (non-removable) storage. Files saved to the external storage are world-readable and can be modified by the user when they enable USB mass storage to transfer files on a computer.

注意:保存在外部存储(SD卡)上的文件对所有的应用程序都是可读的,而保存在内部存储的文件默认对别的应用程序是不可访问的

Caution: External files can disappear if the user mounts the external storage on a computer or removes the media, and there's no security enforced upon files you save to the external storage. All applications can read and write files placed on the external storage and the user can remove them.

4.2、Checking media availability
Before you do any work with the external storage, you should always call getExternalStorageState() to check whether the media is available. The media might be mounted to a computer, missing, read-only, or in some other state. For example, here's how you can check the availability:

boolean mExternalStorageAvailable = false;
boolean mExternalStorageWriteable = false;
String state = Environment.getExternalStorageState();

if (Environment.MEDIA_MOUNTED.equals(state)) {
    /* We can read and write the media*/
    mExternalStorageAvailable = mExternalStorageWriteable = true;
} else if (Environment.MEDIA_MOUNTED_READ_ONLY.equals(state)) {
    /* We can only read the media*/
    mExternalStorageAvailable = true;
    mExternalStorageWriteable = false;
} else {
    /* Something else is wrong. It may be one of many other states, but all we need*/
    /*  to know is we can neither read nor write*/
    mExternalStorageAvailable = mExternalStorageWriteable = false;
}

This example checks whether the external storage is available to read and write. ThegetExternalStorageState() method returns other states that you might want to check, such as whether the media is being shared (connected to a computer), is missing entirely, has been removed badly, etc. You can use these to notify the user with more information when your application needs to access the media.
4.3、Accessing files on external storage

If you're using API Level 8 or greater, use getExternalFilesDir() to open a File that represents the external storage directory where you should save your files. This method takes a type parameter that specifies the type of subdirectory you want, such as DIRECTORY_MUSIC and DIRECTORY_RINGTONES (pass null to receive the root of your application's file directory). This method will create the appropriate directory if necessary. By specifying the type of directory, you ensure that the Android's media scanner will properly categorize your files in the system (for example, ringtones are identified as ringtones and not music). If the user uninstalls your application, this directory and all its contents will be deleted.

If you're using API Level 7 or lower, use getExternalStorageDirectory(), to open a File representing the root of the external storage. You should then write your data in the following directory:

/Android/data/<package_name>/files/ 

The <package_name> is your Java-style package name, such as "com.example.android.app". If the user's device is running API Level 8 or greater and they uninstall your application, this directory and all its contents will be deleted.

4.4、Saving shared files

f you want to save files that are not specific to your application and that should not be deleted when your application is uninstalled, save them to one of the public directories on the external storage. These directories lay at the root of the external storage, such as Music/Pictures/Ringtones/, and others.

In API Level 8 or greater, usegetExternalStoragePublicDirectory(), passing it the type of public directory you want, such as DIRECTORY_MUSIC,DIRECTORY_PICTURESDIRECTORY_RINGTONES, or others. This method will create the appropriate directory if necessary.

If you're using API Level 7 or lower, usegetExternalStorageDirectory() to open a File that represents the root of the external storage, then save your shared files in one of the following directories:

  • Music/ - Media scanner classifies all media found here as user music.
  • Podcasts/ - Media scanner classifies all media found here as a podcast.
  • Ringtones/ - Media scanner classifies all media found here as a ringtone.
  • Alarms/ - Media scanner classifies all media found here as an alarm sound.
  • Notifications/ - Media scanner classifies all media found here as a notification sound.
  • Pictures/ - All photos (excluding those taken with the camera).
  • Movies/ - All movies (excluding those taken with the camcorder).
  • Download/ - Miscellaneous downloads.
4.5、Saving cache files

f you're using API Level 8 or greater, use getExternalCacheDir() to open a File that represents the external storage directory where you should save cache files. If the user uninstalls your application, these files will be automatically deleted. However, during the life of your application, you should manage these cache files and remove those that aren't needed in order to preserve file space.

If you're using API Level 7 or lower, use getExternalStorageDirectory() to open a File that represents the root of the external storage, then write your cache data in the following directory:

/Android/data/<package_name>/cache/ 

The <package_name> is your Java-style package name, such as "com.example.android.app".

4.6、EMMC存储
很多手机现在都有EMMC存储(一般是2G),一些手机并没有挂载在getExternalStorageDirectory()这个节点上(该节点用于挂载外部sdcard了).
而是挂载到如下"/mnt/emmc"的节点上,另外一些手机把EMMC存储直接挂载到了getExternalStorageDirectory()这个节点上,而对于真正的外部sdcard挂载到了"/mnt/sdcard2"这个节点上
因此当我们存储一个文件时首先应该存在getExternalStorageDirectory()这个节点上,其次是"/mnt/sdcard2"这个节点上,再次是"/mnt/emmc",最后才是手机的内部存储(即“/data”区域)。关于此一些封装请参照《Android中一些数据存储函数的封装》
4.7、一些重要的函数
Android中,关于外部存储的重要函数,请参考《Android中关于外部存储的一些重要函数》;关于内部存储的重要函数,请参考《Android中关于内部存储的一些重要函数》。关于数据存储的一些封装请参照《Android中一些数据存储函数的封装》
五、数据库存储

Android provides full support for SQLite databases. Any databases you create will be accessible by name to any class in the application, but not outside the application.

The recommended method to create a new SQLite database is to create a subclass of SQLiteOpenHelper and override the onCreate() method, in which you can execute a SQLite command to create tables in the database. For example:


public class DictionaryOpenHelper extends SQLiteOpenHelper {

private static final int DATABASE_VERSION = 2;
private static final String DICTIONARY_TABLE_NAME = "dictionary";
private static final String DICTIONARY_TABLE_CREATE =
"CREATE TABLE " + DICTIONARY_TABLE_NAME + " (" +
KEY_WORD + " TEXT, " +
KEY_DEFINITION + " TEXT);";
DictionaryOpenHelper(Context context) {
super(context, DATABASE_NAME, null, DATABASE_VERSION);
}
@Override
public void onCreate(SQLiteDatabase db) {
db.execSQL(DICTIONARY_TABLE_CREATE);
}
}



You can then get an instance of your SQLiteOpenHelper implementation using the constructor you've defined. To write to and read from the database, call getWritableDatabase() and getReadableDatabase(), respectively. These both return a SQLiteDatabase object that represents the database and provides methods for SQLite operations.

Android does not impose any limitations beyond the standard SQLite concepts. We do recommend including an autoincrement value key field that can be used as a unique ID to quickly find a record. This is not required for private data, but if you implement acontent provider, you must include a unique ID using theBaseColumns._ID constant.

You can execute SQLite queries using the SQLiteDatabasequery() methods, which accept various query parameters, such as the table to query, the projection, selection, columns, grouping, and others. For complex queries, such as those that require column aliases, you should use SQLiteQueryBuilder, which provides several convienent methods for building queries.

Every SQLite query will return a Cursor that points to all the rows found by the query. The Cursor is always the mechanism with which you can navigate results from a database query and read rows and columns.

For sample apps that demonstrate how to use SQLite databases in Android, see the Note Pad and Searchable Dictionary applications.

Database debugging

The Android SDK includes a sqlite3 database tool that allows you to browse table contents, run SQL commands, and perform other useful functions on SQLite databases. See Examining sqlite3 databases from a remote shell to learn how to run this tool.

六、网络存储

You can use the network (when it's available) to store and retrieve data on your own web-based services. To do network operations, use classes in the following packages:

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