有了数据存储 API,您可以使用内部存储器存储数据。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。本节介绍这个存储私有数据的 API,它使用 android.content.Context.openFileInput
、openFileOutput
和 getCacheDir()
来高速缓存数据,而不是永久地存储。
清单 20 中的代码片段展示了如何从内部私有存储器读取数据。使得存储器为私有的方法是对 openFileOutput()
使用MODE_PRIVATE
。
/** * 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(); } } |
清单 21 中的代码片段展示了如何从内部私有存储器读取数据;注意 openFileInput()
的使用。
/** * 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; } |
清单 22 展示了如何从内部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(); } } |
现在可以来看为公共数据使用外部存储器了。
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; } |
清单 24 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; } |
清单 25 展示了如何写到外部存储器,以存储公共数据。
/** * 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(); } } } |
清单 26 展示了如何从外部存储器读取数据。
/** * 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; } |
清单 27 中的代码片段展示了如何从外部内存删除文件。
/** * 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(参见 清单 28)。
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/> |
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。
清单 29 展示了一个帮助器方法,它返回到内部内存中高速缓存的路径。
/** * 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; } |
清单 30 展示了一个帮助器方法,它返回到外部内存中高速缓存的路径。
/** * 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; } |
通过使用示例应用程序,您现在应该很好地理解了如何为公共数据使用设备的外部存储器。
感谢http://hi.baidu.com/304580500/blog/item/538a5bb0f4a6ce4008230244.html