Android基础#45：Android数据存储之文件存储数据
内容简介：
Android系统当然支持文件系统。在Android系统中，除了Preference文件外，普通的文件读取方式，和java几乎一样。需要注意的是，
Android系统中的文件的存储路径和读取权限。
Android系统中，既提供了App的私有的存储路径，也提供了诸如sdcard相关的公共路径的数据读取接口。例如，
openFileInput()，openFileOutput()方法，getCacheDir()，getExternalFilesDir等等，
示例：
这里，给出一些代码示例，分别针对内部存储器，外部存储器进行操作：
１. 内部数据存储器的操作例子，主要调用了openFileOutput， openFileInput。
这组例子中，实现了下面三个函数：
writeInternalStoragePrivate：写数据到内部存储器
readInternalStoragePrivate： 从内部存储器读数据到指定的buffer中
deleteInternalStoragePrivate：从内部存储器删除数据
/* 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); //将content数组的内容写到filename指定的文件中。 fos.close(); } catch (FileNotFoundException e) { e.printStackTrace(); } catch (IOException e) { e.printStackTrace(); }}//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;}/** * Delete internal private file * @param filename - the filename to delete */public void deleteInternalStoragePrivate(String filename) { File file = getFileStreamPath(filename); if (file != null) { file.delete(); }}说明：
openFileInput()，openFileOutput()是ContextWrapper类的成员函数，所以可以直接在Activity中调用，因为Activity是ContextWrapper的子类。
２. 外部数据存储器的操作例子，主要用到了下面这些函数：
getExternalStorageState()
getExternalFilesDir()
getExternalStorageDirectory()
getExternalStoragePublicDirectory()
这组例子中，实现了下面几个函数：
isExternalStorageAvailable： 外部存储器是否可用
isExternalStorageReadOnly： 外部存储器是否只可读
writeToExternalStoragePublic：写数据到外部存储器
readExternallStoragePublic： 从外部存储器读数据到指定的buffer中
deleteExternalStoragePublicFile：删除外部存储器代码：
/** * 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;}/** * 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;}/** * 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(); } }}/** * 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;}/** * 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(); }}注意： 需要添加外部存储器需要特殊的权限 WRITE_EXTERNAL_STORAGE：
３. 高速缓存的操作示例：
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。
相关方法：
getCacheDir： 得到内部高速缓存
getExternalCacheDir：得到外部高速缓存
代码：
/** * 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; }/** * 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 cachedirectory */public String getExternalCacheDirectory() { String extCacheDirPath = null; File cacheDir = getExternalCacheDir(); if (cacheDir != null) { extCacheDirPath = cacheDir.getPath(); } return extCacheDirPath; }