开发一个将图像下载到用户手机的应用程序。下载部分正常工作，图像存储在＆＃34; / Android / data // files /＆＃34;。我首先检查文件是否存在，如果没有，那么我想下载它。但是当我运行应用程序时，它会继续抛出NullpointerException。是因为＆＃34; / Android / data // files /＆＃34; 在此期间不存在第一次运行时间？如果是，我如何解决这个问题。
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
testtv = (TextView) findViewById(R.id.testtv);
String imagename = "/img11.png";
File image = new File(Environment.getExternalStorageDirectory()
.getAbsolutePath()
+ "/Android/data/com.id.imagedownloader/files" + imagename);
// testtv.setText(image.toString());
if (image.exists()) {
testtv.setText("file exists");
} else {
Boolean result = isDownloadManagerAvailable(getApplicationContext());
if (result) {
downloadFile(imagename);
}
}
}
@SuppressLint("NewApi")
public void downloadFile(String imagename) {
// TODO Auto-generated method stub
String DownloadUrl = "http://testing16.comlu.com/images/" + imagename;
DownloadManager.Request request = new DownloadManager.Request(
Uri.parse(DownloadUrl));
request.setDescription("sample file for testing"); // appears the same
// in Notification
// bar while
// downloading
request.setTitle("Test Title");
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) {
request.allowScanningByMediaScanner();
request.setNotificationVisibility(DownloadManager.Request.VISIBILITY_VISIBLE_NOTIFY_COMPLETED);
}
String fileName = DownloadUrl.substring(
DownloadUrl.lastIndexOf('/') + 1, DownloadUrl.length());
request.setDestinationInExternalFilesDir(getApplicationContext(), null,
fileName);
// get download service and enqueue file
DownloadManager manager = (DownloadManager) getSystemService(Context.DOWNLOAD_SERVICE);
manager.enqueue(request);
}
public static boolean isDownloadManagerAvailable(Context context) {
try {
if (Build.VERSION.SDK_INT < Build.VERSION_CODES.GINGERBREAD) {
return false;
}
Intent intent = new Intent(Intent.ACTION_MAIN);
intent.addCategory(Intent.CATEGORY_LAUNCHER);
intent.setClassName("com.android.providers.downloads.ui",
"com.android.providers.downloads.ui.DownloadList");
List<ResolveInfo> list = context.getPackageManager()
.queryIntentActivities(intent,
PackageManager.MATCH_DEFAULT_ONLY);
return list.size() > 0;
} catch (Exception e) {
return false;
}
}
logcat的
11-03 14:13:35.821: E/AndroidRuntime(771): FATAL EXCEPTION: main
11-03 14:13:35.821: E/AndroidRuntime(771): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.id.imagedownloader/com.id.imagedownloader.MainActivity}: java.lang.NullPointerException: file
11-03 14:13:35.821: E/AndroidRuntime(771): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1955)
11-03 14:13:35.821: E/AndroidRuntime(771): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1980)
11-03 14:13:35.821: E/AndroidRuntime(771): at android.app.ActivityThread.access$600(ActivityThread.java:122)
11-03 14:13:35.821: E/AndroidRuntime(771): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1146)
11-03 14:13:35.821: E/AndroidRuntime(771): at android.os.Handler.dispatchMessage(Handler.java:99)
11-03 14:13:35.821: E/AndroidRuntime(771): at android.os.Looper.loop(Looper.java:137)
11-03 14:13:35.821: E/AndroidRuntime(771): at android.app.ActivityThread.main(ActivityThread.java:4340)
11-03 14:13:35.821: E/AndroidRuntime(771): at java.lang.reflect.Method.invokeNative(Native Method)
11-03 14:13:35.821: E/AndroidRuntime(771): at java.lang.reflect.Method.invoke(Method.java:511)
11-03 14:13:35.821: E/AndroidRuntime(771): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
11-03 14:13:35.821: E/AndroidRuntime(771): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
11-03 14:13:35.821: E/AndroidRuntime(771): at dalvik.system.NativeStart.main(Native Method)
11-03 14:13:35.821: E/AndroidRuntime(771): Caused by: java.lang.NullPointerException: file
11-03 14:13:35.821: E/AndroidRuntime(771): at android.net.Uri.fromFile(Uri.java:441)
11-03 14:13:35.821: E/AndroidRuntime(771): at android.app.DownloadManager$Request.setDestinationFromBase(DownloadManager.java:504)
11-03 14:13:35.821: E/AndroidRuntime(771): at android.app.DownloadManager$Request.setDestinationInExternalFilesDir(DownloadManager.java:466)
11-03 14:13:35.821: E/AndroidRuntime(771): at com.id.imagedownloader.MainActivity.downloadFile(MainActivity.java:63)
11-03 14:13:35.821: E/AndroidRuntime(771): at com.id.imagedownloader.MainActivity.onCreate(MainActivity.java:40)
11-03 14:13:35.821: E/AndroidRuntime(771): at android.app.Activity.performCreate(Activity.java:4465)
11-03 14:13:35.821: E/AndroidRuntime(771): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1049)
11-03 14:13:35.821: E/AndroidRuntime(771): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1919)
11-03 14:13:35.821: E/AndroidRuntime(771): ... 11 more
答案 0 :(得分：0)
将下载文件的本地目标设置为应用程序外部文件目录中的路径
request.setDestinationInExternalFilesDir(context,Environment.DIRECTORY_DOWNLOADS,image);
如果父文件夹不存在，请不要忘记使用mkdirs（）创建目录
答案 1 :(得分：0)
setDestinationInExternalFilesDir需要dirType参数。您应该像以下一样使用它：
request.setDestinationInExternalFilesDir(getApplicationContext(), Environment.DIRECTORY_DOWNLOADS,
fileName);
答案 2 :(得分：-1)
使用此功能，您可以从字节数组创建文件并检查是否存在。尝试这样的事情：
/**
* Create new file from byte array
* @param path
* @param array source
* @throws IOException
*/
public static void writeFile(String path, byte[] array) throws IOException {
File file = new File(path);
if (!file.exists() && array != null) {
BufferedOutputStream bos = new BufferedOutputStream(new FileOutputStream(file));
bos.write(array);
bos.flush();
bos.close();
} else {
// the file exists...
}
}
希望它对你有所帮助!!