添加权限：

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
    <uses-permission android:name="android.permission.MOUNT_UNMOUNT_FILESYSTEMS"/>  
    /** 获取SD卡路径 **/
    private static String getSDPath() {
        String sdcardPath = null;
        boolean sdCardExist = Environment.getExternalStorageState().equals(
                android.os.Environment.MEDIA_MOUNTED);  //判断sd卡是否存在
        if (sdCardExist) {
            sdcardPath = Environment.getExternalStorageDirectory();//获取根目录
        }
        if (sdcardPath != null) {
            return sdcardPath;
        } else {
            return "";
        }
    }

  解决方法：获取根目录的代码改为：
  sdcardPath = Environment.getExternalStorageDirectory().getAbsolutePath();

这样就可以了。

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附文件缓存类：

package com.net.utilities;

import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.util.Arrays;
import java.util.Comparator;

import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.Environment;
import android.os.StatFs;
import android.util.Log;

public class FileOperationUtil {
	private static final String TAG = "FileSaveUtil";
    private static final String CACHE_DIRECTORY = "CacheFile";
                                                            
    private static final int MB = 1024*1024;
    private static final int MAX_CACHE_SIZE = 10 * MB;
    private static final int LEAST_SIZE_OF_SDCARD = 10 * MB;
    
    /** 从缓存中获取图片 **/
    public static Bitmap getImage(final String imageUrl) {   
        final String path = getCacheDirectory() + "/" + convertUrlToFileName(imageUrl);
       // Log.i(TAG, "getImage filepath:" + path);
       // Log.i(TAG, "getImage url:" + url);
        File file = new File(path);
        if (file.exists()) {
//        	Log.i(TAG, "getImage file exists");
            Bitmap bmp = BitmapFactory.decodeFile(path);
            if (bmp == null) {
                file.delete();
            } else {
            	updateFileTime(path);
                return bmp;
            }
        }
        return null;
    }
                                                                
    /** 将图片存入文件缓存 **/
    public static void saveBitmap(String imageUrl, Bitmap bm ) {
        if (bm == null) {
            return;
        }
        //判断sdcard上的空间
        if (getFreeSpaceOfSdcard() < LEAST_SIZE_OF_SDCARD) {
            //SD空间不足
            return;
        }
        String filename = convertUrlToFileName(imageUrl);
        String dir = getCacheDirectory();
        File dirFile = new File(dir);
        if (!dirFile.exists()){
        	if(!dirFile.mkdirs()){
        		Log.w(TAG, "create cache file directorys failed");
        	}
        }
        File file = new File(dir +"/" + filename);
        try {
            file.createNewFile();
            OutputStream outStream = new FileOutputStream(file);
            bm.compress(Bitmap.CompressFormat.JPEG, 100, outStream);
            outStream.flush();
            outStream.close();
        } catch (FileNotFoundException e) {
            Log.w(TAG, "FileNotFoundException");
        } catch (IOException e) {
            Log.w(TAG, "IOException");
        }
    }
                                                                
    /**
     * 计算存储目录下的文件大小，
     * 当文件总大小大于指定的MAX_CACHE_SIZE或者sdcard剩余空间小于指定的LEAST_SIZE_OF_SDCARD
     * 那么删除40%最近没有被使用的文件
     */
    public static boolean removeExtraCache() {
        File dir = new File(getCacheDirectory());
        File[] files = dir.listFiles();
        if (files == null) {
            return true;
        }
        if (!android.os.Environment.getExternalStorageState().equals(
                android.os.Environment.MEDIA_MOUNTED)) {
            return false;
        }
                                                            
        int dirSize = 0;
        for (int i = 0; i < files.length; i++) {
        	dirSize += files[i].length();
        }
                                                            
        if (dirSize > MAX_CACHE_SIZE || getFreeSpaceOfSdcard() < LEAST_SIZE_OF_SDCARD) {
            int removeNum = (int) ((0.4 * files.length) + 1);
            /* 根据文件的最后修改时间进行升序排序 */
            Arrays.sort(files, new Comparator<File>() {
            	@Override
            	 public int compare(File file1, File file2) {
                     if (file1.lastModified() > file2.lastModified()) {
                         return 1;
                     } else if (file1.lastModified() == file2.lastModified()) {
                         return 0;
                     } else {
                         return -1;
                     }
                 }
			});
            for (int i = 0; i < removeNum; i++) {
            	files[i].delete();
            }
            return true;
        }else{
        	return false;
        }
    }
                                                                
    /** 修改文件的最后修改时间 **/
    public static void updateFileTime(String path) {
        File file = new File(path);
        long newModifiedTime = System.currentTimeMillis();
        file.setLastModified(newModifiedTime);
    }
                                                                
    /** 计算sdcard上的剩余空间 **/
    private static int getFreeSpaceOfSdcard() {
        StatFs stat = new StatFs(Environment.getExternalStorageDirectory().getPath());
        double sdFreeSize = ((double)stat.getAvailableBlocks() * (double) stat.getBlockSize());
        return (int) sdFreeSize;
    }
                                                                
    private static String convertUrlToFileName(String url) {
        String[] strs = url.split("/");
        String savedImageName = strs[strs.length - 1];
        return savedImageName;
    }
                                                                
    /** 获得缓存目录 **/
    private static String getCacheDirectory() {
        String dir = getSDPath() + "/" + CACHE_DIRECTORY;
        return dir;
    }
     
    /** 获取SD卡路径 **/
    private static String getSDPath() {
        String sdcardPath = null;
        boolean sdCardExist = Environment.getExternalStorageState().equals(
                android.os.Environment.MEDIA_MOUNTED);  //判断sd卡是否存在
        if (sdCardExist) {
            sdcardPath = Environment.getExternalStorageDirectory().getAbsolutePath();  //获取根目录
        }
        if (sdcardPath != null) {
            return sdcardPath;
        } else {
            return "";
        }
    }
}

SD卡创建文件夹失败，解决办法及文件缓存