Android Java - 如何从URL下载zip文件？

Question

嘿我做了一个新的项目，需要你从我的Dropbox下载一些文件。我添加了一个名为DownloadFile的新类，它具有下载文件的代码。出于某种原因，当我点击下载时，应用程序崩溃。谢谢。

继承人的DownloadFile：

package com.Matt7262.download.app; 

    import android.support.v7.app.ActionBarActivity; 
    import android.os.Bundle; 
    import android.view.Menu; 
    import android.view.MenuItem; 
    //Chat bot library 
    import org.alicebot.ab.Chat; 
    import org.alicebot.ab.Bot; 

    import android.view.View; 
    import android.widget.Button; 
    import android.os.Environment; 
    import android.widget.TextView; 
    import android.widget.Toast; 

    import java.io.InputStream; 
    import java.net.URL; 
    import java.io.DataInputStream; 
    import java.io.DataOutputStream; 
    import java.io.File; 
    import java.io.FileNotFoundException; 
    import java.io.FileOutputStream; 
    import java.io.IOException; 
    import java.net.HttpURLConnection; 
    import java.net.MalformedURLException; 
    import android.widget.ProgressBar; 
    import java.net.URL; 
    import java.net.URLConnection; 
    import android.app.Activity; 
    import android.app.ProgressDialog; 
    import android.os.Environment; 
    import android.view.View.OnClickListener; 

    public class DownloadFile extends ActionBarActivity{ 

    public void updateProgress(int currentSize, int totalSize) 
    { 
     Toast.makeText(getApplicationContext(), "Loading Files...", 
       Toast.LENGTH_SHORT).show(); 
    } 

    public void Download() 
    { 
     try { 
      //set the download URL, a url that points to a file on the internet 
      //this is the file to be downloaded 
      URL url = new URL("https://dl.dropboxusercontent.com/shz/9cyfz0b45mj6szr/7pBuupNz3N/xecta?token_hash=AAEs9cDFswt98D1IhLnab4dHwhwh5z2Lmhq_N6H-2M0LWg&top_level_offset=6"); 

      //create the new connection 
      HttpURLConnection urlConnection = (HttpURLConnection) url.openConnection(); 

      //set up some things on the connection 
      urlConnection.setRequestMethod("GET"); 
      urlConnection.setDoOutput(true); 

      //and connect! 
      urlConnection.connect(); 

      //set the path where we want to save the file 
      //in this case, going to save it on the root directory of the 
      //sd card. 
      File SDCardRoot = Environment.getExternalStorageDirectory(); 
      //create a new file, specifying the path, and the filename 
      //which we want to save the file as. 
      File file = new File(SDCardRoot,"hello.zip"); 

      //this will be used to write the downloaded data into the file we created 
      FileOutputStream fileOutput = new FileOutputStream(file); 

      //this will be used in reading the data from the internet 
      InputStream inputStream = urlConnection.getInputStream(); 

      //this is the total size of the file 
      int totalSize = urlConnection.getContentLength(); 
      //variable to store total downloaded bytes 
      int downloadedSize = 0; 

      //create a buffer... 
      byte[] buffer = new byte[1024]; 
      int bufferLength = 0; //used to store a temporary size of the buffer 

      //now, read through the input buffer and write the contents to the file 
      while ((bufferLength = inputStream.read(buffer)) > 0) { 
       //add the data in the buffer to the file in the file output stream (the file on the sd card 
       fileOutput.write(buffer, 0, bufferLength); 
       //add up the size so we know how much is downloaded 
       downloadedSize += bufferLength; 
       //this is where you would do something to report the prgress, like this maybe 
       updateProgress(downloadedSize, totalSize); 

      } 
      //close the output stream when done 
      fileOutput.close(); 

      //catch some possible errors... 
     } catch (MalformedURLException e) { 
      e.printStackTrace(); 
     } catch (IOException e) { 
      e.printStackTrace(); 
     } 
    } 
    } 

MainActivity：

package com.Matt7262.download.app; 

import android.support.v7.app.ActionBarActivity; 
import android.os.Bundle; 
import android.view.Menu; 
import android.view.MenuItem; 
//Chat bot library 
import org.alicebot.ab.Chat; 
import org.alicebot.ab.Bot; 

import android.view.View; 
import android.widget.Button; 
import android.os.Environment; 
import android.widget.TextView; 
import android.widget.Toast; 

import android.os.AsyncTask; 

public class MainActivity extends ActionBarActivity { 

    TextView input; 
    String dPath = Environment.getExternalStorageDirectory().getAbsolutePath() + "/Download"; 
    private DownloadFile df; 

    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_main); 
    } 

    //EditText mEdit = (EditText)findViewById(R.id.editText1); 

    public void buttonOnClick(View v) 
    { 
     input = (TextView) findViewById(R.id.editText1); 

     String dbPath = Environment.getExternalStorageDirectory().getAbsolutePath() + "/Download/Ab"; 

     Button button=(Button) v; 

     //Creating bot 
     String botname="xecta"; 
     String path= dbPath; 
     Bot xecta = new Bot(botname, path); 

     Chat chatSession = new Chat(xecta); 

     String request = input.getText().toString(); 
     String response = chatSession.multisentenceRespond(request); 
     ((Button) v).setText(response); 
    } 

    public void onClickDownload(View view) 
    { 
     df.Download(); 
    } 

    @Override 
    public boolean onCreateOptionsMenu(Menu menu) { 

     // Inflate the menu; this adds items to the action bar if it is present. 
     getMenuInflater().inflate(R.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(); 
     if (id == R.id.action_settings) { 
      return true; 
     } 
     return super.onOptionsItemSelected(item); 
    } 

    /*public void updateProgress(int currentSize, int totalSize) 
    { 
     Toast.makeText(getApplicationContext(), "Retrieving Files...", 
       Toast.LENGTH_SHORT).show(); 
    }/* 

    /*public void Download() 
    { 
     try { 
      //set the download URL, a url that points to a file on the internet 
      //this is the file to be downloaded 
      URL url = new URL("https://dl.dropboxusercontent.com/shz/9cyfz0b45mj6szr/7pBuupNz3N/xecta?token_hash=AAEs9cDFswt98D1IhLnab4dHwhwh5z2Lmhq_N6H-2M0LWg&top_level_offset=6"); 

      //create the new connection 
      HttpURLConnection urlConnection = (HttpURLConnection) url.openConnection(); 

      //set up some things on the connection 
      urlConnection.setRequestMethod("GET"); 
      urlConnection.setDoOutput(true); 

      //and connect! 
      urlConnection.connect(); 

      //set the path where we want to save the file 
      //in this case, going to save it on the root directory of the 
      //sd card. 
      File SDCardRoot = Environment.getExternalStorageDirectory(); 
      //create a new file, specifying the path, and the filename 
      //which we want to save the file as. 
      File file = new File(SDCardRoot,"hello.zip"); 

      //this will be used to write the downloaded data into the file we created 
      FileOutputStream fileOutput = new FileOutputStream(file); 

      //this will be used in reading the data from the internet 
      InputStream inputStream = urlConnection.getInputStream(); 

      //this is the total size of the file 
      int totalSize = urlConnection.getContentLength(); 
      //variable to store total downloaded bytes 
      int downloadedSize = 0; 

      //create a buffer... 
      byte[] buffer = new byte[1024]; 
      int bufferLength = 0; //used to store a temporary size of the buffer 

      //now, read through the input buffer and write the contents to the file 
      while ((bufferLength = inputStream.read(buffer)) > 0) { 
       //add the data in the buffer to the file in the file output stream (the file on the sd card 
       fileOutput.write(buffer, 0, bufferLength); 
       //add up the size so we know how much is downloaded 
       downloadedSize += bufferLength; 
       //this is where you would do something to report the prgress, like this maybe 
       updateProgress(downloadedSize, totalSize); 

      } 
      //close the output stream when done 
      fileOutput.close(); 

     //catch some possible errors... 
     } catch (MalformedURLException e) { 
      e.printStackTrace(); 
     } catch (IOException e) { 
      e.printStackTrace(); 
     } 
    }*/ 


} 

我觉得这是我走出的logcat的：

logcat

Answer 1

您声明DownloadFile对象但没有初始化它。

private DownloadFile df; 
df.Download(); // Throws NPE 

不要忘记初始化它。

private DownloadFile df = new DownloadFile(); 
df.Download(); 

编辑：

现在你初始化对象，避免了NPE，但这个时候你得到一个NetworkOnMainThreadException。 Honeycomb版本之后，Android不允许您在主线程上执行网络操作。您可以使用AsyncTask来解决这个问题。