理解 Android 本地数据存储 API 利用首选项、SQLite 和内部及外部内存 API
http://www.ibm.com/developerworks/cn/xml/x-androidstorage/
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存储应用程序首选项本节介绍 Preferences API 和屏幕。Android API 提供很多方式处理首选项。其中一种方式是直接使用 SharedPreferences，并使用您自己的屏幕设计和首选项管理。第二种方法是使用 PreferenceActivity。PreferenceActivity 自动负责首选项如何呈现在屏幕上（默认情况下，看起来跟系统首选项一样），并通过使用 SharedPreferences 在用户与每个首选项交互时自动存储或保存首选项。
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Preferences 屏幕的 XML 声明
<?xml version="1.0" encoding="utf-8"?><PreferenceScreen xmlns:android="http://schemas.android.com/apk/res/android" android:id="@+id/prefs_screen" android:key="preferencescreen" > <PreferenceCategory android:title="Assets"> <EditTextPreference android:key="@string/prefs_assetname_friendslist_key" android:title="Friends List" android:summary="Please enter filename" android:defaultValue="friends.txt" /> <EditTextPreference android:key="@string/prefs_assetname_picture_key" android:title="Picture" android:summary="Please enter filename" android:defaultValue="pict2.jpg" /> </PreferenceCategory> <PreferenceCategory android:title="Auto Settings"> <CheckBoxPreference android:key="@string/prefs_autodelete_key" android:title="Delete at Startup" android:summary="Check to clear at startup" android:defaultValue="false" /> </PreferenceCategory></PreferenceScreen>
?PreferenceScreen 包含 EditTextPreference 的两个实例、一个 CheckBoxPreference 和两个由 PreferenceCategory 定义的类别组（一个用于 Asset，另一个用于 Auto Settings）。
使用 SharedPreferences?
/////////////////////////////////////////////////////////////// The following methods show how to use the SharedPreferences//////////////////////////////////////////////////////////////** * Retrieves the Auto delete preference * @return the value of auto delete */public boolean prefsGetAutoDelete() { boolean v = false; SharedPreferences sprefs = PreferenceManager.getDefaultSharedPreferences(appContext); String key = appContext.getString(R.string.prefs_autodelete_key); try { v = sprefs.getBoolean(key, false); } catch (ClassCastException e) { } return v;} /** * Sets the auto delete preference * @param v the value to set */public void prefsSetAutoDelete(boolean v) { SharedPreferences sprefs = PreferenceManager.getDefaultSharedPreferences(appContext); Editor e = sprefs.edit(); String key = appContext.getString(R.string.prefs_autodelete_key); e.putBoolean(key, v); e.commit();}?
MainActivity 插入到数据库中String fname = prefsGetFilename();if (fname != null && fname.length() > 0) { buffer = getAsset(fname); // Parse the JSON file String friendslist = new String(buffer); final JSONObject json = new JSONObject(friendslist); JSONArray d = json.getJSONArray("data"); int l = d.length(); for (int i2=0; i2<l; i2++) { JSONObject o = d.getJSONObject(i2); String n = o.getString("name"); String id = o.getString("id"); dbHelper.insert(id, n); } // Only the original owner thread can touch its views MainActivity.this.runOnUiThread(new Runnable() { public void run() { friendsArrayAdapter.notifyDataSetChanged(); } }); }MainActivity Select All 和将数据绑定到 ListViewfinal ArrayList<Friend> dbFriends = dbHelper.listSelectAll();if (dbFriends != null) { // Only the original owner thread can touch its views MainActivity.this.runOnUiThread(new Runnable() { public void run() { friendsArrayAdapter = new FriendsArrayAdapter( MainActivity.this, R.layout.rowlayout, dbFriends); listView.setAdapter(friendsArrayAdapter); friendsArrayAdapter.notifyDataSetChanged(); } });} ?从本地私有存储器读取数据??? /** * Writes content to internal storage making the content private to * the application. The method can be easily changed to take the MODE * as argument and let the caller dictate the visibility: * MODE_PRIVATE, MODE_WORLD_WRITEABLE, MODE_WORLD_READABLE, etc. * * @param filename - the name of the file to create * @param content - the content to write */public void writeInternalStoragePrivate( String filename, byte[] content) { try { //MODE_PRIVATE creates/replaces a file and makes // it private to your application. Other modes: // MODE_WORLD_WRITEABLE // MODE_WORLD_READABLE // MODE_APPEND FileOutputStream fos = openFileOutput(filename, Context.MODE_PRIVATE); fos.write(content); fos.close(); } catch (FileNotFoundException e) { e.printStackTrace(); } catch (IOException e) { e.printStackTrace(); }}?从内部私有存储器读取数据/** * Reads a file from internal storage * @param filename the file to read from * @return the file content */public byte[] readInternalStoragePrivate(String filename) { int len = 1024; byte[] buffer = new byte[len]; try { FileInputStream fis = openFileInput(filename); ByteArrayOutputStream baos = new ByteArrayOutputStream(); int nrb = fis.read(buffer, 0, len); // read up to len bytes while (nrb != -1) { baos.write(buffer, 0, nrb); nrb = fis.read(buffer, 0, len); } buffer = baos.toByteArray(); fis.close(); } catch (FileNotFoundException e) { e.printStackTrace(); } catch (IOException e) { e.printStackTrace(); } return buffer;}?
从本地私有存储器删除数据/** * Delete internal private file * @param filename - the filename to delete */public void deleteInternalStoragePrivate(String filename) { File file = getFileStreamPath(filename); if (file != null) { file.delete(); }}?为公共数据使用设备的外部存储器
有了数据存储 API，您可以使用外部存储器存储数据。信息可以是私有的，您可以有选择地让其他应用程序对之具有读或写的访问权限。本节您将对此 API 进行编程，以便使用包括 getExternalStorageState()、getExternalFilesDir()、getExternalStorageDirectory() 和 getExternalStoragePublicDirectory() 在内的很多 API 来存储公共数据。您为公共数据使用下面的路径：/Android/data/<package_name>/files/。
在使用外部存储器之前，必须看看它是否可用，是否可写。下面两个代码片段展示了测试这些条件的帮助器方法。清单 23 测试外部存储器是否可用。
/** * Helper Method to Test if external Storage is Available */public boolean isExternalStorageAvailable() { boolean state = false; String extStorageState = Environment.getExternalStorageState(); if (Environment.MEDIA_MOUNTED.equals(extStorageState)) { state = true; } return state;}?测试外部存储器是否只可读/** * Helper Method to Test if external Storage is read only */public boolean isExternalStorageReadOnly() { boolean state = false; String extStorageState = Environment.getExternalStorageState(); if (Environment.MEDIA_MOUNTED_READ_ONLY.equals(extStorageState)) { state = true; } return state;}?写到外部内存/** * Write to external public directory * @param filename - the filename to write to * @param content - the content to write */public void writeToExternalStoragePublic(String filename, byte[] content) { // API Level 7 or lower, use getExternalStorageDirectory() // to open a File that represents the root of the external // storage, but writing to root is not recommended, and instead // application should write to application-specific directory, as shown below. String packageName = this.getPackageName(); String path = "/Android/data/" + packageName + "/files/"; if (isExternalStorageAvailable() && !isExternalStorageReadOnly()) { try { File file = new File(path, filename); file.mkdirs(); FileOutputStream fos = new FileOutputStream(file); fos.write(content); fos.close(); } catch (FileNotFoundException e) { e.printStackTrace(); } catch (IOException e) { e.printStackTrace(); } }}?从外部内存读取数据/** * Reads a file from internal storage * @param filename - the filename to read from * @return the file contents */public byte[] readExternallStoragePublic(String filename) { int len = 1024; byte[] buffer = new byte[len]; String packageName = this.getPackageName(); String path = "/Android/data/" + packageName + "/files/"; if (!isExternalStorageReadOnly()) { try { File file = new File(path, filename); FileInputStream fis = new FileInputStream(file); ByteArrayOutputStream baos = new ByteArrayOutputStream(); int nrb = fis.read(buffer, 0, len); //read up to len bytes while (nrb != -1) { baos.write(buffer, 0, nrb); nrb = fis.read(buffer, 0, len); } buffer = baos.toByteArray(); fis.close(); } catch (FileNotFoundException e) { e.printStackTrace(); } catch (IOException e) { e.printStackTrace(); } } return buffer;}外部内存删除文件/** * Delete external public file * @param filename - the filename to write to */void deleteExternalStoragePublicFile(String filename) { String packageName = this.getPackageName(); String path = "/Android/data/" + packageName + "/files/"+filename; File file = new File(path, filename); if (file != null) { file.delete(); }}?处理外部存储器需要特殊的权限 WRITE_EXTERNAL_STORAGE，它通过 AndroidManifest.xml 请求得到
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
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外部存储 API 通过根据文件类型（比如 Pictures、Ringtones）将文件存储在预先确定的目录中，允许您公共地存储文件。本文没有介绍这种方法，但是您应该熟悉它。此外，记住外部存储器中的文件任何时候都可能消失。
如果您具有不需要长期永久保存的临时文件，那么可以将这些文件存储在高速缓存中。高速缓存是一种特殊的内存，可以用于存储中小型数据（少于兆字节），但是您一定要知道，取决于有多少内存可用，高速缓存的内容任何时候都可能被清除。
/** * Helper method to retrieve the absolute path to the application * specific internal cache directory on the file system. These files * will be ones that get deleted when the application is uninstalled or when * the device runs low on storage. There is no guarantee when these * files will be deleted. * * Note: This uses a Level 8+ API. * * @return the absolute path to the application specific cache * directory */public String getInternalCacheDirectory() { String cacheDirPath = null; File cacheDir = getCacheDir(); if (cacheDir != null) { cacheDirPath = cacheDir.getPath(); } return cacheDirPath; }检索到外部内存高速缓存的路径 /** * Helper method to retrieve the absolute path to the application * specific external cache directory on the file system. These files * will be ones that get deleted when the application is uninstalled or when * the device runs low on storage. There is no guarantee when these * files will be deleted. * * Note: This uses a Level 8+ API. * * @return the absolute path to the application specific cache * directory */public String getExternalCacheDirectory() { String extCacheDirPath = null; File cacheDir = getExternalCacheDir(); if (cacheDir != null) { extCacheDirPath = cacheDir.getPath(); } return extCacheDirPath; }