android读写SD卡上的文件

androidManifest.xml的配置文件：

!-- 在SD卡中创建与删除文件权限 -->
  <uses-permission android:name="android.permission.MOUNT_UNMOUNT_FILESYSTEMS"/>
  <!-- 向SD卡写入数据权限 -->
  <uses-permission android:name="android.permission.WRITE_EXITERNAL_STORAGE"/>

package com.example.sdcarddemo;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.InputStreamReader;
import java.io.RandomAccessFile;
import java.util.RandomAccess;

import android.os.Bundle;
import android.os.Environment;
import android.app.Activity;
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 ="/SDCardDemo.txt";
  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    //获取两个按钮
    Button readButton = (Button) findViewById(R.id.readbtn);
    Button writeButton = (Button) findViewById(R.id.writebtn);
    //获取两个文本框
    final EditText edit1 = (EditText) findViewById(R.id.edit1);
    final EditText edit2 = (EditText) findViewById(R.id.edit2);
    //为两个按钮绑定事件监听器
    writeButton.setOnClickListener(new OnClickListener() {
      
      @Override
      public void onClick(View arg0) {
        // TODO Auto-generated method stub
        //将edit1中的内容写入文件中
        write(edit1.getText().toString());
        edit1.setText("");
      }
    });
    readButton.setOnClickListener(new OnClickListener() {
      
      @Override
      public void onClick(View arg0) {
        // TODO Auto-generated method stub
        //读取指定文件中的内容，并显示出来
        edit2.setText(read());
      }
    });
  }
  /**
   * Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)
   * 如果手机已插入SD卡，且应用程序具有读写SD卡的能力，上面语句返true
   * */
  private String read(){
    try {
      //如果手机插入SD卡，而且应用程序具有访问SD的权限
      if(Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)){
        //获取SD卡对应的储存目录
        File sdCardDir = Environment.getExternalStorageDirectory();
        //获取指定文件的输入流
        FileInputStream fis = new FileInputStream(sdCardDir.getCanonicalPath() + FILE_NAME);
        //将指定输入流包装成bufferedReader
        BufferedReader br = new BufferedReader(new InputStreamReader(fis));
        StringBuilder aBuilder = new StringBuilder("");
        String line = null;
        while((line = br.readLine()) != null){
          aBuilder.append(line);
        }
        //关闭资源
        br.close();
        return aBuilder.toString();
      }
    } catch (Exception e) {
      // TODO: handle exception
    }
    return null;
  }
  /**
   * 向SD卡的文件写入内容
   * 使用RandomAccessFile向Sd卡指定文件追加内容
   * 如果使用FileOutputStream向指定文件写入数据，
   * FileOutputStream会把原有文件的内容清空，不能追缴文件
   * 
   * */
  private void write(String content){
    try {
      //如果手机插入了SD卡，而且应用程序具有访问SD卡的权限
      if(Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)){
        //获取SD卡的目录
        File sdCardDir = Environment.getExternalStorageDirectory();
        File targetFile = new File(sdCardDir.getCanonicalPath() + FILE_NAME);
        //以指定文件创建RandomAccessFile对象
        RandomAccessFile raf = new RandomAccessFile(targetFile, "rw");
        //将文件记录指针移动到最后
        raf.seek(targetFile.length());
        //输出文件内容
        raf.write(content.getBytes());
        //关闭RandAccessFile
        raf.close();
      }
    } catch (Exception e) {
      // TODO: handle exception
    }
  }
  @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;
  }

}