我正在使用以下代码从服务器下载文件，然后将其写入SD卡的根目录，一切正常：

package com.downloader;

import java.io.File;
import java.io.FileOutputStream;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.URL;

import android.os.Environment;
import android.util.Log;

public class Downloader {

    public void DownloadFile(String fileURL, String fileName) {
        try {
            File root = Environment.getExternalStorageDirectory();
            URL u = new URL(fileURL);
            HttpURLConnection c = (HttpURLConnection) u.openConnection();
            c.setRequestMethod("GET");
            c.setDoOutput(true);
            c.connect();
            FileOutputStream f = new FileOutputStream(new File(root, fileName));

            InputStream in = c.getInputStream();

            byte[] buffer = new byte[1024];
            int len1 = 0;
            while ((len1 = in.read(buffer)) > 0) {
                f.write(buffer, 0, len1);
            }
            f.close();
        } catch (Exception e) {
            Log.d("Downloader", e.getMessage());
        }

    }
}

但是，使用 Environment.getExternalStorageDirectory();表示文件将始终写入root /mnt/sdcard。是否可以指定某个文件夹来写入文件？

例如： /mnt/sdcard/myapp/downloads

File sdCard = Environment.getExternalStorageDirectory();
File dir = new File (sdCard.getAbsolutePath() + "/dir1/dir2");
dir.mkdirs();
File file = new File(dir, "filename");

FileOutputStream f = new FileOutputStream(file);
...

向Android清单添加权限

将此WRITE_EXTERNAL_STORAGE权限添加到您的应用程序清单中。

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="your.company.package"
    android:versionCode="1"
    android:versionName="0.1">
    <application android:icon="@drawable/icon" android:label="@string/app_name">
        <!-- ... -->
    </application>
    <uses-sdk android:minSdkVersion="7" />
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
</manifest> 

检查外部存储设备的可用性

您应该始终先检查可用性。外部存储设备上的android官方文档的摘录。

boolean mExternalStorageAvailable = false;
boolean mExternalStorageWriteable = false;
String state = Environment.getExternalStorageState();

if (Environment.MEDIA_MOUNTED.equals(state)) {
    // We can read and write the media
    mExternalStorageAvailable = mExternalStorageWriteable = true;
} else if (Environment.MEDIA_MOUNTED_READ_ONLY.equals(state)) {
    // We can only read the media
    mExternalStorageAvailable = true;
    mExternalStorageWriteable = false;
} else {
    // Something else is wrong. It may be one of many other states, but all we need
    //  to know is we can neither read nor write
    mExternalStorageAvailable = mExternalStorageWriteable = false;
}

使用文件编写器

最后但并非最不重要的是，忘记了，FileOutputStream而改用a FileWriter。有关该类的更多信息，请参见FileWriter javadoc。您可能想要在此处添加一些其他错误处理来通知用户。

// get external storage file reference
FileWriter writer = new FileWriter(getExternalStorageDirectory()); 
// Writes the content to the file
writer.write("This\n is\n an\n example\n"); 
writer.flush();
writer.close();

在这里找到答案-http: //mytechead.wordpress.com/2014/01/30/android-create-a-file-and-write-to-external-storage/

它说，

/**

* Method to check if user has permissions to write on external storage or not

*/

public static boolean canWriteOnExternalStorage() {
   // get the state of your external storage
   String state = Environment.getExternalStorageState();
   if (Environment.MEDIA_MOUNTED.equals(state)) {
    // if storage is mounted return true
      Log.v("sTag", "Yes, can write to external storage.");
      return true;
   }
   return false;
}

然后让我们使用此代码实际写入外部存储：

// get the path to sdcard
File sdcard = Environment.getExternalStorageDirectory();
// to this path add a new directory path
File dir = new File(sdcard.getAbsolutePath() + "/your-dir-name/");
// create this directory if not already created
dir.mkdir();
// create the file in which we will write the contents
File file = new File(dir, "My-File-Name.txt");
FileOutputStream os = outStream = new FileOutputStream(file);
String data = "This is the content of my file";
os.write(data.getBytes());
os.close();

就是这样。如果现在访问/ sdcard / your-dir-name /文件夹，您将看到一个名为-My-File-Name.txt的文件，其内容与代码中指定的相同。

PS：-您需要以下权限-

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

为了将文件下载到SDCard中的“下载”或“音乐”文件夹中

File downlodDir = Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOWNLOADS);// or DIRECTORY_PICTURES

并且不要忘记在清单中添加这些权限

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