求教！android的SD卡访问权限，已经在AndroidManifest里设置了，位置也提到最高了，可为什么还是访问不了？该如何解决

求教！android的SD卡访问权限，已经在AndroidManifest里设置了，位置也提到最高了，可为什么还是访问不了？
问题：已经在AndroidManifest.xml 里设置了访问SD卡的权限<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
但是运行程序的错误提示是：permission denied 没有访问权限！ 
这是为什么呢？
代码：package napo.utils;


import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.util.ArrayList;
import java.util.List;


import napo.model.Mp3Info;


import android.os.Environment;


public class FileUtils {
private String SDCardRoot;


public FileUtils() {
SDCardRoot = Environment.getExternalStorageDirectory()
.getAbsolutePath() + File.separator;
}
  //在SD卡上创建文件，文件名和那个目录
public File createFileInSDCard(String fileName, String dir)
throws IOException {
File file = new File(SDCardRoot + dir + File.separator + fileName);
System.out.print("file-->" + file);
file.createNewFile();
return file;
}
//在SD卡上创建一个目录
public File createSDDir(String dir) {
File dirFile = new File(SDCardRoot + dir + File.separator);
System.out.println(dirFile.mkdirs());
return dirFile;
}
//判断SD卡上的文件是否
public boolean isFileExist(String fileName, String path) {
File file = new File(SDCardRoot + path + File.separator + fileName);
return file.exists();
}


// 将一个inputstream里面的数据写入到SD卡中。
public File write2SDFromInput(String path, String fileName,
InputStream input) {
File file = null;
OutputStream output = null;
try {
createSDDir(path);
file = createFileInSDCard(fileName, path);
output = new FileOutputStream(file);
byte buffer[] = new byte[4 * 1024];
int temp;
while ((temp = input.read(buffer)) != -1) {
output.write(buffer, 0, temp);
}
output.flush();
} catch (Exception e) {
e.printStackTrace();
} finally {
try {
output.close();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
return file;
}


/**
* 读取目录中的Mp3文件的名字和大小
*/
public List<Mp3Info> getMp3Files(String path) {
List<Mp3Info> mp3Infos = new ArrayList<Mp3Info>();
File file = new File(SDCardRoot + File.separator + path);
// 返回当前文件夹中所有的对象
File[] files = file.listFiles();
for (int i = 0; i < files.length; i++) {
if (files[i].getName().endsWith("mp3")) {
Mp3Info mp3Info = new Mp3Info();
mp3Info.setMp3Name(files[i].getName());
mp3Info.setMp3Size(files[i].length() + "");
mp3Infos.add(mp3Info);
}
}
return mp3Infos;


}
}


log：
05-24 09:05:40.022: I/ActivityManager(70): Displayed activity your.napo/.Mp3player: 1571 ms (total 1571 ms)
05-24 09:05:41.852: I/System.out(550): false
05-24 09:05:41.852: W/System.err(550): java.io.IOException: Permission denied
05-24 09:05:41.852: W/System.err(550): at java.io.File.createNewFileImpl(Native Method)
05-24 09:05:41.862: W/System.err(550): at java.io.File.createNewFile(File.java:1160)
05-24 09:05:41.862: W/System.err(550): at napo.utils.FileUtils.createFileInSDCard(FileUtils.java:27)
05-24 09:05:41.862: W/System.err(550): at napo.utils.FileUtils.write2SDFromInput(FileUtils.java:49)
05-24 09:05:41.862: W/System.err(550): at napo.download.HttpDownload.downFile(HttpDownload.java:49)
05-24 09:05:41.862: W/System.err(550): at your.napo.service.DownloadService$DownloadThread.run(DownloadService.java:36)
05-24 09:05:41.862: W/System.err(550): at java.lang.Thread.run(Thread.java:1096)
05-24 09:05:41.862: W/System.err(550): java.lang.NullPointerException