21、从头学Android之Android的数据存储--SD卡-优快云博客

上一节我们学习了Android的数据存储采用File,但是这样的数据是存储在应用程序内的，那么也就是说这样的存储的文件大小还是有一定要限制的，有时候我们需要存储更大的文件，比如电影等，这就用到了我们的SDSard存储卡。Android也为我们提供了SDCard的一些相关操作。Environment这个类就可以实现这个功能。

Environmet类

常用常量

String MEDIA_MOUNTED	当前Android的外部存储器可读可写
String MEDIA_MOUNTED_READ_ONLY	当前Android的外部存储器只读

常用方法

方法名称	描述
public static File getDataDirectory ()	获得Android下的data文件夹的目录
public static File getDownloadCacheDirectory ()	获得Android Download/Cache 内容的目录
public static File getExternalStorageDirectory ()	获得Android外部存储器也就是SDCard的目录
public static String getExternalStorageState ()	获得Android外部存储器的当前状态
public static File getRootDirectory ()	获得Android下的root文件夹的目录

要想实现对SDCard的读取操作，只需要按以下几个步骤就行：

1、先判断这台手机设备上是否有SDCard且具有读写SDCard的权限

Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)

2、调用Environment.getExternalStorageDirectory()获得到外部存储器的目录

3、使用IO流对外部存储器进行文件的读写

4、别忘记了要在AndroidMainfest.xml中添加权限哦

特别声明：以下操作必须要在如下权限下完成：

<uses-permission android:name="android.permission.MOUNT_UNMOUNT_FILESYSTEMS" />  <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />

实例：读写SDCard上的文件

package com.jiahui.sdcarddemo; import java.io.File; import java.io.FileInputStream; import java.io.FileNotFoundException; import java.io.FileOutputStream; import java.io.IOException; import java.io.InputStream; import java.io.OutputStream; import java.io.RandomAccessFile; import android.app.Activity; import android.os.Bundle; import android.os.Environment; import android.view.View; import android.widget.Button; import android.widget.EditText; import android.widget.TextView; public class SDCardDemoActivity extends Activity { private Button btnWrite, btnRead; private EditText edtContent; private TextView tvResult; private static final String FILE_NAME = "test.txt"; public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.main); edtContent = (EditText) this.findViewById(R.id.edtContent); tvResult = (TextView) this.findViewById(R.id.tvResult); btnRead = (Button) this.findViewById(R.id.btnRead); btnWrite = (Button) this.findViewById(R.id.btnWrite); btnWrite.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View v) { System.out.println("往SDCard里写数据"); // 先判断是否有SDCard System.out.println("Environment.getExternalStorageState():" + Environment.getExternalStorageState()); System.out.println("Environment.MEDIA_MOUNTED:" + Environment.MEDIA_MOUNTED); if (Environment.getExternalStorageState().equals( Environment.MEDIA_MOUNTED)) { String content = edtContent.getText().toString(); try { // 获取SDcard路径 File sdCardDir = Environment .getExternalStorageDirectory(); // SDCard目录：/mnt/sdcard String sdcardPath = sdCardDir.getAbsolutePath(); System.out.println("sdCardDir.getAbsolutePath()" + sdCardDir.getAbsolutePath()); File file = new File(sdCardDir, FILE_NAME); // File file = new File(sdcardPath // + File.separator + FILE_NAME); // 以指定文件创建RandomAccessFile对象 RandomAccessFile raf = new RandomAccessFile(file, "rw"); // 将文件记录指针移动最后 raf.seek(file.length()); // 输出文件内容 raf.write(content.getBytes()); raf.close(); } catch (Exception e) { // TODO: handle exception } } } }); btnRead.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View v) { if (Environment.getExternalStorageState().equals( Environment.MEDIA_MOUNTED)) { // 获取SDcard路径 StringBuilder sb = new StringBuilder(); try { File sdCardDir = Environment .getExternalStorageDirectory(); File file = new File(sdCardDir, FILE_NAME); InputStream inputStream = new FileInputStream(file); int len = 0; byte[] buffer = new byte[1024]; while ((len = inputStream.read(buffer)) != -1) { sb.append(new String(buffer, 0, len)); } tvResult.setText(sb.toString()); // 关闭流 inputStream.close(); } catch (Exception e) { // TODO Auto-generated catch block e.printStackTrace(); } } } }); } }

实现效果