android学习笔记47——读写SD卡上的文件

读写SD卡上的文件

通过Context的openFileInput、openFileOutput来打开文件输入流、输出流时，程序打开的都是应用程序的数据文件夹里的文件，其存储的文件大小可能都比较有限——手机内存所限。

SD卡可以更好的存、取应用程序的大小文件数据。SD卡可以大大扩充手机的存储能力。

读、写SD卡上的文件的操作步骤如下：

1.调用Environment的getExternalStorageState()方法判断手机上是否插入了SD卡， 并且应用程序具有读写SD卡的权限。例如使用如下代码：

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

2.调用Environment的getExternalStorageDirectory()方法来获取外部存储器，也就是SD卡的目录；

3.使用FileInputStream、FileOutputStream、FileReader/FileWriter读、写SD卡里面的文件。

注意：

应用程序读、写SD卡的文件有如下两个注意点：

1.手机上应该已插入SD卡。对于模拟器来说，可通过mksdcard命令来创建虚拟存储卡。

2.为了读、写SD卡上的数据，必须在应用程序的清单文件(AndroidMainfest.xml)中添加读写SD卡的权限.

 Eg：

实例如下：

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布局文件==》 <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"     xmlns:tools="http://schemas.android.com/tools"     android:layout_width="match_parent"     android:layout_height="match_parent"     android:orientation="vertical"     tools:context=".MainActivity" >       <EditText         android:id="@+id/edit1"         android:layout_width="match_parent"         android:layout_height="wrap_content" />       <Button         android:id="@+id/btnWrite"         android:layout_width="wrap_content"         android:layout_height="wrap_content"         android:text="write" />       <EditText         android:id="@+id/edit2"         android:layout_width="match_parent"         android:layout_height="wrap_content" />       <Button         android:id="@+id/btnRead"         android:layout_width="wrap_content"         android:layout_height="wrap_content"         android:text="read" />   </LinearLayout>   代码实现==》

 

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package com.example.mysdcard;   import java.io.BufferedReader; import java.io.File; import java.io.FileReader; import java.io.RandomAccessFile;   import android.os.Bundle; import android.os.Environment; import android.app.Activity; import android.util.Log; import android.view.Menu; import android.view.View; import android.view.View.OnClickListener; import android.widget.Button; import android.widget.EditText;   public class MainActivity extends Activity {     final String FILE_NAME = "/crazyit";     EditText Edit1;     EditText Edit2;       @Override     protected void onCreate(Bundle savedInstanceState)     {         super.onCreate(savedInstanceState);         setContentView(R.layout.activity_main);           Button btnWrite = (Button) this.findViewById(R.id.btnWrite);         Button btnRead = (Button) this.findViewById(R.id.btnRead);         Edit1 = (EditText) this.findViewById(R.id.edit1);         Edit2 = (EditText) this.findViewById(R.id.edit2);         btnWrite.setOnClickListener(new MyButtonClick());         btnRead.setOnClickListener(new MyButtonClick());     }       private class MyButtonClick implements OnClickListener     {         @Override         public void onClick(View v)         {             switch (v.getId())             {             case R.id.btnWrite:                 Log.i("swg", "write==" + Edit1.getText());                 write(Edit1.getText().toString());                 Edit1.setText("");                 break;             case R.id.btnRead:                 Log.i("swg", "read==" + read());                 Edit2.setText(read());                 break;             }         }           private String read()         {             try             {                 File file = new File(Environment.getExternalStorageDirectory(), FILE_NAME);                 BufferedReader br = new BufferedReader(new FileReader(file));                 String readline = "";                 StringBuffer sb = new StringBuffer();                 while ((readline = br.readLine()) != null)                 {                     System.out.println("readline:" + readline);                     sb.append(readline);                 }                 br.close();                 System.out.println("读取成功：" + sb.toString());             } catch (Exception e)             {                 e.printStackTrace();             }             return null;         }           private void write(String context)         {             try             {                 Log.i("swg", "write");                 // 如果手机插入SD卡，且应用程序具有访问权限                 boolean value =Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED);                 Log.i("swg", "value==" + value);                 if (Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED))                 {                     Log.i("swg", "sd exist ok");                     // 获取SD卡对应的存储目录                     File sdcardDir = Environment.getExternalStorageDirectory();                     String filePath = sdcardDir.getCanonicalPath() + FILE_NAME;                     Log.i("swg", filePath);                     File targerFile = new File(filePath);                     // 以拟定文件创建RandomAccessFile对象                     RandomAccessFile raf = new RandomAccessFile(targerFile, "rw");                     raf.seek(targerFile.length());                     raf.write(context.getBytes());                     raf.close();                 } else                     Log.i("swg", "sd is not op");             } catch (Exception e)             {                 e.printStackTrace();             }         }     }       @Override     public boolean onCreateOptionsMenu(Menu menu)     {         // Inflate the menu; this adds items to the action bar if it is present.         getMenuInflater().inflate(R.menu.main, menu);         return true;     }   }