我试图用asyncTask下载一个文件，但没有工作，没有错误消息或什么都没有，只是不下载文件。。。我尝试了所有的东西，但似乎有一段时间没有进入。。。有人知道问题出在哪里吗？我在手机上测试过，网址也可以。
class DownloadFileFromURL extends AsyncTask<String, String, String> {
/**
* Before starting background thread
* */
@Override
protected void onPreExecute() {
super.onPreExecute();
System.out.println("Starting download");
}
/**
* Downloading file in background thread
* */
@Override
protected String doInBackground(String... f_url) {
int count;
try {
String root = Environment.getExternalStorageDirectory().toString();
System.out.println("Downloading");
URL url = new URL(f_url[0]);
URLConnection conection = url.openConnection();
conection.connect();
// getting file length
int lenghtOfFile = conection.getContentLength();
// input stream to read file - with 8k buffer
InputStream input = new BufferedInputStream(url.openStream(), 8192);
// Output stream to write file
OutputStream output = new FileOutputStream(root+"/downloadedfile.jpg");
byte data[] = new byte[1024];
long total = 0;
while ((count = input.read(data)) != -1) {
total += count;
// writing data to file
output.write(data, 0, count);
}
// flushing output
output.flush();
// closing streams
output.close();
input.close();
} catch (Exception e) {
Log.e("Error: ", e.getMessage());
}
return null;
}
/**
* After completing background task
* **/
@Override
protected void onPostExecute(String file_url) {
System.out.println("Downloaded");
}
}
我刚刚运行了您的代码，它对我来说很好。图像已下载到SD卡中。
需要注意的是，请确保您在AndroidManifest.xml中设置了这些权限：
<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
以下是我得到的日志（注意我添加了一个ProgressDialog）：
03-21 16:53:46.422 21017-21017/com.imagedownload.danielnugent.imagedownload D/Activity﹕ #1 setTransGradationModeColor false
03-21 16:53:56.211 21017-21017/com.imagedownload.danielnugent.imagedownload D/Activity﹕ #1 setTransGradationModeColor false
03-21 16:54:06.441 21017-21017/com.imagedownload.danielnugent.imagedownload I/System.out﹕ Starting download
03-21 16:54:06.441 21017-21017/com.imagedownload.danielnugent.imagedownload D/Dialog﹕ checkMirrorLinkEnabled returns : false
03-21 16:54:06.441 21017-21017/com.imagedownload.danielnugent.imagedownload D/Dialog﹕ showing allowed
03-21 16:54:06.461 21017-25126/com.imagedownload.danielnugent.imagedownload I/System.out﹕ Downloading
03-21 16:54:06.461 21017-21017/com.imagedownload.danielnugent.imagedownload D/Activity﹕ #1 setTransGradationModeColor false
03-21 16:54:06.481 21017-21017/com.imagedownload.danielnugent.imagedownload D/ProgressBar﹕ updateDrawableBounds: left = 0
03-21 16:54:06.481 21017-21017/com.imagedownload.danielnugent.imagedownload D/ProgressBar﹕ updateDrawableBounds: top = 0
03-21 16:54:06.491 21017-21017/com.imagedownload.danielnugent.imagedownload D/ProgressBar﹕ updateDrawableBounds: right = 144
03-21 16:54:06.491 21017-21017/com.imagedownload.danielnugent.imagedownload D/ProgressBar﹕ updateDrawableBounds: bottom = 144
03-21 16:54:11.596 21017-21017/com.imagedownload.danielnugent.imagedownload I/System.out﹕ Downloaded
为了以防万一这会有用，下面是对我有用的完整MainActivity.java代码。（url是占位符）：
import android.app.ProgressDialog;
import android.os.AsyncTask;
import android.os.Environment;
import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.Menu;
import android.view.MenuItem;
import java.io.BufferedInputStream;
import java.io.FileOutputStream;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.URL;
import java.net.URLConnection;
public class MainActivity extends ActionBarActivity {
private ProgressDialog pDialog;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
new DownloadFileFromURL().execute("http://www.example.com/IMG.jpg");
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.menu_main, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId();
//noinspection SimplifiableIfStatement
if (id == R.id.action_settings) {
return true;
}
return super.onOptionsItemSelected(item);
}
class DownloadFileFromURL extends AsyncTask<String, String, String> {
/**
* Before starting background thread
* */
@Override
protected void onPreExecute() {
super.onPreExecute();
System.out.println("Starting download");
pDialog = new ProgressDialog(MainActivity.this);
pDialog.setMessage("Loading... Please wait...");
pDialog.setIndeterminate(false);
pDialog.setCancelable(false);
pDialog.show();
}
/**
* Downloading file in background thread
* */
@Override
protected String doInBackground(String... f_url) {
int count;
try {
String root = Environment.getExternalStorageDirectory().toString();
System.out.println("Downloading");
URL url = new URL(f_url[0]);
URLConnection conection = url.openConnection();
conection.connect();
// getting file length
int lenghtOfFile = conection.getContentLength();
// input stream to read file - with 8k buffer
InputStream input = new BufferedInputStream(url.openStream(), 8192);
// Output stream to write file
OutputStream output = new FileOutputStream(root+"/downloadedfile.jpg");
byte data[] = new byte[1024];
long total = 0;
while ((count = input.read(data)) != -1) {
total += count;
// writing data to file
output.write(data, 0, count);
}
// flushing output
output.flush();
// closing streams
output.close();
input.close();
} catch (Exception e) {
Log.e("Error: ", e.getMessage());
}
return null;
}
/**
* After completing background task
* **/
@Override
protected void onPostExecute(String file_url) {
System.out.println("Downloaded");
pDialog.dismiss();
}
}
}